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Tiger

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For other uses, see Tiger (disambiguation).

Tiger
A Bengal Tiger (P. tigris tigris) in India's Bandhavgarh National Park.
Conservation status
Endangered (IUCN 3.1)[1]
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Family: Felidae Genus: Panthera Species: P. tigris
Binomial name
Panthera tigris (Linnaeus, 1758)
Historical distribution of tigers (pale yellow) and 2006 (green).[2]
Subspecies
P. t. bengalensis P. t. corbetti Panthera tigris jacksoni P. t. sumatrae Panthera tigris altaica Panthera tigris amoyensis †P. t. balica †P. t. sondaica
Synonyms
Felis tigris Linnaeus, 1758[3] Tigris striatus Severtzov, 1858 Tigris regalis Gray, 1867

The tiger (Panthera tigris) is a member of the Felidae family; the largest of the four "big cats" in the genus Panthera.^[4] Native to much of eastern and southern Asia, the tiger is an apex predator and an obligate carnivore. Reaching up to 4 metres (13 ft) in total length and weighing up to 300 kilograms (660 pounds), the larger tiger subspecies are comparable in size to the biggest extinct felids.^[5][6] Aside from their great bulk and power, their most recognizable feature is the pattern of dark vertical stripes that overlays near-white to reddish-orange fur, with lighter underparts. The largest subspecies of tiger is the Siberian tiger.

Highly adaptable, tigers range from the Siberian taiga, to open grasslands, to tropical mangrove swamps. They are territorial and generally solitary animals, often requiring large contiguous areas of habitat that support their prey demands. This, coupled with the fact that they are endemic to some of the more densely populated places on earth, has caused significant conflicts with humans. Of the nine subspecies of modern tiger, three are extinct and the remaining six are classified as endangered, some critically so. The primary direct causes are habitat destruction and fragmentation, and hunting. Their historical range, which once reached from Mesopotamia and the Caucasus through most of South and East Asia, has been radically reduced. While all surviving species are under formal protection, poaching, habitat destruction and inbreeding depression continue to be threats.

Nonetheless, tigers are among the most recognizable and popular of the world's charismatic megafauna. They have featured prominently in ancient mythology and folklore, and continue to be depicted in modern films and literature. Tigers appear on many flags and coats of arms, as mascots for sporting teams, and as the national animal of several Asian nations.

Naming and etymology

The word "tiger" is taken from the Greek word "tigris", which is possibly derived from a Persian source meaning "arrow", a reference to the animal's speed and also the origin for the name of the River Tigris.^[7][8] In American English, "Tigress" was first recorded in 1611. It was one of the many species originally described, as Felis tigris, by Linnaeus in his 18th century work, Systema Naturae.^[3][9] The generic component of its scientific designation, Panthera tigris, is often presumed to derive from Greek pan- ("all") and theron ("beast"), but this may be a folk etymology. Although it came into English through the classical languages, panthera is probably of East Asian origin, meaning "the yellowish animal," or "whitish-yellow".^[10]

A group of tigers^[11] is rare (see below), but when seen together is termed a 'streak' or an 'ambush'.

Range of the tiger including the western part 1900 and 1990

Range

In the historical past, tigers were widespread in Asia, from the Caucasus and the Caspian Sea, to Siberia and Indonesia. During the 19th century the striped cats completely vanished from western Asia, and became restricted to isolated pockets in the remaining parts of their range. Today, this fragmented relic range extends from India in the west to China and Southeast Asia in the east. The northern limit is close to the Amur River in south eastern Siberia. The only large island inhabited by tigers today is Sumatra. Tigers vanished from Java and Bali during the 20th century, and in Borneo are known only from fossil remains.

Physical characteristics, taxonomy and evolution

The oldest remains of a tiger-like cat, called Panthera palaeosinensis, have been found in China and Java. This species lived about 2 million years ago, at the beginning of the Pleistocene, and was smaller than a modern tiger. The earliest fossils of true tigers are known from Java, and are between 1.6 and 1.8 million years old. Distinct fossils from the early and middle Pleistocene were also discovered in deposits from China, and Sumatra. A subspecies called the Trinil tiger (Panthera tigris trinilensis) lived about 1.2 million years ago and is known fossils found at Trinil in Java.^[12]

Tigers first reached India and northern Asia in the late Pleistocene, reaching eastern Beringia (but not the American Continent), Japan, and Sakhalin. Fossils found in Japan indicate that the local tigers were, like the surviving island subspecies, smaller than the mainland forms. This may be due to the phenomenon in which body size is related to environmental space (see insular dwarfism), or perhaps the availability of prey. Until the Holocene, tigers also lived in Borneo, as well as on the island of Palawan in the Philippines.^[13]

Physical characteristics

Siberian tiger

Tigers are perhaps the most recognisable of all the cats (with the possible exception of the lion). They typically have rusty-reddish to brown-rusty coats, a whitish medial and ventral area, a white "fringe" that surrounds the face, and stripes that vary from brown or gray to pure black. The form and density of stripes differs between subspecies (as well as the ground coloration of the fur; for instance, Siberian tigers are usually paler than other tiger subspecies), but most tigers have over 100 stripes. The pattern of stripes is unique to each animal, and thus could potentially be used to identify individuals, much in the same way as fingerprints are used to identify people. This is not, however, a preferred method of identification, due to the difficulty of recording the stripe pattern of a wild tiger. It seems likely that the function of stripes is camouflage, serving to help tigers conceal themselves amongst the dappled shadows and long grass of their environment as they stalk their prey. The stripe pattern is found on a tiger's skin and if shaved, its distinctive camouflage pattern would be preserved. Like other big cats, tigers have a white spot on the backs of their ears.

Skeleton

Tigers have the additional distinction of being the heaviest cats found in the wild.^[14] They also have powerfully built legs and shoulders, with the result that they, like lions, have the ability to pull down prey substantially heavier than themselves. However, the subspecies differ markedly in size, tending to increase proportionally with latitude, as predicted by Bergmann's Rule. Thus, large male Siberian Tigers (Panthera tigris altaica) can reach a total length of 3.5 m "over curves" (3.3 m. "between pegs") and a weight of 306 kilograms,^[15] which is considerably larger than the sizes reached by island-dwelling tigers such as the Sumatran, the smallest living subspecies with a body weight of only 75-140 kg.^[15] Tigresses are smaller than the males in each subspecies, although the size difference between male and female tigers tends to be more pronounced in the larger subspecies of tiger, with males weighing up to 1.7 times as much as the females.^[16] In addition, male tigers have wider forepaw pads than females. This difference is often used by biologists in determining the gender of tigers when observing their tracks.^[17] The skull of the tiger is very similar to that of the lion, though the frontal region is usually not as depressed or flattened, with a slightly longer postorbital region. The lion's skull has broader nasal openings. However, due to the amount of skull variation in the two species, usually, only the structure of the lower jaw can be used as a reliable indicator of species.^[18]

Subspecies

There are eight recent subspecies of tiger, two of which are extinct. Their historical range (severely diminished today) ran through Bangladesh, Siberia, Iran, Afghanistan, India, China, and southeast Asia, including some Indonesian islands. The surviving subspecies, in descending order of wild population, are:

Bengal tiger

• The Bengal tiger or the Royal Bengal tiger (Panthera tigris tigris) is found in parts of India, Bangladesh, Nepal, Bhutan, and Burma. It lives in varied habitats: grasslands, subtropical and tropical rainforests, scrub forests, wet and dry deciduous forests, and mangroves. Males in the wild usually weigh 205 to 227 kg (450–500 lb), while the average female will weigh about 141 kg.^[19] However, the northern Indian and the Nepalese Bengal tigers are somewhat bulkier than those found in the south of the Indian Subcontinent, with males averaging around 235 kg (518 lb).^[19] While conservationists already believed the population to be below 2,000,^[20] the most recent audit by the Indian Government's National Tiger Conservation Authority has estimated the number at just 1,411 wild tigers (1165–1657 allowing for statistical error), a drop of 60% in the past decade.^[21] Since 1972, there has been a massive wildlife conservation project, known as Project Tiger, to protect the Bengal tiger. The project is considered as one of the most successful wildlife conservation programs^{[citation needed]}, though at least one Tiger Reserve (Sariska Tiger Reserve) has lost its entire tiger population to poaching.^[22]

Indochinese tiger

• The Indochinese tiger (Panthera tigris corbetti), also called Corbett's tiger, is found in Cambodia, China, Laos, Burma, Thailand, and Vietnam. These tigers are smaller and darker than Bengal tigers: Males weigh from 150–190 kg (330–420 lb) while females are smaller at 110–140 kg (242–308 lb). Their preferred habitat is forests in mountainous or hilly regions. Estimates of the Indochinese tiger population vary between 1,200 to 1,800, with only several hundred left in the wild. All existing populations are at extreme risk from poaching, prey depletion as a result of poaching of primary prey species such as deer and wild pigs, habitat fragmentation and inbreeding. In Vietnam, almost three-quarters of the tigers killed provide stock for Chinese pharmacies.

Malayan tiger

• The Malayan tiger (Panthera tigris jacksoni), exclusively found in the southern part of the Malay Peninsula, was not considered a subspecies in its own right until 2004. The new classification came about after a study by Luo et al. from the Laboratory of Genomic Diversity Study,^[23] part of the National Cancer Institute of the United States. Recent counts showed there are 600–800 tigers in the wild, making it the third largest tiger population, behind the Bengal tiger and the Indochinese tiger. The Malayan tiger is the smallest of the mainland tiger subspecies, and the second smallest living subspecies, with males averaging about 120 kg and females about 100 kg in weight. The Malayan tiger is a national icon in Malaysia, appearing on its coat of arms and in logos of Malaysian institutions, such as Maybank.
  

Sumatran tiger

• The Sumatran tiger (Panthera tigris sumatrae) is found only on the Indonesian island of Sumatra, and is critically endangered.^[24] It is the smallest of all living tiger subspecies, with adult males weighing between 100–140 kg (220–308 lb) and females 75–110 kg (154–242 lb).^[25] Their small size is an adaptation to the thick, dense forests of the island of Sumatra where they reside, as well as the smaller-sized prey. The wild population is estimated at between 400 and 500, seen chiefly in the island's national parks. Recent genetic testing has revealed the presence of unique genetic markers, indicating that it may develop into a separate species,^[specify] if it does not go extinct.^[26] This has led to suggestions that Sumatran tigers should have greater priority for conservation than any other subspecies. While habitat destruction is the main threat to existing tiger population (logging continues even in the supposedly protected national parks), 66 tigers were recorded as being shot and killed between 1998 and 2000, or nearly 20% of the total population.

Siberian Tiger

• The Siberian tiger (Panthera tigris altaica), also known as the Amur, Manchurian, Altaic, Korean or North China tiger, is confined to the Amur-Ussuri region of Primorsky Krai and Khabarovsk Krai in far eastern Siberia, where it is now protected. Considered the largest subspecies, with a head and body length of 190–230 cm (the tail of a tiger is 60–110 cm long) and an average weight of around 227 kg (500 lb) for males,^[19] the Amur tiger is also noted for its thick coat, distinguished by a paler golden hue and fewer stripes. The heaviest wild Siberian tiger on record weighed in at 384 kg,^[27] but according to Mazak these giants are not confirmed via reliable references.^[15] Even so, a six-month old Siberian tiger can be as big as a fully grown leopard. The last two censuses (1996 and 2005) found 450–500 Amur tigers within their single, and more or less continuous, range making it one of the largest undivided tiger populations in the world. Genetic research in 2009 demonstrated that the Siberian tiger, and the western "Caspian tiger" (once thought to have been a separate subspecies that became extinct in the wild in the late 1950s^[28][29]) are actually the same subspecies, since the separation of the two populations may have occurred as recently as the past century due to human intervention.^[30]

South China tiger

• The South China tiger (Panthera tigris amoyensis), also known as the Amoy or Xiamen tiger, is the most critically endangered subspecies of tiger and is listed as one of the 10 most endangered animals in the world.^{[31][clarification needed]} One of the smaller tiger subspecies, the length of the South China tiger ranges from 2.2–2.6 m (87–104 in) for both males and females. Males weigh between 127 and 177 kg (280–390 lb) while females weigh between 100 and 118 kg (220–260 lb). From 1983 to 2007, no South China tigers were sighted.^[32] In 2007 a farmer spotted a tiger and handed in photographs to the authorities as proof.^[32][33] The photographs in question, however, were later exposed as fake, copied from a Chinese calendar and photoshopped, and the “sighting” turned into a massive scandal.^{[34] [35] [36]}

In 1977, the Chinese government passed a law banning the killing of wild tigers, but this may have been too late to save the subspecies, since it is possibly already extinct in the wild. There are currently 59 known captive South China tigers, all within China, but these are known to be descended from only six animals. Thus, the genetic diversity required to maintain the subspecies may no longer exist. Currently, there are breeding efforts to reintroduce these tigers to the wild.

A hunted down Balinese tiger

Extinct subspecies

• The Balinese tiger (Panthera tigris balica) was limited to the island of Bali. They were the smallest of all tiger subspecies, with a weight of 90–100 kg in males and 65–80 kg in females.^[15] These tigers were hunted to extinction—the last Balinese tiger is thought to have been killed at Sumbar Kima, West Bali on 27 September 1937; this was an adult female. No Balinese tiger was ever held in captivity. The tiger still plays an important role in Balinese Hinduism.

A photograph of a Javan tiger.

• The Javan tiger (Panthera tigris sondaica) was limited to the Indonesian island of Java. It now seems likely that this subspecies became extinct in the 1980s, as a result of hunting and habitat destruction, but the extinction of this subspecies was extremely probable from the 1950s onwards (when it is thought that fewer than 25 tigers remained in the wild). The last confirmed specimen was sighted in 1979, but there were a few reported sightings during the 1990s.^[37][38] With a weight of 100-141 kg for males and 75-115 kg for females, the Javan tiger was one of the smaller subspecies, approximately the same size as the Sumatran tiger^{[citation needed]}

Hybrids

Further information: Panthera hybrid, liger and tigon

Hybridization among the big cats, including the tiger, was first conceptualized in the 19th century, when zoos were particularly interested in the pursuit of finding oddities to display for financial gain.^[39] Lions have been known to breed with tigers (most often the Amur and Bengal subspecies) to create hybrids called ligers and tigons.^[40] Such hybrids were once commonly bred in zoos, but this is now discouraged due to the emphasis on conserving species and subspecies. Hybrids are still bred in private menageries and in zoos in China.

The liger is a cross between a male lion and a tigress.^[41] Because the lion sire passes on a growth-promoting gene, but the corresponding growth-inhibiting gene from the female tiger is absent, ligers grow far larger than either parent. They share physical and behavioural qualities of both parent species (spots and stripes on a sandy background). Male ligers are sterile, but female ligers are often fertile. Males have about a 50% chance of having a mane, but, even if they do, their manes will be only around half the size of that of a pure lion. Ligers are typically between 10 to 12 feet in length, and can be between 800 and 1,000 pounds or more.^[41]

The less common tigon is a cross between the lioness and the male tiger.^[42]

Colour variations

White tigers

Main article: White tiger

There is a well-known mutation that produces the white tiger, technically known as chinchilla albinistic,^[43] an animal which is rare in the wild, but widely bred in zoos due to its popularity. Breeding of white tigers will often lead to inbreeding (as the trait is recessive). Many initiatives have taken place in white and orange tiger mating in an attempt to remedy the issue, often mixing subspecies in the process. Such inbreeding has led to white tigers having a greater likelihood of being born with physical defects, such as cleft palates and scoliosis (curvature of the spine).^[44][45] Furthermore, white tigers are prone to having crossed eyes (a condition known as strabismus). Even apparently healthy white tigers generally do not live as long as their orange counterparts.

A pair of white tigers at the Singapore Zoo

Recordings of white tigers were first made in the early 19th century.^[46] They can only occur when both parents carry the rare gene found in white tigers; this gene has been calculated to occur in only one in every 10,000 births. The white tiger is not a separate sub-species, but only a colour variation; since the only white tigers that have been observed in the wild have been Bengal tigers^[47] (and all white tigers in captivity are at least part Bengal), it is commonly thought that the recessive gene that causes the white colouring is probably carried only by Bengal tigers, although the reasons for this are not known.^[44][48] Nor are they in any way more endangered than tigers are generally, this being a common misconception. Another misconception is that white tigers are albinos, despite the fact that pigment is evident in the white tiger's stripes. They are distinct not only because of their white hue; they also have blue eyes and pink noses.

A rare golden tabby/strawberry tiger at the Buffalo Zoo

Golden tabby tigers

Main article: Golden tabby

In addition, another recessive gene may create a very unusual "golden tabby" colour variation, sometimes known as "strawberry". Golden tabby tigers have light gold fur, pale legs and faint orange stripes. Their fur tends to be much thicker than normal.^[49] There are extremely few golden tabby tigers in captivity, around 30 in all. Like white tigers, strawberry tigers are invariably at least part Bengal. Both white and golden tabby tigers tend to be larger than average Bengal tigers.

Other colour variations

There are also unconfirmed reports of a "blue" or slate-coloured tiger, the Maltese Tiger, and largely or totally black tigers, and these are assumed, if real, to be intermittent mutations rather than distinct species.^[43]

Biology and behaviour

Territorial behavior

Tigers are essentially solitary and territorial animals. The size of a tiger's home range mainly depends on prey abundance, and, in the case of male tigers, on access to females. A tigress may have a territory of 20 square kilometres while the territories of males are much larger, covering 60–100 km². The ranges of males tend to overlap those of several females.

Tigers for the most part are solitary animals

The relationships between individuals can be quite complex, and it appears that there is no set "rule" that tigers follow with regards to territorial rights and infringing territories. For instance, although for the most part tigers avoid each other, both male and female tigers have been documented sharing kills. For instance, George Schaller observed a male tiger share a kill with two females and four cubs. Females are often reluctant to let males near their cubs, but Schaller saw that these females made no effort to protect or keep their cubs from the male, suggesting that the male might have been the father of the cubs. In contrast to male lions, male tigers will allow the females and cubs to feed on the kill first. Furthermore, tigers seem to behave relatively amicably when sharing kills, in contrast to lions, which tend to squabble and fight. Unrelated tigers have also been observed feeding on prey together. The following quotation is from Stephen Mills' book Tiger, as he describes an event witnessed by Valmik Thapar and Fateh Singh Rathore in Ranthambhore:^[50]

A dominant tigress they called Padmini killed a 250 kg (550-lb) male nilgai - a very large antelope. They found her at the kill just after dawn with her three 14-month-old cubs and they watched uninterrupted for the next ten hours. During this period the family was joined by two adult females and one adult male - all offspring from Padmini's previous litters and by two unrelated tigers, one female the other unidentified. By three o'clock there were no fewer than nine tigers round the kill.

A tiger is capable of jumping to almost twice its height

When young female tigers first establish a territory, they tend to do so fairly close to their mother's area. The overlap between the female and her mother's territory tends to wane with increasing time. Males, however, wander further than their female counterparts, and set out at a younger age to eke out their own area. A young male will acquire territory either by seeking out a range devoid of other male tigers, or by living as a transient in another male's territory, until he is old and strong enough to challenge the resident male. The highest mortality rate (30-35% per year) amongst adult tigers occurs for young male tigers who have just left their natal area, seeking out territories of their own.^[51]

Male tigers are generally more intolerant of other males within their territory than females are of other females. For the most part, however, territorial disputes are usually solved by displays of intimidation, rather than outright aggression. Several such incidents have been observed, in which the subordinate tiger yielded defeat by rolling onto its back, showing its belly in a submissive posture.^[52] Once dominance has been established, a male may actually tolerate a subordinate within his range, as long as they do not live in too close quarters.^[51] The most violent disputes tend to occur between two males when a female is in oestrus, and may result in the death of one of the males, although this is actually a relatively rare occurrence.^[51][53]

To identify his territory, the male marks trees by spraying of urine and anal gland secretions, as well as marking trails with scat. Males show a grimacing face, called the Flehmen response, when identifying a female's reproductive condition by sniffing their urine markings.

Tigers have been studied in the wild using a variety of techniques. The populations of tigers were estimated in the past using plaster casts of their pugmarks. This method was found faulty^[54] and attempts were made to use camera trapping instead. Newer techniques based on DNA from their scat are also being evaluated. Radio collaring has also been a popular approach to tracking them for study in the wild.

Tiger dentition. The large canines are used to make the killing bite, but they tear meat when feeding using the carnassial teeth

Hunting and diet

In the wild, tigers mostly feed on larger and medium sized animals. Sambar, gaur, domestic buffalo, chital, boar, and nilgai are the tiger's favored prey in India. Sometimes, they also prey on leopards, pythons, sloth bears and crocodiles. In Siberia the main prey species are Mandchurian elk, wild boar, Sika Deer, Moose, roe deer, and musk deer. In Sumatra Sambar, Muntjac, wild boar, and Malayan Tapir are preyed on. In the former Caspian tiger's range, prey included Saiga Antelope, camels, Caucasian Wisent, yak, and wild horses. Like many predators, they are opportunistic and will eat much smaller prey, such as monkeys, peafowls, hares, and fish.

Adult elephants are too large to serve as common prey, but conflicts between tigers and elephants do sometimes take place. A case where a tiger killed an adult Indian Rhinoceros has been observed.^[55] Young elephant and rhino calves are occasionally taken. Tigers also sometimes prey on domestic animals such as dogs, cows, horses, and donkeys. These individuals are termed cattle-lifters or cattle-killers in contrast to typical game-killers.^[56]

A South China tiger of the Save China's Tigers project with his blesbuck kill

Old tigers, or those wounded and rendered incapable of catching their natural prey, have turned into man-eaters; this pattern has recurred frequently across India. An exceptional case is that of the Sundarbans, where healthy tigers prey upon fishermen and villagers in search of forest produce, humans thereby forming a minor part of the tiger's diet.^[57] Tigers will occasionally eat vegetation for dietary fiber, the fruit of the Slow Match Tree being favoured.^[56]

Tigers usually hunt at night.^[58] They generally hunt alone and ambush their prey as most other cats do, overpowering them from any angle, using their body size and strength to knock large prey off balance. Even with their great masses, tigers can reach speeds of about 49-65 kilometres per hour (35-40 miles per hour), although they can only do so in short bursts, since they have relatively little stamina; consequently, tigers must be relatively close to their prey before they break their cover. Tigers have great leaping ability; horizontal leaps of up to 10 metres have been reported, although leaps of around half this amount are more typical. However, only one in twenty hunts ends in a successful kill.^[58]

Tigers' extremely strong jaws and sharp teeth make them superb predators.

When hunting large prey, tigers prefer to bite the throat and use their forelimbs to hold onto the prey, bringing it to the ground. The tiger remains latched onto the neck until its prey dies of strangulation.^[59] By this method, gaurs and water buffalos weighing over a ton have been killed by tigers weighing about a sixth as much.^[60] With small prey, the tiger bites the nape, often breaking the spinal cord, piercing the windpipe, or severing the jugular vein or common carotid artery.^[61] Though rarely observed, some tigers have been recorded to kill prey by swiping with their paws, which are powerful enough to smash the skulls of domestic cattle,^[56] and break the backs of sloth bears.^[62]

During the 1980s, a tiger named "Genghis" in Ranthambhore National Park was observed frequently hunting prey through deep lake water,^[63] a pattern of behaviour that had not been previously witnessed in over 200 years of observations. Moreover, he appeared to be extraordinarily successful for a tiger, with as many as 20% of hunts ending in a kill.

Reproduction

A Siberian tigress with a cub at Buffalo Zoo

Mating can occur all year round, but is generally more common between November and April.^[64] A female is only receptive for a few days and mating is frequent during that time period. A pair will copulate frequently and noisily, like other cats. The gestation period is 16 weeks. The litter size usually consists of around 3–4 cubs of about 1 kg (2 lb) each, which are born blind and helpless. The females rear them alone, sheltering them in dens such as thickets and rocky crevices. The father of the cubs generally takes no part in rearing them. Unrelated wandering male tigers may even kill cubs to make the female receptive, since the tigress may give birth to another litter within 5 months if the cubs of the previous litter are lost.^[64] The mortality rate of tiger cubs is fairly high - approximately half do not survive to be more than two years old.^[64]

There is generally a dominant cub in each litter, which tends to be male but may be of either sex.^[63] This cub generally dominates its siblings during play and tends to be more active, leaving its mother earlier than usual. At 8 weeks, the cubs are ready to follow their mother out of the den, although they don't travel with her as she roams her territory until they are older. The cubs become independent around 18 months of age, but it is not until they are around 2–2½ years old that they leave their mother. Females reach sexual maturity at 3–4 years, whereas males reach sexual maturity at 4–5 years.^[64]

Over the course of her life, a female tiger will give birth to an approximately equal number of male and female cubs. Tigers breed well in captivity, and the captive population in the United States may rival the wild population of the world.^[65]

Interspecific predatory relationships

Tiger hunted by wild dogs as illustrated in Samuel Howett & Edward Orme, Hand Coloured, Aquatint Engravings, Published London 1807

Tigers may kill such formidable predators as leopards, pythons and even crocodiles on occasion,^[66][67][68] although predators typically avoid one another. When seized by a crocodile, a tiger will strike at the reptile's eyes with its paws.^[56] Leopards dodge competition from tigers by hunting in different times of the day and hunting different prey.^[55] With relatively abundant prey, tigers and leopards were seen to successfully coexist without competitive exclusion or inter-species dominance hierarchies that may be more common to the savanna.^[69] Tigers have been known to suppress wolf populations in areas where the two species coexist.^[70][71] Dhole packs have been observed to attack and kill tigers in disputes over food, though not usually without heavy losses.^[62] Siberian tigers and brown bears can be competitors and usually avoid confrontation; however, tigers will kill bear cubs and even some adults on occasion. Bears (Asiatic black bears and brown bears) make up 5-8% of the tiger's diet in the Russian Far East.^[15] Some bears emerging from hibernation will try to steal tigers' kills, although the tiger will sometimes defend its kill. Sloth bears are quite aggressive and will sometimes drive younger aged tigers away from their kills, although in most of cases Bengal tigers prey on sloth bears.^[15]

A tiger swimming at Six Flags Great Adventure in Jackson Township, New Jersey

Habitat

Typical tiger country has three main features: It will always have good cover, it will always be close to water and plenty of prey. Bengal Tigers live in all types of forests, including Wet, Evergreen, semi-evergreen of Assam and eastern Bengal; the mangrove forest of Ganges Delta; The deciduous forest of Nepal and thorn forests of the Western Ghats. Compared to the lion, the tiger prefers denser vegetation, for which its camouflage is ideally suited, and where a single predator is not at a disadvantage compared to a pride. Among the big cats, only the tiger and jaguar are strong swimmers; tigers are often found bathing in ponds, lakes, and rivers. Unlike other cats, which tend to avoid water, tigers actively seek it out. During the extreme heat of the day, they are often to be found cooling off in pools. Tigers are excellent swimmers and can swim up to 4 miles. Tigers are often to be found carrying their dead prey across lakes.

Conservation efforts

Main article: Tiger conservation

Tiger headcount in 1990

Poaching for fur and destruction of habitat have greatly reduced tiger populations in the wild. At the start of the 20th century, it is estimated there were over 100,000 tigers in the world but the population has dwindled to about 2,000 in the wild.^[72] Some estimates suggest the population is even lower, with some at less than 2,500 mature breeding individuals, with no subpopulation containing more than 250 mature breeding individuals.^[73] The threat of extinction is mitigated somewhat by the presence of some 20,000 tigers currently in captivity, although parts of the captive population, such as the 4-5,000 animals in China's commercial tiger farms, are of low genetic diversity.

India

Main article: Project Tiger

India harbors the largest population of wild tigers in the world, along with one of the highest human populations. A major concerted conservation effort known as Project Tiger has been underway since 1973, spearheaded by Indira Gandhi. The fundamental accomplishment has been the establishment of over 25 well-monitored tiger reserves in reclaimed land where human development is categorically forbidden. The program has been credited with tripling the number of wild Bengal tigers from roughly 1,200 in 1973 to over 3,500 in the 1990s, though the reports of the Indian government are occasionally met with some skepticism.^{[citation needed]} A recently passed tribal Bill, which allows tribal populations to reside inside designated tiger sanctuaries, may have impacts on the continuing success of the program.^{[citation needed]}

A tiger census carried out over 2007, whose report was published on February 12, 2008 stated that the wild tiger population in India has come down to approximately 1,411. It is noted in the report that the decrease of tiger population can be attributed directly to poaching.^[74]

Russia

The Siberian tiger was on the brink of extinction with only about 40 animals in the wild in the 1940s. Under the Soviet Union, anti-poaching controls were strict and a network of protected zones (zapovedniks) were instituted, leading to a rise in the population to several hundred. Poaching again became a problem in the 1990s, when the economy of Russia collapsed, local hunters had access to a formerly sealed off lucrative Chinese market, and logging in the region increased. While an improvement in the local economy has led to greater resources being invested in conservation efforts, an increase of economic activity has led to an increased rate of development and deforestation. The major obstacle in preserving the species is the enormous territory individual tigers require (up to 450 km² needed by a single female).^[14] Current conservation efforts are led by local governments and NGO's in consort with international organizations, such as the World Wide Fund and the Wildlife Conservation Society.^[14] The competitive exclusion of wolves by tigers has been used by Russian conservationists to convince hunters in the Far East to tolerate the big cats, as they limit ungulate populations less than wolves, and are effective in controlling the latter's numbers.^[75] Currently, there are about 400-550 animals in the wild.

Tibet

In Tibet, tiger and leopard pelts have traditionally been used in various ceremonies and costumes. In January 2006 the Dalai Lama preached a ruling against using, selling, or buying wild animals, their products, or derivatives. It has yet to be seen whether this will result in a long-term slump in the demand for poached tiger and leopard skins.^[76][77][78]

Rewilding

The first attempt at rewilding was by Indian conservationist Billy Arjan Singh, who reared a zoo-born tigress named Tara, and released her in the wilds of Dudhwa National Park in 1978. This was soon followed by a large number of people being eaten by a tigress who was later shot. Government officials claim that this tigress was Tara, an assertion hotly contested by Singh and conservationists. Later on, this rewilding gained further disrepute when it was found that the local gene pool had been sullied by Tara's introduction as she was partly Siberian tiger, a fact not known at the time of release, ostensibly due to poor record-keeping at Twycross Zoo, where she had been raised.^{[79][80][81][82][83][84][85][86][87][88]}

Save China's Tigers

Main article: Save China's Tigers

The organisation Save China's Tigers, working with the Wildlife Research Centre of the State Forestry Administration of China and the Chinese Tigers South Africa Trust, secured an agreement on the reintroduction of Chinese tigers into the wild. The agreement, which was signed in Beijing on 26 November 2002, calls for the establishment of a Chinese tiger conservation model through the creation of a pilot reserve in China where indigenous wildlife, including the South China Tiger, will be reintroduced. Save China's Tigers aims to rewild the critically endangered South China Tiger by bringing a few captive-bred individuals to South Africa for rehabilitation training for them to regain their hunting instincts. At the same time, a pilot reserve in China is being set-up and the Tigers will be relocated and release back in China when the reserve in China is ready.^[89] The offspring of the trained tigers will be released into the pilot reserves in China, while the original animals will stay in South Africa to continue breeding.^[90]

The reason South Africa was chosen is because it is able to provide expertise and resources, land and game for the South China tigers. The South China Tigers of the project has since been successfully rewilded and are fully capable of hunting and surviving on their own.^[91] This project is also very successful in the breeding of these rewilded South China Tigers and 5 cubs have been born in the project, these cubs of the 2nd generation would be able to learn their survival skills from their successfully rewilded mothers directly.^[92]

Relation with humans

Tiger hunting on elephant-back, India, early 19th century

Tiger as prey

Main article: Tiger hunting

The tiger has been one of the Big Five game animals of Asia. Tiger hunting took place on a large scale in the early nineteenth and twentieth centuries, being a recognised and admired sport by the British in colonial India as well as the maharajas and aristocratic class of the erstwhile princely states of pre-independence India. Tiger hunting was done by some hunters on foot; others sat up on machans with a goat or buffalo tied out as bait; yet others on elephant-back.^[93] In some cases, villagers beating drums were organised to drive the animals into the killing zone. Elaborate instructions were available for the skinning of tigers and there were taxidermists who specialised in the preparation of tiger skins.

Man-eating tigers

Main article: Man-eating tigers

Stereographic photograph (1903) of a captured man-eating tiger in the Calcutta zoo; the tiger had claimed 200 human victims.

Although humans are not regular prey for tigers, they have killed more people than any other cat, particularly in areas where population growth, logging, and farming have put pressure on tiger habitats. Most man-eating tigers are old and missing teeth, acquiring a taste for humans because of their inability to capture preferred prey.^[94] Almost all tigers that are identified as man-eaters are quickly captured, shot, or poisoned. Unlike man-eating leopards, even established man-eating tigers will seldom enter human settlements, usually remaining at village outskirts.^[95] Nevertheless, attacks in human villages do occur.^[96] Man-eaters have been a particular problem in India and Bangladesh, especially in Kumaon, Garhwal and the Sundarbans mangrove swamps of Bengal, where some healthy tigers have been known to hunt humans. Because of rapid habitat loss due to climate change, tiger attacks have increased in the Sundarbans.^[97]

Traditional Asian medicine

Instructions for tiger skinning

Many people in China have a belief that various tiger parts have medicinal properties, including as pain killers and aphrodisiacs.^[98] There is no scientific evidence to support these beliefs. The use of tiger parts in pharmaceutical drugs in China is already banned, and the government has made some offenses in connection with tiger poaching punishable by death. Furthermore, all trade in tiger parts is illegal under the Convention on International Trade in Endangered Species of Wild Fauna and Flora and a domestic trade ban has been in place in China since 1993. Still, there are a number of tiger farms in the country specializing in breeding the cats for profit. It is estimated that between 4,000 and 5,000 captive-bred, semi-tame animals live in these farms today.^[99][100]

As pets

The Association of Zoos and Aquariums estimates that up to 12,000 tigers are being kept as private pets in the USA, significantly more than the world's entire wild population.^[101] 4,000 are believed to be in captivity in Texas alone.^[101]

Part of the reason for America's enormous tiger population relates to legislation. Only nineteen states have banned private ownership of tigers, fifteen require only a licence, and sixteen states have no regulations at all.^[101]

The success of breeding programmes at American zoos and circuses led to an overabundance of cubs in the 1980s and 1990s, which drove down prices for the animals.^[101] The SPCA estimate there are now 500 lions, tigers and other big cats in private ownership just in the Houston area.^[101]

In the 1983 film Scarface, the protagonist, Tony Montana, aspires to obtaining all the exterior trappings of the American Dream, which in the character's opinion included keeping a pet tiger on his property

19th century painting of a tiger by Kuniyoshi Utagawa

Cultural depictions

The tiger replaces the lion as King of the Beasts in cultures of eastern Asia,^[102] representing royalty, fearlessness and wrath.^[103] Its forehead has a marking which resembles the Chinese character 王, which means "king"; consequently, many cartoon depictions of tigers in China and Korea are drawn with 王 on their forehead.^{[citation needed]}

Of great importance in Chinese myth and culture, the tiger is one of the 12 Chinese zodiac animals. Also in various Chinese art and martial art, the tiger is depicted as an earth symbol and equal rival of the Chinese dragon- the two representing matter and spirit respectively. In fact, the Southern Chinese martial art Hung Ga is based on the movements of the Tiger and the Crane. In Imperial China, a tiger was the personification of war and often represented the highest army general (or present day defense secretary),^[103] while the emperor and empress were represented by a dragon and phoenix, respectively. The White Tiger (Chinese: 白虎; pinyin: Bái Hǔ) is one of the Four Symbols of the Chinese constellations. It is sometimes called the White Tiger of the West (西方白虎), and it represents the west and the autumn season.^[103]

In Buddhism, it is also one of the Three Senseless Creatures, symbolizing anger, with the monkey representing greed and the deer lovesickness.^[103]

The Tungusic people considered the Siberian tiger a near-deity and often referred to it as "Grandfather" or "Old man". The Udege and Nanai called it "Amba". The Manchu considered the Siberian tiger as Hu Lin, the king.^[17]

The widely worshiped Hindu goddess Durga, an aspect of Devi-Parvati, is a ten-armed warrior who rides the tigress (or lioness) Damon into battle. In southern India the god Aiyappa was associated with a tiger.^[104]

The weretiger replaces the werewolf in shapeshifting folklore in Asia;^[105] in India they were evil sorcerers while in Indonesia and Malaysia they were somewhat more benign.^[106]

The tiger continues to be a subject in literature; both Rudyard Kipling, in The Jungle Book, and William Blake, in Songs of Experience, depict the tiger as a menacing and fearful animal. In The Jungle Book, the tiger, Shere Khan, is the wicked mortal enemy of the protagonist, Mowgli. However, other depictions are more benign: Tigger, the tiger from A. A. Milne's Winnie-the-Pooh stories, is cuddly and likable. In the Man Booker Prize winning novel "Life of Pi," the protagonist, Pi Patel, sole human survivor of a ship wreck in the Pacific Ocean, befriends another survivor: a large Bengal Tiger. The famous comic strip Calvin and Hobbes features Calvin and his stuffed tiger, Hobbes. A tiger is also featured on the cover of the popular cereal Frosted Flakes (also marketed as "Frosties") bearing the name "Tony the Tiger".

Sala fighting the tiger, the symbol of Hoysala Empire at Belur, Karnataka, India

The Tiger is the national animal of Bangladesh, Nepal, India^[107] (Bengal Tiger)^[108] Malaysia (Malayan Tiger), North Korea and South Korea (Siberian Tiger).

World's favourite animal

In a poll conducted by Animal Planet, the tiger was voted the world's favourite animal, narrowly beating the dog. More than 50,000 viewers from 73 countries voted in the poll. Tigers received 21% of the vote, dogs 20%, dolphins 13%, horses 10%, lions 9%, snakes 8%, followed by elephants, chimpanzees, orangutans and whales.^{[109][110][111][112]}

Animal behaviourist Candy d'Sa, who worked with Animal Planet on the list, said: "We can relate to the tiger, as it is fierce and commanding on the outside, but noble and discerning on the inside".^[109]

Callum Rankine, international species officer at the World Wildlife Federation conservation charity, said the result gave him hope. "If people are voting tigers as their favourite animal, it means they recognise their importance, and hopefully the need to ensure their survival," he said.^[109]

Gallery

Picture of Felis tigris (Panthera tigris) subspecies unknown	Dervish with a lion and a tiger. Mughal painting, c. 1650	Bengal tiger	Sumatran tiger
Siberian Tiger	Bengal tiger cooling off at Bandhavghar, India	Vibrissae of a Tiger at Chester Zoo	Captive Bengal tiger at the Bannarghetta National Park, India
The Hindu goddess Durga riding a tiger - painting in Orissa, India	A toy showing a tiger pouncing on a redcoat (British soldier). This belonged to Tippu Sultan who was popularly known as the Tiger of Mysore.	Tiger Coat of Arms of the Jewish Autonomous Oblast in Russia

See also

• 21st Century Tiger, information about tigers and conservation projects
• Siegfried & Roy, two famous tamers of tigers
• Tiger Temple, a Buddhist temple in Thailand famous for its tame tigers

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References

• Brakefield, T. (1993). Big cats kingdom of might, Voyageur press.
• Dr. Tony Hare. (2001) Animal Habitats P. 172 ISBN 0-8160-4594-1
• Kothari, Ashok S. & Chhapgar, Boman F. (eds). 2005. The Treasures of Indian Wildlife. Bombay Natural History Society and Oxford University Press, Mumbai.
• Mazák, V. (1981). Panthera tigris. (PDF). Mammalian Species, 152: 1-8. American Society of Mammalogists.
• Nowak, Ronald M. (1999) Walker's Mammals of the World. Johns Hopkins University Press. ISBN 0-8018-5789-9
• Sankhala, K. (1997), Der indische Tiger und sein Reich, Bechtermuenz Verlag, ISBN 3-86047-734-X Abridged German translation of Return of the Tiger, Lustre Press, 1993.
• Seidensticker, John. (1999) Riding the Tiger. Tiger Conservation in Human-dominated Landscapes Cambridge University Press. ISBN 0521648351

The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing. It is the only fully functional GNSS in the world, can be used freely by anyone, anywhere, and is often used by civilians for navigation purposes. It uses a constellation of between 24 and 32 medium Earth orbit satellites that transmit precise radiowave signals, which allow GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR GPS. Although NAVSTAR is not an acronym,^[1] a few backronyms have been created for it.^[2]

Since it became fully operational on April 27, 1995, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, tracking and surveillance, and hobbies such as geocaching. Also, the precise time reference is used in many applications including the scientific study of earthquakes and as a required time synchronization method for cellular network protocols such as the IS-95 standard for CDMA.

History

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology that GPS relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system.

The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.

After Korean Air Lines Flight 007 was shot down in 1983 after straying into the USSR's prohibited airspace,^[3] President Ronald Reagan issued a directive making GPS freely available for civilian use as a common good.^[4] The satellites were launched between 1989 and 1993.

Initially the highest quality signal was reserved for military use, while the signal available for civilian use was intentionally degraded ("Selective Availability", SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m.

Of crucial importance for the function of GPS is the placement of atomic clocks in the satellites, first proposed by Friedwardt Winterberg in 1955.^[5] Only then can the required position accuracy be reached.

Timeline

• In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.

Satellite numbers^[6][7][8]

Block	Launch Period	Satellites launched	Currently in service
I	1978–1985	10+11	0
II	1985–1990	9	0
IIA	1990–1997	19	13
IIR	1997–2004	12+11	12
IIR-M	2005–2009	7+12	6
IIF	2009–2011	0+102	0
IIIA	2014–?	0+123	0
IIIB		0+83	0
IIIC		0+163	0
Total		59+21+122+363	31
1Failed 2In preparation 3Planned. (Last update: 16 December 2008)

• In 1978 the first experimental Block-I GPS satellite was launched.
• In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace due to navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS would be made available for civilian uses once it was completed.^[9][10]
• By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
• On February 14, 1989, the first modern Block-II satellite was launched.
• In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.
• By December 1993 the GPS achieved initial operational capability.^[11]
• By January 17, 1994 a complete constellation of 24 satellites was in orbit.
• Full Operational Capability was declared by NAVSTAR in April 1995.
• In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive^[12] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
• In 1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety.
• On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.
• In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.
• In 2004, U.S. President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.
• November 2004, QUALCOMM announced successful tests of Assisted-GPS for mobile phones.^[13]
• In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.
• On September 14, 2007, the aging mainframe-based Ground Segment Control System was transitioned to the new Architecture Evolution Plan.^[14]
• The most recent launch was on March 15, 2008.^[15] The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December 10, 1990.^[16]
• On May 19, 2009, the U. S. Government Accountability Office issued a report warning that the some GPS satellites could fail as soon as 2010.^[17]
• On May 21, 2009, the Air Force Space Command allayed fears of GPS system failure saying "There's only a small risk we will not continue to exceed our performance standard"^[18]

Basic concept of GPS

A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac). The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receiver's location. The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.

It might seem three satellites are enough to solve for position, since space has three dimensions. However, even a very small clock error multiplied by the very large speed of light^[19]—the speed at which satellite signals propagate—results in a large positional error. Therefore receivers use four or more satellites to solve for x, y, z, and t, which is used to correct the receiver's clock. While most GPS applications use the computed location only and effectively hide the very accurately computed time, it is used in a few specialized GPS applications such as time transfer, traffic signal timing, and synchronization of cell phone base stations.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible (see ^[20], Chapters 7 and 8 of ^[21], and ^[22]).

Position calculation introduction

To provide an introductory description of how a GPS receiver works, measurement errors will be ignored in this section. Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the indicated transit time, . of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as . Knowing the distance from GPS receiver to a satellite and the position of a satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres, if they intersect in more than one point, intersect in a circle. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below.

Two sphere surfaces intersecting in a circle

The article, trilateration, shows mathematically that the surfaces of two spheres, intersecting in more than one point, intersect in a circle.

A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivable that they could intersect at zero points, one point, or in the very special case in which the centers of the three spheres are colinear (i.e. all three on the same straight line) the sphere surface could intersect the entire circumference of the circle. Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, shows this intersection. The two intersections are marked with dots. Again trilateration clearly shows this mathematically. The correct position of the GPS receiver is the intersection that is closest to the surface of the earth for automobiles and other near-Earth vehicles. The correct position of the GPS receiver is also the intersection which is closest to the surface of the sphere corresponding to the fourth satellite. (The two intersections are symmetrical with respect to the plane containing the three satellites. If the three satellites are not in the same orbital plane, the plane containing the three satellites will not be a vertical plane passing through the center of the Earth. In this case one of the intersections will be closer to the earth than the other. The near-Earth intersection will be the correct position for the case of a near-Earth vehicle. The intersection which is farthest from Earth may be the correct position for space vehicles.)

Correcting a GPS receiver's clock

The method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.

It is likely that the surfaces of the three spheres intersect, since the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, since any clock error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let denote the pseudorange of the fourth satellite. Let . Note that is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of

(correct time) - (time indicated by the receiver's on-board clock),

and the GPS receiver clock can be advanced if is positive or delayed if is negative.

System detail

Unlaunched GPS satellite on display at the San Diego Aerospace museum

System segmentation

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).^[23]

Space segment

See also: GPS satellite and List of GPS satellite launches

A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.

The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes,^[24] but this was modified to six planes with four satellites each.^[25] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.^[26] The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).^[27] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.^[28]

Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within line of sight (12,900 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day.^[29] The ground track of each satellite therefore repeats each (sidereal) day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

As of March 2008^[update],^[30] there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.^[31]

Control segment

The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).^[32] The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Then 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.^[33]

Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked 'unhealthy', so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.

User segment

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).

The user's GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007^[update], receivers typically have between 12 and 20 channels.^[34]

A typical OEM GPS receiver module measuring 15×17 mm.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

A typical GPS receiver with integrated antenna.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol, or the newer and less widely used NMEA 2000.^[35] Although these protocols are officially defined by the NMEA,^[36] references to these protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

Navigation signals

GPS broadcast signal

Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-week, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The messages are sent in frames, each taking 30 seconds to transmit 1500 bits.

Transmission of each 30 second frame begins precisely on the minute and half minute as indicated by the satellite's atomic clock according to Satellite message format. Each frame contains 5 subframes of length 6 seconds and with 300 bits. Each subframe contains 10 words of 30 bits with length 0.6 seconds each.

Words 1 and 2 of every subframe have the same type of data. The first word is the telemetry word which indicates the beginning of a subframe and is used by the receiver to synch with the navigation message. The second word is the HOW or handover word and it contains timing information which enables the receiver to identify the subframe and provides the time the next subframe was sent.

Words 3 through 10 of subframe 1 contain data describing the satellite clock and its relationship to GPS time. Words 3 through 10 of subframes 2 and 3, contain the ephemeris data, giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate.

The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC). Words 3 through 10 of subframes 4 and 5 contain a new part of the almanac. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the entire almanac from a single satellite.^[37] The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. The second purpose is for relating time derived from the GPS (called GPS time) to the international time standard of UTC. Finally, the almanac allows a single-frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver. An important thing to note about navigation data is that each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites.

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses a code division multiple access (CDMA) spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual message data. The message data is transmitted at 50 bits per second. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code (a so-called Gold code) at 1.023 million chips per second, and the precise (P) code at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.^[38] The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Satellite frequencies

• L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.
• L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
• L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
• L4 (1379.913 MHz): Being studied for additional ionospheric correction.
• L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009.^[39]

C/A code

[edit] Demodulation and decoding

Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code.

Since all of the satellite signals are modulated onto the same L1 carrier frequency, there is a need to separate the signals after demodulation. This is done by assigning each satellite a unique pseudorandom sequence known as a Gold code, and the signals are decoded, after demodulation, using modulo 2 addition of the Gold codes corresponding to satellites n₁ through n_k, where k is the number of channels in the GPS receiver and n₁ through n_k are the pseudorandom numbers associated with the satellites. The results of these modulo 2 additions are the 50 bit/s navigation messages from satellites n₁ through n_k. The Gold codes used in GPS are a sequence of 1023 bits with a period of one millisecond. These Gold codes are highly mutually orthogonal, so that it is unlikely that one satellite signal will be misinterpreted as another. As well, the Gold codes have good auto-correlation properties.^[40]

There are 1025 different Gold codes of length 1023 bits, but only 32 are used. These Gold codes are quite often referred to as pseudo random noise since they contain no data and are said to look like random sequences^[41]. However, this may be misleading since they are actually deterministic sequences.

If the almanac information has previously been acquired, the receiver picks which satellites to listen for by their PRNs. If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern.

The receiver uses the C/A Gold code with the same PRN number as the satellite to compute an offset, O, that generates the best correlation. The offset, O, is computed in a trial and error manner. The 1023 bits of the satellite PRN signal are compared with the receiver PRN signal. If correlation is not achieved, the 1023 bits of the receiver's internally generated PRN code are shifted by one bit relative to the satellite's PRN code and the signals are again compared. This process is repeated until correlation is achieved or all 1023 possible cases have been tried (see "How a GPS Receiver Gets a Lock"). If all 1023 cases have been tried without achieving correlation, the frequency oscillator is offset to the next value and the process is repeated.

Since the carrier frequency received can vary due to Doppler shift, the points where received PRN sequences begin may not differ from O by an exact integral number of milliseconds. Because of this, carrier frequency tracking along with PRN code tracking are used to determine when the received satellite's PRN code begins (see "How a GPS Receiver Gets a Lock"). Unlike the earlier computation of offset in which trials of all 1023 offsets could potentially be required, the tracking to maintain lock usually requires shifting of half a pulse width or less. To perform this tracking, the receiver observes two quantities, phase error and received frequency offset. The correlation of the received PRN code with respect to the receiver generated PRN code is computed to determine if the bits of the two signals are misaligned. Comparisons with correlation computed with receiver generated PRN code shifted half a pulse width early and half a pulse width late (see section 1.4.2.4 of ^[21]) are used to estimate adjustment required. The amount of adjustment required for maximum correlation is used in estimating phase error. Received frequency offset from the frequency generated by the receiver provides an estimate of phase rate error. The command for the frequency generator and any further PRN code shifting required are computed as a function of the phase error and the phase rate error in accordance with the control law used. The Doppler velocity is computed as a function of the frequency offset from the carrier nominal frequency. The Doppler velocity is the velocity component along the line of sight of the receiver relative to the satellite.

As the receiver continues to read successive PRN sequences, it will encounter a sudden change in the phase of the 1023 bit received PRN signal. This indicates the beginning of a data bit of the navigation message (see section 1.4.2.5 of ^[21]). This enables the receiver to begin reading the 20 millisecond bits of the navigation message. Each subframe of the navigation frame begins with a Telemetry Word which enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. Also each subframe of the navigation frame is identified by bits in the handover word (HOW) thereby enabling the receiver to determine which subframe (see section 1.4.2.6 of ^[21] and section 2.5.4 of "Essentials of Satellite Navigation Compendium"). There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data before computing the intersections of sphere surfaces.

After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange. The receiver is potentially capable of getting a new pseudorange measurement at the beginning of each subframe or every 6 seconds.

Then the orbital position data, or ephemeris, from the navigation message is used to calculate precisely where the satellite was at the start of the message. A more sensitive receiver will potentially acquire the ephemeris data more quickly than a less sensitive receiver, especially in a noisy environment.^[42]

This process is repeated for each satellite to which the receiver is listening.

Carrier phase tracking (surveying)

Utilizing the navigation message to measure pseudorange has been discussed. Another method that is used in GPS surveying applications is carrier phase tracking. The period of the carrier frequency times the speed of light gives the wave length, which is about 0.19 meters for the L1 carrier. With a 1% of wave length accuracy in detecting the leading edge, this component of pseudorange error might be as low as 2 millimeters. This compares to 3 meters for the C/A code and 0.3 meters for the P code.

However, this 2 millimeter accuracy requires measuring the total phase, that is the total number of wave lengths plus the fractional wavelength. This requires specially equipped receivers. This method has many applications in the field of surveying.

We now describe a method which could potentially be used to estimate the position of receiver 2 given the position of receiver 1 using triple differencing followed by numerical root finding, and a mathematical technique called least squares. A detailed discussion of the errors is omitted in order to avoid detracting from the description of the methodology. In this description differences are taken in the order of differencing between satellites, differencing between receivers, and differencing between epochs. This should not be construed to mean that this is the only order which can be used. Indeed other orders of taking differences are equally valid.

The satellite carrier total phase can be measured with ambiguity as to the number of cycles as described in CARRIER PHASE MEASUREMENT and CARRIER BEAT PHASE. Let denote the phase of the carrier of satellite j measured by receiver i at time . This notation has been chosen so as to make it clear what the subscripts i, j, and k mean. In view of the fact that the receiver, satellite, and time come in alphabetical order as arguments of and to strike a balance between readability and conciseness, let so as to have a concise abbreviation. Also we define three functions, : which perform differences between receivers, satellites, and time points respectively. Each of these functions has a linear combination of variables with three subscripts as its argument. These three functions are defined below. If is a function of the three integer arguments, i, j, and k then it is a valid argument for the functions, : , with the values defined as

,

, and

.

Also if are valid arguments for the three functions and a and b are constants then is a valid argument with values defined as

,

, and

,

Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch as shown in BETWEEN-SATELLITE DIFFERENCING. This difference is designated as

Double differencing can be performed by taking the differences of the between satellite difference observed by receiver 1 with that observed by receiver 2. The satellite clock errors will be approximately eliminated by this between receiver differencing. This double difference is designated as .

Triple differencing can be performed by taking the difference of double differencing performed at time with that performed at time . This will eliminate the ambiguity associated with the integral number of wave lengths in carrier phase provided this ambiguity does not change with time. Thus the triple difference result has eliminated all or practically all clock bias errors and the integer ambiguity. Also errors associated with atmospheric delay and satellite ephemeris have been significantly reduced. This triple difference is designated as .

Triple difference results can be used to estimate unknown variables. For example if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs quite possibly will be sufficient to solve for the three components of position of receiver 2. This may require the use of a numerical procedure such as one of those found in the chapter on root finding and nonlinear sets of equations in Numerical Recipes ^[43]. Also see Preview of Root Finding. To use such a numerical method, an initial approximation of the position of receiver 2 is required. This initial value could probably be provided by a position approximation based on the navigation message and the intersection of sphere surfaces. Although multidimensional numerical root finding can have problems, this disadvantage may be overcome with this good initial estimate. This procedure using three time pairs and a fairly good initial value followed by iteration will result in one observed triple difference result for receiver 2 position. Greater accuracy may be obtained by processing triple difference results for additional sets of three independent time pairs. This will result in an over determined system with multiple solutons. To get estimates for an over determined system, least squares can be used. The least squares procedure determines the position of receiver 2 which best fits the observed triple difference results for receiver 2 positions under the criterion of minimizing the sum of the squares.

Position calculation advanced

Before providing a more mathematical description of position calculation, the introductory material on this topics is reviewed. To describe the basic concept of how a GPS receiver works, the errors are at first ignored. Using messages received from four satellites, the GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as where the subscript i denotes which satellite and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the indicated transit time, . of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as . Knowing the distance from GPS receiver to a satellite and the position of a satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres if they intersect in more than one point intersect in a circle. We are here excluding the unrealistic case for GPS purposes of two coincident spheres. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below depicting this which hopefully will aid the reader in visualizing this intersection. Two points at which the surfaces of the spheres intersect are clearly marked on the figure. The distance between these two points is the diameter of the circle of intersection. If you are not convinced of this, consider how a side view of the intersecting spheres would look. This view would look exactly the same as the figure because of the symmetry of the spheres. And in fact a view from any horizontal direction would look exactly the same. This should make it clear to the reader that the surfaces of the two spheres actually do intersect in a circle.

Two sphere surfaces intersecting in a circle

The article, trilateration, shows mathematically how the equation for a circle is determined. A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivable that they could intersect in 0 or 1 point. We are here excluding the unrealistic case for GPS purposes of three colinear (lying on same straight line) sphere centers. Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, is shown below to aid in visualizing this intersection. Again trilateration clearly shows this mathematically. The correct position of the GPS receiver is the one that is closest to the fourth sphere. This paragraph has described the basic concept of GPS while ignoring errors. The next problem is how to process the messages when errors are present.

Surface of a sphere intersecting a circle (i.e., the edge of a disk) at two points

Let denote the clock error or bias, the amount by which the receiver's clock is slow. The GPS receiver has four unknowns, the three components of GPS receiver position and the clock bias . The equation of the sphere surfaces are given by

, Another useful form of these equations is in terms of the pseudoranges, which are simply the ranges approximated based on GPS receiver clock's indicated (i.e. uncorrected) time so that . Then the equations becomes:

. Two of the most important methods of computing GPS receiver position and clock bias are (1) trilateration followed by one dimensional numerical root finding and (2) multidimensional Newton-Raphson calculations. These two methods along with their advantages are discussed.

• The receiver can solve by trilateration followed by one dimensional numerical root finding.^[43] This method involves using trilateration to determine the intersection of the surfaces of three spheres. It is clearly shown in trilateration that the surfaces of three spheres intersect in 0, 1, or 2 points. In the usual case of two intersections, the solution which is nearest the surface of the sphere corresponding to the fourth satellite is chosen. The surface of the earth can also sometimes be used instead, especially in the case of civilian GPS receivers since it is illegal in the United States to track vehicles of more than 60,000 feet (18,000 m) in altitude. The bias, is then computed as a function of the distance from the solution to the surface of the sphere corresponding to the fourth satellite. To determine what function to use for computing see the chapter on root finding in ^[43] or the preview. Using an updated received time based on this bias, new spheres are computed and the process is repeated. This repetition is continued until the distance from the valid trilateration solution is sufficiently close to the surface of the sphere corresponding to the fourth satellite. One advantage of this method is that it involves one dimensional as opposed to multidimensional numerical root finding.

• The receiver can utilize a multidimensional root finding method such as the Newton-Raphson method.^[43] Linearize around an approximate solution, say from iteration k, then solve four linear equations derived from the quadratic equations above to obtain . The radii are large and so the sphere surfaces are close to flat.^[44][45] This near flatness may cause the iterative procedure to converge rapidly in the case where is near the correct value and the primary change is in the values of , since in this case the problem is merely to find the intersection of nearly flat surfaces and thus close to a linear problem. However when is changing significantly, this near flatness does not appear to be advantageous in producing rapid convergence, since in this case these near flat surfaces will be moving as the spheres expand and contract. This possible fast convergence is an advantage of this method. Also it has been claimed that this method is the "typical" method used by GPS receivers.^[46][47][48] A disadvantage of this multidimensional root finding method as compared to single dimensional root findiing is that according to,^[43] "There are no good general methods for solving systems of more than one nonlinear equations." For a more detailed description of the mathematics see Multidimensional Newton Raphson.

• Other methods include:

1. Solving for the intersection of the expanding signals form light cones in 4-space cones
2. Solving for the intersection of hyperboloids determined by the time difference of signals received from satellites utilizing multilateration,
3. Solving the equations in accordance with .^[46][47][49]

• When more than four satellites are available, a decision must be made on whether to use the four best or more than four taking into considerations such factors as number of channels, processing capability, and geometric dilution of precision. Using more than four results in an over-determined system of equations with no unique solution, which must be solved by least-squares or a similar technique. If all visible satellites are used, the results are always at least as good as using the four best, and usually better. Also the errors in results can be estimated through the residuals. ^[50] With each combination of four or more satellites, a geometric dilution of precision (GDOP) vector can be calculated, based on the relative sky positions of the satellites used.^[51][50] As more satellites are picked up, pseudoranges from more combinations of four satellites can be processed to add more estimates to the location and clock offset. The receiver then determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a specific coordinate system such as latitude and longitude, using the WGS 84 geodetic datum or a local system specific to a country.^[52]

• Finally, results from other positioning systems such as GLONASS or the upcoming Galileo can be used in the fit, or used to double check the result. (By design, these systems use the same bands, so much of the receiver circuitry can be shared, though the decoding is different.)

P(Y) code

Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.^{[citation needed]} In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective.

Error sources and analysis

Sources of User Equivalent Range Errors (UERE)

Source	Effect
Signal Arrival C/A	± 3 m
Signal Arrival P(Y)	± 0.3 m
Ionospheric effects	± 5 m
Ephemeris errors	± 2.5 m
Satellite clock errors	± 2 m
Multipath distortion	± 1 m
Tropospheric effects	± 0.5 m
C/A	± 6,7 m
P(Y)	± 6,0 m

User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, , of about 1 meter. The standard deviations, , for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e. RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error. Electronics errors are one of several accuracy-degrading effects outlined in the table above. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally.(see summary table near end of "Sources of Errors in GPS")

Error Diagram Showing Relation of Indicated Receiver Position, Intersection of Sphere Surfaces, and True Receiver Position in Terms of Pseudorange Errors, PDOP, and Numerical Errors

The term user equivalent range error (UERE) refers to the standard deviation of a component of the error in the distance from receiver to a satellite. The standard deviation of the error in receiver position, , is computed by multiplying PDOP (Position Dilution Of Precision) by , the standard deviation of the user equivalent range errors. is computed by taking the square root of the sum of the squares of the individual component standard deviations.

PDOP is computed as a function of receiver and satellite positions. Consider the unit vectors pointing from the receiver to the satellites. Connecting the tails of these unit vectors forms a tetrahedron. PDOP is sometimes approximated as being inversely proportional to the tetrahedron volume^[45]. Also a more detailed description of how to calculate PDOP is given in the section, Geometric dilution of precision computation (DOP).

is given by = 6.7 meters for the C/A code. The standard deviation of the error in estimated receiver position, , is given by for the C/A code. The error diagram to the right shows the inter relationship of indicated receiver position, true receiver position, and the intersection of the four sphere surfaces.

Signal arrival time measurement

The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about one percent of a bit pulse width, , or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate at the speed of light, this represents an error of about 3 meters.

This component of position accuracy can be improved by a factor of 10 using the higher-chiprate P(Y) signal. Assuming the same one percent of bit pulse width accuracy, the high-frequency P(Y) signal results in an accuracy of or about 30 centimeters.

Atmospheric effects

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer (see airmass). Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Because ionospheric delay affects the speed of microwave signals differently depending on their frequency — a characteristic known as dispersion - delays measured on two or more frequency bands can be used to measure dispersion, and this measurement can then be used to estimate the delay at each frequency.^[53] Some military and expensive survey-grade civilian receivers measure the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave. (see Atmospheric Effects in "Sources of Errors in GPS")

The effects of the ionosphere generally change slowly, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems (SBAS) such as WAAS (available in North America and Hawaii), EGNOS (Europe and Asia) or MSAS (Japan), which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect both is more localized and changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.^{[citation needed]}

Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct, either by applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter.^{[citation needed]}

Multipath effects

GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas (e.g. a choke ring antenna) may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

Ephemeris and clock errors

While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. If a fast Time To First Fix (TTFF) is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices.^[54] See also Assisted GPS.

The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.

These problems tend to be very small, but may add up to a few meters (10s of feet) of inaccuracy.^[55]

Geometric dilution of precision computation (DOP)

As a first step in computing DOP, consider the unit vector from the receiver to satellite i with components,

where the distance from receiver to the satellite, , is given by

and where denote the position of the receiver and denote the position of satellite i. Formulate the matrix A as

The first three elements of each row of A are the components of a unit vector from the receiver to the indicated satellite. The elements in the fourth column are c where c denotes the speed of light. Formulate the matrix, Q, as

This computation is in accordance with "Section 1.4.2 of PRINCIPLES OF SATELLITE POSITIONING" where the weighting matrix, P, has been set to the identity matrix.

The elements of the Q matrix are designated as

The Greek letter is used quite often where we have used d. However the elements of the Q matrix do not represent variances and covariances as they are defined in probability and statistics. Instead they are strictly geometric terms. Therefore d as in dilution of precision is used. PDOP, TDOP and GDOP are given by

,

, and

in agreement with "Section 1.4.9 of PRINCIPLES OF SATELLITE POSITIONING".

The horizontal dilution of precision, , and the vertical dilution of precision, , are both dependent on the coordinate system used. To correspond to the local horizon plane and the local vertical, x, y, and z should denote positions in either a North, East, Down coordinate system or a South, East, Up coordinate system.

Derivation of DOP equations

The equations for computing the geometric dilution of precision terms have been described in the previous section. This section describes the derivation of these equations. The method used here is similar to that used in "Global Positioning System (preview) by Parkinson and Spiker"

Consider the position error vector, e, defined as the vector from the intersection of the four sphere surfaces corresponding to the pseudoranges to the true position of the receiver. where bold denotes a vector and denote unit vectors along the x, y, and z axes respectively. Let denote the time error, the true time minus the receiver indicated time. Assume that the mean value of the three components of e and are zero.

where are the errors in pseudoranges 1 through 4 respectively. This equation comes from linearizing the equation relating pseudoranges to receiver position, satellite positions, and receiver clock errors as shown in [2] . Multiplying both sides by there results

.

Transposing both sides

.

Post multiplying the matrices on both sides of equation (2) by the corresponding matrices in equation (3), there results

.

Taking the expected value of both sides and taking the non-random matrices outside the expectation operator, E, there results

.

Assuming the pseudorange errors are uncorrelated and have the same variance, the covariance matrix on the right side can be expressed as a scalar times the identity matrix. Thus

since

Note: since

Substituting for there follows

From equation (7), it follows that the variances of indicated receiver position and time are

and

.

The remaining position and time error variance terms follow in a straightforward manner.

Selective availability

GPS includes a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.

SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the US military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.

Before it was turned off in 2000, typical SA errors were 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President Clinton in 2000.

Another restriction on GPS, antispoofing, remains on. This encrypts the P-code so that it cannot be mimicked by an enemy transmitter sending false information. Few civilian receivers have ever used the P-code, and the accuracy attainable with the public C/A code is so much better than originally expected (especially with DGPS) that the antispoof policy has relatively little effect on most civilian users. Turning off antispoof would primarily benefit surveyors and some scientists who need extremely precise positions for experiments such as tracking the motion of a tectonic plate.

DGPS services are widely available from both commercial and government sources. The latter include WAAS and the US Coast Guard's network of LF marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.

During the 1990-91 Gulf War, the shortage of military GPS units caused many troops and their families to buy readily available civilian units. This significantly impeded the US military's own battlefield use of GPS, so the military made the decision to turn off SA for the duration of the war.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The amount of error added was "set to zero"^[56] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the US military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.^[56]

Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,^[57] have stated that it is not intended to be reintroduced.

One interesting side effect of the Selective Availability hardware is the capability to correct the frequency of the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 × 10^-13 (one in five trillion). This represented a significant improvement over the raw accuracy of the clocks.^{[citation needed]}

On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,^[58] eventually making the policy permanent.^[59]

Relativity

Satellite clocks are slowed by their orbital speed but sped up by their distance out of the Earth's gravitational well.

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45.9 microseconds (μs) per day, because they have a higher gravitational potential than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7.2 μs per day. When combined, the discrepancy is about 38 microseconds per day; a difference of 4.465 parts in 10¹⁰.^[60] To account for this, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.^[61] Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was first proposed by Friedwardt Winterberg in 1955.^[62]

Sagnac distortion

GPS observation processing must also compensate for the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. A Lorentz transformation is thus applied to convert from the inertial system to the ECEF system. The resulting signal run time correction has opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.^[63]

Possible sources of interference

Natural sources

Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.

Space weather degrades GPS operation in two ways, direct interference by solar radio burst noise in the same frequency band ^[64] or by scattering of the GPS radio signal in ionospheric irregularities referred to as scintillation ^[65]. Both forms of degradation follow the 11 year solar cycle and are a maximum at sunspot maximum although they can occur at anytime. Solar radio bursts are associated with solar flares and their impact can affect reception over the half of the Earth facing the sun. Scintillation occurs most frequently at tropical latitudes where it is a night time phenomenon. It occurs less frequently at high latitudes or mid-latitudes where magnetic storms can lead to scintillation ^[66]. In addition to producing scintillation, magnetic storms can produce strong ionospheric gradients that degrade the accuracy of SBAS systems ^[67].

Artificial sources

In automotive GPS receivers, metallic features in windshields,^[68] such as defrosters, or car window tinting films^[69] can act as a Faraday cage, degrading reception just inside the car.

Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well documented case, the entire harbor of Moss Landing, California was unable to receive GPS signals due to unintentional jamming caused by malfunctioning TV antenna preamplifiers.^[70][71] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in the online magazine Phrack.^[72]

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military claimed to destroy six GPS jammers during the Iraq War, including one that was destroyed ironically with a GPS-guided bomb.^[73] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007.^[74]

Some countries allow the use of GPS repeaters to allow for the reception of GPS signals indoors and in obscured locations, however, under EU and UK laws, the use of these is prohibited as the signals can cause interference to other GPS receivers that may receive data from both GPS satellites and the repeater.

Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to deal with the interference. The first is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction".^[75] Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in some receivers, which is designed to provide a warning to the user if jamming or another problem is detected. The U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain its lock on the encrypted GPS signals during interference which causes civilian receivers to lose lock.^[76]

Accuracy enhancement

Augmentation

Main article: GNSS Augmentation

Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.

Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.

Precise monitoring

The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

After SA, which has been turned off, the largest error in GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.

Receivers with decryption keys can decode the P(Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not available to the public. Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization, below). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period one one-thousandth of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.

Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

Timekeeping

While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (TAI - GPS = 19 seconds) with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2009 is 15 seconds due to the leap second added to UTC December 31 2008. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, given the current rate of change of the Earth's rotation (with one leap second introduced approximately every 18 months), should be sufficient to last until approximately year 2300.

As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21 1999 (00:00:19 TAI on August 22 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field, which only repeats every 8,192 weeks (157 years), thus lasting until year 2137 (157 years after GPS week zero).

Modernization

Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995,^[77] the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS. Announcements from the U.S. Vice President and the White House in 1998 initiated these changes, and in 2000 the U.S. Congress authorized the effort, referring to it as GPS III.

The project aims to improve the accuracy and availability for all users and involves a new control segment (called GPS OCX), new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) of the L2C code is expected in 2008.^[78] A goal of 2013 has been established for the entire program, with incentives offered to the contractors if they can complete it by 2011 (See GPS signals).

Applications

The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.

Military

The military applications of GPS span many purposes:

• Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies. The GPS-receivers that commanders and soldiers use are respectively called the Commanders Digital Assistant and the Soldier Digital Assistant.^{[79][80][81][82]}
• Target tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile.^{[citation needed]} These weapon systems pass GPS co-ordinates of targets to precision-guided munitions to allow them to engage the targets accurately. Military aircraft, particularly those used in air-to-ground roles use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be looked up in Google Earth^{[citation needed]}).
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions. Artillery projectiles with embedded GPS receivers able to withstand accelerations of 12,000G have been developed for use in 155 mm howitzers.^[83]
• Search and Rescue: Downed pilots can be located faster if they have a GPS receiver.
• Reconnaissance and Map Creation: The military use GPS extensively to aid mapping and reconnaissance.
• The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an Electro-Magnetic Pulse (EMP) sensor (W-sensor) which form a major portion of the United States Nuclear Detonation Detection System.^[84][85]

Civilian

See also: GNSS applications and GPS navigation device

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS: absolute location, relative movement, and time transfer.

The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation. The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize clocks to exacting standards enables time transfer, which is critical in large communication and observation systems. An example is CDMA digital cellular. Each base station has a GPS timing receiver to synchronize its spreading codes with other base stations to facilitate inter-cell hand off and support hybrid GPS/CDMA positioning of mobiles for emergency calls and other applications. Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.

The US Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 km (60,000 ft) altitude and 515 m/s (1,000 knots) ^[86] are classified as munitions (weapons) for which US State Department export licenses are required. These parameters are clearly chosen to prevent use of a receiver in a ballistic missile. It would not prevent use in a cruise missile since their altitudes and speeds are similar to those of ordinary aircraft.

This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A code and cannot correct for SA, etc.

Disabling operation above these limits exempts the receiver from classification as a munition. Different vendors have interpreted these limitations differently. The rule specifies operation above 18 km and 515 m/s, but some receivers stop operating at 18 km even when stationary. This has caused problems with some amateur radio balloon launches as they regularly reach 100,000 feet (30km).

GPS tours are also an example of civilian use. The GPS is used to determine which content to display. For instance, when approaching a monument it would tell you about the monument.

GPS functionality has now started to move into mobile phones en masse. The first handsets with integrated GPS were launched already in the late 1990’s, and were available for broader consumer availability on networks such as those run by Nextel, Sprint and Verizon in 2002 in response to US FCC mandates for handset positioning in emergency calls. Capabilities for access by third party software developers to these features were slower in coming, with Nextel opening up those APIs upon launch to any developer, Sprint following in 2006, and Verizon soon thereafter.

Awards

Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:

• Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).
• Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel.

One GPS developer, Roger L. Easton, received the National Medal of Technology on February 13, 2006 at the White House.^[87]

On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

Other systems

Main article: Global Navigation Satellite System

Other satellite navigation systems in use or various states of development include:

• Beidou – China's regional system that China has proposed to expand into a global system named COMPASS.
• Galileo – a proposed global system being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia, South Korea, and Ukraine, planned to be operational by 2013.
• GLONASS – Russia's global system which is being restored to full availability in partnership with India.
• Indian Regional Navigational Satellite System (IRNSS) – India's proposed regional system.
• QZSS – Japanese proposed regional system covering Japan only.

Multidimensional Newton-Raphson for GPS

This section provides a more detailed discussion of the equations used in the second method described in Position calculation advanced. The linearized equations are developed using the appropriate partial derivatives and the algorithm is described. In ^[45] the same method is discussed but the equations are not shown. Let and denote the true coordinates of GPS receiver position at time, . Let denote the unknown clock error or bias, the amount by which the receiver's clock is slow. Let the coordinates of each satellite, and the time the message was sent, be , let the GPS clock's indicated received time be and c be the speed of light. The pseudorange is computed as . Assume the message travels at the speed of light, then the pseudorange satisfies the equation,

When an approximate solution, rather than the exact solution, is used in equation 1, there is a residual, . Transforming to the right hand side of the equation there results,

A solution will have been found when is zero or sufficiently close to zero for .

In order to linearize equation 2, the partial derivatives are computed as

where

.

Linearizing the right hand side of equation 2 about the approximate solution, there results

where is the residual due to linearization which is in addition to the residual, , due to an approximate solution.

In order to drive closer to zero choose the values such that

That is choose the values

such that the residual in equation 2 changes by approximately .

Let

Substituting and transposing to the left hand side of the equation, there results

Equations 6 provide a set of four linear equations in four unknowns, the delta terms. They are in a form for solution. Using the values of and determined by this linear equation solution,

is evaluated using

Then set in equations 2 through 6, plug the terms

from equations 7 into equations 2, set in equations 7, and reevaluate the residuals in equations 2. This procedure is repeated until the residuals are sufficiently small in magnitude.

Osman Sagar

Osman Sagar popularly known as Gandipet, is an artificial lake in the Indian city of Hyderabad. The lake is around 46 km², and the reservoir is around 29 km².

History

Osman sagar was created by damming the Musi river in 1920, for providing drinking water source for Hyderabad, and also saving the city from floods, witnessed Hyderabad suffered in 1908. It was during the reign of The Last Nizam of Hyderabad, Osman Ali Khan, hence the name.

A princely guest house called Sagar Mahal, overlooking the lake, now a heritage building, was built as a summer resort of the last Nizam. It is located on the banks and has the best view of the lake. Andhra Pradesh Tourism Department[1], currently, runs the place as a resort.

It still serves the city as one of its drinking water sources.

The breeze of the lake is very pleasant, and has been popular with the locals since The Nizam's time.

It is a popular tourist destination, especially after the rainy season when the reservoir is full, and its parks, resorts, amusement park are a major attraction. This lake has serve a drinking water for hyderabad but now it was using for resort due to increase in population, it was not sufficient for water supply to Hydreabad.

It is located close to another lake, Himayat Sagar.

Images of the lake

Hussain Sagar

Hussain Sagar is a lake in Hyderabad, India built by Hazrat Hussain Shah Wali in 1562, during the rule of Ibrahim Quli Qutb Shah. It was a lake of 24 kilometres built on a tributary of the River Musi to meet the water and irrigation needs of the city. There is a large monolithic statue of the Gautam Buddha in the middle of the lake which was erected in 1992.^[1]

Tank Bund

Tank bund road

The Tank Bund dams Hussain Sagar Lake on the eastern side and connects the twin cities of Hyderabad and Secunderabad. There are 33 statues of famous people of Andhra Pradesh erected by former chief minister N.T. Rama Rao. ^[2] This boulevard has beautiful gardens on both sides, with well laid foot paths and benches, and a good volume of traffic running in between. This road gets busy during the evenings as people come here to enjoy the view and cool air from the lake and to relax at gardens. Moving further from the Tank bund road towards the other side one can connect with the Necklace road, which is another popular boulevard which many consider as a twin to the Tank bund. The sailing club is present close to the Tank bund, where many sailors participate in events held at te lake throughout the year.

Parallel to the Tank bund, is the Lower Tank Bund road which was primarily built to reduce traffic congestion. This lower Tank Bund road is heralded by the well known Missamma temple, the sprawling green Indira Park, Bharat Sevashram Sangha, Ramakrishna Mission and Snow world. An important flyover Telugu talli flyover rises from the Lower tank bund road and flies across the end of the Tank bund road.

Landmarks

Ganesh immersion in the lake

The aesthetically built Andhra Pradesh Secretariat buildings, NTR Memorial, Lumbini Park, Prasads IMAX, Hyderabad Boat Club, Sri Venkateswara Temple (Birla Mandir) and Telugu Thalli Flyover are on the Southern side. The Secunderabad Sailing Club, Sanjeevaiah park, Hotel Marriott and Hazrat Saidani Ma Saheba tomb are on the northern side of the lake. Necklace road connects these two sides along the western banks. The Railway line between Hyderabad and Secunderabad runs parallel to this road. Raj Bhavan, the residence of Andhra Pradesh Governor is on the western banks.

• Boat club and rides are provided by the A.P. tourism department.
• Lumbini Park features a musical fountain and well landscaped garden.
• Necklace road has been opened to public which passes round the Hussain Sagar Lake, has many recreational facilities and gardens.
• NTR Gardens on the Necklace road is a good place to hangout in the evenings for the young as well as the old.
• Annual immersion of Lord Ganesh idols after the 10 day Ganesh Chaturthi celebrations on Ananta Chaturdashi. Locally this event is called Ganesh Nimajjanam

Panorama of Important Landmarks around Hussain sagar.

Once the source of drinking water for the twin cities of Hyderabad and Secunderabad, the lake's current condition is far from desirable. Since the 1980s and 1990s the immersion of Ganesh idols during the festival of Ganesh Chaturthi has led to the further pollution of the polluted lake. Currently there are numerous^[3] environmentalist groups and government agencies that are trying to improve the condition of the lake.

Necklace road station

Transport

Hussain sagar has a MMTS Train station at Necklace road.

The state-owned APSRTC runs the city bus service, connecting all the major centres of the city.

CHARMINAR

Charminar (Hindi: चार मीनार, Telugu: చార్ మినార్, Urdu: چار مینار, meaning "Four Towers" or "Mosque of the four minarets") is one of the most important monuments in the city of Hyderabad, capital of the state of Andhra Pradesh, India.

Contents [hide] 1 History 2 Construction 3 Notes 4 See also 5 External links

[edit] History

Muhammad Quli Qutb Shah built the monument in 1591 shortly after he had shifted his capital from Golkonda to what now is known as Hyderabad^[1]. Legend has it that the building honours a promise Quli Qutb Shah made to Allah. He built this famous structure because one reason which is very much valid is the elimination of plague disease from this city. The masjid became popularly known as Charminar because of its four (Farsi char = four) minarets (Minar (Arabic manara) = spire/tower), which possibly honour the first four caliphs of Islam. The actual masjid occupies the top floor of the four-story structure. Madame Blavatsky reports that each of the floors was meant for a separate branch of learning - before the structure was transformed by the imperial British administration into a warehouse for opium and liqueurs.^[2] There is a legend that an underground tunnel connects the palace at Golconda to Charminar to give the Qutb Shahi royal family an escape route should they need it during a siege. However, the exact location of the tunnel is unknown.

In 1591 while laying the foundation of Charminar, Quli prayed: Oh God, bestow unto this city peace and prosperity. Let millions of men of all castes, creeds and religions make it their abode. Like fishes in the water. True to the legend, the city blossomed into a synthesis of two cultures.^{[citation needed]}

Construction

The Charminar is a beautiful and impressive square monument. Each side measures 20 m, and each of the corners has a tall, pointed minaret. These four gracefully carved minarets soar to a height of 48.7 m above the ground, commanding the landscape for miles around. Each minaret has four stories, marked by a delicately carved ring around the minaret. Unlike the Taj Mahal, Charminar's four fluted minarets of Charminar are built into the main structure. Inside the minarets 149 winding steps guide the visitor to the top floor, the highest point one can reach, which provides a panoramic view of the city.

Each side of the structure opens into a plaza through giant arches that overlook four major thoroughfares, which once upon a time were royal roads. The arches dwarf other features of the building except the minarets. Each arch is 11 m wide and rises 20 m to the pinnacle from the plinth, which is a large table raised seven or eight feet from the ground with steps that go up to it. Today, the four arches each have a clock, which was installed in 1889.

A vault that appears from inside like a dome, supports two galleries within the Charminar, one over another, and above those a terrace that serves as a roof, bordered with a stone balcony. The main gallery has 45 covered prayer spaces with a large open space in front to accommodate more people for Friday prayers.

Muhammad Quli Qutb Shah built the Charminar.

Built with granite and lime mortar, Charminar is a fine example of the Cazia style of architecture^[3]. Locally available granite, sand and lime were used in the construction of QutUb Shahi monuments including Charminar. Lime used for the plaster seems to have been specifically ground and treated to create a durable stucco. Generally shell, lime, jaggery, white of egg, etc. are known to enhance the binding property of lime. The SiO2 /CaO ratio in Charminar’s mortar and plaster (1.61-2.25) indicates that the engineers at that time were probably aware of the necessity of having a higher SiO2 content but were not sure of the value that maximizes the strength of lime cement; today, the usual practice is to have a ratio of 3.0 to 1.

It is said that, during the Mughal Governorship between Qutb Shahi and Asaf Jahi rule, the South Western minaret "fell to pieces" after being struck by lightning, but "was forthwith repaired" at a cost of Rs 60,000.^{[citation needed]} In 1824, the monument was replastered at a cost of Rs 100,000.

This graceful monument is very beautiful on the inside, and is particularly known for its carvings and mouldings. The painstaking details result in a graceful, lace-like look. The Charminar looks particularly spectacular at night when it is illuminated.

The monument overlooks another beautiful and grand mosque called Makkah Masjid. The area surrounding Charminar is also known by same name. A thriving market still lies around the Charminar, attracting people and merchandise of every description. In its heyday, the Charminar market had some 14,000 shops; today the market around the Charminar is crowded with shops which sell glass bangles in rainbow colours.

RADIO

Radio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. Information is carried by systematically changing (modulating) some property of the radiated waves, such as amplitude, frequency, or phase. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.

Etymology

Originally, radio or radiotelegraphy was called "wireless telegraphy", which was shortened to "wireless". The prefix radio- in the sense of wireless transmission, was first recorded in the word radioconductor, coined by the French physicist Édouard Branly in 1897 and based on the verb to radiate (in Latin "radius" means "spoke of a wheel, beam of light, ray"). "Radio" as a noun is said to have been coined by advertising expert Waldo Warren (White 1944). The word appears in a 1907 article by Lee De Forest, was adopted by the United States Navy in 1912 and became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun "broadcasting" itself came from an agricultural term, meaning "scattering seeds".) The term was then adopted by other languages in Europe and Asia, although British Commonwealth countries continued to use the term "wireless" until the mid-20th century.

In recent years the term "wireless" has gained renewed popularity through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network (WLAN), WiFi and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS. Today, the term "radio" often refers to the actual transceiver device or chip, whereas "wireless" refers to the system and/or method used for radio communication, hence one talks about radio transceivers and Radio Frequency Identification (RFID), but about wireless devices and wireless sensor networks.

Processes

Radio systems used for communications will have the following elements. With more than 100 years of development, each process is implemented by a wide range of methods, specialized for different communications purposes.

Each system contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to an antenna; this structure converts the rapidly-changing alternating current into an electromagnetic wave that can move through free space.

Electromagnetic waves travel through space either directly, or have their path altered by reflection, refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion (the inverse-square law); some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible; this is the fundamental limit to the range of radio communications.

The electromagnetic wave is intercepted by a receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is "tuned" to respond preferentially to the desired signals, and reject undesired signals.

Early radio systems relied entirely on the energy collected by an antenna to produce signals for the operator. Radio became more useful after the invention of electronic devices such as the vacuum tube and later the transistor, which made it possible to amplify weak signals. Today radio systems are used for applications from walkie-talkie children's toys to the control of space vehicles, as well as for broadcasting, and many other applications.

Invention

Main article: Invention of radio

The meaning and usage of the word "radio" has developed in parallel with developments within the field and can be seen to have three distinct phases: electromagnetic waves and experimentation; wireless communication and technical development; and radio broadcasting and commercialization. Many individuals -- inventors, engineers, developers, businessmen -- contributed to produce the modern idea of radio and thus the origins and 'invention' are multiple and controversial.

Development from a laboratory demonstration to commercial utility spanned several decades and required the efforts of many practitioners. Thomas Edison applied in 1885 to the U.S. Patent Office for a patent on a wireless telegraphy system which anticipated later developments in the field. The patent was granted as Patent # 465971 on December 29, 1891, and Guglielmo Marconi felt it necessary to purchase rights to the Edison wireless telegraphy patent as a foundation stone of his own subsequent work in wireless telegraphy.

Tesla demonstrating wireless transmissions during his high frequency and potential lecture of 1891. After continued research, Tesla presented the fundamentals of radio in 1893.

In 1893, in St. Louis, Missouri, Nikola Tesla made devices for his experiments with electricity. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated in detail the principles of his wireless work.^[1] The descriptions contained all the elements that were later incorporated into radio systems before the development of the vacuum tube. He initially experimented with magnetic receivers, unlike the coherers (detecting devices consisting of tubes filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884) used by Guglielmo Marconi and other early experimenters.^[2]

The first radio could not transmit sound or speech and was called the "wireless telegraph." The first public demonstration of wireless telegraphy took place in the lecture theater of the Oxford University Museum of Natural History on August 14, 1894, carried out by Professor Oliver Lodge and Alexander Muirhead. During the demonstration a radio signal was sent from the neighboring Clarendon laboratory building, and received by apparatus in the lecture theater.

In 1895 Alexander Stepanovich Popov built his first radio receiver, which contained a coherer. Further refined as a lightning detector, it was presented to the Russian Physical and Chemical Society on May 7, 1895. A depiction of Popov's lightning detector was printed in the Journal of the Russian Physical and Chemical Society the same year. Popov's receiver was created on the improved basis of Lodge's receiver, and originally intended for reproduction of its experiments.

Development

Telephone Herald in Budapest, Hungary (1901).

In 1896, Marconi was awarded the British patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for, for radio. In 1897 he established the world's first radio station on the Isle of Wight, England. Marconi opened the world's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people.

The next great invention was the vacuum tube detector, invented by Westinghouse engineers. On Christmas Eve, 1906, Reginald Fessenden used a synchronous rotary-spark transmitter for the first radio program broadcast, from Ocean Bluff-Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible. This was, for all intents and purposes, the first transmission of what is now known as amplitude modulation or AM radio. The first radio news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan, which survives today as all-news format station WWJ under ownership of the CBS network. The first college radio station began broadcasting on October 14, 1920, from Union College, Schenectady, New York under the personal call letters of Wendell King, an African-American student at the school.^[3] That month 2ADD, later renamed WRUC in 1940, aired what is believed to be the first public entertainment broadcast in the United States, a series of Thursday night concerts initially heard within a 100-mile (160 km) radius and later for a 1,000-mile (1,600 km) radius. In November 1920, it aired the first broadcast of a sporting event.^[3][4] At 9 pm on August 27, 1920, Sociedad Radio Argentina aired a live performance of Richard Wagner's Parsifal opera from the Coliseo Theater in downtown Buenos Aires, only about twenty homes in the city had a receiver to tune in. Meanwhile, regular entertainment broadcasts commenced in 1922 from the Marconi Research Centre at Writtle, England.

American girl listens to radio during the Great Depression.

One of the first developments in the early 20th century (1900-1959) was that aircraft used commercial AM radio stations for navigation. This continued until the early 1960s when VOR systems finally became widespread (though AM stations are still marked on U.S. aviation charts). In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. By the end of the decade, they were established commercial modes. Radio was used to transmit pictures visible as television as early as the 1920s. Commercial television transmissions started in North America and Europe in the 1940s. In 1954, Regency introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5 V Battery".

In 1960, Sony introduced its first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was durable, because there were no tubes to burn out. Over the next 20 years, transistors replaced tubes almost completely except for very high-power uses. By 1963 color television was being regularly transmitted commercially, and the first (radio) communication satellite, Telstar, was launched. In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links. In the 1970s, LORAN became the premier radio navigation system. Soon, the U.S. Navy experimented with satellite navigation, culminating in the invention and launch of the GPS constellation in 1987. In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software-defined radio that can be programmed to be virtually any radio by changing its software program. Digital transmissions began to be applied to broadcasting in the late 1990s.

Uses of radio

Early uses were maritime, for sending telegraphic messages using Morse code between ships and land. The earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors.

Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once it discovered that its submarine cables had been tapped by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began from San Jose in 1909^[5], and became feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detection and locating of aircraft and ships by the use of radar (RAdio Detection And Ranging).

Today, radio takes many forms, including wireless networks and mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among methods of dramatic presentation in that it used only sound. For more, see radio programming.

Audio

A Fisher 500 AM/FM hi-fi receiver from 1959.

AM radio uses amplitude modulation, in which the amplitude of the transmitted signal is made proportional to the sound amplitude captured (transduced) by the microphone, while the transmitted frequency remains unchanged. Transmissions are affected by static and interference because lightning and other sources of radio emissions on the same frequency add their amplitudes to the original transmitted amplitude. In the early part of the 20th century, American AM radio stations broadcast with powers as high as 500 kW, and some could be heard worldwide; these stations' transmitters were commandeered for military use by the US Government during World War II. Currently, the maximum broadcast power for a civilian AM radio station in the United States and Canada is 50 kW, and the majority of stations that emit signals this powerful were grandfathered in; these include WGN (AM), WJR, KGA at 50 kW. In 1986 KTNN received the last granted 50,000 watt license. These 50 kW stations are generally called "clear channel" stations (Not to be confused with the Clear channel radio conglomerate), because within North America each of these stations has exclusive use of its broadcast frequency throughout part or all of the broadcast day.

Bush House, home of the BBC World Service.

FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone causes the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. Due to its need for a wider bandwidth, FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines, hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in Long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.

High power is useful in penetrating buildings, diffracting around hills, and refracting for some distance beyond the horizon. Consequently, 100,000 watt FM stations can regularly be heard up to 100 miles (160 km) away, and farther (e.g., 150 miles, 240 km) if there are no competing signals. A few old, "grandfathered" stations do not conform to these power rules. WBCT-FM (93.7) in Grand Rapids, Michigan, USA, runs 320,000 watts ERP, and can increase to 500,000 watts ERP by the terms of its original license. Such a huge power level does not usually help to increase range as much as one might expect, because VHF frequencies travel in nearly straight lines over the horizon and off into space. Nevertheless, when there were fewer FM stations competing, this station could be heard near Bloomington, Illinois, USA, almost 300 miles (500 km) away.^{[citation needed]}

Pure One Classic- DAB Digital Radio from 2008

FM subcarrier services are secondary signals transmitted in a "piggyback" fashion along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the sub-channel program might be an alternate foreign language radio program for various ethnic groups. Sub-carriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically re-tune themselves to the same channel in a different district by using sub-bands.

Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (or kilometres) away, even though they are using VHF.

Marine voice radios can use single sideband voice (SSB) in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges. Narrowband FM sacrifices fidelity to make more channels available within the radio spectrum, by using a smaller range of radio frequencies, usually with five kHz of deviation, versus the 75 kHz used by commercial FM broadcasts, and 25 kHz used for TV sound.

Government, police, fire and commercial voice services also use narrowband FM on special frequencies. Early police radios used AM receivers to receive one-way dispatches.

Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. On an AM radio SSB sounds like ducks quacking, or the adults in a Charlie Brown cartoon. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.

TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances. Commercial services such as XM, WorldSpace and Sirius offer encrypted digital Satellite radio.

Telephony

Mobile phones transmit to a local cell site (transmitter/receiver) that ultimately connects to the public switched telephone network (PSTN) through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. Recent developments in Sweden (such as DROPme) allow for the instant downloading of digital material from a radio broadcast (such as a song) to a mobile phone.

Satellite phones use satellites rather than cell towers to communicate.

Video

Television sends the picture as AM and the sound as AM or FM, with the sound carrier a fixed frequency (4.5 MHz in the NTSC system) away from the video carrier. Analog television also uses a vestigial sideband on the video carrier to reduce the bandwidth required.

Digital television uses 8VSB modulation in North America (under the ATSC digital television standard), and COFDM modulation elsewhere in the world (using the DVB-T standard). A Reed–Solomon error correction code adds redundant correction codes and allows reliable reception during moderate data loss. Although many current and future codecs can be sent in the MPEG-2 transport stream container format, as of 2006 most systems use a standard-definition format almost identical to DVD: MPEG-2 video in Anamorphic widescreen and MPEG layer 2 (MP2) audio. High-definition television is possible simply by using a higher-resolution picture, but H.264/AVC is being considered as a replacement video codec in some regions for its improved compression. With the compression and improved modulation involved, a single "channel" can contain a high-definition program and several standard-definition programs.

Navigation

All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.

Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators. LORAN systems also used time-of-flight radio signals, but from radio stations on the ground. VOR (Very High Frequency Omnidirectional Range), systems (used by aircraft), have an antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing or radial from the station, thus establishing a line of position. An aircraft can get readings from two VORs and locate its position at the intersection of the two radials, known as a "fix." When the VOR station is collocated with DME (Distance Measuring Equipment), the aircraft can determine its bearing and range from the station, thus providing a fix from only one ground station. Such stations are called VOR/DMEs. The military operates a similar system of navaids, called TACANs, which are often built into VOR stations. Such stations are called VORTACs. Because TACANs include distance measuring equipment, VOR/DME and VORTAC stations are identical in navigation potential to civil aircraft.

Radar

Radar (Radio Detection And Ranging) detects objects at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface. Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft.

General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.

Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the Doppler Effect to separate moving vehicles from clutter. Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more. Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the Doppler Effect to measure wind speeds.

Data (digital radio)

Most new radio systems are digital, see also: Digital TV, Satellite Radio, Digital Audio Broadcasting. The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful