What is Global Positioning System (GPS)?
GPS stands for Global Positioning System, and it is a space-based radio navigation system that delivers very accurate navigation pulses to users on or near the Earth. Every 12 hours, 24 major satellites in 6 orbits circle Earth in the Navstar GPS of the United States. Furthermore, Russia operates the GLONASS (Global Navigation Satellite System) constellation, and the European Union approved funding in 2007 for the launch of 30 satellites to build its version of GPS, known as Galileo, which began operations in 2016.
China launched two satellites in 2000 and another in 2003 as part of the BeiDou (“Big Dipper”) local navigation system. China, which had only a limited role in Galileo, revealed intentions in 2006 to expand BeiDou into a full GPS service called the BeiDou Navigation System. China launched a series of 14 second-generation satellites known as BeiDou-2, or Compass, in 2007 to provide services in the country. BeiDou-3, a third-generation constellation of 30 satellites, was launched in 2020 and already provides global coverage.
A GPS receiver operated by a user on Earth measures the time it takes radio signals from four or more satellites to reach its location, calculates the distance to each satellite, and derives the user’s longitude, latitude, and altitude based on this calculation. The Navstar constellation was designed for military use by the US Department of Defense, but a less precise version of the service is now provided for free to civilian users all around the world.
The basic civilian service will locate a receiver within 10 meters (33 feet) of its true location, while other augmentation techniques can pinpoint the location within 1 cm (0.4 inches). GPS has expanded much beyond its original military function and has sparked a revolution in personal and commercial navigation, due to its accuracy and ubiquity. Battlefield missiles and artillery projectiles, like the US space shuttle and the International Space Station, as well as commercial jetliners and private planes, use GPS signals to establish their positions and velocities. GPS location benefits ambulance fleets, family automobiles, and railroad locomotives, as well as farm tractors, ocean liners, hikers, and even golfers.
Many GPS receivers are the size of a pocket calculator and run on disposable batteries, while GPS computer chips the size of a baby’s fingernail are found in wristwatches, cellular phones, and personal digital assistants.
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How Global Positioning System works?
GPS satellites orbit the Earth twice a day in a precise orbit. Each satellite sends out a unique signal with orbital data that GPS devices can decode and use to calculate the satellite’s precise location. GPS receivers employ this information, together with trilateration, to determine a user’s precise location. The GPS receiver uses the time it takes to receive a broadcast signal to compute the distance between each satellite. The receiver can calculate a user’s position and show it electronically using distance measurements from a couple of additional satellites, allowing you to measure your running route, map a golf course, find your way home, or embark on an adventure anyplace.
The triangulation theory underpins GPS’s unrivaled navigational capabilities. A GPS receiver triangulates by measuring the time it takes a satellite signal to travel from space to Earth—less than a tenth of a second. The time is then multiplied by the speed of a radio wave, which is 300,000 km (186,000 miles) per second, to get the distance between it and the satellite. This places the receiver on the surface of an imaginary sphere with a radius equal to the satellite’s distance. When signals from three further satellites are handled in the same way, the receiver’s built-in computer calculates the location where all four spheres cross, determining the user’s current longitude, latitude, and altitude.
(Three satellites would ordinarily provide an unambiguous three-dimensional fix, but at least four are used in practice to compensate for receiver clock inaccuracies.) The receiver also determines current velocity (speed and direction) by measuring the instantaneous Doppler effect shifts caused by the movements of the same four satellites.
Even though a satellite signal just takes a fraction of a second to reach Earth, a lot can happen in that time. Satellite signals may be slowed and distorted by electrically charged particles in the ionosphere and density fluctuations in the troposphere, for example. For GPS users, these factors can result in positioning mistakes, which can be exacerbated by timing errors in GPS receiver clocks. Relativistic time dilations, a phenomenon in which a satellite’s clock and a receiver’s clock, positioned in separate gravitational fields and traveling at different speeds, tick at different rates, may contribute to additional inaccuracies. Finally, the reduced precision of the civilian C/A-code pulse is the single greatest cause of inaccuracy for Navstar users.
However, there are a variety of augmentation methods available to improve the accuracy of both military and civilian systems. Differential GPS techniques can be used when pinpoint precision is required for location information. Differential navigation relies on a stationary “base station” that sits at a predetermined location on the ground and continuously examines the signals emitted by GPS satellites in its field of vision. The system then calculates and transmits real-time navigation corrections to roving receivers in the area. In effect, each wandering receiver subtracts its position solution from the base station’s solution, removing any statistical inaccuracies that may exist between the two. The United States Coast Guard maintains a network of such base stations and sends corrections via radio beacons across the country.
Other differential corrections are encoded in commercial radio stations’ usual broadcasts. Precision farming has become a frequent term in agriculture as a result of farmers hearing these broadcasts being able to direct their field equipment with amazing accuracy.
Another method of GPS augmentation is to employ the carrier waves that carry the navigation pulses from the satellites to Earth. Because the carrier wave is more than 1,000 times shorter than the basic navigation pulses, this “carrier-aided” technique can reduce navigation errors to less than 1 cm in the correct circumstances (0.4 inches). The shorter length and significantly higher number of carrier waves impinging on the receiver’s antenna each second account for the vastly enhanced accuracy. Geosynchronous overlays are another enhancement approach. GPS payloads are “piggybacked” onto commercial communication satellites in geostationary orbit, which is about 35,000 kilometers (22,000 miles) above Earth.
The Navstar system
The space segment, the control segment, and the user segment are the three key segments of the Navstar GPS. The Navstar constellation in orbit around Earth makes up the space component. In 1978, an experimental Block I model was launched as the first satellite. Over the next decade, nine additional of these developmental satellites were launched, followed by 23 heavier and more capable Block II production models launched between 1989 and 1993. The 24th Block II satellite was launched in 1994, completing the GPS constellation, which now consists of two dozen Block II satellites marching in a single line in six circular orbits around Earth (plus three spares orbiting in reserve).
Most spots on Earth may see at least five satellites at any given moment because of their orbits. Newer Block II satellites have been launched to replace previous types since 1994. The first Block III satellite was launched in 2018. The last launch of ten Block III satellites was slated for 2023.
A typical Block II satellite weighs around 900 kg (2,000 pounds) and measures around 17 meters (56 feet) broad when its solar panels are unfurled. The winglike solar arrays that generate electrical power from sunlight, the 12 helical antennae that communicate navigation pulses to ground users, and the long, spearlike radio antenna that picks up orders from control engineers are all important components. As a satellite rides through its 12-hour orbit, its main body pivots, and the solar arrays swivel, maintaining the navigation antennae pointed toward Earth’s center and the solar arrays perpendicular to the Sun’s beams.
The control segment consists of a Master Control Station at a United States Air Force base in Colorado, as well as four unmanned monitoring stations located around the world: Hawaii and Kwajalein Atoll in the Pacific Ocean, Diego Garcia in the Indian Ocean, and Ascension Island in the Atlantic Ocean. To check for orbital changes, each monitoring station tracks all of the GPS satellites in its field of view. Gravitational pulls from the Moon and Sun, the Earth’s non spherical form, and the pressure of solar radiation creates variations in satellite orbits.
The Master Control Station processes this data, and the revised orbital information is rapidly communicated to the satellites via enormous ground antennas. The satellites within a given ring drift too far from their original configuration every eighteen months on average and must be forced back with onboard thrusters fired by ground control.
The millions of GPS receivers that pick up and decode satellite signals make up the user segment. There are hundreds of distinct types of GPS receivers in use; some are built for use in automobiles, trucks, submarines, ships, aircraft, and orbiting satellites, while others are designed for personal navigation.