Global Positioning System facts for kids
A Global Positioning System, also known as GPS, is a system of satellites designed to help navigate on the Earth, in the air, and on water.
A GPS receiver shows where it is. It may also show how fast it is moving, which direction it is going, how high it is, and maybe how fast it is going up or down. Many GPS receivers have information about places. GPSs for automobiles have travel data like road maps, hotels, restaurants, and service stations. GPSs for boats contain nautical charts of harbors, marinas, shallow water, rocks, and waterways. Other GPS receivers are made for air navigation, hiking and backpacking, bicycling, or many other activities. The majority are in smartphones.
Most GPS receivers can record where they have been, and help plan a journey. While traveling a planned journey, it predicts the time to the next destination.
How it works
A GPS unit takes radio signals from satellites in space in orbit around the Earth. There are 31 satellites which are 20,200 kilometres (12,600 mi) above the Earth. The orbital period is 11 hours and 58 minutes. Each circle is 26,600 kilometres (16,500 mi) radius due to the Earth's radius. Far from the North Pole and South Pole, a GPS unit can receive signals from 6 to 12 satellites at once. Each satellite contains an atomic clock which is carefully set by NORAD several times every day.
The radio signals contain information about the time and position of the satellite, including its ephemeris. The GPS receiver subtracts the current time from the time the signal was sent. The difference is how long ago the signal was sent. The time difference multiplied by the speed of light is the distance to the satellite. The GPS unit uses trigonometry to calculate where it is from each satellite's position and distance. Usually there must be at least four satellites to solve the geometric equations. A GPS receiver can calculate its position many times in one second.
Many inexpensive consumer receivers are accurate to 20 metres (66 ft) almost anywhere on the Earth.
A GPS unit can usually also calculate its current speed. Cheap ones like in a mobile phone do this by comparing present position with recent position. Expensive ones like in an airliner use the Doppler effect and are very accurate.
The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including classified engineering design studies from the 1960s. The U.S. Department of Defense developed the system, which originally used 24 satellites. It was initially developed for use by the United States military and became fully operational in 1995. Civilian use was allowed from the 1980s.
The work of Gladys West is credited as instrumental in the development of computational techniques for detecting satellite positions with the precision needed for GPS.
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.
In 1955, Friedwardt Winterberg proposed a test of general relativity – detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites.
Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth's observers to run 38 microseconds faster per day than the clocks on the Earth. The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day (6 mi/d). This was corrected for in the design of GPS.
Some GPS receivers are separate units with their own power and display. These were the majority in the 20th century. Military receivers then displayed only the geographic coordinates, or some had no display but only gave the coordinates to another machine.
Now, the majority of GPS receivers are part of mobile phones, and many are built into wristwatches, cars and other devices. The GPS part of a mobile phone is small and usually poor, but the phone also uses mobile base stations and Wi-Fi signals to help. This is called aGPS or "augmented GPS".
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Global Positioning System Facts for Kids. Kiddle Encyclopedia.