File:GPS These are GPS receivers. People carry them to learn where they are and plan where and how to go to the next place.
A Global Positioning System, also known as GPS, is a system designed to help navigate on the Earth, in the air, and on water.
A GPS receiver shows where it is. It also shows 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 GPSs for planes, hiking and backpacking, bicycling, or many other activities.
Most GPS receivers 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
File:ConstellationGPS.gif GPS satellites circle the earth in four planes, plus a group over the equator. This example shows the number of satellites visible to a GPS receiver at 45° North in blue. Red satellites are blocked by the Earth.
A GPS unit gets radio signals from satellites in space circling the Earth. There are about 30 satellites 20,200 kilometres (12,600 mi) above the Earth. (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 very good time and position of the satellite. 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. A GPS receiver calculates its speed and direction by using its change in position and change in time.
Many inexpensive consumer receivers are accurate to 20 metres (66 ft) most anywhere on the Earth.
Sometimes military and costly commercial receivers are accurate to 30 centimetres (12 in).
History
The system was created by the United States Department of Defense. In the beginning, it was only used by the U.S. military, but in 1983 President Ronald Reagan made an order to allow anyone to use the system.
Sometimes GPS receivers are part of cell phones, wrist watches, and cars.
Other systems
There are other systems that act in the same way. One was put in space by Russia, called GLONASS. Another that is not yet done is Galileo, built by the European Union.
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