Trilateration in GPS

GPS units use trilateration to pinpoint a position on the Earth's surface

Navigation instrument on map, close-up
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Global Positioning System units use the mathematical technique of trilateration to determine user position, speed, and elevation. GPS units constantly receive and analyze radio signals from several GPS satellites. They use these signals to calculate the precise distance or range to each satellite being tracked.

How Trilateration Works

Trilateration is a sophisticated version of triangulation. Data from a single satellite pinpoints a position to a large area of the earth's surface. Adding data from a second satellite narrows the position down to the region where the two spheres of satellite data overlap. Adding data from a third satellite provides a relatively accurate position, and all the GPS units require three satellites for an accurate placement. Data from a fourth satellite— or more than four satellites—enhances precision and determines accurate elevation or, in the case of aircraft, altitude. GPS receivers routinely track four to seven satellites or even more simultaneously and use trilateration to analyze the information. 

The U.S. Department of Defense maintains the 24 satellites that relay data worldwide. Your GPS device can remain in touch with at least four satellites no matter where you are on earth, even in wooded areas or major metropolises with tall buildings. Each satellite orbits the earth twice a day, regularly sending signals to earth, at an altitude of about 12,500 miles. Satellites run on solar energy and have backup batteries.  

GPS History 

GPS was introduced in 1978 with the launch of the first satellite. It was controlled and used solely by the military until the 1980s. The full fleet of 24 active satellites controlled by the U.S. wasn't in place until 1994. 

When GPS Fails 

When a GPS navigator receives insufficient satellite data because it's not able to track enough satellites, trilateration fails. The navigator notifies the user rather than provide incorrect position information. Satellites also sometimes fail temporarily because signals move too slowly due to factors in the troposphere and ionosphere. Signals might also ping off certain formations and structures on earth, causing a trilateration error.