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What is trilateration?

A GPS receiver uses trilateration (a more complex version of triangulation) to determine its position on the surface of the earth by timing signals from three satellites in the Global Positioning System. The GPS is a network of satellites that orbit the earth and send a signal to GPS receivers providing precise details of the receiver's location, the time of day, and the speed the device is moving in relation to the three satellites.

Each satellite in the GPS constellation sends out periodic signals along with a time signal. These are received by GPS devices, which then calculate the distance between the device and each satellite based on the delay between the time the signal was sent and the time when it was received. The signals travel at the speed of light, but there is a delay because the satellites are at an altitude of tens of thousands of kilometers above the earth.

Once a GPS device has distances for at least three satellites, it can perform the trilateration calculations. Trilateration works in a similar way to pinpointing your position on a map knowing the precise distance from three different landmarks using a pair of compasses. Where the three circles centered on each of the landmarks overlap is your location given the radius of each circle is your distance from each landmark.

In the GPS version, the calculations are carried out in three-dimensions with an imaginary set of 3D compasses so that your location is where three spheres of radius given by the distance to each of three satellites overlap. If the GPS device can see a fourth satellite, then the measurements can be double-checked.

The calculation process happens very quickly, allowing the GPS device to pinpoint its location, altitude (if it is in an aircraft), speed and direction.

The transmissions are timed to begin precisely on the minute and the half minute as indicated by the satellite's atomic clock. The first part of the GPS signal tells the receiver the relationship between the satellite's clock and GPS time. The next chunk of data gives the receiver the satellite's precise orbit information.