The Precision of GPS

A typical hand-held GPS receiver calculates position by using the signals from at least four or more Satellites. In order to work properly, four satellites are needed to insure a very accurate local time and position. In other words, the hand-held GPS uses four measurements to solve for four variables: x, y, z, and t.

Every satellite sends signals that travel at the speed of light through outer space, and slightly slower through the atmosphere. The hand-held device then uses the arrival time of the signal to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry.