How GPS Works

The basis of GPS is triangulation from satellites. Position is calculated from distance measurements (ranges) to satellites. Mathematically we need four satellite ranges to determine exact position. Three ranges are enough if we reject ridiculous answers or use other tricks.

To triangulate, a GPS receiver measures distance using the travel time of radio signals. Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite. To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time. By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light and you've got distance.

To measure travel time, GPS needs very accurate timing which it achieves with some tricks. Accurate timing is the key to measuring distance to satellites. Satellites are accurate because they have atomic clocks on board. Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove errors.

Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. To use the satellites as references for range measurements we need to know exactly where they are. GPS satellites are so high up their orbits are very predictable. Minor variations in their orbits are measured by the Department of Defense. The error information is sent to the satellites, to be transmitted along with the timing signals.

Finally you must correct for any delays the signal experiences as it travels through the atmosphere. The earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors. Some errors can be factored out using mathematics and modeling. The configuration of the satellites in the sky can magnify other errors. Differential GPS can eliminate almost all error.

What's your GPS application? Find the right equipment by determining your GPS needs.