How GPS works

Many of us rely on GPS in the mountains from time to time, but when asked to explain it, we reveal that although we certainly know what it does, many of us do not know how it does it, explained as follows.

The USA is committed to maintaining the availability of at least 24 GPS satellites, 95% of the time. The U.S. Space Force has been flying 31 operational GPS satellites for well over a decade. They fly in medium Earth orbit at an altitude of approximately 20,200 km, each circling the Earth twice a day. Each satellite always knows exactly where it is in terms of latitude, longitude, and altitude. At precisely the same instant in time, at one second intervals, each satellite transmits a timing pulse, coded with the transmitting satellite’s location. Each transmits on its own individual frequency, to ensure that they are all distinguishable from one another.

Your GPS microchip receives the signals from those GPS satellites in the hemisphere above which are in clear line of sight. The next step is to measure the minute differences in the times of arrival of these pulses from each satellite. Then, using some very complex three-dimensional digital geometry and algebra, your GPS microchip calculates its own latitude, longitude and altitude.

The reason why GPS is slightly less accurate when calculating altitude than latitude and longitude is that moving a GPS device laterally will always produce slightly greater differences in the arrival times of the pulses from different satellites than moving it vertically, and the greater these differences, the greater the accuracy. At sea, altitude errors are unavoidable. GPS ignores tidal changes, since it has to provide consistent land data based on mean sea level.



Return to the top of this page, to the Home Page or to the Blog Index.