How GPS works

by Mel Owen

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 knows exactly where it is at all times, 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 that satellite’s location. Each transmits on its individual frequency, to ensure that they are all distinguishable one from the other.

Your GPS microchip receives the signals from those GPS satellites which are ‘visible’ (not obscured by buildings, trees, hills). 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 that 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.

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