The Global Positioning System, also called NavStar, that we use was built by the US military and has been fully operational since 1995. Many modern real time GPS tracking device receivers use a combination of both GPS and the Russian GLONASS satellites or Chinese BEIDOU for improved coverage and accuracy.
The real time GPS vehicle tracking system currently has 31 active satellites in orbits inclined 55 degrees to the equator. The satellites orbit about 20,000km from the earth’s surface and make two orbits per day. The orbits are designed so that there are always 6 satellites in view, from most places on the earth. The real time tracking device uses a lot of complex technology, but the concept is simple.
The real time tracking device receiver gets a signal from each GPS satellite. The satellites transmit the exact time the signals are sent. By subtracting the time the signal was transmitted from the time it was received, the real time GPS tracking device can tell how far it is from each satellite. The real time GPS tracking device receiver also knows the exact position in the sky of the satellites, at the moment they sent their signals. So given the travel time of the GPS signals from three satellites and their exact position in the sky, the real time GPS tracking device receiver can determine your position in three dimensions – east, north, and altitude then show on real time GPS tracking system.
There is a complication. To calculate the time the GPS signals took to arrive, the real time GPS tracking device receiver needs to know the time very accurately. The GPS satellites have atomic clocks that keep very precise time, but it’s not feasible to equip a real time GPS tracking device receiver with an atomic clock. However, if the real time tracking device receiver uses the signal from a fourth satellite it can solve an equation that lets it determine the exact time, without needing an atomic clock.
If the real time tracking device receiver is only able to get signals from 3 satellites, you can still get your position, but it will be less accurate. As we noted above, the real time tracking device receiver needs 4 satellites to work out your position in 3-dimensions. If only 3 satellites are available, the real time GPS tracking device receiver can get an approximate position by making the assumption that you are at mean sea level. If you really are at the mean sea level, the position will be reasonably accurate. However, if you are in the mountains, the 2-D fix could be hundreds of meters off.
A modern real time GPS tracking device receiver will typically track all of the available satellites simultaneously, but only a selection of them will be used to calculate your position and used for real time GPS fleet tracking.