There are 24 GPS satellites orbiting the Earth in six orbits (in groups of four). There are usually no more than a dozen satellites on the same side of the Earth. Most GPS receivers can track 8 to 12 satellites. At least 3 satellites are required to calculate LAT/LONG (2 dimensional) coordinates. Add one more and you can calculate the three dimensional coordinates. For a given location, the GPS receiver knows which satellites are nearby at that moment because it is constantly receiving updates from them.

　　Some consumer GPS devices have two to five parallel channels to receive satellite signals. Because up to 12 satellites may be visible (the average is 8), this means that the GPS receiver must visit each satellite in order to get information about each satellite.

　　Most GPS receivers on the market are 12-channel and 20-channel models (UTSU Mio269, etc.), which allow them to continuously track the information of each satellite. Advantages of the 12-channel receiver include fast cold start and initialization of satellite information, as well as better reception in forested areas. Generally, 20-channel receivers do not need external antennas, but in enclosed Spaces such as cabins and carriages, external antennas are still needed to ensure accurate reception of GPS signals

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