1. Satellite trajectory

　　There are 24 GPS satellites orbiting the earth along six orbits (every four satellites in a group). Generally, no more than 12 satellites are 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 (2D) coordinates. Add another one to calculate 3D coordinates. For a given position, the GPS receiver knows which satellites are nearby at this time, because it continuously receives update signals from satellites.

　　2. Parallel channel

　　Some consumer GPS devices have 2-5 parallel channels to receive satellite signals. Because at most 12 satellites may be visible (the average is 8), this means that the GPS receiver must access each satellite in order to obtain information about each satellite.

　　Most GPS receivers on the market are 12 parallel channels, which allows them to continuously track the information of each satellite. The advantages of 12 channel receivers include fast cold start and initialization of satellite information, and they can have better reception effect in forest areas. Generally, a 12 channel receiver does not need an external antenna unless you are in a closed space, such as a cabin or carriage.

　　3. Positioning time

　　This refers to the time it takes for your GPS receiver to determine its current position when you restart it. For a 12 channel receiver, if you are near the last positioning position, the positioning time is generally 3-5 minutes in cold start and 15-30 seconds in hot start, while for a 2-channel receiver, the cold start time is more than 15 minutes, and the hot start time is 2-5 minutes.

　　4. Positioning accuracy

　　The horizontal positioning accuracy of most GPS receivers is about 5m~10m, but this is only when SA is not turned on.

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