1、 Satellite  trajectory

Currently, GPS has 24 satellites orbiting the Earth along six  orbits, and there are generally not more than 12 satellites on the same side of  the Earth. Most GPS receivers can track 8-12 satellites. Calculating  two-dimensional coordinates requires at least three satellites, and adding one  satellite can calculate three-dimensional coordinates. For a specific location,  the receiver knows which satellites are nearby, and the principle is to  continuously receive signals from the satellites for updates.

2、 Parallel  channel

Generally, ordinary GPS devices receive satellite signals through  2-5 parallel channels. According to the rule that up to 12 satellites may be on  the same side, the GPS receiver must sequentially access each satellite to  obtain relevant data information.

Most high-end GPS receivers are 12 parallel channels, which allows  them to track the information of each satellite at once. Its advantages include  fast cold start and initialization of satellite information, especially in  forest areas where better reception performance can be achieved. Generally, a 12  channel receiver does not require an external antenna, unless in a closed space,  such as a cabin or carriage.

3、 Positioning  time

The positioning time is generally the time it takes to determine  the current position when restarting the GPS receiver. For a 12 channel  receiver, if it is located near the last positioning position, the cold start  positioning time is about 4 minutes and the hot start is about 20 seconds, while  for a 2 channel receiver, if the cold start exceeds 16 minutes, the hot start  also takes 4 minutes.

4、 Positioning  accuracy

When SA is not turned on, the horizontal positioning accuracy of  most GPS receivers is around 10m.

5、 DGPS  function

In order to minimize the impact of SA and atmospheric refraction,  there is a device called DGPS transmitter. It is a fixed GPS receiver that, when  receiving satellite signals, can accurately determine the exact time at which  the satellite signals are transmitted in theory. Then, it compares it with the  actual transmission time and calculates the difference, which is very close to  the impact of SA and atmospheric refraction. When sent out, other GPS receivers  can use it to obtain a more accurate position reading.

Many GPS device providers have set up DGPS transmitters in some  regions for their customers to use for free, as long as the GPS receivers  purchased by customers have DGPS functionality.

6、 Signal  interference

To achieve good positioning, a GPS receiver requires at least 3  satellites to be visible. If you are in a dense jungle, canyon, or on a street  lined with tall buildings, you may not be able to connect with enough  satellites, so you can only locate two-dimensional coordinates. Similarly, if  inside a building, it may not be possible to update the location, and some GPS  receivers have separate antennas that can be attached to the windshield, or an  external antenna that can be placed on the roof, which helps the receiver  receive more satellite signals

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