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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