1. DGPS function

　　To minimize the effects of SA and atmospheric refraction, there is a device called a DGPS transmitter. It is a fixed GPS receiver (set within a radius of 100km to 200km of a survey site) to receive the signal from the satellite, it knows exactly what the precise time the satellite signal is theoretically transmitted to, and then correlates it with the actual transmission time. Compare, then calculate the "difference", which is very close to the effect of SA and atmospheric refraction, and send this difference so that other GPS receivers can use it to get a more accurate position reading (5m to 10m or less error).

　　Many GPS equipment providers set up DGPS transmitters in some areas for their customers to use free of charge, as long as the GPS receivers purchased by customers have DGPS capabilities.

　　2. Signal interference

　　To give you a good fix, a GPS receiver needs at least 3-5 satellites to be visible. If you're in a canyon or on a street lined with tall buildings, or in a dense jungle, you may not be able to communicate with enough satellites to be able to locate or get only two-dimensional coordinates. Also, if you're inside a building, you may not be able to update your location, 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 you receivers get more satellite signals.

　　3. Physical indicators

　　When purchasing GPS equipment, physical indicators such as size, weight, display screen, waterproof, shockproof, dustproof performance, high temperature resistance, and power consumption must be considered.

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Basic knowledge of GPS satellite positioning system