The GPS system consists of three parts

　　The space part is the general term for GPS artificial satellites. The average height of the artificial satellite is about 20200Km. Its orbit is an ellipse, and the earth is at a focus of the ellipse; The operation cycle is about 12 hours. Nearly 30 navigation satellites, some of which are standby satellites, are unevenly distributed on six orbital planes with an inclination of about 55 °. The U.S. military can adjust the number of working satellites through ground control. In the GPS system, the GPS satellite is a dynamic known point, and all the navigation and positioning information of the user end is calculated according to the "ephemeris" sent by the dynamic known point. GPS ephemeris is actually a series of real-time state parameters that describe the movement and orbit of GPS satellites. The broadcast ephemeris received by the civil GPS module is a prediction ephemeris for real-time data processing directly broadcast by GPS satellites to users through navigation messages in the form of spread spectrum communication. The civil pseudo-random code C/A ephemeris and military P-code ephemeris are broadcast on different carriers at different rates.

　　For the whole GPS system, the ground control part is actually the core of the whole system. The ephemeris for navigation and positioning broadcast by all GPS satellites are provided by five monitoring stations distributed on the ground. The ground system is responsible for monitoring GPS signals, collecting data, calculating and injecting navigation messages, status diagnosis, orbit correction, etc. It is the massive data processing of the ground monitoring system that makes the GPS system operate accurately.

　　The GPS positioning module is often called the user part. Like a "radio", it receives and demodulates satellite broadcast C/A code signals, with a frequency of 1575.42MHz. The GPS module does not broadcast signals and belongs to passive positioning. By calculating the pseudo distance between each satellite and the receiver, the longitude, latitude, altitude and time correction of the receiver are obtained by using the range intersection method. The characteristic is that the point position speed is fast, but the error is large. The initial positioning module requires at least 4 satellites to participate in the calculation, which is called 3D positioning. Three satellites can achieve 2D positioning, but the accuracy is poor. The GPS module continuously outputs the positioning information and auxiliary information in NMEA format through the serial communication port for the receiver to select applications.

　　The price index of GPS module performance mainly includes reception sensitivity, positioning time, position accuracy, power consumption, time accuracy, etc. The module boot positioning time varies greatly in different boot modes. Generally speaking, the cold start time refers to the condition that no data helpful for positioning is stored in the module, including ephemeris, time, etc., which is generally nominal within 1 minute; Warm start time means that there is a relatively new satellite ephemeris in the module (generally no more than 2 hours), but the time deviation is large, usually within 45 seconds; The hot start time refers to the condition that the shutdown does not exceed 20 minutes and the RTC time error is very small. Generally, the nominal value is within 10 seconds; Recapturing time is like a car drilling through a tunnel and recapturing satellites when it leaves the tunnel. Generally, the nominal value is less than 4 seconds.

　　If the module is placed for a long time after positioning, or the module is transported hundreds of kilometers away after positioning, there is an ephemeris inside the module, but this ephemeris is wrong or has no reference significance. In these cases, the positioning time may take several minutes or even longer. Therefore, when the GPS module leaves the factory, the ephemeris and other data inside the module should be cleared, so that the customer can quickly locate the module by cold start.

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