Components of a GPS system

　　GPS system consists of three parts: space part, control part and user part.

　　The space part is a general term for GPS satellites. The average height of the artificial satellite is about 20200Km, the orbit is an ellipse, and the earth is located at a focus of the ellipse; the operation period is about 12 hours. Nearly 30 navigation satellites are unevenly distributed on the six orbital planes with an inclination of about 55°, some of which are spare satellites. The US military can adjust the number of working satellites through the ground control part. In the GPS system, the GPS satellite is a dynamic known point, and all the navigation and positioning information of the user terminal is calculated based on the "ephemeris" sent by this dynamic known point. GPS ephemeris is actually a series of real-time state parameters describing the movement and orbit of GPS satellites. The broadcast ephemeris received by the civil GPS module is the forecast ephemeris directly broadcast to the user by the GPS satellite through the navigation message in the spread spectrum communication mode for real-time data processing, and broadcasts the civil pseudo ephemeris at different rates on different carriers. Random code C/A code ephemeris and military P code ephemeris.

　　For the whole GPS system, the ground control part is actually the core of the whole system. The ephemerides used for navigation and positioning broadcast by all GPS satellites are provided by 5 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 enables the GPS system to operate accurately.

　　The GPS positioning module we often say is called the user part. It receives and demodulates the broadcast C/A code signal of the satellite like a "radio", and the intermediate frequency is 1575.42MHz. The GPS module does not broadcast signals and belongs to passive positioning. By calculating the pseudo-distance with each satellite, the four parameters of longitude, latitude, altitude and time correction of the receiver are obtained by using the distance rendezvous method. The module for initial positioning requires at least 4 satellites to participate in the calculation, which is called 3D positioning. 3 satellites can achieve 2D positioning, but the accuracy is not good. The GPS module continuously outputs the positioning information and auxiliary information in NMEA format through the serial communication port for the receiver to choose the application.

　　The most expensive indicators of GPS module performance mainly include receiving sensitivity, positioning time, position accuracy, power consumption, time accuracy, etc. The module startup positioning time is very different in different startup modes. Generally speaking, the cold start time refers to the situation that the module does not save any data that is helpful for positioning, including ephemeris, time, etc., generally nominally within 1 minute; the warm start time means that there are newer satellites inside the module Ephemeris (generally no more than 2 hours), but the time deviation is very large, generally within 45 seconds; the hot start time refers to the case where the shutdown does not exceed 20 minutes, and the RTC time error is very small. Typically nominally within 10 seconds; the recapture time is like a car going through a tunnel and recapturing a satellite as it exits the tunnel. Generally nominal within 4 seconds.

　　If the module is placed for a long time after positioning, or the module is transported to a place 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 can take several minutes or more. Therefore, the general GPS module should clear the ephemeris and other data inside the module when it leaves the factory, so that the customer can quickly locate the module by cold start after getting the module.

　　The positioning accuracy can be investigated under static and dynamic conditions, and the effect of dynamic positioning is better than that of static positioning. The nominal positioning parameters of the GPS module are measured in a completely open sky with good satellite signals. Therefore, it is difficult to achieve the nominal positioning time and positioning accuracy in conventional tests. There are two common description methods for horizontal positioning accuracy: one is ?m CEP, that is, the circle probability error, which means that the measured point has a 50% probability of being located in a circle with the real coordinates as the center and ?m as the radius; The second is ?m 2DRMS, that is, 2 times the horizontal mean square root error, which means that the measured point has a probability of about 95.5% to be located in a circle with the real coordinates as the center and ?m as the radius.

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