The GPS positioning module, which we often refer to as the user part, receives and demodulates satellite broadcast C/A code signals like a "radio", with a center frequency of 1575.42MHz. The GPS module does not broadcast signals and belongs to passive positioning. By calculating the pseudo distance with each satellite and using the distance intersection method, the receiver's four parameters of longitude, latitude, altitude, and time correction are obtained. 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, called 3D positioning. Three satellites can achieve 2D positioning, but the accuracy is poor. The GPS module continuously outputs positioning information and auxiliary information in NMEA format through the serial communication port for the receiver to choose applications.

　　The * price indicators of GPS module performance mainly include reception sensitivity, positioning time, position accuracy, power consumption, time accuracy, etc. The positioning time of module startup varies greatly under different startup modes. Generally speaking, the cold start time refers to the situation where there is no data stored inside the module that helps with positioning, including ephemeris, time, etc., which is usually nominal within 1 minute; Warm start time refers to the time when there is a relatively new satellite ephemeris inside the module (usually no more than 2 hours), but the time deviation is significant, usually within 45 seconds nominal; The hot start time refers to the situation where the shutdown does not exceed twenty minutes and the RTC time error is very small. Generally, the nominal time is within 10 seconds; Recapturing time is like a car drilling through a tunnel and capturing satellites again when it exits. Generally, the nominal time is within 4 seconds.

　　If the module is stored for a long time after positioning, or if the module is transported hundreds of kilometers away after positioning, there will be an ephemeris inside the module, but this ephemeris is incorrect or has no reference significance. In these cases, the positioning time may take several minutes or even longer. So generally, when GPS modules leave the factory, they need to clear the ephemeris and other data inside the module, so that customers can quickly locate them through cold start after receiving the module.

　　The positioning accuracy can be evaluated in both static and dynamic situations, and the dynamic positioning effect is superior to static positioning. The nominal positioning parameters of the GPS module refer to measurements made under fully open skies with excellent satellite signals. So it is difficult to achieve the nominal positioning time and positioning accuracy in conventional testing. There are two common ways to describe horizontal positioning accuracy: first? MCEP, namely Circular error probable, means that 50% probability of the measured point is located in a circle with the true coordinate as the center? Within a circle with a radius of m; The second is? M2DRMS, that is, twice the water average root mean square error, means that there is a 95.5% probability that the measured point is located in a circle with the true coordinate as the center? Within a circle with a radius of m.

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