Civil GPS

　　Generally, L1 carrier of GPS system is used for civil GPS, and the frequency is 1575.42 MHz. Two different pseudo-random noise codes, C/A code and P code, are loaded on this carrier frequency in a phase modulation manner. C/A code is a distance measurement code for civil use. The code length is 1023 symbols, i.e. 1023 times of jumping from digital zero to digital 1. The 1023 symbols repeat 1000 times per second, i.e. 1.023MHz, or once every millionth of a second. P code is a military code with a very long code length and a code speed of 10.23MHz, that is, it jumps once every millionth of a second.

　　The GPS receiver calculates the time from satellite to receiver by comparing the symbol jitter, and then converts it into distance. Obviously, the time accuracy of P code is 10 times higher, and the distance accuracy is 10 times higher. The time accuracy of modern signal processing technology to calculate the symbol jitter is one percent of the code width. The distance converted from one millionth of a second is 300 meters, and its one percent is 3 meters. The precision of P code is one tenth of this value, namely 0.3m. In other words, when calculating the actual distance between a satellite and the receiver, the theoretical accuracy of C/A code is 3 meters.

　　The receiver "knows" the distance between itself and the satellite, and cannot calculate its position, because it does not know the position of the satellite when transmitting radio waves. Therefore, a 50Hz navigation message is also loaded on the satellite carrier. This navigation message includes: satellite track parameters, clock parameters, orbit correction parameters, correction value of atmospheric refraction of GPS signals, etc. GPS receiver calculates the position of a satellite in space at a certain time through these parameters, then determines the distance between itself and the satellite, and then calculates its actual position. The navigation message has a total length of 1500 bits, and each cycle is 30 seconds in the case of 50Hz transmission.

　　It may be boring, but this is one of the countless "why" hidden behind each GPS technical parameter. When you compare the technical parameters of various models of GPS receivers in GARMIN and MAGELLAN, you will find that the cold start time of all models of GPS receivers (that is, GPS knows nothing about the satellite and its location) is about 45 seconds. It is not that the manufacturer is lazy to improve the GPS performance, but it has reached the technical limit: 30 seconds out of 45 seconds are used to receive navigation messages, and the remaining 15 seconds are used to calculate the receiver position.

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