In the absence of noise interference, comprehensive considerations such as ranging accuracy, signal bandwidth, required power, and different satellite recognition, the global positioning system uses pseudo -random code ranging technology. The pseudo -random code is also called a pseudo -noise code. It is a binary sequence that can be determined and can be reused and replicated, and has random statistical characteristics. In the occasion of deep space communication, the use of pseudo -random coding signals can achieve low signal -to -noise ratio, which greatly improves the reliability of communication, and can realize the code distribution of multiple address communication. In addition, the use of pseudo -random coding signals can achieve high -performance confidential communication. These characteristics are in line with the technical requirements of the GPS system.

　　According to the general result of the signal detection theory, the best receiver of the ranging is a related receiver under white noise conditions with a uniform power spectrum. This receiving method is to use the copy signal of the transmitting signal (referred to as a local signal) and the receiving signal and the sum of the noise of the noise, and then determine the distance of the target by measuring the position of the maximum value of the relevant function. From the perspective of related receiving methods, the ranging signal is required to have the self -related characteristics similar to white noise. Pseudo -random code ranging technology is the manifestation of this idea.

　　The transmission of signal transmission with a pseudo -random code needs to detect the maximum value of the related output. This can only be detected by moving the local code by code. Considering that the detection is performed after the integration is performed, the integration time should not be too short, so it takes a certain time to detect the maximum related output, that is, it takes a certain capture time. In the case of not knowing the distance to be tested and the difference in the standing clock, the longer the code, the longer the required capture time. In order to shorten the capture time, the GPS satellite also broadcasts a short code, namely C/A code, also known as rental code. Because the C/A code uses two Golder codes composed of the same code sequence with good interdependence characteristics, which keeps synchronized with the P code. Therefore, after capturing the C/A code, the P code can be easily captured.

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