What are the GPS signals

　　The user part of GPS consists of GPS receiver, data processing software and corresponding user equipment such as computer meteorological instruments. Its function is to receive signals from GPS satellites and use these signals for navigation and positioning. The above three parts together form a complete GPS system.

　　GPS satellites transmit carrier signals of two frequencies, namely the L1 carrier with a frequency of 1575.42MHz and the L2 carrier with a frequency of 1227.60HMz. Their frequencies are 154 times and 120 times that of the fundamental frequency of 10.23MHz, respectively, and their wavelengths are 19.03cm. and 24.42cm. A variety of signals are modulated on L1 and L2 respectively. These signals mainly include:

　　C/A code

　　The C/A code is also called the coarse acquisition code. It is modulated on the L1 carrier and is a 1MHz pseudo-random noise code (PRN code) with a code length of 1023 bits (period of 1ms). Since the C/A codes of each satellite are different, we often use their PRN numbers to distinguish them. The C/A code is a main signal used by ordinary users to determine the distance between the station and the satellite.

　　P code

　　The P code, also known as the fine code, is modulated on the L1 and L2 carriers and is a 10MHz pseudo-random noise code with a period of seven days. When implementing AS, the P code and the W code are added modulo-2 to generate a confidential Y code. At this time, the general user cannot use the P code for navigation and positioning.

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