The Global Positioning System (GPS) is a space based radio navigation system originally developed by the US Air Force. Although GPS was originally developed for use as a military positioning system, it also has an important impact on civil society. In fact, you can now use GPS receivers in vehicles, ships, and even mobile phones. The GPS navigation system consists of 24 groups of satellites, all of which transmit multiple signals in L1 and L2 bands. Through 1.57542 GHz L1 frequency band, each group of satellites generates 1.023 Mchips BPSK (binary phase shift) spread spectrum signal. Spread spectrum sequences use virtual random number (PN) sequences called C/A (coarse acquisition) codes. Although the spread spectrum sequence is 1.023 Mchips, the actual signal data transmission rate is 50 Hz [1]. In the original deployment of the system, the accuracy error of the GPS receiver can reach more than 20~30 meters. This error is caused by the random frequency error added by the US military for security reasons. However, this error signal source, called Selective Availability, was canceled on May 2, 2000. Today, the maximum error of the receiver does not exceed 5 meters, while the general error has been reduced to 1~2 meters.

　　Whether in L1 or L2 (1.2276 GHz) bands, GPS satellites will generate so-called "P code" auxiliary signals. This signal is a modulated signal of 10.23 Mbps BPSK, and also uses PN sequence as spread spectrum code. The military carries out more accurate positioning operations through the transmission of P code. In the L1 band, the P code is transmitted 90 degrees out of phase through the C/A code to ensure that the two kinds of signal numbers can be measured on the same carrier [2]. P code can reach - 163 dBW signal power in L1 band; Up to - 166 dBW in L2 band. Relatively speaking, if the C/A code on the earth surface is used, the minimum broadcast power of - 160 dBW can be achieved in the L1 band.

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