1.1 Introduction to GPS

　　GPS (Global Positioning System) is a new generation of satellite navigation, time service and positioning system developed by the United States in the 1970s. Each of the 24 dedicated GPS satellites has its own atomic clock, which can broadcast accurate UTC standard time to the ground 24/7. In the data stream decoded by many universal GPS decoding chips, in addition to location information, it also contains time information (year, month, day, hour, minute, second) and PPS (Pulse Second). PPS identifies the starting point of time information, and its accuracy can reach microsecond level.

　　1.2 Introduction to timing method

　　NTP is a protocol used to synchronize computer time. Its synchronization clock source is not limited to the network time server, but also includes clock devices, such as quartz clock, atomic clock, GPS receiver, etc. The NTP server software abstracts these clock sources into corresponding data structures, corresponding to different memory addresses, and synchronizes the computer clock by reading the information in the address and performing statistical algorithm processing.

　　There are mainly three time calibration schemes using GPS as the synchronous clock source: pulse synchronization mode, serial synchronization mode and integrated mode. The GPSD timing scheme adopted in this paper is a comprehensive way. The comparison of the three methods is shown in Table 1.

　　1.3 Integrated timing based on GPSD

　　GPSD (GPS Daemon) is a daemon software used to process the data decoded by the GPS receiving unit. The specific process of GPSD based integrated timing is shown in Figure 1. The GPS antenna receives the GPS signal, transmits it to the G591 chip for decoding, and outputs the data and PPS signal in NMEA0183 protocol format every second. After the MAX 232 completes the level conversion, it transmits them to the computer via the RXD and DCD ends of the serial port respectively; After processing, GPSD software writes accurate time information to specific memory segments; NTP server software reads the time information in this address segment by sharing memory, and then completes the work of correcting the system clock.

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Four major GPS systems in the world