Satellite timing is a timing method for transmitting or broadcasting standard time signals by means of navigation satellites. It has been widely used for its low cost, high precision and simple implementation.

At present, the commonly used navigation satellites are China's Beidou navigation system and the US GPS. Satellite timing and location are combined, and accurate user timing can be achieved on the basis of precise positioning.

The GPS satellite timing device is composed of an antenna, a radio frequency unit, a signal processing unit, a data processing unit and an output interface unit.

At present, the GPS satellite timing device has a variety of time synchronization interface standards to achieve the transmission time. For wide area distributed networks, when the satellite time service receiver receives standard time, it is necessary to publish this time to every part of the system.

Commonly used time synchronization interface has time encoding, typical time codes such as IRIG-B code, DC and AC codes. The AC code (AC) signal is modulated and the transmission distance is far away. The time message interface is also used in short distance, and the time is transmitted through the RS232 serial port. Optical fiber is superior to electromagnetic interference and has become a popular means of time transfer.

NTP network time synchronization uses network protocol to realize the time synchronization of computers. Now it has been applied more and more widely. Through the network to determine a number of nodes as the clock source website, in order to achieve synchronization with UTC time, to provide users with unified, standard time transfer service. The complexity of time synchronization on the Internet is taken into account at the very beginning of the NTP time protocol design. Therefore, in the actual application, when the clock source is effective, time correction and tracking can be realized. If there is a network failure, it can still maintain the stability of time. Ensure that the network maintains precise time synchronization in a certain period of time. Therefore, the NTP mechanism based on UDP/IP's hierarchical time distribution model can satisfy different Internet environments. The recommended GPS satellite timer is SYN2151.

With the improvement of the accuracy of time synchronization, NTP network timing ms level accuracy has been unable to meet the demand in many fields which require higher synchronization accuracy. In this case, IEEE 1588 PTP has attracted the attention of many users. The full name of PTP is "the precision clock synchronization protocol standard of network measurement and control system". Taking the SYN2411 IEEE1588 master clock as an example, the PTP has high timing accuracy and can reach ns level, but it must have hardware circuit support to use master-slave matching. There are two ways of timing in the PTP protocol, one is MAC mode, the other is UDP mode, the accuracy is the same, and the UDP mode is commonly used.

Time message refers to the time information extracted and corrected by the timing device after receiving the reference time source and output according to a certain format. For example, the SYN2306C GPS Beidou timing navigation receiver, the time message is usually combined with the second pulse. Its timing principle is shown in the diagram. The general message information is output at a certain time interval after the second pulse pulse. The message message contains the time corresponding to the second pulse, the rise time of the second pulse is generally less than 5ns, and the number of NS levels can be provided by the rising edge of the second pulse precision. Timing accuracy.

At present, the NEMA protocol is the most widely used in time messages. The commands related to time in NEMA protocol mainly include commands such as GGA, GLL and RMC.