Do you know satellite timing? This article explains the GPS Beidou satellite timing service in detail.
Do you know satellite timing? This article explains the GPS Beidou satellite timing service in detail
What is satellite timing?
The timing equipment obtains standard time information from the signals of Beidou navigation satellites or GPS navigation satellites, and transmits this information to the equipment that needs time information (computers, main controllers, sampling equipment, RTUs, etc.) through various types of interfaces. Time calibration of a single device or time synchronization of multiple systems can be achieved. This process is called satellite timing.
The working principle of satellite timing
Both GPS satellites and Beidou satellites are equipped with atomic clocks (cesium clocks or rubidium clocks). With an accurate clock, coupled with the continuous correction of the ground station, the time of the satellite system will be very accurate. The satellite will broadcast a time in its own message, and the signal edge that broadcasts this time is strictly corresponding to this time value. By measuring this edge, an accurate changing edge can be recovered locally, which is synchronized with the launch time. The navigation message provides the "week number" at the current moment. This week number is counted from the start time of Beidou or GPS system. In addition, the current week can be known by calculating the information of the pseudo-random code modulated on the carrier. Seconds, with this information, the timing function can be realized.
Common time service method
The current mainstream time synchronization signals and interface methods include 1PPS/1PPM, IRIG-B code, RS-232 serial port and NTP network timing. The timing accuracy of 1PPS/1PPM pulse and IRIG-B code can reach the order of nanoseconds, and the accuracy of RS-232 and NTP timing can reach the order of milliseconds in general. 1PPS/1PPM and IRIG-B code and RS-232 all need special interface and cable, and NTP way can adopt the way of network.
a) 1PPS/1PPM time service mode This format time signal outputs a pulse signal every second or every minute. Obviously, the pulse output does not contain specific time information.
b) B code timing mode IRIG has several coding standards A, B, D, E, G, H. Among them, IRIG-B coding is the most used in time synchronization applications, with DC code (BC level offset), AC code (1kHz sinusoidal carrier amplitude modulation) and other formats. The IRIG-B signal outputs one frame per second, and each frame is one second long. There are 100 symbols in a frame, and symbols of different pulse widths represent binary 0, 1 and position flags.
c) RS-232 serial port time service mode time output Send a string of date and time messages expressed in ASCII code through EIA standard serial interface. Rich information such as parity, clock status, and diagnosis results can be inserted into the time message. This method can be used on the computer to intuitively see the current time information and correct the computer time at any time, which is very convenient to use.
d) Network time service method Network time service uses the NTP protocol to transmit uniform and standard time on the Internet. The specific implementation plan is to designate several time servers on the network to provide users with timing services, and these time servers should be able to compare with each other to improve accuracy. All PCs, servers and other devices in the local area network are synchronized with the time server through the network. The NTP protocol automatically determines the network delay and compensates for the time of the obtained data. In this way, the time of the LAN equipment is kept uniform and accurate.
Commonly used computer timing methods
At present, the computer keeps the time in a single way. When the computer is turned off, the crystal on the motherboard runs on battery power to keep the time. After the computer is turned on, the time is counted by the software program installed in the computer's BIOS. The crystal on the main board will drift in long-term operation, and the software timing will be affected by the system program or other application software programs and cause larger errors. Therefore, it is difficult for the computer itself to guarantee its time accuracy, nor can it guarantee the accuracy of time synchronization between multiple computers.
In order to solve this problem, the high-precision clock source of the satellite positioning system is used as a time reference to realize the precise modification and synchronization of the clock on a global scale. The data obtained from the satellite positioning module and the second pulse signal are transmitted to the computer through the data interface. After reading the data processing with the timing software on the computer, the precise time pulse mark and time data are obtained. When the second pulse arrives, the received time data is processed and the computer system time is updated. This method can ensure the time accuracy of a single computer at any time, and can also achieve the purpose of high-precision synchronization of time between multiple computers.