Application and principle of NTP clock server


Application and principle of NTP clock server
Application and principle of NTP clock server

Anhui Jingzhun Electronic Technology Co., LtdOfficial micro blog——ahjzsz

Principle of Beidou time service

Time service means that the receiver obtains the clock difference between the local time and the Beidou standard time in some way, and then adjusts the local clock to control the time difference in a certain precision range.

Satellite navigation system usually consists of three parts: navigation time service satellite, ground detection, correction and maintenance system and user receiver. For the Beidou-1 local satellite system, the ground detection center should help users complete the positioning and timing synchronization together. Beidou timing system diagram 1

In the Beidou navigation system, time service users can obtain high clock accuracy by constantly verifying the clock difference according to the broadcast or positioning information of satellites; according to the time sequence characteristics of broadcast or navigation messages, high-precision synchronous second pulse 1PPS signal can be obtained through the counter, which is used for synchronous operation of multi-channel data acquisition and control in the same or different places.

“Beidou-1” provides users with two kinds of time service: one-way time service and two-way time service. The accuracy of one-way time service is 100 ns, and that of two-way time service is 20 ns. In the one-way time service mode, the user does not need to interact with the ground center station, but only needs to receive the Beidou broadcast message signal, and independently obtain the clock difference between the local time and the Beidou standard time to realize time synchronization; in the two-way time service mode, the user and the central station carry out mutual information, and send the time service application signal to the central station, and the central station calculates the time difference of the user, Then, the outgoing signal is transmitted to the user via satellite, and the user adjusts the local clock to align with the standard time signal according to this time.

**1.1 one way time service**

Beidou time is the standard Beidou time accurately maintained by the central control station, and the user clock time is the clock face time of the user clock. If there is a clock difference between the two clock when they are not synchronized, the Beidou time and the user clock time have the same readings, but their occurrence times are different.

The one-way time service for each superframe of the outgoing broadcast signal of the ground center station is that the user can independently calculate the clock error and correct the local time by receiving the Beidou broadcast message information, so as to synchronize the local time with the Beidou time. The first frame data segment in the cycle sends the standard Beidou time (day, hour, minute signal and time correction data) and the position information of the satellite. At the same time, the time scale information is modulated in the outbound signal by a special way, and the transmission delay from the central station to the satellite, the delay from the satellite to the user machine and other delays (such as troposphere, ionosphere, Sagnac effect, etc.) and then transmitted to the user, that is, the local clock time of the user is the time of observing the satellite, The time delay between the received signal and the local signal is measured by the user, and then calculated according to the satellite position information, delay correction information in the navigation message and the receiver’s own position information in advance.

Generally speaking, for a fixed user with known precise coordinates, precise time measurement or synchronization can be realized by observing one satellite. If two or more satellites are observed, more observations are provided and the robustness of timing is improved.

**1.2 two way time service**

All the information processing of bidirectional time service is carried out in the central control station, and the user only needs to return the received time scale signal. For the convenience of explanation, a simplified model is given: the time scale signal st0 is sent by the central station system at t0 time. The time scale signal arrives at the satellite after delay, and it is transmitted by the satellite transponder to the time service user. The processing of the received signal by the user can also be regarded as signal forwarding. The satellite forwards the received signal and passes through the space The propagation delay is transmitted back to the central station system. That is, st0, the time scale signal representing time T0, finally returns to the central station system at t0 + + +. The time difference between the time of receiving the time scale signal and the transmitting time of the central station system is used to obtain the bidirectional propagation delay + + +. The unidirectional propagation delay from the central station to the user is obtained by dividing it by 2. The central station sends the one-way propagation delay to the user, timing the time scale signal received by the user and the unidirectional propagation delay, calculates the difference between the local clock and the central control system time, and corrects the local clock to synchronize with the central control system time.

1.3 comparison between two-way time service and one-way time service

(1) From the principle of two-way time service and one-way time service, it can be seen from the introduction that the main difference between two-way time service and one-way time service lies in the way of obtaining the propagation delay from the central station system to the user machine: one way time service uses the satellite position information broadcast by the system to calculate the one-way propagation delay, satellite position error and modeling error (tropospheric mode) independently according to a certain calculation model The two-way time service does not need to know the user’s position and satellite’s position, and can be obtained by dividing the two-way propagation time, which reflects all kinds of delay information more accurately, so the estimation accuracy is higher. In Beidou system, the system design value of one-way time service accuracy is 100ns, and that of bidirectional time service is 20ns. The performance of actual time service user is usually better than this index.

(2) One way time service needs to calculate the location of the user in advance. If the location is unknown, it needs to send a location request to obtain the location information. There is no need to know the location of the user, and all processing is completed by the central station system.

(3) Due to the passive mode of one-way time service, it does not occupy the system capacity (location information needs to be obtained). The two-way time service is based on the interaction with the central station, so it will occupy the system capacity and be limited.

2. Characteristics of Beidou time service

(1) The accuracy of Beidou time service is better than 20 ~ 100ns;
(2) Time service system and equipment work stably and reliably with little interference;
(3) Multiple output modes;
(4) Portable and low consumption
(5) Application scope: aviation and navigation, land transportation, scientific investigation, polar exploration, equipment inspection, system monitoring, etc.

Application of Beidou time service

At present, the distribution of each sending end and receiving end in the power system is widespread and scattered. There are real-time clocks in the automation device, so the inherent error is hard to avoid. With the increase of operation time, the accumulated error will become larger and larger, and the correct time measurement function will be lost. How to realize the time synchronization of the real-time clock and achieve the time unification of the whole network has been the goal of the power system. If a Taipei bucket timing is installed at each end, the high precision of Beidou timing machine can ensure that the relative error between time signal and UTC is not more than 20-100ns. The high-precision time synchronization within the coverage of the satellite is of great value in power system detection and measurement.

In practical application, the precise synchronization of remote or local multi-channel data acquisition and control using satellite time service signal is mainly to use satellite signal receiver to get PPS second pulse signal or use this signal to get ppm, PPH pulse signal, synchronously start multi-channel data analog-to-digital converter ADC, digital control analog-to-digital converter, turn on or off synchronously It can also be used to measure and judge, make accurate time tag, such as various fault location in power system. In time service equipment, the receiver sends out 1PPS second pulse and positioning and clock information every second. PPS second pulse signal has a strict time relationship with the external data information. In use, it is possible to realize time conversion.

In recent years, with the improvement of the technical level of power grid operation, most of the substations adopt integrated automation scheme. Remote centralized control and operation not only improves labor productivity, but also reduces the possibility of misoperation. Therefore, the time synchronization requirement of real-time clock is the most basic requirement of substation automation system. In order to ensure the safety and economic operation of power grid, various automation devices based on computer technology and communication technology are widely used. The normal operation and function of these devices are also inseparable from the unified time benchmark of the whole network.

Beidou satellite time service technology has been gradually mature, and has penetrated into all fields and aspects of daily production and life. Beidou satellite timing equipment, which is stable, reliable, portable and low-cost, can realize accurate clock timing and synchronous data acquisition and control, and has broad application prospects in industrial control time synchronization. The use of navigation satellites for object positioning, clock timing and synchronous data acquisition and control can achieve the degree of accuracy that traditional measurement and control methods can not achieve. It is increasingly widely used in navigation and aviation, land transportation, scientific investigation, polar exploration, geographical survey, weather forecast, equipment inspection, system monitoring and so on.

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