As the basic physical unit with the highest measurement accuracy, time plays an increasingly important role in social economy, national defense security, and science and technology. Satellite navigation system timing has the characteristics of wide range, low cost and high accuracy, and has been widely used. The time synchronization of the system is very important for many key infrastructures related to national economic and social security, and the effective operation of communication systems, power systems, and financial systems all depend on high-precision time synchronization. In mobile communications, precision timing is required to ensure the synchronous operation of base stations, and power grids have strict requirements for time and frequency in order to effectively transmit and distribute power. The timing service of the Beidou Navigation Satellite System can be effectively applied to the communication, power and financial systems to ensure the safe and stable operation of the system.
      The Beidou standard service can achieve a timing accuracy of 10-20 nanoseconds, but the new generation of mobile communications, Internet of Things, distributed observation, measurement and detection and other fields have put forward nanosecond or even sub-nanosecond accuracy requirements for time. In addition to providing basic navigation and positioning timing services, the BDS-3 global navigation satellite system also integrates satellite-based augmentation and PPP-B2B precision point positioning functions to realize short message communication and international search and rescue services. Among them, the ppp-b2b service information is broadcast to China and surrounding areas through the ppp-b2b signals of three GEO satellites. The user station recovers high-precision satellite orbit and clock error products by receiving correction information such as orbit, clock deviation and inter-symbol deviation relative to the broadcast ephemeris, which can realize real-time high-precision navigation and positioning.
      As of 2020, the third generation of Beidou satellite system has been completed, Beidou timing accuracy and GPS timing accuracy is comparable, in many important industries in many countries have begun to choose to use their own Beidou satellite timing or GPS Beidou dual-mode timing scheme, which not only ensures the accuracy and stability of timing, but also ensures that in a special period, the satellite timing is not interrupted by others.

     The principle of timing device of satellite navigation system
       Satellite navigation system timing device timing includes one-way timing and two-way timing two-way, two-way timing for the authorized service device are one-way timing, that is, by the receiver autonomous calculation to complete the timing, its principle is as follows: satellite coordinates and satellite clock can be calculated according to the broadcast ephemeris, ionospheric delay can be weakened to centimeter-level by dual-frequency, tropospheric delay can also be corrected accurate to centimeter-level, earth rotation and relativistic effect can be accurately calculated according to the formula. If the coordinates of the receiving point are known, the only unknown is the receiver clock error, and only one satellite is needed to calculate the receiver clock error; If the receiver coordinates are unknown, at least 4 satellites are required, and then the receiver coordinates and clock error are calculated using the least squares method. Based on the calculated receiver clock error, the receiver corrects its time to the exact UTC time and accurately outputs a 1PPS pulse signal for the user's use.

 From the above timing principle, it can be seen that the timing accuracy is the synthesis of the accuracy of the main influencing factors such as the orbit (i.e., satellite coordinates), satellite clock aberration, pseudorange, ionospheric delay, tropospheric delay, etc. To analyze the timing accuracy of the timing device of the satellite navigation system, it is necessary to analyze the influence of the comprehensive factors of each satellite navigation system on the time accuracy.
1. The timing accuracy of the satellite navigation system timing device is affected by the accuracy of the broadcast ephemeris and the satellite clock error
   Atomic clocks made by using the principle of fixed frequency oscillations of atoms that are not affected by pressure and temperature have high precision, such as quartz clocks, rubidium atomic clocks, hydrogen atomic clocks, etc. Navigation satellites are generally equipped with high-precision anchor atomic clocks and hydrogen atomic clocks, which are very stable. The timing device of the satellite navigation system receives the broadcast ephemeris, and the GPS satellite clock error accuracy provided by the broadcast ephemeris is about 5ns.
2. The influence of ionosphere and troposphere on the timing accuracy of the timing device of the satellite navigation system
  The influence of the ionosphere and troposphere is related to the altitude angle of the satellite, and the influence of the troposphere can reach more than 20 meters above the altitude angle of 10 degrees, and the ionosphere can exceed 50 meters at the altitude angle of more than 10 degrees. For dual-frequency receivers, the combination of dual-frequency ionospheric elimination can be used to eliminate the influence of the ionosphere, and the ionospheric difference after elimination is only centimeter-level; The troposphere can be accurately corrected to the centimeter level using models; The corrected residuals of the Earth's autobiography are also only centimeter-level; Relativistic effects can be corrected to negligible with formulas. Therefore, the distance residuals after the elimination of the ionosphere, troposphere, earth autobiography and relativistic effects through dual-frequency ionospheric depletion, tropospheric correction, earth autobiography and relativistic effects should be about 0.1m, which is negligible. For single-frequency receivers, since the electrical layer cannot be eliminated, it can be corrected to 60% by model correction. If the ionospheric delay is 40m, the corrected ionospheric residuals are 16m, and the reduction time is 53.370ns. For users with sub-microsecond timing accuracy, a single-frequency BDS timing device receiver can be used.

     3. The timing accuracy of the BDS timing device is affected by the pseudorange accuracy
From the timing principle, it can be seen that the pseudorange F also affects the timing accuracy. The pseudorange is generated by the C/A code or the P code, taking GPS as an example, the distance corresponding to the width of the C/A code element is 293.052m, if the receiver ranging accuracy is 1/100 of a code element, then the ranging accuracy is 2.930m, and the accuracy of the P code can reach 0.290mMo In the actual measurement, the pseudorange will be affected by noise. Through the above analysis, the following conclusions can be drawn: First, the premise of satellite navigation system timing is first positioning, and only by accurately knowing the coordinates of the receiver can the timing be completed. Second, for dual-frequency GNSS receivers, the timing accuracy of the timing device is mainly affected by the accuracy of satellite orbit, satellite clock error and pseudorange measurement. For ravines and sheltered places, due to the small number of visible satellites for single-system timing, the receiver coordinates cannot be calculated, and GNSS multi-system timing can be used to increase the number of visible satellites to speed up the timing.

Features:
Ultra-precise time synchronization for time transfer applications
PPS input internal delay auto-calibration 
CGGTTS V2E compliant 
Track all visible GNSS signals (GPS, GLONASS, Galileo, BeiDou, NAVIC)
High-precision, low-noise measurements 
Unique interference monitoring and anti-interference 
Powerful web interface and recording tools

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