Research On Key Technologies Of High Precision Inter-station Two-way Time And Frequency Transfer

Time and frequency transfer between stations is the key to achieving large-scale time-frequency system. It is the basis for the establishment of UTC/TAI and the core of the implementation of satellite navigation, positioning and time service. TWSTFT is one of the most important time and frequency transfer technologies. It is a hot topic to further correct its two-way non-reciprocity to improve the accuracy and its time and frequency transfer stability. At the same time, two-way Optic-fiber time and frequency transfer is considered to bring a new era to time and frequency transfer. Based on the frame of Multi-GEO satellite TWSTFT in Chinese Satellite Navigation System and the time and frequency transfer technology that may be used in the future for satellite navigation, the following research results has been achieved:(1) To correct for the non-reciprocity in TWSTFT introduced by satellite’s motion relative to the ground, a model of two-way satellite delay (Light Time) in the inertial system is proposed, and an iterative calculation process based on ephemeris data and TWSTFT measurement is proposed also, called’Light Time Solution’. To take TWSTFT measurements with the links established between each pair of three stations the measured data of three ground stations as an example, the satellite daily periodic motion relative to the ground introduces daily fluctuations in the non-reciprocity of TWSTFT with the peak to peak value as 453ps, the average of-161ps. If not corrected, daily fluctuations will limit TWSTFT time transfer accuracy in the order of 0.2ns and limit its frequency stability (Allan Deviation) to 1.05×10-14, with the averaging time of 0.5day. The triangle closure verification test with the there TWSTFT links measurements shows that:the triangle closure of the non-reciprocities with daily fluctuations of the peak-to-peak value as 128ps calculated by Light Time Solution method agrees well with the triangle closure of with the there TWSTFT links measurements with daily fluctuations of the peak-to-peak value as 147ps.(2) In order to improve two-way satellite transfer frequency stability, it is proposed to separate Allan variance of the measurement noise with Allan variance of clock error in the 2-GEO satellite TWSTFT frame. Theoretical analysis shows that, the parallel TWSTFT links with 2 GEO satellites are with independent transmit and receive equipments, and measurement noises of the two links are uncorrelated with each other and the measured values contain the same clock error. It is possible to observe the Allan variance of the clock error left side of the measurement noises. Further analysis of the measured data shows that with single-link measurement noise Allan deviation (square root of Allan variance) up to as 3×10-10τ-1 (τ for averaging time), it has been achieved to observe the Allan deviation of clock error as 7×10-12τ0.65 which shows improvement of TWSTFT frequency stability with two orders of magnitude.(3) In order to improve the stability of two-way satellite time transfer and ensure the integrity of the clock difference measurements, a multi-link dynamic weighted average method is proposed in the N-GEO satellite TWSTFT frame. Dynamic weighted average based on the newly proposed dynamic time variance to characterize the time transfer stability, integrate N-GEO satellite TWSTFT measurements to suppress measurement noise (particularly non-stationary measurement noise) effects. It achieves dynamic time deviation (DTDEV) improvement of TWSTFT of the ratio as 1/√N, while ensuring the integrity of the clock difference measurement. With the above theoretical analysis, the simulation results show that:when one link measurement noise show non-stationary as standard deviation with sudden change, cycle or linearly increasing as 100 percent of the non-stationary case, dynamic weight average method has shown improvement against with the standard weighted average method based on standard time variance up to 43%,11%,12% with DTDEV. The 2-GEO satellite TWSTFT measurement analysis shows that the improvement of 30%.In other words, the improvement ratio is 0.7 is achieved against with the original measurements, close to the theoretical optimal value 1/√2.(4) To find optimal frequency in the radio domain to transmit and to achieve time and frequency transfer in the Optic-Fiber intensity modulation direct detection system, a detailed analysis of the effect of fiber dispersion on frequency stability of the frequency signal transmitted, the principle of transfer frequency selection is given; the two-way Optic-Fiber time and frequency transfer based on pseudorandom noise code and radio carrier phase measurements is proposed. Radio carrier cycle integer ambiguity solving method is proposed based on the derivation of the propagation speed of the pseudo-random code and carrier in the Optic-Fiber system. The common clock indoor experiment with lkm rolled fiber and the optic-fiber time and frequency transfer units with 10MHz PRN code and 30MHz carrier radio frequency show that the time and frequency transfer with frequency stability (Allan Deviation) of 1.57×10-12τ-1 (τ is averaging time,1s≤τ≤104s) and time stability (Time Deviation) of 20ps (τ=100s) is achieved.