Time-frequency synchronization for distributed phase coherent network based on optically carried satellite navigation signals

This paper proposes a bidirectional time-frequency synchronization system based on optically carried satellite navigation signals for point-to-point and point-to-multipoint clock synchronization among distributed coherent receiving nodes on the ground. The system consists of a self-developed modem, an electrical amplifier (EA), a directly modulated laser (DML), an optical fiber, and a photodetector (PD), with transmission between two nodes conducted via a single optical fiber link. By leveraging the spread spectrum gain of satellite navigation signal waveform, the modem transmission power is reduced to as low as −65 dBm. High-precision time-frequency synchronization is achieved by using pseudo-random noise (PRN) code phase to assist carrier phase, enabling the calculation and synchronization of time differences between any two nodes in the distributed coherent reception network. Based on the target time difference estimation, we investigate and compare the closed-loop control performance of PID, second-order phase-locked loop (PLL), and third-order PLL algorithms. Among these, the PID algorithm features independent parameters, directly compensates the phase error with rapid response, and its derivative term mitigates overshoot. Simulation results indicate the PID algorithm exhibits the fastest convergence speed and the smallest oscillation amplitude after stabilization. Consequently, the PID control algorithm combined with a low-pass filter (LPF) algorithm is ultimately adopted, and a digital-to-analog converter (DAC) is used to tune the secondary node’s crystal oscillator frequency to achieve point-to-point time-frequency synchronization. Experiments demonstrated that under distributed node clock diversity, the system achieves the best time synchronization stability of 1.89 ps, with frequency stability reaching ≤3.00 × 10 −12 at 1 s and ≤3.04 × 10 −15 at 10 3 s. Both theoretical analysis and experimental results validate the correctness and effectiveness of the proposed method.