A Hybrid Time Synchronization Optimization Scheme for Onboard Payloads with Independent Clock

Crystal oscillators are typically selected as payload clocks, and onboard periodic time synchronization is necessary to ensure long-term stability. Time synchronization method varies for payloads with different time precision requirements, which are categorized into high precision time requirement (HPTR) and general precision time requirement (GPTR) payloads. As presented in the case of China Seismo-Electromagnetic Satellite (CSES), HPTR payloads typically employ pulse per second (PPS) signals for synchronization, whereas GPTR payloads employ onboard time broadcasts via CAN bus. Time inconsistency is unavoidable due to the differences in time synchronization methods and payload timing systems. The sources of delay in timestamp transmission are analyzed in detail, and a hybrid synchronization optimization scheme is proposed to measure the delays with two separate models. The first model is designed to minimize the internal delay of payload time synchronization system, which includes a finite state machine(FSM) and a hardware input trigger signal. The second model is designed to measure timestamp transmission delay for calculation during post-processing, by connecting to both second pulse signal and CAN bus. Performed on one of the CSES payload prototype, experiment results indicate that the optimized time difference is expected to be less than 1 ms. This hybrid optimization scheme effectively improves time consistency without additional PPS resource, which is practical and low-cost.