Time Synchronization Maintenance Strategy for VLEO Satellites Operating Under GNSS Outage in the Intersatellite Data Relay System

Abstract

Very Low Earth Orbit (VLEO) satellites, leveraging orbital advantages, have found extensive applications in direct-to-cell (D2C) communication, high-resolution optical and radar remote sensing. The intersatellite data relay system (IDRS) can offer wide-area coverage and flexible service, promising to provide round-the-clock and global data transmission for the VLEO constellation. However, under GNSS outage, the VLEO satellite’s ephemeris extrapolation errors can reach tens of kilometers, while local clock drift accumulates, posing significant challenges for maintaining long-term time synchronization. Considering communication resources of the IDRS are limited, we proposed two synchronization maintenance methods, aiming at balancing the synchronization performance and signaling overhead. Specifically, based on restarting interrupts, the first method achieves favorable synchronization performance with low signaling overhead. Based on error differential and initial error compensation, the second method breaks through the performance bottleneck and is suitable for high-reliability communication. We derived synchronization performance of both methods theoretically and obtained closed-form solutions which align with the Monte Carlo results. Numerical simulations indicate that, compared with the baseline algorithms, the proposed methods significantly reduce the collision ratio while maintaining low signaling overhead, making them more suitable for maintaining long-term time synchronization in the large-scale VLEO constellation. We also analyzed adaptability of the proposed methods across different scenarios to guide practical deployment.