Impact on the bias propagation in the integrity monitoring of filter-based real-time LEO satellite clock determination

With the growing demand for real-time Low Earth Orbit (LEO) satellite clock products for LEO-augmented Positioning, Navigation, and Timing (PNT) services, efficient filter-based LEO satellite clock determination followed by an ultra-short clock prediction is the most thought-after approach. While the accuracy of LEO real-time products is paramount, it is equally important to apply Integrity Monitoring (IM) techniques to ensure their reliability in practical applications. In this study, an IM algorithm for filter-based LEO satellite clock and orbit determination is proposed, taking into consideration the bias propagation. The algorithm is tested using Sentinel-3B onboard GNSS observations together with the National Centre for Space Studies (CNES) real-time products. Key factors influencing the clock (and orbit) Protection Levels (PLs), including various errors in the observation domain like the observation noise, the GNSS real-time product noise and biases, are analyzed. The results demonstrated that bias propagation plays a dominant role in the computed PLs. When assuming an integrity risk of 1 × 10⁻ 5 , increasing the overbounding Standard Deviation (STD) of the zenith-referenced phase noise and GNSS real-time Signal-In-Space Ranging Errors (SISREs) from 0.005 m to 0.05 m only raises the LEO satellite clock PLs from the ns-level to less than 20 ns, and the orbital PLs from the decimeter to meter level. However, when the bias propagation and accumulation effects of the real-time GNSS products are considered, e.g., assuming an overbounding bias of 0.02 m, the clock PLs can increase to beyond 50 ns. This demonstrates the significant contribution of the bias propagation to filter-based LEO satellite clock and orbital PLs.