Integrity monitoring of LEO satellite POD under poor observation continuities: simulated scenarios using sentinel-6A data

Abstract Low earth orbit (LEO) satellites will enhance the Global Navigation Satellite Systems (GNSSs) in future positioning, navigation, and timing services. In addition to achieving high orbital accuracy of the LEO satellites, ensuring the reliability and safety of the near-real-time (NRT) LEO satellite precise orbit determination (POD) is equally essential for maintaining integrity in LEO-augmented positioning and timing, yet it is less studied. Compared to the favorable data conditions of scientific LEO satellites in post-processing mode, GNSS observations collected by navigation-oriented LEO satellites may encounter significant discontinuities due to not only tracking problems, but also transmission delays and interruptions within potential data downlinking for NRT ground-based POD, posing challenges to its integrity monitoring (IM). In particular, the protection level (PL) during observation gaps may encounter large increases and strongly harm the IM availability. This study investigates the algorithm calculating the NRT LEO satellite PLs under various observation gaps, followed by different lengths of observation tails based on the reduced-dynamic model. Experiments are conducted using seven consecutive days of Sentinel-6 A real GNSS data, assuming several hours of observation gaps per day. The strategy estimating (RD) and not estimating (CD) piecewise-constant stochastic accelerations in the POD process are both tested. While the former benefits the POD accuracy in complete data conditions, the latter exhibited its advantage in reducing the PLs within large data gaps. Results showed that during observation discontinuities, the RD strategy yields significantly degraded PLs in all three directions, which increase rapidly with gap duration. In the 9 h gap case, the along-track PL exceeds 20 m, while under nominal conditions, it remains around 0.51 m. In contrast, the CD strategy achieves more stable PLs, i.e. below 2 m even during a 9 h gap, although its convergence after observation recovery is slower. Furthermore, the study derives suitable thresholds of alert limits to bound 99% of the PLs under various gap conditions, providing a reference for LEO integrity assessment under such conditions.