Abstract
The availability of orbit information with high precision and low latency is a key requirement for many Earth-observation missions, predominantly in the field of radio occultation. Traditionally, precise orbit determination solutions of low-Earth orbit (LEO) satellites are obtained offline on ground after downloading GNSS measurements and auxiliary spacecraft data to the processing center. The latency of this processing depends on the frequency of LEO downlink contacts and the availability of precise GNSS orbit and clock products required for the orbit determination process. These dependencies can be removed by computing the precise orbit determination solution on board the satellite using GNSS broadcast ephemerides. In this study, both real data and simulated measurements from a representative LEO satellite are processed in a flight-proven Kalman-filter algorithm. The paper studies the use of GPS, Galileo and BeiDou-3 for real-time orbit determination in different combinations with simulated measurements. Results show that use of dual-frequency observations and broadcast ephemerides of Galileo and BeiDou-3 leads to a significant reduction of 3D rms orbit errors compared to GPS-only processing. An onboard navigation accuracy of about one decimeter can be achieved without external augmentation data, which opens up new prospects for conducting relevant parts of the science data processing in future space missions directly on board a LEO satellite.
Highlights
Earth-observation missions in low Earth orbit (LEO) commonly rely on the high-precision orbit information to fulfill their scientific objectives
The real-time navigation accuracy of lowEarth orbit (LEO) satellites that can be achieved with multi-GNSS observations and broadcast ephemerides has been evaluated using real GPS observations collected onboard the Swarm-C satellite as well as simulated GPS, Galileo, and BeiDou-3 measurements
In the case of Galileo, the two satellites (E14, E18) in eccentric orbits are still marked as unhealthy. Even though their ranging signals are of high quality, their broadcast ephemerides show a slightly degraded performance and the satellites are not included in the Galileo almanac
Summary
Earth-observation missions in low Earth orbit (LEO) commonly rely on the high-precision orbit information to fulfill their scientific objectives. Less favorable results with meter-level 3D errors were obtained in most studies and flight trials of onboard orbit determination using unaugmented GPS due to the limited quality of its broadcast ephemerides (Gong et al, 2019; Montenbruck et al, 2008; Reichert et al, 2002; Wang et al, 2015) To cope with this limitation in GPS-based onboard POD, the transmission of precise GNSS orbit and clock information via a geostationary satellite link has already been proposed almost two decades ago (Reichert et al, 2002; Toral et al, 2006), but not been demonstrated in actual space missions so far. This paper is an extension of a previous study with significantly enhanced measurement simulation and navigation filter settings (Hauschild & Montenbruck, 2020)
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