Abstract
The feasibility of precise real-time orbit determination of low earth orbit satellites using onboard GNSS observations is assessed using six months of flight data from the Sentinel-6A mission. Based on offline processing of dual-constellation pseudorange and carrier phase measurements as well as broadcast ephemerides in a sequential filter with a reduced dynamic force model, navigation solutions with a representative position error of 10 cm (3D RMS) are achieved. The overall performance is largely enabled by the superior quality of the Galileo broadcast ephemerides, which exhibits a two- to three-times smaller signal-in-space-range error than GPS and allows for geodetic-grade GNSS real-time orbit determination without a need for external correction services. Compared to GPS-only processing, a roughly two-times better navigation accuracy is achieved in a Galileo-only or mixed GPS/Galileo processing. On the other hand, GPS tracking offers a useful complement and additional robustness in view of a still incomplete Galileo constellation. Furthermore, it provides improved autonomy of the navigation process through the availability of earth orientation parameters in the new civil navigation message of the L2C signal. Overall, GNSS-based onboard orbit determination can now reach a similar performance as the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) navigation system. It lends itself as a viable alternative for future remote sensing missions.
Highlights
Sentinel-6A “Michael Freilich,” known as Jason Continuity of Service (Jason-CS), is the latest satellite of the European earth observation program (Donlon et al 2021)
While the ground-collected navigation data cover all data transmitted by all global navigation satellite system (GNSS) satellites on a day of interest, they are fully representative of the GPS/Galileo orbit, clock offset, group delay, and earth orientation parameters (EOPs) data available onboard for those GNSS satellites tracked by the PODRIX receiver
As a reference for the performance assessment and initial filter tuning, precise orbit determination results based on ambiguity-fixed processing of GPS and Galileo observations from the PODRIX receiver were used (Montenbruck et al 2021)
Summary
Sentinel-6A “Michael Freilich,” known as Jason Continuity of Service (Jason-CS), is the latest satellite of the European earth observation program (Donlon et al 2021). The primary payload comprises the Poseidon-4 Ku/C-band altimeter and the advanced microwave radiometer-climate quality (AMR-C). Altimeter processing relies on highly accurate knowledge of the orbital position of the spacecraft, its altitude, to determine the mean height of the sea level. Sentinel-6A precise orbit determination (POD) is supported by two independent sensors, namely. The PODRIX receiver likewise provides a real-time onboard navigation solution, which is mainly intended for platform support rather than science data processing and confined to an accuracy of roughly 3 m. The achieved performance reflects a trade-off between performance, algorithmic complexity, and robustness and reflects typical accuracy needs for attitude and orbit control systems of LEO satellites
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