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.
This paper presents and discusses some of the enhancements in robustness against jamming and spoofing using unique features of the cryptographically protected Galileo PRS signals. Different techniques exploiting these PRS properties have been implemented and evaluated on the German national PRS test receiver (PROOF). It is shown that the used HDDM interference mitigation method is already as good as the stateof-the-art in high-end GNSS receivers for OS signals. Then the benefits of cryptographically protected signals are discussed with a demonstration how trusted PRS signals can be used to detect and mitigate spoofing attacks also in comparison to one potential Open Service Navigation Message Authentication (OS-NMA) configuration.
Over the last decade, chip-scale atomic clocks (CSACs) have emerged as stable time and frequency references with small size, weight, and power (SWaP). While the short-term stability of these devices clearly outperforms other oscillators with similar power consumption, their stability over longer time intervals is notably limited by frequency noise. Such long-term deviations can effectively be compensated by disciplining the clock with respect to a stable time and frequency reference such as Coordinated Universal Time (UTC) or a time scale based on GNSS observations. In view of the limited accuracy of GPS pseudorange observations and broadcast ephemerides, the performance of GNSS-disciplined atomic clocks is commonly limited to the few-nanosecond level. For further improvement, this study combines the use of carrier phase-based precise-point-positioning (PPP) techniques and high-performance broadcast ephemerides to discipline the phase of a CSAC with respect to GNSS broadcast time. Making use of a dual-frequency, dual-constellation GPS/Galileo receiver, a sub-nanosecond time interval error with respect to a national UTC timing laboratory is demonstrated over time intervals from 1 s to several days.
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