Precise real‐time navigation of LEO satellites using GNSS broadcast ephemerides

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.

Position, Navigation, and Timing (PNT) Through Low Earth Orbit (LEO) Satellites: A Survey on Current Status, Challenges, and Opportunities

“…These hybrid solutions combine the best of both techniques, but they increase the time latency. On the other hand, a technique that has gained attention in recent years [99] is the use of precise GNSS algorithms with broadcast ephemerides. The GNSS-POD with broadcast ephemerides allows precise positioning without the need for complementary ground-tospace links, thus reducing the latency issue.…”

Performance assessment of GNSS-based real-time navigation for the Sentinel-6 spacecraft

“…Spacecraft operated at lower altitudes and experiencing higher drag forces would typically require an increased level of empirical forces, which then reduces the dynamical constraints on the resulting orbit. Nevertheless, independent simulations of GNSS-based real-time navigation (Hauschild and Montenbruck 2021) suggest that a better than 0.1 mm/s along-track velocity accuracy can even be reached for spacecraft orbiting at down to 400 km altitude.…”

“…These advantages are likewise of interest for GNSS-based onboard navigation of LEO satellites, which largely relies on the use of un-augmented broadcast ephemerides. Notable benefits of Galileo for this application have been earlier predicted in a simulation study of Hauschild and Montenbruck (2021) but lack a practical confirmation so far. Using actual flight data from the Sentinel-6A satellite now allows to reliably demonstrate the feasibility of 1 dm (3D RMS) real-time navigation using joint GPS and Galileo observations and shows that GNSS offers a viable alternative to DORIS for real-time orbit determination of LEO satellites.…”

Precise Onboard Time Synchronization for LEO Satellites

“…Data processing is done with RTNAV (Hauschild & Montenbruck, 2021;Montenbruck & Ramos-Bosch, 2008), a multi-GNSS-capable software for LEO satellite navigation in a simulated real-time mode. It is based on an extended Kalman filter (EKF), using pseudoranges and carrier phases as measurement input and a reduced-dynamics orbit model (Wu et al, 1991) for propagation of the satellite's state vector.…”

“…GNSS has been well proven and established for orbit determination in LEO for many years. It has been shown previously that dual-frequency, multi-constellation GNSS orbit determination in LEO using a carrier-phase approach with a reduced dynamics orbit model and broadcast ephemerides can achieve standard deviations of around one decimeter without external augmentation data (Hauschild & Montenbruck, 2021).…”