Bart Kevers, Fugro Innovation and Technology
James Bartle, Fugro GB Marine Limited
Jonathan Ward, Fugro Innovation and Technology
Gencer Yilmaz, Fugro Innovation and Technology
Scoot Glazier, Fugro Australia Pty Ltd
Bart Kevers, Mr., Fugro Innovation and Technology
Satellites in Low Earth Orbit (LEO) are commonly equipped with GNSS (Global Navigation Satellite Systems) receivers to obtain real-time positioning, velocity, and time (PVT). Although the highest possible PVT accuracy is obtained by post-processing the raw GNSS data in facilities on the ground, recent advancements in space applications drive the need for precise onboard orbit determination (P2OD) achieving sub-dm level accuracy in real-time with advanced filters, signals from multiple constellations, and PPP (Precise Point Positioning).
SpaceStar is a GNSS navigation software solution providing real-time onboard high accuracy PVT for satellites in LEO based on the PPP (Precise Point Positioning) technique. PPP is a GNSS augmentation technique which uses orbit and clock corrections to enable users to obtain absolute high accuracy positioning. PPP is used extensively for precise Earth-based navigation and was demonstrated onboard a satellite in LEO in 2022 by Fugro on the Loft Orbital YAM-3 satellite.
This abstract summarizes how Fugro has continued to develop and utilise the PPP technique to estimate real-time nominal positioning at sub-decimetre levels of accuracy in LEO on the NorSat-TD satellite, launched in May 2023. At the time of writing, SpaceStar is still active on NorSat-TD, having generated GBs of data and counting. The system architecture used to deliver PPP-enabling corrections to LEO and the architecture onboard is described. The initial results from the demonstration are presented and discussed, followed by a description of future developments and experimentation that is planned to improve performance going forward.
On Norsat-TD SpaceStar is running on a Software Defined Radio (SDR). The SDR receives Fugro’s orbit & clock corrections over L-band by interfacing with a dedicated antenna. It also receives raw GNSS measurements from the onboard GNSS receiver, which in turn receives radiofrequency (RF) signals from its GNSS antenna. Both GNSS and L-band antenna boresight are pointing to empty space in nominal attitude.
The position output generated by SpaceStar in orbit is referenced against a post-processed solution to assess its accuracy. The solution tooling has been validated against ESA’s precise orbits on Sentinel-3A datasets, resulting in Root-Mean-Square (RMS) errors of 5.2, 4.8, 3.7 cm in radial, along-track, and cross-track respectively.
SpaceStar’s in-orbit PPP performance onboard NorSat-TD shows RMS errors at sub decimetre level. During correction outages there is generally no significant performance degradation over that period. A tighter correlation exists between the quality of GNSS observations and the stability of the SpaceStar output. Periods of lower quality observations cause relatively more noise in the SpaceStar output. After complete measurement outages, SpaceStar has demonstrated to reconverge within 20-25 minutes.
To further illustrate the relative performance gain achieved using SpaceStar, a comparison is made with the stand-alone output of the onboard GNSS receiver, where both the receiver and SpaceStar are referenced against the post-processed solution. The comparison shows roughly an order of magnitude improvement compared to stand-alone GNSS receiver output.
Fugro continues its efforts to improve performance by iteration on design and experimentation with different configurations. Research will also be conducted into the feasibility of doing PPP with integer ambiguity resolution (PPP-IAR), and a laser ranging campaign is also planned as an additional method which can be used to verify accuracy.
A key finding that is of increasing importance is the integrity of the computed PVT solution. SpaceStar’s PPP engine has shown to output reliable PVT solutions in events with enhanced signal interference, while the stand-alone GNSS solution has demonstrated to be susceptible to outlying outputs. SpaceStar’s integrity is also evident in case of spoofing events. Where the GNSS receiver outputs a spoofed position, SpaceStar’s positioning software rejects the spoofed observations, demonstrating great reliability of its solution. On top of that, the possibility to include navigation message authentication (NMA) as part of SpaceStar’s services will be evaluated to build further on PVT integrity.
In conclusion, SpaceStar’s PPP performance results in typical errors of 10 cm given the quality of observations it receives onboard NorSat-TD, which is over an order of magnitude better than the stand-alone receiver output. Furthermore, it has demonstrated its enhanced PVT solution integrity compared to the stand-alone GNSS receiver onboard.