Real Time Navigation Algorithms for LEO Small Satellites using COTS GNSS

Abstract: Many satellites in LEO use a GNSS-based navigation system, taking advantage of the GNSS constellations in MEO to enhance navigation capabilities. The thesis work focused on developing the software enabling GNSS-based navigation for Hemeria's future small satellites operating in LEO. Real time algorithms were developed to perform on board accurate frame conversions based on the International Earth Rotation and Reference Systems Service 2010 convention. The implementation was tested and independently cross-validated using space mechanics libraries and data from Observatory of Weights and Measures of Paris. The method was based on the Earth Orientation Parameters, and the balance between autonomy and accuracy was assessed. In pursuit of an affordable navigation system, a commercial Off-The-Shelf GNSS receiver was used. The navigation solution, derived from single frequency measurements, suffered from a systematic bias caused by the ionosphere. Mitigation strategies with ionospheric corrections were included in the IONOS simulator. Several attitude control modes were simulated so the effect of spacecraft tumbling versus the availability of GNSS measurements in a degraded situation was quantified.  Through this thesis, algorithms for GNSS-based navigation of Hemeria's small satellites in LEO were developed. The results showed the successful implementation of real time frame conversions at the metre level at best. It was found that the implementation allowed an improved autonomy of the frame conversion at the decimetre level of accuracy over a period of two months.