Investigating RTK using Geostationary Satellites and IRNSS

Abstract

The IRNSS and SBAS constellations have geostationary satellites in the space segment. Geostationary satellites provide additional observations and are always visible to a given user. However, due to their small line-of-sight velocities, geostationary satellites have two unique challenges: Doppler collision and observability, both of which can affect their use in an RTK solution. The first phase of this research is aimed at understanding Doppler collision. It is a unique phenomenon in GNSS where tracking errors are introduced in the measurements due to cross-correlation between two or more satellites. Doppler collisions affect geostationary satellites for longer durations and the error resembles code multipath. If not mitigated, Doppler collision could have an impact on the ability to use code measurements of geostationary satellites in RTK positioning. This research describes likely conditions for Doppler collision, derives a Doppler collision error envelope for geostationary pseudorange measurements, and then demonstrates the effect using simulated and live signals. The second phase of this research presents the effect of Doppler collision on an RTK solution using geostationary satellites, with emphasis on ambiguity convergence time. Multiple mitigation techniques such as de-weighting of geostationary observations and use of narrow correlator are proposed to reduce the impact of Doppler collision. iii The third phase talks about the observability of a geostationary satellite. The relatively static nature of geostationary satellites leads to poor observability and has a direct impact on the convergence of ambiguities. The poor observability can limit the use of standalone constellations such as IRNSS in an RTK solution. Finally, an investigation is conducted on both hardware-simulated and live data of IRNSS to understand the impact of Doppler collision and observability. Mitigation methods are applied, and the improvement in the code measurement error and the convergence of ambiguities is presented. Overall, this thesis is aimed at addressing some of the key issues arising from the use of geostationary satellites in an RTK solution so that a multi-constellation RTK solution progresses one step closer to the possibility of an all-constellation RTK solution, including IRNSS.