Real-Time Precise Orbit Determination With Undifferenced Ambiguity Resolution for LuTan-1A Satellite Using Onboard GPS/BDS-3 Observations

The rapid growth of space-based activities has greatly heightened the need for accurate, real-time tracking of low Earth orbit (LEO) satellite positions. Currently, the real-time precise orbit determination (RTPOD) for LEO satellites typically relies on ambiguity-float solutions derived from onboard Global Navigation Satellite System (GNSS) observations. This research aims to perform RTPOD with undifferenced ambiguity resolution for the LuTan-1A satellite based on the square root information filter. The influence of the GPS/BDS-3 combination on ambiguity resolution and LEO RTPOD performance is also investigated. For the purpose of enabling the ambiguity resolution of the LuTan-1A satellite, the proposed method adopts a globally distributed network to conduct the epoch-by-epoch estimation of the GPS/BDS-3 real-time satellite clock and phase bias. The RTPOD results of the LuTan-1A satellite reveal that the three schemes, namely GPS-only, BDS-3-only, and GPS+BDS-3, have an ambiguity fixing rate of 96.7%, 89.7%, and 99.8%, and the time to first fix of 3.7, 11.0, and 1.0 min, respectively. The dual-system combination of GPS and BDS-3 brings about more available GNSS satellites, thereby resulting in an evident improvement in ambiguity resolution performance. Furthermore, the application of undifferenced ambiguity resolution enables more accurate and robust RTPOD results for the LuTan-1A satellite. With regard to convergence time, thanks to the ambiguity fixing, a reduction of more than 60% is accomplished for the GPS-only and GPS+BDS-3, and 23% for BDS-3-only. Additionally, the LEO three-dimensional orbit accuracy for the ambiguity-fixed solution of the GPS-only, BDS-3-only, and GPS+BDS-3 schemes are 2.6, 2.8, and 1.7 cm, respectively, which are 38.1%, 35.7%, and 49.0% better than the corresponding ambiguity-float solutions.