Precise and Operational Orbit Determination with Onboard GPS

Abstract

URRENT and future LEO missions are trending to incorporate Global Positioning System (GPS) on-board for orbit determination purposes. As a result of this trend, there are two scenarios arising: one of precise orbit determination applications and the other where the GPS tracking data is the main source of operational orbit determination with moderate accuracy. The first scenario represents a more or less manual process where the highest available accuracy is obtained, whereas the second scenario focuses mainly on operational aspects like reliability, solution stability, availability and much less on the orbit determination’s performance. The relatively inexpensive implementation of an on-board GPS receiver in comparison to traditional ranging from ground stations makes this solution very attractive operationally. Also the availability of GPS constellation solutions in near real time with almost unquestionable availability (IGS rapid and ultra-rapid solutions) and very high accuracy allow for operational GPS orbit determination systems to be implemented somewhat easily. Still the trend to use GPS as the primary tracking source is slow due to the computational complexity of the telemetry decommutation and GPS data preprocessing, maintaining this GPS based solution as back-up rather than as the primary for operational orbit determination. GMV has analyzed the different potential scenarios for orbit determination (precise and operational) in relation to the complexity of processing and operational reliability as part of a flight dynamics facility and also from the orbit determination performance point of view. These scenarios cover a range from very precise orbit determination for altimetry missions, where full rate GPS data is combined with DORIS and SLR data, to moderate accuracy where just pseudo-range and single frequency GPS combinations are considered. Different International GNSS Service (IGS) GPS products are incorporated into these scenarios to determine the degradation that is to be expected by relying on the less accurate products (IGS rapid and ultra-rapid solutions in comparison to the final orbits) when near real time applications are needed. Different mission profiles (CHAMP and JASON-1) are investigated to determine the impact of the orbit and attitude dynamics and satellite configuration on the final orbit determination’s performance. The main goal is to have a picture of the potential system capabilities for a given tracking scenario configuration that allow for the optimization of the mission design from the orbit determination point of view. The analysis also focuses on long tracking data periods to see the capabilities of the software (focusPOD) to operationally maintain the uninterrupted orbit determination process in a configuration where there is data coming from the satellite telemetry, ancillary data coming from the GPS constellation (IGS) and environmental data like solar activity and Earth orientation.