GPS Orbit Determination for Micro-Satellites – The PROBA-2 Flight Experience

The PROBA-2 microsatellite of the European Space Agency (ESA) is equipped with a novel single frequency global positioning system (GPS) receiver, which combines low resource requirements with a high navigation performance. Aside from supporting the spacecraft and mission operations, the Phoenix GPS receiver on PROBA-2 is used to study realtime and offline navigation using single-frequency GPS measurements. A 1 m accuracy is targeted for both applications which has so far been the domain of dual-frequency receivers. Following a mission overview, the achievable performance for ground-based orbit determination and real-time onboard navigation is discussed for different processing concepts based on flight data collected during the first half year of mission operations. It is shown that a 1 m or better accuracy can be achieved in either case, despite sub-optimal GPS tracking conditions induced by the mission specific attitude profile. Besides self-consistency checks and comparisons with independent GPS based orbit determination solutions, the accuracy is evaluated using satellite laser ranging (SLR) measurements as an external reference. The results show that single-frequency GPS tracking is sufficient to meet the navigation requirements of even advanced remote sensing missions in low Earth orbit, if their accuracy potential is properly exploited

Autonomous and Precise Navigation of the PROBA-2 Spacecraft

PROBA-2 is the second technology demonstration mission within the project for onboard autonomy of the European Space Agency (ESA). Besides other instruments and sensors, the micro-satellite will be equipped with two new types of global positioning system (GPS) receivers. These will support the spacecraft operations and demonstrate recent advances in the field of autonomous real-time navigation and offline orbit determination for micro-satellites. The paper provides an overview of the key PROBA-2 navigation elements and discusses their scope and capabilities. Special attention is given to the Phoenix-XNS miniature GPS receiver and its embedded navigation function which are presented along with a discussion of the employed filtering and processing algorithms. The impact of PROBA-2 attitude changes on the GPS tracking is analyzed and the employed strategies for minimizing possible outages are presented. Hardware-in-the loop simulations in a signal simulator testbed are used to demonstrate the feasibility of 1 m level real-time navigation using a single-frequency GPS receiver and to demonstrate the overall robustness of the PROBA-2 onboard navigation