Multi-Receiver Precise Baseline Determination: Coupled Baseline an Attitude Estimation with a Low-Cost Off-The-Shelf GNSS Receiver

Satellite-based formation flying Earth observations are crucial for environmental monitoring and sustainable use of our Earth’s resources. One example is the German mission TanDEM-X, for which the twin satellites TerraSAR-X and TanDEM-X are working in unison to derive highly precise digital elevation models. To interpret the measurement data of such satellites correctly, the post-processing algorithms depend on precise millimeter-level knowledge of their relative position, known as their baseline. One technique for deriving estimates of the required precision is Precise Baseline Determination (PBD) based on measurements from Global Navigation Satellite Systems (GNSS). While the latest advancements in PBD are suitable for meeting the current precision requirements of non-agile missions, future Synthetic Aperture Radar (SAR) projects will require the same precision for agile mission profiles. Additionally, the PBD for agile missions depends on precise knowledge of the satellite’s attitude. Usually, the PBD and the attitude estimation are performed independently from each other. Whereas PBD utilizes GNSS observations with one receiver per spacecraft, the attitude is estimated via star trackers and inertial measurement units. This paper presents a novel approach of simultaneous GNSS-based baseline and attitude estimation, named Multi Receiver Precise Baseline Determination (MR-PBD). Two benefits are expected. Firstly, the achieved precision of the inter-spacecraft baseline estimate will improve, since multiple receivers are used per spacecraft. Secondly, the estimation delivers an additional attitude product, which can be used to improve the existing star-tracker-based attitude estimation. To validate the approach, the observations of a low-cost and low-power GNSS receiver (u-blox ZED-F9P) are utilized and plugged into a Spirent signal generator. By applying MR-PBD a baseline estimate was achieved that is 15% to 40% more precise than the single receiver PBD by utilizing 3 to 12 receivers per spacecraft.