PRISMA Relative Orbit Determination using GPS Measurements

The Swedish PRISMA (Prototype Research Instruments and Space Mission technology Advancement) mission is a technologic demonstration from the Swedish National Space Board (SNSB) and the Swedish Space Corporation (SSC). It will provide the demonstration and validation of different sensors, as well as navigation and guidance algorithms, and strategies for rendezvous and advanced formation flying. The aim is to prepare future missions such as Proba-3, Darwin, SMART-OLEV, which rely on formation flying. The launch of the PRISMA satellites is expected in the beginning of 2010. The mission has two spacecraft, called Main (or Mango) and Target (or Tango). Initially attached at launch, they will first be operated as a single combined unit, and then be separated for the execution of autonomous rendezvous and formation flying experiments. The on-board GPS-based absolute and relative navigation system is contributed by the German Aerospace Center (DLR). It consists of two redundant single-frequency Phoenix-S receivers on each spacecraft and a dedicated navigation software residing on the Main on-board computer. The DLR’s GPS system is not only a sensor and navigation experiment, but it also provides fundamental navigation system functionality for the formation as a whole. Among other experiments, PRISMA supports DLR’s Spaceborne Autonomous Formation Flying Experiment (SAFE). The French National Space Agency (CNES) contribution to PRISMA consists of a Formation Flying RF (FFRF) sensor, funded in collaboration with the Spanish Centre for the Development of Industrial Technology (CDTI), and its dedicated guidance, navigation and control (GNC) software. The FFRF instruments are developed by Thales Alenia Space. This will be calibrated and validated during flight experiment open and closed loops. This contribution is a part of the Formation Flying In Orbit Ranging Demonstration (FFIORD) experiment. The reference for FFRF sensor calibration and validation will be given by the on-ground precise relative orbit determination based on GPS code and phase single-frequency measurements. Although the primary on-ground absolute and relative orbit determination tasks will be performed by DLR during mission operations, a CNES relative precise orbit determination will also be done for backup and internal needs of FFRF sensor calibration. A pre-flight CNES/ DLR hardware-in-the-loop joint validation of the FFRF and GPS systems was performed, in a context as representative as possible to the PRISMA mission, using a Spirent GPS signal simulator. This article presents conditions of the simulations, the algorithm used and the results of CNES relative orbit determination obtained during this joint validation. The expected orbit determination accuracy is evaluated by comparing CNES precise orbit with the simulated reference. A comparison is also done with regards to the FFRF relative position solution. The results are in total agreement with the required specifications for the FFRF validation and calibration. To validate CNES algorithms for PRISMA, an additional test with in-flight GPS measurements from Gravity Recovery and Climate Experiment (GRACE) experiment was performed. GPS flight data collected during GRACE satellites closest approach in the swap maneuver are used in a configuration representative of PRISMA formation geometry. The results presented in this article fully confirm the simulated performance.