Functional model for spacecraft formation flying using non-dedicated GPS/Galileo receivers

There is trend in spacecraft engineering toward distributed systems where a number of smaller spacecraft work as a larger multi-instrumented satellite. However, in order to make the small satellites work together as a single large platform, the precise relative positions and orientations of the elements of the formation have to be estimated. Global Navigation Satellite System (GNSS) receivers can be utilized to provide baseline estimates with centimeter to millimeter level accuracy. The legitimate spaceborne receivers can not be applied on smaller satellites due to various restrictions which are discussed in this paper, and therefore non-dedicated receivers are lately being used. However, to achieve precise relative positioning in situations with high dynamics and/or large receiver clock offsets, the standard functional model for GNSS-based relative positioning can not be applied. In the presented hardware-in-the-loop simulation of the Proba-3 spacecraft flying in formation, the error for the standard functional model can up to 16 centimeters. A new functional model is introduced that can correct the time-varying offset error and achieve the mm-level accuracy of GNSS even with non-dedicated GNSS receivers.