LOW COST RELATIVE NAVIGATION SENSING: GNSS-LIKE DEVICES HOSTED ON DEPLOYED TETHERS

摘要

Currently, the possibility to perform very precise GNSS-based relative navigation determination among two or more spacecraft in rendezvous, docking and/or formation flying scenarios is deemed as the baseline for several current and future missions. Due to the required centimetre-level accuracy, the Pseudorange measurements accuracy is not sufficient, and Carrier Phase measurements shall be employed. Those algorithms making exclusive use of Carrier Phase measurements are usually called "Kinematic" algorithms, and require for dedicated Integer Ambiguity Resolution algorithms. Obviously, this relative navigation determination concept is only suitable being the spacecraft in the range of the GNSS constellations coverage. In situations where the GNSS constellations do not provide coverage (from MEO/GEO orbits to non-Earth planetary orbits and passing through inter-planetary missions), systems based on RF signal combining emitters and receivers on-board the same platform have been proposed and are currently being studied (i.e. DARWIN RF subsystem). Nevertheless, DARWIN scenario takes advantage of the multi-spacecraft scenario characteristic (four, at least, spacecraft hosting an emitter are required to provide 3-dimensional navigation). Innovative approaches are required when considering three or just two spacecraft in formation flying. This paper illustrates the concept and supplies simulation based accuracy results of a full GNSS-like based relative navigation determination with RF emitters/receivers hosted on board two spacecraft flying in close formation or performing a RDV and docking manoeuvre, a target and a chaser respectively named. To fulfil concept requirements, at least three RF pseudolites shall be placed on-board or around the target spacecraft so to provide the required RF coverage meant for the chaser spacecraft. A key point to achieve the required system performance is related to the relative geometry between the emitting pseudolites and the receiving antennas. The ideal being the emitters uniformly distributed in the receiver antenna field of view. It is analysed how to achieve this spatial distribution, that cannot be accomplished by placing multiple emitters on-board the same vehicle structure (characteristic spacecraft dimension will be much smaller than the inter-spacecraft distance), by using a tether based deployed pseudolites system. Tether deployment mechanisms, particularly gravity gradient assisted, are assessed in the paper, in order to achieve the best RF emitter pseudolites distribution for a maximum formation flying navigation determination accuracy.