The objective of the study is to define efficient navigation concepts for multi-revolution Solar Electric Propulsion transfers covering the large variety of mission requirements and constraints. These concepts shall build on the experience gained with Smart-1. The use of the new developments shall be included when appropriate in terms of operational load, transfer duration, on-board complexity, mass and cost as well as mission robustness.

Multi-revolution Solar Electric Propulsion (SEP) transfer from the low altitude launcher injection orbit to the final operational orbit is seriously considered or already base-lined for telecom (NeoSat, Electra, Alphabus-extension), Navigation (Galileo next generation) and Earth Observation satellites. With SMART-1 significant experience has been gained to carry out these operations by a fully ground-based approach using conventional radiometric measurements. Meanwhile the boundary conditions for such operations have changed considerably. Engine technology and characteristics have evolved. On-board GNSS based navigation has become standard. Developments are on-going to extend the use of GNSS to higher altitudes even up to GEO. Operations driving mission and spacecraft constraints (attitude control, eclipse handling, parallel payload operations) are different from SMART-1. Several promising operational concepts shall be defined in terms of: - on-board equipment - navigation tasks to be performed on-board by the GNC system - navigation tasks to be performed on ground - tasks to be performed on-ground to support the on-board equipment and GNC system - Interface between the ground and on-board system The concepts shall cover the entire range between pure ground-based and full autonomous approaches. The study shall pay special attention to hybrid concepts with an efficient sharing between ground and on-board functions. It shall include a concept where the orbit is determined and propagated on-board using GNSS and where the thrust direction profile and switching times are calculated optimally on-ground and up-linked to the spacecraft not in absolute times, but relative to the perigee passage time, which is known on-board. It is expected that the synchronization of the thrust with the perigee passage time allows to increase the interval of ground updates from 4 days for Smart-1 to several weeks. For fine targeting to the station position, a few free drift revolutions with small trim manoeuvres should be foreseen at the end. The following aspects of each concept shall be analysed: - navigation performance in terms of fuel usage and targeting accuracy - timeline of operations - ground operational load - on-board processing load - mission robustness - cost for on-board equipment and GNC algorithms - cost for ground system development and operations Pro's and Con's of the different concepts will be identified, quantified and compared to come up with recommendations for the future missions.