Finally, after having extensively investigated the close-range region, on the 17th of November we started drifting apart from Beesat-4. The objective of the following days was to demonstrate the primary goal of AVANTI: the capability to approach a noncooperative target from 10km to 1km fully autonomously.
To meet this goal we benefited from a new release of the BIROS AOCS software that solved the reliability issues previously experienced. Thus on the 19th of November we re-initialized our onboard navigation filter introducing errors typical of the accuracy that two-line-elements (TLE) products present. We could not directly use the broadcasted TLE, since they were strongly deteriorated due to the last weeks we spent in close vicinity of Beesat-4, which made impossible for the tracking stations to separate the signals emitted by the two spacecraft.
Our new target was to achieve 1km of mean separation within the following four days, with a final relative orbit of 100m and 60m of magnitude of respectively relative eccentricity and inclination vectors. Afterwards, we wanted to set the onboard controller in “station-keeping”, to maitain such relative orbit.
AVANTI maneuver planner and attitude selection logic
Once received from ground a target relative orbit to be reached at a given time, the AVANTI system autnomously plans the necessary maneuvers to achieve it with a minimum possible delta-v consumption. The maneuver planning is subject to the constraint to avoid scheduling maneuvers when BIROS is in sight of a ground station during supported high-rate downlink passes. In these cases, in fact, the required Earth-poining attitude mode (EPM) is not compatible with the attitude required to perform maneuvers (TFM: thruster firing mode). At latest after 4 maneuvers, the planner updates the remaining maneuver plan till the end-point. This behaviour is required to “close-the-loop”, that is to adjust the control authority on the current relative navigation solution, which includes the effects of the maneuver execution errors and potential changes in the differential aerodinamic drag.
Relative semi-major axis and mean longitude profiles during the rendezvous
The delta-v minimum strategy produced smoothly distributed changes in the relative semi-major axis across the whole reconfiguration time horizon (grey intermediate targets in the plot). Vertical dashed green lines mark the times of the maneuvers executed during the rendezvous.
Another peculiarity of the AVANTI maneuver planner is that it computes reconfiguration profiles able to guarantee the passive-safety of the trajectory throughout the complete transition phase. Therefore the resultant relative trajectory presents the well-known shifting-spiral shape. The size of the spiral is step-wise reduced to acquire the final aimed relative orbit.
Relative trajectory of BIROS with respect to BEESAT-4 during the autonomous approach
Indeed fully autonomous rendezvous towards a noncooperative target making use of angles-only vision-based measurements is now demonstrated!