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22.11.16 - Close approach to 80m



Starting from a few hundred meters, a challenging approach to a few dozen meters was successfully exercised last week. Not really an autonomous rendezvous though: the main objective was rather to collect relevant experience at mid to close range.

Rendezvous with BEESAT-4

At this distance, the vision-based navigation system has indeed to face three major problems:

  • The target object becomes so bright that no star is visible anymore in the background if the electronic shutter of the camera is used: as a result, the accuracy of the measured line of sight is heavily degraded. The navigation has in fact to rely on the onboard estimate of the attitude which is not as precise as the direct estimation of the attitude of the camera using the visible stars.
  • Due to the limited field of view of the camera (12° x 18°), a small error of the onboard navigation solution can lead to a loss of visual tracking. An error of 10m in the relative state estimate can in the worst case translate into 6° error at a distance of 100m, so that the camera may point in the wrong direction in the presence of navigation uncertainty.
  • Our concept of passive safety using a proper phasing of the relative e/i vectors induces a spiraling relative motion. At close distance, a very peculiar spacecraft attitude profile is thus needed to follow the target, which in turns results in large variations of the drag.

Some reliabilities problems on the spacecraft have prevented us to perform this approach autonomously (these problems will be solved soon with a software patch of the AOCS). As a consequence, this approach has been performed with the ground segment in the loop, resulting in a pretty large control cycle of about 12 hours (time necessary to download the images, process them, compute a new guidance plan and upload it to the spacecraft). Nevertheless, we were able to reduce the separation by less than hundred meters and we are glad to recognize BEESAT-4 in one picture: its cubic shape together with its two antennas (check it here) are now clearly visible! This is the ultimate proof that we were following the proper object.

Complete approach                         BEESAT-4 finally spotted  
 

 


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