Rendezvous with a satellite
A unique experiment – with a very special goal – has been devised by the German Space Operations Center (GSOC) of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The AVANTI experiment (Autonomous Visual Approach Navigation and Target Identification) is intended to demonstrate how a satellite can detect a spacecraft in space and approach it autonomously. This capability will be necessary in the future for capturing old and inactive satellites, as well as space debris, and bringing them into a safe orbit. To this end, scientists are using the recently launched BIROS satellite and the BEESAT-4 pico-satellite that it deployed. The technology for the experiment is based on BIROS, which is meant to represent the 'catcher' satellite. The small BEESAT-4 acts as an 'inactive' satellite in the experiment.
AVANTI will fully autonomously calculate an optimised flight path for BIROS so that it can approach the target object. "With AVANTI, we are trying to prove that it is possible to safely approach inactive or non-cooperative objects in motion at a greater to medium distance. The BIROS and BEESAT-4 satellites are perfect for this experiment. AVANTI only requires a simple sensor system for relative navigation, which is why we can use the star camera, which is already on BIROS, as a monocular camera," explains Gabriella Gaias, project manager at GSOC. "A particular challenge is that BIROS is in a low Earth orbit. As a result, both satellites are eclipsed by Earth at regular intervals, so that BEESAT-4 can no longer be seen by the camera."
DLR researchers are using improved algorithms for the steering, navigation and control of the 'catcher'. One after the other, AVANTI performs a series of measurements. Then, the star camera acquires images of the target area. An image processing program analyses the recordings, identifies the flying object and measures the bearing to the object. Subsequently, the relative real-time navigation algorithm is fed with information from the bearing measurements and the calibrated flight manoeuvre data with which the relative movement of BEESAT-4 can be calculated. The resulting relative position and speed of the target object can ultimately be used for manoeuvre planning to program a safe and efficient flight path to reach BEESAT-4.
The danger posed by passive and non-cooperative projectiles in space has been acknowledged for some time. As recently as 2009, the collision of two satellites raised awareness in the global space community of how dangerous space debris, in particular old satellites, can be. With AVANTI, GSOC is continuing the ARGON experiment, which scientists successfully completed in 2012. At that time, the technology was on the Swedish PRISMA mission. In contrast to ARGON, AVANTI can conduct its tasks completely autonomously and without additional information from the small satellite. In addition to the low Earth orbit in which the experiment is taking place, the conditions for AVANTI are much more demanding than in its predecessor mission.