A cosmic game of cat and mouse
For the first time in the history of space exploration, scientists at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have demonstrated in a real space experiment how a satellite can approach a counterpart by fully autonomously, making use of only optical or vision-based navigation. This highly sophisticated technology was demonstrated on the Earth observation satellite BIROS (Bi-spectral InfraRed Optical System) as part of the AVANTI experiment (Autonomous Visual Approach Navigation and Target Identification), during which the satellite was able to draw to within 50 metres of its target object.
BEESAT-4, a picosatellite dispatched by BIROS last September, was used as a target satellite in the experiment. "We have, for the first time ever, demonstrated how a satellite can autonomously recognise and approach a flying object without using GPS data from the target satellite or requiring commands from the control centre," says Thomas Terzibaschian from the DLR Institute of Optical Sensor Systems, confirming the experiment's success. BIROS was fitted with a spring-loaded deployer mechanism that ejected the picosatellite at a velocity of 1.5 metres per second. In addition, DLR scientists and engineers equipped the microsatellite BIROS with a propulsion system that enables the washing machine-sized, roughly 130-kilogram satellite to rapidly execute any slew manoeuvres. During the AVANTI experiment, BIROS learned how to specifically modify its trajectory and use a camera head of the star trackerto search for and 'keep an eye' on the picosatellite. Unlike in conventional orbital manoeuvres, the satellite did not receive any commands from the control centre in Oberpfaffenhofen, relying instead on the on board AVANTI experiment programs. Equipping satellites with this capability could become extremely important in the future, as it would allow them to detect and later capture old and inactive satellites and space debris.
Approach to almost 50 metres
"In the AVANTI experiment, we demonstrated for the first time that merely one passive camera is sufficient to enable the autonomous approach of one satellite toward a non-cooperative object," says Gabriella Gaias from Space Operations and Astronaut Training at DLR. The image data was used to identify the target satellite and monitor its position. This then enabled the calculation and execution of the thrust manoeuvre needed to complete the approach. All of the steps were fully autonomous and took place on board BIROS. During the experiment, BIROS approached its target cube satellite – which has an edge length of just 10 centimetres – to within 50 metres. "This distance was specified as a minimum threshold for security reasons as the satellite does not have any additional and redundant sensor systems that would enable precise measurement of the relative positions. We also had to allow for a certain inaccuracy of the control system," adds Gaias.