As a further step on today’s way of technology to completely autonomous satellites a satellite’s attitude determination and control system (ADCS) – an essential subsystem – must work autonomously not only during nominal phases of the mission but also in unexpected situations or emergency cases. These include, among others, situations during which the satellite’s main ADCS is corrupt itself or during which the main ADCS’s capability does not suffice any more, e.g. when the satellite is spinning and tumbling uncontrolled with high rates.

To face those situations in the future we imagine a sensor system which is autonomously able to (re)acquire a satellite’s attitude under above mentioned stress conditions and which should also be affordable for smaller satellites and missions. In our opinion the best approach would be an horizon acquisition sensor system, because it would – unlike many other attitude determination systems (e.g. sun sensors, star cameras etc.) – work in nearly every imaginable case for those reasons: the central body’s (in most cases the earth’s) surface looks different to the dark space even during eclipse and it is only hardly probable – nearly impossible – that the satellite would spin and tumble in a mode during which the central body is never visible.

So the sensor system HORACE, which will be newly developed, detects the horizon in video-data-frames provided by a camera using image processing algorithms and calculates a vector to the center of the 2D-projection of the earth. We want to use kind of “normal” or “off-the-shelf” cameras as optical sensors to firstly keep the expenses low but also secondly to test the capability of our algorithms. So in a future version with even more powerful and generic algorithms the HORACE system would work with any image-data and does not need its own camera – as especially earth-observing satellites already include one as payload. Also implemented in future versions would be calculations of spinning rates and rotation matrixes of the satellite’s movement to so predict the nadir pointing vector and pass this data to stabilizing actuators.

From today’s perspective the HORACE-mission on REXUS 15/16 so could be seen as the first small step on a way, which hopefully leads to an autonomously working ADCS, capable enough for emergency cases, what itself would fulfill a great step to completely autonomous satellites.