In Orbit Performance of a Fully Autonomous Star Tracker

The Department of Automation at DTU has developed the Advanced Stellar Compass (ASC), a fully autonomous star tracker, for use as high precision attitude reference onboard spacecrafts. The ASC is composed of a CCD-based camera and a powerful microprocessor containing star catalogue, image-analysis software and a search engine. The unit autonomously performs all tasks necessary to calculate the inertial attitude from a star image. To allow for flexible attitude manoeuvres, the ASC can, simultaneously, drive from one to four cameras, efficiently removing dropouts from, e.g., sun blinding of one camera. The key features of the ASC are fast and extremely robust arcsecond accuracy in a miniature design. Due to the attractive ratios of performance to mass, power and cost, the ASC has been selected as the baseline attitude sensor for a suite of missions by the major space agencies of the world. Generally, it is difficult to test and verify the true robustness and accuracy of a star tracker on ground. This is caused by the fact that only real-sky tests offer high fidelity stimulation of the sensor, while the atmosphere instabilities result in a dominant noise source intrinsically limiting the achievable accuracy. This paper compares in-orbit performance with ground test starting with a brief summary of the ASC configurations on the missions referenced, i.e. Teamsat, ASTRID2 and Oersted. Then the in-orbit operation and performance of the ASC with special emphasis on robustness and closed loop operation aspects is discussed based on specific attitude manoeuvres. This is followed by a comparison between ground-test and in orbit operation in situations where the instrument for some reason has to operate close to bright objects. For reference, simulations and in-orbit performance of stray light baffles are given. Finally, the accuracy over the lifetime is discussed. The importance of the last topic is augmented by factors such as radiation, aging by direct sun exposure and non-stellar objects.