Absolute attitude determination system for a spherical air bearing testbed

Three degree of freedom (3-DOF) ACS satellite testbed systems are essential hardware-in-The-loop testing facilities for the construction of a wide range of intelligent AOCS algorithms. Intrepid is a 3-DOF spherical air-bearing satellite testbed custom designed at the Surrey Space Centre, UK, and installed on-site at Airbus Defence and Space in Friedrichshafen, Germany for the Orbital Flight Dynamics and AOCS / GNC Department. Satellites require long-Term stable and short-Term high resolution attitude measurements, which are fused on-board to estimate the attitude. The on-Testbed IMU provides high resolution attitude information, but has the drawback of not knowing the initial attitude and also drifting over time. This paper concerns the development of an absolute attitude determination system (AADS) that utilises an infra-red camera, a series of LEDs positioned across the table-Top, and intelligent software to independently determine the absolute attitude of the table-Top. The system thus provides long-Term stable attitude measurements and overcomes the shortcomings of the IMU. The newly installed AADS software contains vision processing and triangulation algorithms that provide real-Time absolute attitude information to the host computer. The vanilla system can achieve attitude determination accuracy (during slew manoeuvres) with an offset of slightly over 0.2° and standard deviations of 0.33° and 0.74° in roll and pitch respectively. The software can calibrate itself compensating for intrinsic and extrinsic offsets including incorrectly placed LEDs and a misaligned camera system. A sub-pixel feature detection technique is utilised to enhance the accuracy of the system. It is shown that by modifying the position of the LEDs or modifying the placement of the camera, optimal attitude accuracies along different axes can be obtained. This paper will detail the main operation of the system from both a hardware and software perspective and provide experimental results during operation.