ESA GNC Conference Papers Repository

The Star Tracker (STR) based attitude estimation is being generalized for any kind of mission like Earth observation, scientific mission with accurate inertial pointing, interplanetary, formation flying or even telecom in GEO orbit. The classical mono-STR solution may be enhanced by considering:<br> · several classical autonomous star-trackers whose outputs are processed within the spacecraft central computer (Multi Star-Tracker, shortly MSTR); <br> · several optical heads (including their proximity electronics) processed within a dedicated electronics which delivers to the ACS a composite attitude measurement (Multi Heads Star-Tracker, shortly MHST).<br> The MHST/MSTR based attitude estimation is now proposed to enhance the classical STR based attitude estimation by providing:<br> · better attitude restitution accuracy about the 3 satellites axes thanks to the data from several Optical Heads (OH),<br> · tolerance to high rate dynamics conditions or inertial pointing,<br> · tolerance to the blinding of one OH,<br> · simplification of the sensor suite (to get rid of gyros or to limit the use to “coarse gyros”),<br> · cost effectiveness (mainly for MHST solution).<br> The use of MHST leads to a design of innovative estimation concept and FDIR at sensor and ACS level. It requires a greater interaction between ACS designer and unit supplier to achieve an optimal ACS design for a given mission (implementation selection related to field of view (FOV), satellite FDIR, satellite guidance to avoid STR blinding, assisted re-acquisition with the help of other sensors). The system/ACS shall have a clear visibility on MHST operational behaviour, especially at algorithms and software levels, due to the fact that MHST concept: · becomes the sole attitude sensor if a gyro-less configuration is selected,<br> · may require a huge data exchange at a high rate with ACS depending on the architecture,<br> · requires some mechanical alignment and stability performances between the optical heads,<br> · requires some precise thermal environment for optimizing the performance,<br> · shares the “authority” with ACS in particular for the Optical Head management.<br> The paper summarizes the main results from the Technical Research Program “Attitude Determination Tailored to Multiple Heads Star Tracker” awarded by ESA to Thales Alenia Space. The objective of the study has been to prepare the initial development effort at ACS level prior to the practical implementation of the MHST concept within future missions. The study is still in progress and then only partial results will be provided. The paper address in detail the attitude estimation issues like data-fusion, calibration and attitude observers and other topics at system level like failure detection and validation. Based on the available measurements (attitude only, attitude and angular rate), and the commanded torque, the attitude observer reconstructs the spacecraft state variables such as the predicted attitude and angular rate, the perturbing torque and other additional states. The attitude observer (designed according to the Extended Kalman Filter approach) is based on a model of the plant implementing the correction and multiple prediction scheme useful to reduce the effects of delays. A Gyroless solution is then proposed for the “Classical” LEO Earth pointing mission. A Gyro-star-tracker (GyroSTR) solution is proposed for the “Agile/Accurate” LEO Earth pointing mission. The design description of these MSTR/MHST based attitude determination is presented including their advantages, drawbacks and constraints, and their performances in nominal and degraded configuration.