The Reuven Ramaty High-Energy Solar Spectroscopic Imager RHESSI spacecraft spins at about 15 rpm around an axis close to Sun center. Precise knowledge of the pointing and the roll angle of the rotating spacecraft is needed in order to reconstruct images with 2 are see resolution using the modulation patterns seen on each of the detectors behind the bi-grid rotating collimators. Therefore, the aspect system consists of two subsystems of sensors, the Solar Aspect System (SAS) and Roll Angle System (RAS). The measured data are sent to the Aspect Data Processor (ADP), where a data compression of about a factor of 1000 and a formatting into telemetry packets takes place. The transmitted data consist of 'solar limb' data from the SAS and 'star event' data from the RAS. Knowing the geometry of all features of the SAS, the position of the Sun center, with respect to a spacecraft fixed coordinate system, can be reconstructed. Similarly, a list of position angle marks can be generated by fitting of the star events and subsequent comparison with a star catalog. Integrating this information allows correcting and interpolating the roll angles that provides a precision of < 0.4 are see (pitch and yaw) and <1 are min (roll).
