M. Karovska, T. Aldcroft, and R. Cameron
SAO
The AXAF Ground Aspect Determination System (GADS) pipeline is a collection of tools which process aspect telemetry and generate aspect data products. It produces a three-axis aspect solution in J2000 celestial coordinates, to provide both image reconstruction and celestial location for X-ray data, with better than 1 arcsec accuracy. The aspect solution is calculated from telemetry data from the AXAF aspect camera (ACA) and inertial reference units (IRU), plus associated calibration and alignment data. The ACA telemetry typically consists of pixelated images of 5 stars and 3 fiducial lights, at 2.050 second intervals. The fiducial light images are transported into the ACA through relay optics from selectively-powered fiducial lights around each X-ray detector focal plane. The absolute celestial coordinates of the stars, and their apparent positions in the aspect camera field of view, determines the absolute pointing of the AXAF aspect camera and spacecraft. The relative motion between the star and fiducial light images allows the absolute knowledge of the aspect camera pointing to be transferred to the X-ray focal plane, providing an absolute celestial coordinate system for detected X-ray events. The centroid of each star or fid light image is calculated using a PSF-fitting algorithm, where the appropriate PSF is interpolated from a database of PSFs which have been subsampled on a finer pixel grid than the ACA pixel size. The database of PSFs is obtained from ground calibration or raytracing. The IRUs provide 4 axes of integrated angular rate telemetry at 0.25625 second intervals, from which 3 orthogonal axis rate data are derived. The star centroids are combined with the IRU rate data in a Kalman filter, to provide a time-varying optimal aspect solution and associated covariance matrix, assuming known noise models for the aspect camera and IRU data.