Industrial Automatic Control Systems and Controllers Annotation << Back Error Modelling of Pyramid Star Identification Algorithm used in Small Spacecraft Star Trackers Emanov D.V. Star tracker’s attitude estimation error is comprised of multiple factors: errors of optical systems, imaging sensor noise and method errors of algorithms. At the same time it is required to have an estimation of required star centroids in fi eld of view when designing star tracker’s optical system and star catalog to correctly select maximum star magnitude which star tracker should be able to detect. In this work attitude estimation errors of pyramid algorithm, coupled with QUEST (QUaternion ESTimator) algorithm, are experimentally evaluated. The attitude estimation error function of centroid number and centroid coordinates estimation error is modelled. It is shown that availability of more than 20 centroids as pyramid algorithm input guarantees spacecraft attitude estimation error under 30 arcseconds, given centroid coordinates estimation error is 0,5 pixels, and if some spikes are acceptable, even 1 pixel. Furthermore, it is shown that probability of star tracker failure is under 0,01 if more than 20 centroids are available as pyramid algorithm input. Therefore, requirements for star tracker optical system are specified. Keywords: Star tracker, astrometry, attitude estimation, pyramid algorithm, method error. DOI: 10.25791/asu.5.2025.1586 Pp. 32-39.