Implementation of an in-orbit calibration is an effective approach to reduce position error of individual star spot down to arcsec on star tracker. However, without knowledge of variance errors of some error sources, the reduction of such errors to what extent is unclear leading to in-orbit correction inefficiency. Based on Cramer Rao Lower Bound(CRLB) theory, we obtain the minimum variance error of position and make an accurate definition of precision windowed track mode for in-orbit calibration. After that, the error constraints on proper motion, velocity aberration, drift in focal length and other factors are studied in details. Imposing proper restrictions on those parameters, in-orbit correction of star tracker could approach CRLB accuracy.
KEYWORDS: Stars, Space operations, Monte Carlo methods, Signal to noise ratio, Motion models, Charge-coupled devices, Error analysis, Algorithm development, Detection and tracking algorithms, Motion measurement
The influence of acceleration resulting from spacecraft maneuvering on star spot positioning and hereafter dynamic motion compensation technology are addressed. Firstly the pattern of the smeared star-spot image under maneuvering condition and the locating error are investigated. It is found that instead of following a symmetrical shaped pattern, the smeared star spot under acceleration is twisted. Simulation verifies that the position error is far beyond Cramer Rao Lower Bound(CRLB) of photoelectric devices under uniform velocity. Then a novel scheme to derive the more general accurate motion compensation is proposed. In this case, an approximate CRLB is derived to give a baseline to measure the performance of any positioning algorithm and motion compensation technique on star tracker. The theory and corresponding simulations show the novel general compensation approach is better than the conventional compensation strategy and close to the approximate CRLB. Therefore, a CRLB position accuracy of star spot is expected to be realized by using the generalized dynamic compensation method on maneuvering spacecraft.
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