DYNAMICAL BINNING FOR HIGH ANGULAR RATE STAR TRACKING

摘要

Recent advances in CCD and Active Pixel Sensor (APS) technology and the development of efficient star pattern recognition algorithms hold the prospect of realising a star tracker system that can serve as the only on-board attitude sensor. One of the obstacles to be overcome to attain this objective is the loss of performance of star trackers in the presence of high angular rates. Typically, accuracy degradation sets in at rates of 0.1-0.2 deg/s and most sensors have lost track at 1-1.5 deg/s. This is a severe limitation since angular velocities of up to 1 deg/s or higher are not uncommon on spacecraft. This paper describes a concept, called "dynamical binning", for enhancing the robustness of star trackers to high angular rates. Under dynamical conditions, the star image appears to be moving on the pixel array. The photon flux from the star is spread over a large number of pixels thus resulting in a low SNR. The proposed concept aims to de-spread the star image by oversampling and combining the pixel read-outs from successive subsamples in a manner that compensates for the motion of the star image. Dynamical binning is considered here in the context of APS-based star sensors. APSs are well-suited to this technique because they offer greater flexibility in the processing of pixel-level information and because the logic required for dynamical binning can be implemented directly on the same imager chip hosting the pixel array. After describing the dynamical binning algorithm, the paper presents the results of simulations that confirm the gains in SNR afforded by this technique. Implementation aspects are also discussed. In particular it is argued that on-chip implementation of the algorithm is feasible with current technology. Finally, plans for a prototype imager with dynamical binning to be realised in ESTEC are discussed. A patent application for the dynamical binning concept has been filed by ESA.