Calibration and Characterization of Imaging Distortion for Star Trackers
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abstract
Star trackers are primarily used to determine a spacecraft's attitude. One of the most important issues regarding precise attitude determination with star trackers is calibration. It is desired that calibration account for any mechanism which tends to distort, usually relative to the pin hole camera ideal model, the imaged data. These distortion mechanisms can include those related to the sensor including non-planarity and instrument aging and those related to the optics including lens distortion, misalignment, and aberration. In this paper we summarize two approaches for estimating the image distortion. We detail algorithm development for these approaches which show promise for solving the nominal problem of time independent (static) distortion. In addition, the use of recursive estimation approaches along with sufficient data rate allows for estimation of the system performance in real time for slowly time varying (dynamic) distortion. The time varying distortion mechanisms can include instrument aging and temperature effects. Temperature effects are studied in order to gauge the expected system performance and the validity of the calibration approach. The calibration approaches have been evaluated by simulation. In addition, the calibration approaches will be evaluated with experimental testing. Ground-based testing can and has been used for an initial calibration for star trackers prior to launch. However for the purpose of algorithm validation, ground based testing is the best source of experimental testing for characterization of the size and the functional form of the distortion because extensive experiments can be done with independent knowledge of the sensor orientation. These studies are designed to establish and validate an autonomous on-orbit calibration algorithm. Orbital conditions cannot be exactly achieved on the ground; however, the most prominent conditions of the orbital environment can be simulated individually. Slowly time varying temperature effects can be realized in ambient conditions and a larger controlled temperature gradient can be simulated with the use of heaters. In this paper, we will present results from exp erimental night sky testing of prototype star tracker cameras. We will detail the characteristics of the camera and the ability of the calibration algorithms to improve system performance.
