Parameter Interference in Distortion and Alignment Calibration
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Parameter Interference in Distortion and Alignment Calibration Roberto V. F. Lopes! and Malcolm D. Shuster' Abstract Estimation algorithms are presented for the consistent determination inflight of focalplane distortion parameters and alignment parameters for attitude sensing instruments. The ambiguity in the specification of the parameter sets is demonstrated and a simple prescription, called SMARTCAL, is given for removing it for the case of polynomial representations of the focal-plane distortion. We demonstrate that independent calibration of attitude-sensitive scientific instruments by the investigator followed by the independent alignment of the attitude determined system by the attitude support team can lead to random walks in the calibration and alignment parameters of all instruments and sensors. Modalities are presented which avoid these parameter instabilities.
Introduction The practical use of scientific instruments and attitude sensors on spacecraft generally requires that these be recalibrated after launch. In addition, due to changes in the spacecraft structure arising from thermal flexure and zero-gravity effects, the alignment of these devices must also be determined. For unmanned spacecraft, the separation of alignment and distortion corrections is nontrivial. It is made still more complicated by the fact that we commonly represent spacecraft alignment in terms of transformations in three dimensions, while distortions of the focal plane are most conveniently treated in two dimensions. Thus, investigations of the interference of focal-plane distortion and misalignment are compromised by a fundamental difference in treatment. Motivated by these concerns, a previous report [1] developed the representation of rotations in the focal plane. In the present work we develop and test algorithms which apply this representation to the estimation of sensor focalplane distortion parameters and rotational parameters. 'Engenheiro de Desenvolvimento Tecnol6gico, Divisao de Sistemas Espaciais, Instituto Nacional de Pesquisas Espaciasis (INPE), 12201 Sao Jose dos Campos (SP), Brazil. email: [email protected]. 2Director of Research, Acme Spacecraft Company, 13017 Wisteria Drive, Box 328, Germantown, Maryland, 20874. email: [email protected].
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It is not always appreciated that distortion and misalignment are not independent transformations of the focal plane. Therefore, attempts to calibrate sensors both for distortion and misalignment corrections sometimes lead to estimates of the parameters which are not meaningful individually. In ground calibration, when the sensor alignment and the calibration are commonly referred to an optical alignment cube mounted on the sensor, no ambiguity arises. In space, unfortunately, one no longer has any knowledge of the orientation of the optical alignment cube, and the unambiguous separation of alignment and distortion parameters is no longer possible. If one is interested only in representing the transformation of sensor data from the spacecraft to an
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