Abstract
We examine the effectiveness and robustness of a moon-tracking backup mode for modern star trackers. This approach can be used to calculate a three-axis attitude solution from overexposed images of the moon. In examining this problem, we carefully consider the operational conditions under which this capability would be necessary and tailor our algorithmic approach appropriately. We describe a detailed simulation of the moon imaging process and outlinetherequiredimageprocessingstepsrequiredtoextractusefulinformationfromthemoonimages.Theimage processingextractsthemoonoutlineandestimatesthemoonpositionandorientationusinganonlinearleast-squares fit. Under most lighting conditions, our moon-vector estimates have an error of about 0.001 deg, and our sun-vector estimates have an error less than 0.25 deg. The capability of maintaining moderate accuracy attitude tracking in the presence of moon incursions is an important milestone toward a star-tracker-only attitude control system for microsatellites and nanosatellites. Nomenclature A, B, C, D = pixel vertex points a = radius of moon image, pixels b = semiminor axis of terminator curves, pixels CFG = rotation matrix (from G to F) E = overexposure factor e = eccentricity fi = error function fpix = focal length, pixels g = curve-fitting error hG = focus blur impulse response Ithresh = image intensity threshold
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