Spin axis attitude determination accuracy model in presence of biases

A practical model is presented for the effects of biases on the spin axis attitude pointing orientation of a spin-stabilized spacecraft. Biases are induced by spacecraft design limitations, for example, dynamic imbalance and sensor mounting alignment errors, and by environmental effects such as the variations in the infrared Earth radius seen by the Earth sensor. The measurement equations for the sun and Earth sensors are formulated in the presence of the relevant biases. The effects of the biases on the angular measurements are established by means of a small-angle perturbation technique. The propagation of the biases into the resulting attitude solution is derived by means of a realistic single-frame attitude-determination method. The statistical properties of the expected attitude error can then be expressed in terms of the specified input biases. Furthermore, practical approaches for the reconstruction of the biases from the observed measurement residuals are provided. The models are demonstrated using actual sensor data of the Comet Nucleus Tour (CONTOUR) spacecraft collected during its phasing orbits in August 2002. The sensitivities of the sun and Earth sensor measurements and the attitude vector to the relevant biases are analyzed in detail, and the reconstruction of the biases in the infrared Earth radii is illustrated for CONTOUR's final sensor coverage interval.