Abstract A bias error model for the misalignment between mission instrument and optical and inertial attitude sensors is discussed together with a model for drift and scale-factor biases of gyrnscopes. An algorithm to estimate these biases from inorbit calibraiton data is proposed and analysed for an illustrative case of ASTRO-C X-ray Astronomy Satellite. It is shown that some components of the misalignment vectors can be estimated very accurately but others poorly. This is verified by simulation. It is also shown that these know ledges are enough, however, for the attitude control system to eliminate the misslignment-induced pointing error that wouId be inevitable without in-flight calibration.