Aerospace Control Systems

Spacecraft GNC Systems Engineering




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How many axes of misalignment between two star trackers can be estimated?

There are a few who claim that misalignment about the boresights of the two star trackers are not observable or cannot be estimated accurately, and that only the misalignment about an axis perpendicular to the boresight vectors is observable. There is an abundance of evidence to the contrary in the literature. Three relative misalignments can be estimated quite accurately. This was proven in the paper

M. D. Shuster and D. S. Pitone, "Batch Estimation of Spacecraft Sensor Misalignments, II. Absolute Alignment Estimation", Journal of the Astronautical Sciences, Vol. 39, No. 4, Oct–Dec 1991, pp. 547–571.

Aso see the article "General Definition Of Relative Misalignment" cited in the Conference Papers bibliography.

What cannot be estimated from the star tracker data, however, is the absolute misalignment of each star tracker relative some coordinate frame such as a body frame or a payload frame. That would entail six degrees of freedom (two rotation vectors) whereas only three degrees of freedom of misalignment (one rotation vector) are observable.