  Though in-orbit calibration is adopted to reduce position error of individual
star spot down to 0.02pixel on star tracker, little study has been conducted on
the accuracy to what extent for some significant error sources which often
leads to in-orbit correction inefficiency. This study presents the general
theory and estimates of the minimum error constraints, including not only on
position but also on intensity and scale of Gaussian shaped profile based on
Cramer Rao Lower Bound(CRLB) theory. By imposing those constraints on motion,
drift in focal length and so on, margins of in-flight error sources and the
final accuracy of star tracker can be analytically determined before launch.
