Abstract: An autonomous orbit determination system based on two angles and selenocentric distance was studied.One angle was between the lines of sight to a star and selenocentric direction, and the other was between the sun and selenocentric direction.A modified observability analysis method defined by the spectral function was presented through the study on some nonlinear system. And the relationship between the observable degree and the accuracy of the system was proved.The simulation resalts show that the modified method is feasible.

Key words: Spectural radius, Error covariance array, Observable degree, Autonomous celestial navigation, Lunar orbiter