Robust attitude determination without rate gyros

A method is described for obtaining optimal attitude estimation algorithms for spacecraft lacking attitude rate measurement devices (rate gyros). Absence of rate data may result either from intentional design or from unexpected failure of existing gyros. Problems arise since typical attitude estimation is accomplished by filtering both attitude rate and attitude measurements. Rates are nearly always sampled much more densely than are attitudes. Thus, the absence/loss of rate data normally reduces both the total amount of data available and the sampling density (in time) by a substantial fraction. As a result, the sensitivity of the estimates to model uncertainty and to measurement noise increases. In order to maintain accuracy in the attitude estimates, there is increased need for accurate models of the rotational dynamics. The proposed approach is based on the Minimum Model Error (MME) optimal estimation strategy, which has been successfully applied to estimation of poorly-modeled dynamic systems which are relatively sparsely and/or noisily measured. Thus, an MME-based approach directly addresses the problems created by absence of attitude rate measurements.