  A deterministic attitude estimator for a rigid body under an attitude
dependent potential is studied. This estimator requires only a single direction
measurement to a known reference point at each measurement instant. The
measurement cannot completely determine the attitude, but an attitude
estimation scheme based on this measurement is developed; a feasible set
compatible with the measurement is described and it is combined with an
attitude dynamics model to obtain an attitude estimate. The attitude is
globally represented by a rotation matrix, and the uncertainties are described
by ellipsoidal sets. A numerical example for a spacecraft in a circular orbit
is presented.
