The sequence of images obtained from a passive forward-looking forward-traveling sensor provides the optical now on which passive ranging is based. Passive ranging is of interest in the context of nap-of-the-Earth helicopter obstacle avoidance. Ranging can be performed for some chosen features or, in principle, for all points in the field-of-view (field based) of the sensor. Velocity filtering is a track-before-detect algorithm that can be utilized to perform field-based passive ranging. We expand on the theoretical understanding and the performance analysis of the velocity filtering algorithm as applied to optical-flow-based ranging.
