The Mars Exploration Rover (MER) Descent Image Motion Estimation System (DIMES) is the first autonomous machine vision system used to safely land a robotics payload on another planet. DIMES consists of a descent camera and an algorithm for estimating horizontal velocity using image, inertial and altitude measurements. Before DIMES was accepted by MER for inclusion in the mission, its performance was validated through field testing using a manned helicopter to image three Mars analog test sites. Statistical analysis of the resulting 1900+ test cases showed that DIMES met its velocity estimation requirement. This paper describes the DIMES field test approach and associated results.