This work extends the earlier results on passive vision-based tracking and motion estimation of a ground vehicle. The follower small unmanned air vehicle (UAV) is equipped with a single gimbaled pan/tilt camera and a high bandwidth wireless link for video and command transmitting. The objective is for the UAV to maintain a horizontal circular orbit about the target with a predefined radius and to concurrently provide real-time estimation of the target's position, speed and heading. The target velocity estimation problem is formulated such that the recently developed L1 fast adaptive estimator can be applied. We give a rigorous proof of asymptotic stability for the guidance law for the static target case, and provide a reformulation of the control objective for the moving target case so that the existing controller can be applied naturally.