This paper addresses the problem of on-board estimation of the state and/or the velocity components of an unknown leader by open-loop control of a single camera installed on the follower aircraft. Unlike the conventional pan-tilt gimbal, a custom-built gimbal is assumed that permits translational movement of the camera. The only information about the unknown leader is deduced from its bearing and subtended angles in the image plane of the camera. An adaptive estimation algorithm is applied to estimate the state vector and the geometric parameters of the unknown object from visual measurements outside the flight control loop of the follower aircraft. Asymptotic convergence of the estimated parameters to the true ones is proven and shown to be independent of the flight controller. The estimated parameters can be used in the guidance law of the follower aircraft without any modifications to it. The main benefit of the proposed paradigm is in this decoupling of estimation from control.