In this paper we propose a visual estimation algorithm to aid the guidance system of an unmanned aerial vehicle (UAV). Specifically, we are considering a glider UAV trying to reach a target zone. Given that the UAV is falling fast, the scale of the images captured by the on-board camera grows. The vehicle dynamics and the control algorithm cause fast changes in the region seen by the camera and in the way it projects to the image plane. We propose an algorithm based on projective transformations to estimate the target coordinate during the flight trajectory. The proposed vision algorithm provides accurate position information allowing the control system to guide the UAV to the target zone. Simulations have shown the accuracy of the vision algorithm and its usefulness in the considered application.