We present a method for simultaneously locking on to a ground target and estimating the position of an unmanned aerial vehicle (UAV) under countermeasure (CM) conditions, where sensors are prevented from successfully tracking a target. Owing to the limited payload and power of the UAVs, we employ a monocular camera and a global positioning system (GPS) to carry out vision-based simultaneous localization and mapping (SLAM) using both an unscented Kalman filter and a Kalman filter. Since this approach estimates the state of the UAV and the location of the target, we can estimate the position of the target in the image, even in the presence of CMs. Our experiments show that the proposed method successfully locks on to the target and estimates the state of the UAV.