Unmanned aerial vehicle (UAV) shows extraordinary superiority on the battlefield with high mobility, low cost, no casualties, and other advantages, which has changed the form of modern war and made unmanned intelligent war mainstream. For example, using UAVs for reconnaissance provides helpful information for commanders on the battlefield, which has excellent intelligence support value and operational command function. It should be noted that high-precision and real-time target localization is a necessary condition for UAV reconnaissance and target strikes. The airborne electro-optical platform is equipped with high-resolution imaging equipment to recognize the target at a long distance so that the ground target can be tracked and measured without entering the target area. However, in the case of limited observation conditions, such as large inclination angles and small rendezvous angles, the traditional localization method can no longer meet the accuracy requirement. At present, most of the target localization methods to improve accuracy under limited observation conditions can be divided into two basic types. One is based on image matching, and the other is based on geometry. But under the condition of a large inclination angle, the localization method based on image matching is seriously affected by the perspective transformation. Moreover, the results show that limited observation conditions make the design matrix ill-conditioned, and the condition number of the least square method is very large, which seriously affects the accuracy of the geometric localization method. Therefore, improving the target localization accuracy under limited observation conditions is of great significance.

The laser range finder has high measurement accuracy. It is not affected by the limited observation conditions such as a large inclination angle and small rendezvous angle, which makes laser ranging have tremendous application value in the field of ground target localization. In order to improve localization accuracy under limited observation conditions, this paper proposes a global optimization method for ground target localization based on the platform's location and laser ranging. In this method, first, according to the continuous observation data of the static ground target, the weighted error equation in the earth-centered and earth-fixed (ECEF) coordinate system is established. Second, the nonlinear problem is transformed into the problem of the eigenvector solution. Third, all stationary points are found by enumerating seven eigenvectors. Finally, the actual location of the ground target is calculated according to the actual situation. In fact, the critical point of this method is to transform the nonlinear problem into an eigenvector problem through data centralization, singular value decomposition, and other steps. This not only accurately finds the global optimal solution of the equation without iteration or optimization procedure but also improves the efficiency and stability of ground target localization.

The ground target localization system is mainly composed of an airborne electro-optical platform, integrated global position system (GPS) and inertial navigation system (INS), and laser range finder. Furthermore, the platform flies in a circle over the ground target, obtaining the UAV's longitude, latitude, and height in the world geodetic system-1984 coordinate system, as well as the distance between the UAV and the static ground target at different time. The simulation experiment results based on the Monte Carlo method show that the localization accuracy is affected by location error of the airborne electro-optical platform, distance error of laser ranging, and data size of continuous observation (Fig. 5, Fig. 7, and Fig. 8). Furthermore, the flight test results show that the proposed method is feasible and effective under limited observation conditions. The target localization error is less than 30 m when the platform is 5 km away from the target, and the observation angle is 66.42°. Moreover, the operation time can be controlled within 10 ms when the quantity of continuous observation data is within 300 (Table 3 and Table 4).

In view of limited observation conditions such as large inclination angles and small rendezvous angles, this paper proposes a global optimization method to improve the accuracy of ground target localization. According to the location information provided by the integrated navigation system and the distance information provided by the laser range finder, the corresponding measurement model and weighted error model are set up. A fast closed-form solution for nonconvex optimization problems is obtained by deriving equivalent eigenvectors. The simulation and flight experiments results show that the proposed method has the advantages of high localization accuracy, computational efficiency, and robustness compared with the traditional localization method under limited observation conditions, which is of great significance to the reconnaissance and attack of targets on the battlefield.