针对采用捷联式导引头的被动寻的导弹的制导信息估计问题，提出一种适用于非线性系统的可观性增强最优制导律。在固连于目标的虚拟相对直角坐标系中建立非线性运动学模型。依据克拉美-罗下界理论，将视线角速率的时间积分作为系统可观性度量，设计了以相对距离为自变量的、综合考虑系统可观性、命中精度和能量消耗的性能指标，避免了制导方案对剩余飞行时间的依赖。利用变分法求解最优问题，得到了制导律解析解。数值仿真结果表明所提制导方案能够在仅测角条件下实现导弹对运动目标的拦截。与基于线性化模型设计的可观性增强最优制导律相比，所提制导律在拦截运动目标时对系统可观性的增强更为显著，使制导信息估计误差的数值更小、收敛速度更快，能使导弹具有更高的命中精度。

This paper proposes a nonlinear observability-enhancement optimal guidance law for moving targets of the missiles with strapdown seekers. Firstly， a nonlinear relative motion model between the missile and the moving target is constructed based on a virtual relative coordinate system with its origin fixed at the target， so that the linearization process can be avoided under the condition of small-angle assumption. Secondly， the integral of the LOS rate is leveraged as the measure of target observability based on the Cramer-Rao lower bound theory. The performance index to be optimized is designed by considering target observability， guidance accuracy and energy consumption. In addition， the dependence on the time-to-go is removed by utilizing the relative range as the independent variable. The analytical solution for the guidance law is derived by using calculus of variation. The simulation results show that the proposed guidance law can intercept moving targets using bearing-only measurements. Compared with the observability-enhancement optimal guidance laws based on the linearized model， the proposed guidance law can enhance the system observability more significantly， which results in a smaller value of the estimation error of guidance information， faster convergence， and higher hit accuracy of the missile.