Utilizing only bearing angle measurement for guidance and target tracking is common in homing missiles with passive sensors and in underwater passive target tracking. Their control and estimation problems are theoretically interesting and practically signi cant. These problems are nontrivial due to the time-varying system dynamics, the nonlinear measurements and the complex target-pursuit geometry when the target is maneuverable. When considering the homing missile guidance with bearings-only measurements, the proportional navigation guidance is the most popular [11,19]. Unfortunately, the closed loop may demonstrate unsatisfactory performance when the missile and the target are in a collision course near the end of interception [17,6]. The problem is caused by the lack of observability in the range and the range rate. This poses considerable di culty as these two state variables are needed to realize the guidance law, and they are estimated by a state lter. It follows that an e ective guidance law should seek to achieve not only terminal accuracy, but also information content enhancement of essential measurements, in order to o er the lter su cient information to generate consistent estimates. Numerous studies dealing with observability-enhanced guidance control laws in homing missiles were conducted during the past decades [17,6,1,8,9]. One approach [8,9] is to formulate the guidance as an optimal quadratic control problem and to