Analysis of zenith pass problem in a roll-pitch optics seeker

There are mainly two kind of structures for a missile two-axes seeker, which is always mentioned as elevate-azimuth and roll-pitch. The elevate-azimuth structured seeker has been studied for a long time, and it is applied in all sort of missiles. Not only structure design but also control algorithm of the elevate-azimuth seeker are deeply developed. But for elevate-azimuth seeker there are several disadvantages such as big size and large abaxial angle blind. So the elevate-azimuth seeker gimbal angle is designed up to ±65° according to relative references. In modern air war, it is important for a missile to have the ability of “first recognize” and “first attack”. That means the air to air missile seeker is required to be a system with large gimbal angle and large abaxial launch angle. With the mentioned requirement, roll-pitch seeker with its advantages smart size and large gimbal angle is studied widely and deeply. The pitch angle of roll-pitch is up to ±90° while its roll angle up to n × 360°. But there are also some disadvantages for the roll-pitch seeker, the most troublesome one is the zenith pass problem. The zenith pass problem is brought about from the gimbal kinematics, it is said that when target moves in the center of image plane, roll servo command gotten from the kinematics algorithm will go to infinite. Math model of zenith pass problem is proposed in this paper and two ways to solve the problem are brought forward.