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The detecting information of strap down seeker can’t be applied to proportional navigation directly because it is coupled with missile attitude. So the method of line of sight (LOS) rate reconstruction for strap down seeker is proposed. The reconstruction result would be reflected by background noise and any inaccurate metrical information. Thus a LOS rate reconstruction method based on central difference approximation and Central Difference Kalman Filter (CDKF) was proposed to obtain more precise guidance information. Monte Carlo simulations confirmed the validity of the designed method. Introduction Homing missiles use a space stabilized seeker antenna in order to acquire and track the target. In conventional implementations of proportional navigation guidance systems, the degree to which the seeker is stabilized places fundamental limitations on the homing accuracy of the missile . Strap down guidance system has become one of development trends of missile guidance system for its potential advantages of improving the system reliability and reducing cost. However, it also takes some fatal disadvantages. Firstly, only angles relative to body coordinate system can be measured by the strap down seeker. It leads to a high nonlinearity for guidance system. Secondly, more serious measurement noise was introduced in the system because of wider instantaneous field of view. It is difficult to extract inertial LOS rate, which is essential to the design of the guidance law . For these problems, Ying Liu proposed using filter method [3] to obtain the target line of sight angle rate. Tinting Sun establish the mathematical model of strap down optical seeker . Guiyang Zhang establish the second-order of the LOS angular based on the target maneuvering model . Pei Wang proposed using Nonlinear Tracking-Differentiator to estimate the angle rate . Kalman Filter can obtain optimal unbiased estimation for linear system corrupted by white noise. Nevertheless, it cannot be applied to nonlinear system. The EKF expends dynamic model as a Taylor series about the current state estimate, and measurement model about the state prediction, and then linearization is achieved by neglecting second and higher order terms. But truncations decrease estimation precision; Calculation of Jacobian matrix’s are nontrivial in most applications and usually leads to significant implemental difficulty. A new method, CDKF [7-9] without and approximation for the nonlinear system model was proposed. The CDKF can predict the mean and covariance up to, even higher than the second order precision for Gauss distributed random variable. In this paper, we focus on the strap down seeker which can only get LOS in body coordinates. The LOS first-order dynamic model was constructed. A LOS reconstruction filter was designed by using the model via the CDKF method. The results of simulation in MATLAB environment have showed that the method has a good performance on estimating the LOS angle. Meanwhile, the precision of the estimation can be improved. International Conference on Information Sciences, Machinery, Materials and Energy (ICISMME 2015) © 2015. The authors Published by Atlantis Press 1149 The LOS Reconstruction Method We take the ground coordinate as the inertial frame. The definition and transformation of coordinates used in this paper are described as follows. The readers can refer to References . Inertial coordinates I I I Ox y z Body coordinates B B B Ox y z Inertial LOS coordinates L L L Ox y z Body LOS coordinates C C C Ox y z The yaw angle IH q and the pitch angle IV q in the inertial reference frame I S are essential to implement guidance law design. Since the optical system was fixed on the missile’s body, only the yaw angle BH q and pitch angle BV q of LOS in body frame B S can be obtained. It is essential to decouple the missile attitudes information from the detecting information because of the missile attitudes are in flux. The vector of target in inertial LOS coordinates and body LOS coordinates are all ( ,0,0) r R ? According to the transformation matrices among the coordinates, we get the following equation: [ ] [ ] ( ) [ ] ( , , ) ( , ) 0 0 , 0 0 T T T BH BV r IH IV r x y z T T q q R T q q R y θ γ = ⋅ ⋅ = ⋅ (1) The IH q and IV q can be obtained by the following equation [ ] ( ) 3 1 2 arctan arcsin( ) T T IH IV q q a a a = −     (2) Where,

Research on Line of Sight Angle Rate Reconstruction based on Strap down Seeker