Derivation and implementation of an extended Kalman filter for the purpose of estimating the line of sight rates between a non-thrusting, guided spinning projectile and a ground target is investigated. Line of sight rates are required signals to implement the proportional navigation guidance scheme. Using low-cost strapdown sensors, the line of sight rate is not directly measurable but is observable as shown in this paper. The line of sight signal generated by strapdown seekers inherently includes projectile body motion, but with the use of a global positioning system and a strapdown inertial sensor suite, the seeker signal is transformed to the inertial frame and the required guidance signals are generated. Limited laser seeker update rates also preclude direct discrete derivative formulation of the line of sight rates in a spinning airframe, but a method to accomplish this is presented. The various update rates of the sensors onboard the guided projectile are handled within the Kalman Alter by use of a sequential update. The derived filter is simulated using a six degree of freedom simulation to provide the necessary signal inputs to the filter. The simulation also serves to close the guidance, navigation, and control loops and evaluate the entire system architecture. The simulation results are examined to show the successful implementation of the Kalman filter and the utility of a strapdown seeker.
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