A New Strategy of Guidance Command Generation for Re-entry Vehicle

Abstract

Guidance commands for re-entry vehicles are generated based on the line-of-sight (LOS) information. If the guidance command is proper, the vehicle will reach the target point precisely, but the premise is that the autopilot can track guidance command steadily. Usually, the error between guidance command and real state is used to design autopilot. Guidance commands can be in lots of formats, such as acceleration, Euler angles, Euler rate, angle of attack, sideslip angle and so on 1. For re-entry vehicles, Lu 2 derived body acceleration command, Daigoro 3 developed guidance command in the format of angle of attack and sideslip angle. The advantage of Euler angle command is that the feedback of the controlled variables (Euler-angle body attitude) can be directly provided by inertial measurement unit (IMU) or gyros 3. The guidance system will determine the Euler angle commands to be tracked so that the velocity vector is steered according to the guidance law. So designers used to convert angle of attack command and sideslip angle to command of Euler angles 3-5. After command of angle of attack and sideslip angle are generated, the next problem is how to realize the conversion from commands of angle of attack and sideslip angle to command of Euler angles (yaw, pitch, roll). There exists between velocity coordinate, launch coordinate and body coordinate. Based on the information of heading angle, flight path angle, and bank angle (q,s,n), commands of Euler angles can be derived from transformation matrix. Above method is feasible, but the problem need to be emphasized is: heading angle, flight path angle, and bank angle are derived based on the navigation data, the navigation data have error after long time flying during booster phase and middle phase, so the value of heading angle, flight path angle, and bank angle have error. If we use above conversion method, commands of Euler angles have error. Furthermore, computation of the bank angle comprises complicated triangular function. In a real flight process, to alleviate the operation burden, bank angle is set to be zero considering that bank angle is always small. Bank angle is treated as zero factitiously, the calculation process is simplified, but error is brought in. Reentry homing vehicle is equipped with inertial measurement unit and radar seeker, the latter is used for terminal guidance. Dowdle 6 estimated target maneuver using EKF with the on-board active rardar seeker measurements. The measurements were relative range, range rate, line-of-sight (LOS) angles and LOS rates. Balakrishnan 7,8 has estimated LOS rates from passive measurements of LOS angles alone using modified Polar coordinates. With the method of Carlson filter or Kalman filter 9-13 , it is possible that relative distance and velocity between vehicle and target can be estimated precisely. This paper introduces a new strategy of guidance command generation for re-entry vehicle. Transformation matrix from launch coordinate to velocity coordinate is solved relying on the relative position measured by...

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