Guidance laws for a short-range attack missile

Guidance laws are developed for the midcourse guidance of a short range attack missile of the SRAM II class. In this study, the SRAM II mission is assumed to be the following: the missile is launched subsonically at low altitude from an aircraft; it performs a thrusting, climbing turn to point toward the target at burnout; and it coasts in a vertical plane at zero angle of attack to a fixed target on the ground. The optimal control formulation minimizes the total flight time subject to target intercept conditions. An analytical guidance law is developed by neglecting the aerodynamics and creating burnout conditions which enable the zero angle of attack coast to get close to the target. To improve the results of the analytical guidance law, error compensation terms are included in the state differential equations and the final conditions that lead to a control law which can be tuned to improve the performance of the missile and enable it to hit the target. The implicit guidance law is developed by assuming a guidance polynomial, and uses the target intercept constraints to solve for the coefficients of the guidance polynomial. The guidance algorithms are presented in a feedback form and are sufficiently simple for real-time onboard implementation. These guidance laws are applied to a paper missile and the guided trajectories are compared with the numerical optimal solution. Although the guided trajectories are not nearly optimal, low miss distances are achieved. Also, the guidance laws yield final velocities that are comparable with the numerical optimal solution. Finally, it is shown that the miss distances resulting from midcourse guidance can be easily corrected by using the aerodynamic maneuvering capabilities of the missile with a relatively short homing guidance phase.