This paper focuses on avoidance maneuvers for an aircraft from an attacking missile whose guidance strategy is unknown to the aircraft. The proposed algorithm presents the timing of evasive maneuver for the aircraft to be just before the intercept time. That is, the missile is in the final moment of the terminal phase to hit the aircraft. In that situation, the line-of-sight (LOS) rate converges to zero where the missile's guidance law can be approximated as a proportional navigation guidance law with an unknown effective navigation constant. The proposed algorithm provides the equivalent value for the effective navigation constant so that the aircraft maximizes the miss distance (the closest distance between the two vehicles) between two vehicles. The equivalent effective navigation constant is determined by a so-called 'inverse calculation' that comes from the LOS rate dynamics between the aircraft and the missile. The proposed algorithm estimates the target information such as missile's lateral and axial accelerations, dynamics, heading and velocity via an uncertainty and disturbance estimator (UDE). Then, the estimated lateral acceleration sequence data are used to estimate the equivalent effective navigation constant with the least squares method. Finally, the evasive-maneuver timing on the maximum available miss distance for the aircraft can be obtained as a function of an effective navigation constant, which is derived by an adjoint analysis along with Lagrangian interpolation. This study takes a simple structure and estimates the unknown missile guidance dynamics under a simple assumption. Numerical results demonstrate the potential of the proposed algorithm.
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