The suitability of modified proportional navigation, or an equivalent zero effort miss formulation, for strategic intercepts during midcourse guidance, followed by a ballistic coast to the endgame, is addressed. The problem is formulated in terms of relative motion in a general, three-dimensional framework, The proposed guidance law commands thrust 1) along the line of sight unit direction and 2) along the zero effort miss component that is perpendicular to the line of sight. The fatter term is scaled with a guidance gain. If the guidance law is to be suitable for longer range targeting applications with significant ballistic coasting after burnout, zero effort miss computations must account for the different gravitational accelerations experienced by each vehicle, Approximations for the true differential gravity effect, which relieve the burden for direct numerical propagation of the governing equations, are considered, Approximations considered are constant, linear, quadratic, and linearized inverse square models, Theoretical results are applied to a numerical engagement scenario, and the resulting performance is evaluated in terms of the miss distances determined from nonlinear simulation. References: 16
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