Aerospace and Ocean Engineering (ABSTRACT)We consider the problem of optimal evasion when the pursuer is known to employ fixed gain proportional navigation . The performance index is a mcasure of closest approach. The analysis is done for planar motions at constant speed . The kinematics are first linearized around a nominal eollision course. The dynamics of the opponents are modeled by first order systems and their accelerations may be bounded.Three cases are studied : unconstrained optimal evasion ( where the evader is not subjected to any path constraint ) against a single pursuer, optimal evasion with a terminal path angle constraint for the evader and optimal evasion against more than one pursuer.The optimal controls are shown to be 'bang -bang' with the number of switches depending on the pursuer's navigation gain and on the particular constraints of each case.