The initial heading angle of missile and the estimate accuracy of time-to-go considerably influence the missiles guidance performance.Based on angle-of-fall control optimal guidance law of air-to-surface missile,some in-depth research work was carried out.For a specified angle-of-fall,how to select initial heading angle to avoid command saturation was studied.The launch envelope of initial heading angle and terminal impact angle were given.In addition,since the optimal guidance law with angle constraint often achieves a curving ballistic trajectory,the traditional estimate method of time-to-go may bring about large error.Two new methods are proposed in this paper,which can overcome the shortcomings of traditional methods.Simulation results demonstrate the effectiveness of the proposed methods.