Patent US 4,189,747

A tracking system for automatically tracking a designated target in response to video signals provided by a scanning infrared imaging sensor. A gated tracker provides output signals which are proportional to the angular deviation between the target's line of sight and the sensor's optical axis. Tracking gates are manually or automatically adaptable to the target image size so as to provide increased tracking stability and accuracy. Means are further provided for sensing when the target's image is larger than the tracking gates and for automatically expanding the tracking gates to compensate therefor. The gated tracker further comprises normalization means for providing tracker response substantially independent of target image size.