1. In a video tracker for viewing a distant target (12) and background scene (13) to produce a tracking window (10) having a leading edge and a trailing edge: the combination comprising means for enhancing contrast between target video and background video, comprising a first amplifier (80) having an input for receiving digitized video signals produced by the tracker, a positive peak detector (88) and negative peak detector (90) receiving digital signals from the amplifier, addition-division device (93) receiving digital signals from the detectors and supplying an intermediate input digital voltage V.sub.INT back to the input of the amplifier so that peak detector signals are centered on the V.sub.INT signal, and comparator means (98 and 99) for comparing signals derived from the peak detectors with the digital signal produced by the amplifier to provide digitized output signals (100) representing enhanced contrast between the target and background scene; means for producing an analog signal (43) representing a video lock-on target center point, said signal-producing means including a first logic means (32) translating the aforementioned digitized output signals into digital signals representing the distance between a video window leading edge and the target leading edge, a second logic means (34) translating the aforementioned digitized output signals into digital signals representing the distance between the target leading edge and a point midway between the target edges, and means (38, 40 and 42) converting the last mentioned digital signals into a first analog signal (43) representing the target center point; and means for deriving a signal representing changes in size of the target, comprising a target height counter circuit (52) that produces a second analog signal (66) related to target size, a second amplifier (48) summing the first and second analog signals into an output representing one edge of a video window, a third amplifier (50) differencing the first and second analog signals into an output representing another edge of a video window and ramp generator means (72) providing a sloping signal (73) that intersects the outputs from the second and third amplifiers.