1. A radar system comprising: a radar antenna; a transmitter; a receiver; circulator means for coupling said transmitter and receiver to said antenna; an image processor coupled to said receiver; and a detection processor coupled to said image processor, said detection processor having: software for receiving a frame of image data; software for performing a rate of change of variance calculation with respect to at least one pixel in said frame of image data; and software for comparing said calculated rate of change of variance with a predetermined threshold to provide output data.