Abstract: A device for the detection of and defense against low-flying helicopters which are masked or hidden by ground cover has a heat-sensitive sensor and an optical filter to detect heat radiation which is modulated at a frequency corresponding to the frequency of rotation of the helicopter blades. The helicopter motor and drive train provide a heat source, and the detector is positioned above the helicopter so that heat radiation reaching the detector from the helicopter is modulated by the rotating blades so that the detector can distinguish the helicopter heat source from other ground heat sources. The detector can be positioned by deployment from a tank and suspended above an area by a parachute, or may be contained in a remote piloted vehicle. The detector may relay a position signal to ground artillery for combat of the helicopter, or may be equipped with an explosive device which may be deployed after detection of a helicopter.