Microwave Radar at War: "The US also built a series of tail-warning radars, most notably the "AN/APS-13" for fighters. This was a low-power pulse radar set, operating at 67 cm / 450 MHz, that flashed a red light and rang a bell when an enemy aircraft was approaching from the rear. Similar sets, the "AN/APS-16" and "AN/APS-17", were built for bombers."