Monica was a range-only tail warning radar for bombers, introduced by the RAF in June 1942. Officially known as ARI 5664, it operated at the boundary between VHF and UHF frequencies of around 300 MHz. It was developed at the Bomber Support Development Unit in Worcestershire.
Unfortunately for the RAF, the Germans developed a passive radar receiver, Flensburg (FuG 227), which was used by Luftwaffe nightfighters from spring 1944 onward to home in on bombers using Monica. On the morning of 13 July 1944, a Junkers Ju 88G-1 nightfighter equipped with Flensburg mistakenly landed at RAF Woodbridge. After examining the Flensburg equipment, the RAF ordered Monica withdrawn from all Bomber Command aircraft.
Monica was also used by the U.S. Army Air Corps as the AN/APS-13, where - known as Archie - it was used as the radar altimeter for the Little Boy atomic bomb dropped on Hiroshima.