Monica was a range-only tail warning radar for bombers, introduced by the RAF in June 1942.

[citation needed] After the Luftwaffe became aware of Monica from a crashed bomber, German scientists developed a passive radar receiver, named Flensburg (FuG 227).

[4] However, on the morning of 13 July 1944, a 7.Staffel/NJG 2-flown Junkers Ju 88 G-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After examining the Flensburg equipment, the RAF ordered Monica withdrawn from all Bomber Command aircraft.

[5] An AN/APS-13 was used as a radar altimeter during the Atomic bombings of Hiroshima and Nagasaki by the 509th Composite Group, USAAF.