File:ASV Mk II side array on Douglas Digby at Rockcliffe.png

English: This image shows the ASV Mk. II side-looking antennas installed on a Douglas Digby of No. 751 Squadron, RCAF, seen at RCAF Rockcliffe on 1 October 1944.

ASV Mk. I was the first airborne radar system, reaching operational status in 1939. It allowed aircraft to search the surface of the ocean for objects like ships and U-boats. It proved almost useless for this role, but navigators used it extensively for stationkeeping with convoys as well as a navigational tool.

The Mk. II was a cleaned up version, which also added an optional second set of antennas looking to the sides of the aircraft. Because these antennas lay along the top and side of the aircraft they were more streamlined and could be made much larger without effecting performance, which greatly improve detection range. A Mk. II aircraft would fly a search pattern with the side antennas selected, reaching both sides for any objects to appear. When one was seen, they would mark its position on a map, switch the radar to the front-firing antennas, and commence a run on the target.

Examples of the ASV were supplied to the Canadians in August 1940 as part of the Tizard Mission. They began producing sets at Research Enterprises in Toronto, and these were widely used on Canadian and US aircraft. This aircraft, a Douglas Digby (B-18 Bolo) must be one of the earliest Canadian aircraft to be fit with this radar.