 Starting in 1940 , British aircraft were increasingly equipped with the IFF Mk . II system , which allowed radar operators to determine whether a blip on their screen was a friendly aircraft . IFF was a responder that sent out a pulse of radio signal immediately on reception of a radio signal from a radar system . The IFF 's transmission mixed with the radar 's own pulse , causing the blip to stretch out in time from a small peak to an extended rectangular shape . 
