<P> Pip - squeak was a simple radio navigation system used by the British Royal Air Force during the early part of World War II . Pip - squeak used an aircraft's voice radio set to periodically send out a 1 kHz tone which was picked up by ground - based high - frequency direction finding (HFDF, "huff - duff") receivers . Using three HFDF measurements, observers could determine the location of friendly aircraft using triangulation . </P> <P> Pip - squeak was used by fighter aircraft during the Battle of Britain as part of the Dowding system, where it provided the primary means of locating friendly forces, and indirectly providing identification friend or foe (IFF). At the time, radar systems were sited on the shore and did not provide coverage over the inland areas, so IFF systems that produced unique radar images were not always useful for directing interceptions . Pip - squeak was added to provide coverage in these areas . As more radar stations were added and over-land areas became widely covered, pip - squeak was replaced by IFF systems of increasing sophistication . </P> <P> Pip - squeak gets its name from a contemporary comic strip, Pip, Squeak and Wilfred . It was first implemented in the TR. 9D radio . The system was also used by the USAAF, where the equipment was known as RC - 96A . </P> <P> By the middle of 1930, the Air Defence of Great Britain (ADGB) command of the Royal Air Force was planning its response to the threat of air attack . This involved the construction of a large number of acoustic mirrors to provide early warning, along with a network of observer stations that would soon be arranged into the Royal Observer Corps (ROC). The system would provide defence around the London area only, starting on the coast from Suffolk to Sussex with a thin belt of anti-aircraft artillery, a fighter operating area inland, and a second group of guns in or near the city . The system was basically unchanged in operation from its World War I counterpart, but greatly expanded the area allotted to the fighters . </P>

Where did the term pip squeak come from