The Radar Stations and the Observer Corps were fine for the purpose of detecting and tracking hostile aircraft coming in from across the English Channel, but to make the detection system work and function properly the it was essential for the Sector Controllers to have an accurate assessment as to the position of their own fighter aircraft. Without knowing where their aircraft were it would have been impossible to vector them to another combat area or where their aircraft were to in relation to any current attack.
One of the early methods of friendly aircraft detection was being experimented as early as February 1939. A.F.Wilkins and a R.H.A.Carter, scientists at Bawdsey were busy testing apparatus that they had built in an effort to display a different sort of blip on the CRT screens so that the filter rooms could tell if a detected aircraft was one of theirs, or if it was an enemy aircraft. During one of the official tests in March 1939, the operators at Bawdsey, busy checking their screens suddenly picked up an unusual and strange blip on the screens. Instead of the small round blip that was usually displayed, this one was brighter and the shape was elongated. They had picked up the experimental aircraft that was fitted with a new device and they tracked it for many minutes.
The information was immeadiately sent to Bentley Priory Filter Room and it was classified as a confirmed detection of a friendly aircraft. One of those present at 'Bentley" was Lord Chatfield who was Minister for Co-ordination of Defence who exclaimed that this was a great step forward and that radio detection was about to be perfected. ACM Hugh Dowding was also aware of the contribution that IFF could make in aircraft detection, and by early 1940 with the Battle of Barking Creek in which two Hurricanes of 56 Squadron were accidently shot down by Spitfires of 74 Squadron was still fresh in his mind, he was pushing for such an apparatus where he could know the position of all friendly aircraft.
IFF worked reasonably well but it did have its failings. In dogfights it became impossible to distinguish one aircraft from another. Many times friendly aircraft were not able to be detected while at time they were easily seen.
A later method that was introduced was the use of HF/DF, High Frequency Direction Finding to give it its correct name, but to most it was known as "Pip-squeak" the code name given to the apparatus. All British fighters were equipped with a TR9D transmitter receiver. This was the only contact that a pilot had with ground control, although its main drawback was that it was limited to a range of only 40 - 45 miles (64 - 72Km) at about 15,000 feet (4,545m) although under perfect weather conditions this range could be extended further. The unit had two channels. One channel was used for the purpose of voice communication with Sector Control to which the aircraft was attached. Each squadron had its own frequency and always operated with this setting. The other channel was set on a common frequency to all squadrons and was unsuitable for voice communication. It was this channel that was known as "Pip-squeak".
As soon as this channel was selected, the transmitter sounded a shrill fourteen second whistle at about 1,000 cycles, which was received by the Sector Controller.
A sector had three direction finding (D/F) stations that formed a triangle with each corner being approximately thirty miles apart and each being connected by a land line to the stations Fixer Room. Here, on a small map table, the D/F stations were marked and were surrounded by a compass rose. As each of the D/F stations received its bearing from "Pip-squeak" a plotter at the map table could then take a line, with a humble piece of string from each D/F station on the map, and when that station reported the direction from which it picked up the aircraft's signal, the operator of the string aligned it with that direction. Thus, when the three strings were aligned in unison, where they crossed was the position of the aircraft. 
 Peter Flint Dowding and Headquarters Fighter Command 1966 Airlife p33
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