Pip-squeak was a 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.
As more radar stations were added and over-land areas became widely covered, pip-squeak was replaced by IFF systems of increasing sophistication.
The system was basically unchanged in operation from its World War I counterpart, but greatly expanded the area allotted to the fighters.
[1] In tests carried out primarily from Biggin Hill during the mid-1930s, the expanded fighter operating area demonstrated a serious problem keeping track of friendly forces.
In the summer of 1937 he requested that every sector be equipped with three huff-duff sets in order to allow rapid triangulation of the location of fighters.
[1] The Air Staff was slow to respond to Dowding's request due to a shortage of cathode ray tubes (CRTs) used in the huff-duff sets, and by the end of 1937 only five sectors were equipped.
During tests in March 1938, the value of DF as part of the reporting system became very clear to all involved, and on 14 April 1938 the Air Ministry ordered a further 29 sets to equip all sectors.
These were initially delivered without a CRT and required longer to reliably determine direction using a manual radiogoniometer, but were designed to be upgraded as CRTs arrived.
[1] Throughout 1938, the Royal Aircraft Establishment worked on a new version of the TR.9 set, the "D" model, which was designed specifically to aid DF operators.
An active transponder system based around a regenerative receiver had been introduced in 1939,[2] but demonstrated problems with gain settings and had the disadvantage that it could only work with the Chain Home radars.
[3] These problems were addressed in the IFF Mark II, which had an automatic gain control and several internal receivers that could respond to any of the popular radars of the era.
Using the TR.9D, the most common radio during the early stages of the war, there were two available channels, and the frequencies were selected before the mission using swappable crystal oscillators.
In the original system this required them to turn the "wind" knob that moved the single second-hand counter-clockwise around the face of the clock.
The system also automatically switched the radio from voice to pip-squeak channel at the 12 o'clock location; if the pilot was talking he would be cut off.
[6] The later placed the "Master Contactor" in a box in the equipment bay near the radio, and it was pre-set to the correct second-hand location for each section, prior to the mission.
This consisted of a circular plotting board with a map on the top surface marked with the Ordnance Survey National Grid, and a series of compass angles on a protractor around the outer edge.
Things improved with the introduction of the TR.1388 sets, which had several voice channels and much longer range, but pip-squeak still interrupted the pilot when it broadcast.
This meant that information about the location of friendly forces had to be sent back up the chain to Group and Fighter Command Headquarters, increasing the amount of traffic flowing through the system.