Broadcaster Pip-squeak's airborne unit consisted of two primary parts, an oscillator to produce a whistle at , and a mechanical clock with electrical contacts to periodically switch the oscillator and DF broadcast channel on and off. 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. Both the section leader and one other aircraft in the section normally had pip-squeak aboard. Shortly after forming up after a scramble, the squadron leaders would be asked to ready their pip-squeak clocks. 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. There were up to four sections of aircraft in each squadron, although most squadrons had two or three sections at any given time. Each section had its own position for the hand; red section had the 12 o'clock location, yellow was at 9 o'clock, blue at 6 o'clock and green at 3 o'clock. Once the clocks were properly positioned, the sector controller would initiate a countdown,
Synchronize time, 5, 4, 3, 2, 1, mark. At mark, the pilots would turn on the clock, which would start the second hand moving clockwise. When the hand reached the 12 o'clock position the oscillator was automatically turned on, and it turned off again just before the 3 o'clock position, broadcasting for 14 seconds per minute. 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. Red section, having started in the 12 o'clock position, started broadcasting immediately when the system was activated. When it stopped 14 seconds later, yellow section's clock was now reaching the 12 o'clock position and began to broadcast, and so forth. Over a one-minute period, all four sections (if present) squeaked and could be located. A separate radio control switch stopped the radio signal from broadcasting while the clock continued to move. This allowed the pilot to set up the system early in the flight, and then turn it off when better communications were needed, like in combat. The system could then be turned on again at any time, with the clock still in the proper position. Sector Commanders could ask pilots to turn it on by asking "Is your Cockerel crowing?". There were two common versions of pip-squeak, one with the clock located in the cockpit, and a second that used a remote clock system. 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. The "Remote Contactor" display was located in the cockpit, driven by electrical signals from the Master Contactor, whose once-per-second signals powered a
stepper motor driving the second hand. This version had only a single control to turn the clock on and off to start it up at "mark", a separate switch on the radio console allowed the signal to be stopped and started while leaving the clock running.
Plotting Each sector was equipped with three huff-duff sets for determining the location of the pip-squeak radios. Although in theory only two were needed, adding a third offered redundancy as well as helping reduce the chance of errors in the plotting. The stations were positioned approximately apart in as close to an equilateral triangular layout as possible. One of the three stations was co-located at the Sector Control center, with the two remote stations communicating with the center over telephone lines. At the Sector Control, a simple system was used to rapidly take a
"fix". 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. The location of the three stations was represented by small holes drilled into the map. Weighed strings passed through the holes, and could be pulled up and across the map by the plotters. When a report was received from a huff-duff operator, the plotter would pull their string so it lay on the indicated angle; the weight (or elastic cord) on the other end kept the string taut. With the three reports plotted, the strings would normally intersect at a small triangle or star somewhere on the map. This location was read against the Grid. The operators could identify which section they were tracking simply by looking at a
sector clock painted with section colours, as the sections had manually synchronized their clocks with this one. A fourth operator observing the plots would then call in the position to the main operations room. The system required fast operations by all involved, as they had only 14 seconds to make a plot before the next section reported in. When the grid location was passed to the operations room, a marker for that section could be updated on the sector's plotting table. Pip-squeak did not directly produce
identification friend or foe (IFF) information, but served that purpose in practice by allowing the operators to determine which plots were friendly. This might be used, for instance, when reports from radar or ROC observers were tracking friendly forces without knowing it. ==Problems==