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Duxford Radio Society


Imperial War Museum, Duxford, England 



Equipment History Files


22 January 2012


The RAF Pip-Squeak Fighter Location System as used in the Battle of Britain 1940
Summary

Pip-Squeak was the code word for equipment installed in at least two key aircraft in each RAF Sector Station's Flight or Squadron.

When enabled, Pip-Squeak regularly keyed the Hurricane or Spitfire radio transmitter, usually a TR9D, for 14 seconds every minute, to allow ground-based direction finding stations to take bearings and thereby determine the position of the Squadron Leader's aircraft and hence the general location of his Squadron.

 

DRS Pip-Squeak Explainer Notes V2 


Pip–Squeak – The Missing Link

Introduction

By 1939, RAF Fighter Command had the world's first integrated command and control system ready for action.  It was only in February 1935 that Robert Watson-Watt had confirmed that aircraft could be detected by radio waves. This was known as Radio Direction Finding (RDF), later as RAdio Detection And Ranging (RADAR).   All aircraft (both RAF and Luftwaffe) could be tracked when flying over the North Sea and English Channel, by the Chain Home (CH) and Chain Home Low (CHL) radar systems. These covered the coast line from the south of England to Scotland.  However an issue remaining was how to locate the RAF fighters when they were over land, before and after combat.

At the end of the hectic manoeuvres of aerial combat the squadron of highly skilled pilots would have become separated and could well have imagined they were over, say Maidstone, when actually they were over Crawley.  Bearing in mind that the airborne duration of a Spitfire or a Hurricane was around one hour, it was essential that they were able to quickly re-establish their position and return to base.   A Hurricane could carry only 95 gallons of petrol in total and did not carry drop tanks as did the later Mustang.

The answer was Pip-Squeak, the code word for equipment installed in at least two key aircraft in each Sector Stations Flight or Squadron.  Pip-Squeak regularly keyed the aircraft transmitter for a brief period to allow direction finding stations to take a location bearing.

Fighters, at the time of the Battle of Britain, were fitted with TR9D HF Radios (using 2 Volt valves and a dry HT battery).  These had only two crystal controlled transmitter channels, one for normal Radio Telephony (R/T) transmissions and the other for Pip-Squeak.

Three Direction Finding (D/F) stations in each sector, operated by highly skilled WAAFs, would then determine the radio bearings from the Pip-Squeak transmissions for each Squadron’s position.  This information was then passed on to the Sector Operations Control Room via dedicated GPO landlines.  Once their positions had been triangulated on the Fixer Table and converted into a Grid Reference they were passed to the WAAF plotters on the General Situation Map (GSM)

RAF Duxford Sector Operations Room

RAF Duxford Sector Operations Room & GSM

A counter was encoded to show the Squadron number by small flags such as 19 and 242 or W for the Big Wings (up to five Squadrons).  Also included was the number of aircraft and their height in 'Angels' i.e. Angels one-five = 15,000 ft.  These were then placed on the GSM, together with counters showing the incoming Luftwaffe raids.



This information was received via landlines from the Observer Corps Control Centres, together with their Air Raid Number i.e. X = Unidentified or H 15 = Hostile Raid 15, the Number of Bombers and Fighters and height in Angels.  This enabled the Sector Controller to work out the “Tizzy Angle” and vector his Squadron's aircraft on to the in-coming raid.  This saved time and fuel, by not having to operate standing patrols.   In fact, the GSM was just like a giant three-dimensional Chess Board.

The System.  (See Explainer Notes No.3 above)

The Aircraft Master Contactor

This was a clockwork mechanism of high precision with a thermostatically controlled heater ensuring that its temperature remained stable, whatever the height of the aircraft.  The clock was housed in a small wooden box padded with Sorbo rubber to resist shock and vibration and mounted behind the cockpit in a crate.  The clock was wound up and switched to ‘Start’ before takeoff.  By keying the aircraft power supply it would send a 12 volt pulse each second to the stepping motor in the Remote Contactor.

The Remote Contactor

This unit was mounted on the right hand side of the cockpit within reach of the Pilot.  Before take-off, its Bezel/Marker Index was set to the appropriate position for that aircraft, by turning the pointer knob to say  270º = Yellow Leader.   After take-off the Sector Controller would call up all Squadrons to switch on their Pip-Squeak contactors by saying “Synchronise time 5, 4, 3, 2, 1, Mark”.

The pointer on its shaft then rotated at 1 rpm controlled by the stepping motor, which was driven by the pulses from the Master Contactor.  Connected to the other end of the pointer shaft was a notched cam with a make and break contact riding on its rim. This contact opened for fourteen seconds in each minute depending upon where the pointer was initially positioned.

Pip-Squeak was normally activated by request from the Sector Controller saying “Is your Cockerel crowing?”  The pilot would then operate the external switch to automatically turn the TR9D radio to transmit from its normal R/T channel to the special Pip-Squeak channel, for 14 seconds at the appropriate time.  The TR9D could then be turned back to its original channel by switching off the external switch i.e. to Receive. The pointer would continue to turn on the Contactor, ready for Pip-Squeak to be reactivated. 

The Direction Finding Stations

The Pip-Squeak signal was picked up by three direction-finding stations situated about thirty miles from each other.  One was at the Sector Control and the other two were between this station and the coast. The radio receiver at each of these stations, a Marconi DFG12 with BFO, produced a 1 kHz signal from which the transmitting aircraft’s bearing was determined. Comparing the timing of a particular transmission with the station’s Pip-Squeak Synchronised clock, the calling aircraft could be identified e.g. Red leader (1 –14 secs), Yellow = (15 –29 secs), Blue = (30 – 44 secs) and Green = (45 – 59 secs).  There was a one second gap between each of these signals.

The TR9D HF Radio 

At the time of the Battle of Britain the TR9D was being replaced by a more powerful, four channel VHF equipment known as the TR1133.   Due to production difficulties with these new sets, however, the TR9D continued to be used in most aircraft throughout the Battle. Changing the crystals for a different R/T channel enabled the Squadrons to operate in other sectors.  The Sector Controller used a short range Fixed Station TR9D, so that he could communicate with the Squadrons as they returned to their original airfield.

The range of the TR9D transmissions from the aircraft was extended by the use of Relay Stations.  (See the Duxford Radio Journal No.59).

Further information on The Battle of Britain can be found by visiting your local Library, Internet Web sites or watching TV documentaries.

Max G4WEZ  Member 8


Footnote:   I would like to thank the following DRS members for their help and guidance in producing this article.

Denis Willis - For the Pip-Squeak Demonstrator, made from original components, Ken Richards for Graphics, Chris Buckley for Research, Richard Howes for Photography and IWM Museum Assistant Graham Rogers who was extremely helpful with the Duxford Operations Room photography.
Source: Max Westley, G4WEZ, Pip-Squeak - the Missing Link, Duxford Radio Society Journal 60, Autumn/Winter 2010

Acknowledgements:  M. Westley, K. Richards, C. Buckley
Copyright notice: This original article was first published in the Duxford Radio Society Journal No. 60, October 2010.  Copyright reserved by  M. Westley and K. Richards 2010.  This Internet version edited and compiled by R. Howes 2011 & 2012  
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V2.00  created 26-11-2010 modified 22-01-2012