Second World War Aircraft Direction Finding

This image is from the Historic England archive and dates from around 1951. It is a low level aerial photograph of Leckhampton Hill Fort, above the town of Cheltenham. What is interesting, is the image shows a rare glimpse of a RDF or HF/DF station. © Historic England Archive, Harold Wingham Collection (reference HAW/9390/08), permission number 7614.

The start of the twentieth century was a post-industrial technological arms race, and the traces left behind are like a timeline of pioneering developments in an attempt to outsmart our adversaries. Two of those boundary breaking technologies were wireless telegraphy, and aviation. The story of Radio Direction Finding, or RDF, joins these two in pioneering a technology that would help to win the Battle of Britain.

There is a bit of confusion online surrounding DF during the second world war, and I will try to logically clarify my understanding of the systems without going into too much technical detail. Some of the terms include Direction Finding (DF), Radio Direction Finding (RDF), Cathode Ray Direction Finding (CRDF), twin path DF, Watson-Watt, Adcock / Watson-Watt DF, High-Frequency Direction-Finding (HFDF) also known as huff-duff and finally ‘fixing’ through the use of Fixer Stations1Thank you to Ian Brown for this information.. It is the latter term in relation to locating friendly fighter aircraft that I will focus on in this article, and in this context the terms RDF and HFDF are interchangeable, but strictly speaking RDF was the name given to Chain Home radar stations before the term radar was adopted.

Mysterious sites like this have baffled some history sleuths. I hope to dispel some myths and explain what I understand the function to be. Authors photograph 2019.

Direction Finding Background

Aircraft location finding started with the pre-war system of direction finding, pioneered at Orford Ness. Two high powered transmitters would transmit on the same frequency (288.5 kHz), from different but fixed and known locations, at different time intervals, from a highly directional and rotating antenna (6 rpm). The transmitter would broadcast a continuous signal, and a morse letter V (dot dot dot dash) each time the antenna rotated past north. The design of the antenna also meant that the signal was strongest at the poles (0 or 360 and 180 degrees), and weakest (or null) at 90 and 270 degrees. The navigators on board the aircraft would listen for the (…-) and time how long it would take for the signal to fade to null (due east or west). This time was multiplied by six to give the angle of the aircraft (or receiver) from the transmitter. It was a simple system when there were few aircraft in the sky, and accuracy of up to 1 degree was possible, but in unfavourable conditions this could drop to 20 degrees accuracy.

The experimental rotating radio direction finding beacon at Orford Ness formed one half of a 1929 radio navigation system, the second station being at the Royal Aircraft Establishment at Farnborough. Authors photograph, 2017.

The next form of direction finding that took place at the same time, was that to identify the bearing of unknown radio transmissions. A number of organisations undertook this form of direction finding; the General Post Office (GPO) was tasked with locating illegal transmitters, this led to the formation of the Radio Security Service (RSS) who sought illegal transmissions from spies operating in England and across Europe. And just as DF technology developed on land, the Royal Air Force (RAF) developed what is often credited (incorrectly) as the forerunner of RADAR to detect enemy aircraft, the Royal Navy (RN) also operated direction finding equipment for the purposes of detecting enemy submarines and surface vessels. All of these methods relied on tuning in to a transmission, establishing a bearing to it, not only from one station, but from multiple stations so the source could be triangulated. On a slight tangent, it led to the myth across German forces that if they kept their transmissions to sub-30 seconds the enemy (British forces) would not be able to obtain a bearing. This was untrue, and wireless DF operators could often pinpoint a transmission within seconds of it coming on the air.

The Adcock Aerial system used with the Cathode Ray D/F Type AH 6 from the Imperial War Museum archives (reference A 31087). This was a form of naval direction finding station, often located near the coast for direction finding vessels or submarines at sea.

Air Defence

It was as early as the 1930’s that the Royal Air Force (RAF) Air Defence Great Britain (ADGB) Command established the requirement for an early warning system to protect the country from air attack. This initial system consisted of a number of acoustic ‘sound’ mirrors constructed around the south east coast, along with observers from the Royal Observer Corps (ROC). London was the perceived main target, so most protective measures were designed to protect the Capital. Aircraft of the era were relatively slow moving, and noisy, so this method could provide a sufficient early warning to stand-up the anti-aircraft crews who would have formed a number of defensive anti-aircraft gun lines inland. While planes could be detected and tracked as they approached the coast, there was a massive capability gap inland – how do we track our own aircraft sent to intercept the attackers? Radio sets provided ground-to-air and air-to-ground communications, and it was decided to exploit this existing technology to provide friendly force tracking.

30 ft diameter sound mirror at Dungeness constructed in 1930. A parabolic concrete dish with microphone boom still in position. While the dish was fixed, the listener in a room below the dish would have some control over the direction by moving the microphone. Authors photograph, 2018.

Pip Squeak

Fighters of the early 1940’s were fitted with a battery powered 2-channel high-frequency (HF) radio, the TR9. With a range of around 35 miles, and operating between 4.3 MHz and 6 MHz, one channel was utilised for voice communication, and the second as a backup. It was decided to use this secondary channel to broadcast a 1 kHz tone for the purposes of direction finding the friendly aircraft once airborne. This would be done by a network of directional receiver stations dotted around the country, all linked to the Fighter Command network.

The Transmitter-Receiver TR9 consisted of a two-valve radiotelephony transmitter and a six-valve receiver contained in one case. It was designed primarily for use in single-seater fighter aircraft and was intended to provide two-way communication to a range of 35 miles air/ground and 5 miles air-to-air. Frequency coverage was 4300-6000 kc/s (author comment: 4.3-6 MHz) and the entire power supply was derived from an HT dry battery (author comment: HT stands for High Tension, also known as a B battery and was used to provide a high voltage output for vacuum tube radio equipment) and a secondary cell. In single-seat fighters the set was installed behind the pilot’s cockpit The pilot was provided with headphones, microphone and a remote control unit which operated the send-receive switch, receiver tuning and volume control.

Imperial War Museum collection, exhibit COM 496.

To enable this automatic transmission, akin to a beacon, a number of modifications were required. A model-D TR9 radio (TR9D) was introduced into service to facilitate the simultaneous transmission of voice and DF signal. In addition was a mechanical timer and remote contactor. Of these two devices, the timer would facilitate the allocation of a transmission time slot of 14 seconds out of every minute, and the contactor would activate the radio for the broadcast of a 1 kHz signal for ground-to-air direction finding. The facility to accommodate four transmission time slots out of every minute would enable up to four fighter sections within a squadron to be tracked, each one allocated a different time slot. Very similar in principle to modern time division digital signals – just much slower.

Remote Contactor Type 4 as fitted to fighter aircraft. When switched on, the mechanical contactor would activate the transmitter which would broadcast a 1 kHz tone on the secondary channel. The transmission would last 14 seconds out of every minute. Transmit time is indicated by the red portion on the dial. Image from eBay seller bctechnical (accessed 8 December 2019).

Shortly after forming up during 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.

Westley, Max (October 2010). “Pip–Squeak – The Missing Link”Duxford Radio Society Journal.

This form of DF was redundant in 1942 with the increase in overland radar coverage from the Chain Home stations, and with the widespread use of IFF (Interrogate Friend or Foe). This system broadcast a return identifying the aircraft as friendly when interrogate by the radar, semi-automating the process. It also explains how some sites appear not to have been finished or made operational. The DF site at Garway Hill was constructed in 1942 but never became operational, evidence of the demise of the system.

HFDF (Pip-Squeak) Network

The direction finding of friendly aircraft was never intended to work in isolation of other technologies. As with the sound mirrors of the 1930s, radar coverage of the Chain Home network at the time was mainly out to sea covering the approaches to the United Kingdom. This was adequate to provide an early warning, but once aircraft were over land, tracking became difficult. In order to facilitate timely and accurate fighter intercepts of approaching aircraft, it was key to direct airborne fighters to their target. It would take a combination of DF and Chain Home radar to provide the full picture.

The Chain Home radar system, 1939 – 1940.

Working initially independently, each DF station would tune into the transmissions from friendly squadrons. Once a bearing was established, this would have been passed to the immediate sector control room where the bearings from at least two DF stations would have been plotted one the sector plotting table. This would allow a grid square to be identified, that could then be passed to Fighter Control.

A photograph of a DF sector plotting table on display at the RAF Air Defence Museum at Neatishead. The red line visible is the Norfolk coast, and each of the metal rings relate to a recognised DF station. As bearings from each of these sites were called in, a string from each station was laid across the map on the given bearing. There was a bearing ring around each HFDF station place on the board to aid rapid plotting. Where the strings crossed would have indicated a grid square that was then shared with the Fighter Control sector HQ. Photograph by Dave Hill, Flickr user hillcroftdave, dated 12 September 2009. Reproduced under the Creative Commons licence.

DF Plotting Table example

In Fighter Control sector headquarters, an air picture was gathered. All of this information was passed via telephone from visual observers, CH radar operators and RDF (or HFDF) locations. As information was gathered, tactical pieces were placed on a paper map of the sector and each operator responsible for a particular aircraft type and sector would move their groups of aircraft around the board. A reproduction of this is on display at the RAF Duxford museum.

A reproduction of the WW2 Duxford Control Room. At the end of the table are two positions marked RDF 1 and RDF 2. It would have been here that the operators would have received the telephone calls with the locations of friendly fighters from the regional Fighter Command network. Photograph by Peter Veenendaal, Flickr user petervosamer, dated 01 August 2018. Reproduced under the Creative Commons licence.

The British had developed an air defence network that gave them a critical advantage during the Battle of Britain. The Dowding System – named for Fighter Command’s Commander-in-Chief Air Chief Marshal Sir Hugh Dowding – brought together technology, ground defences and fighter aircraft into a unified system of defence. It not only controlled the fighter force, but other elements of the defence network as well, including anti-aircraft guns, searchlights and barrage balloons.The system had a clearly defined chain of command, enabling control of both the flow of intelligence on incoming raids and the communication of orders.

Imperial War Museum article on What was the Dowding System?

The capability of the TR9 radio sets limited their air-to-ground range to 35 miles. This had a direct correlation to the design the HFDF network, as no two stations could be more than 35 miles apart without risking gaps in coverage. In actual fact, the stations would be within around 30 miles of each other, typically located in a group of three, with two subordinate stations and one control site plotting the sector and reporting to sector control at Fighter Command.

Plotting the known sites onto the map, it’s possible to establish some patterns associated with the placement of these sites. A good example is the south west of England, where there are three sites in Devon and Cornwall at Baxworthy, Clyst Honington and Looe, all within the St. Eval fighter command sector.

30 mile rings placed over the three DF sites within the St. Eval sector.