Fighter Direction Tenders

GCI Radars (Neptune/Overlord)

Ground controlled interception radars in operation Neptune / Overlord.  The Allied Invasion of France, June 1944.

Background

This accounts starts by introducing the organisation that made it all possible – the Telecommunications Research Establishment (TRE).  TRE started as a small group of scientists within the Bawdsey Research Group under the British Air Ministry Research Group and was located at Orfordness, Suffolk, England.  Here they worked on the development of a Chain Home (CH) radar which later was to be credited for the survival of the United Kingdom in the 1940 – Battle of Britain.  They were relocated to the Isle of Purbeck at Worth Matravers, Dorset, in May 1940.  In fear of a *”Bruneval style” raid on Dorset, TRE and its supporting Telecommunications Flying Unit (TFU) at RAF Station Hurn, moved on 25^th May, 1942, to Malvern and Defford respectively.  From here, TRE responded successfully to the questions “Can you do it?” and “When can we have it?”  One of the many concerns of those planning the inevitable invasion of the Continent, was the need of forward warning and control radar.  This equipment must be in a position prior to the beach landings of Allied forces – thus the concept evolved of installing suitable radar equipment on floating vessels in the Channel.  These vessels became known as Fighter Direction Tenders (FDTs).

The two Ground Controlled Interception (GCI) radar systems installed on the FDTs were developed by TRE – Air Ministry Experimental Station (AMES) Type 11 and Type 15.  The Type 15 radar antenna was developed at Worth Matravers and built at Christchurch in December 1940.  The AMES Type 11 system was developed by TRE, following the Bruneval raid.  It had many similarities to the sophisticated German “Wurzburg” radar system, parts of which were captured during the raid.

This section covers the development of FDTs and their use in the invasion of the Continent.  Reports are also included on the GCI Radar Convoys that landed on the assault beaches to assume the duties carried out by the FDTs.  TRE was also active in many other interrelated areas of radar systems development in support of the “D-Day” invasion.  The following come to mind: the OBOE navigational system for improved bombing accuracy, used extensively to soften enemy defences in the landing areas; the continuing development in the field of radio countermeasures (electronic warfare) through the use of counter and counter-counter measures to stay one step ahead of the enemy’s systems; the mobile radar systems which would follow the battle line once established on the Continent; and the mobile, personal and aircraft beacons used primarily for photo-recce over-flights.  In their development, it was necessary to ensure that final products were compatible with those used by other Allied forces.

The FDTs in addition to all other applications of radar equipment, had one thing in common – a good percentage of the radar officers and technicians were members of the Royal Canadian Air Force, attached to the Royal Air Force as Radio Direction Finding (RDF) personnel.  Their input is included in the following paragraphs.

*On 27^th February 1942 a British raid at Bruneval on the Cherbourg peninsula captured apparatus from a German Wurzburg radar installation and brought it to Worth Matravers for study by TRE.

Fighter Direction Tenders (FDT)

Fitment and Testing of Fighter Direction Tenders

The concept of using ground radar installations on floating vessels was initially tested during the invasion of Sicily in July 1943.  An RAF mobile GCI unit was temporarily mounted on Landing Ship Tank (LST) 305 for use in the Mediterranean Theatre.  Results were so successful that the AOC-in-C Fighter Command recommended that similar vessels be converted and equipped for landings in Europe. Fighter Command had originally recommended the conversion of four LSTs to be used by Combined Operations.  At a meeting with the Admiralty in November 1943, it was decided that only three vessels were to be equipped to perform interception and raid reporting functions, under the code name “Becky”.  Specifications were prepared and by the middle of February 1944 conversion of the three LSTs had been completed.

The LSTs selected for FDT conversion were part of the 112 made available to the Royal Navy during World War 2 under the US lend-lease programme.  They were built in the USA for use in assault landings on enemy beaches and were originally designed to carry eighteen 30-ton tanks, or 27 three ton trucks and 8 jeeps, or 177 troops.  Upon conversion to the FDT role, the LST would have a displacement of 3,700 tons and a length of 328 feet, comparable in size to a Naval cruiser. The vessel would carry a Naval crew of 8 officers and 92 ratings in addition to 19 Air Force officers and 157 airmen.

The conversion of the three LSTs was carried out by John Brown’s Shipyard on the Clyde in Scotland.  The work included welding shut the bow unloading doors and covering the hatches with armoured plate.  Three hundred tons of pig iron were secured to the main deck to slow roll action of the vessel.  A new deck was laid over the vehicle cargo space to contain the operational accommodation for the Filter Room, Communications Office, Cypher Office, Air Control Room and Radar Receiving Room.  A Direction Finding Office was provided forward for the installation of Naval D/F equipment.  Space was provided at the aft end of the tank deck for the Transmitter Room, Transceiver Rooms, Aircraft D/F, Radio Counter-Measures Office and W/T Storeroom.  A bridge Visual Direction position and a bridge Plot House were constructed above the deck.

Two GCI radar systems were installed on each FDT – a Type 15 GCI forward and a Type 11 GCI amidships.  The radar aerials were 30 ft above water level which limited the accuracy of aircraft height readings  The Type 11 radar on 520 Mc/s were provided as an alternative if the 200 Mc/s band was seriously jammed.  Mk111 IFF (Identification, Friend or Foe) was installed to provide identification only when Air Movements Liaison Section information did not provide the answer.  The IFF was normally left switched off.  Airborne Interception (AI) Beacons were installed on the FDTs to aid the control of night fighters.  Each FDT carried 1.5 metre Mark IV and 10 centimetre Mark VIII A1 Beacons.  Finally, many channels of VHF/R/T and W/T were installed to provide the essential communications.

Sea trials were carried out in the Ailsa Craig area which included radar calibration and communications tests.  It was necessary to raise the Type 11 antenna to eliminate a 20 degree “blind” area either side of the bow.  Otherwise the tests were generally acceptable.

During April 1945, four exercises took  place off the Humber coast with No 12 Fighter Group Sector.  These included the use of *”window” (known as “chaff” by the US Forces).  Exercises were continued in May 1944 with No. 11 Fighter Group Sector and Naval Authorities in the Portsmouth and Portland areas.  By the end of May, the FDTs were ready for action and they sailed from Cowes, on the Isle of Wight, to join the Assault Task Forces at 2200 hours on 5^th June 1944.

*Window – metallic strips cut to a half wavelength of the enemy’s detecting radar systems.  Dispersed from an aircraft, the radar return would simulate a large formation of aircraft.

Fighter Direction Tenders in the Assault

During the assault and until GCI facilities became established ashore, the control of fighters operating in the assault area and the main shipping area would be exercised from the three FDTs.

FDT 216

Off the American beaches (Omaha and Utah) in the western half of the assault area for the control of those British and American fighters detailed to operate therein.

FDT 217

Off the British beaches (Sword, Juno and Gold) in the eastern half of the assault area for the control of those British and American fighters detailed to operate therein.

FDT 13

In the main shipping route for the control of those fighters detailed to operate in that area.

FDT 217 acted as the co-ordinating vessel to order reinforcements, if necessary, depending upon which part of the assault area was being attached.  As dawn broke, the beaches were being bombed by RAF aircraft and the Allied cruisers and destroyers commenced shelling.  All three FDTs commenced radar watch at H-hour (07.25 hours) on 06 June and were immediately in contact with the fighter aircraft providing air defence to the armada.

Ref: Canadians on Radar 1940 – 1945 (George Grande, Sheila Linden, Horace Macaulay)