Site Records

Site Name: RAF Sopley ('AVO') R3 GCI ROTOR Station

OS Grid Ref: SZ162978

RSG site visit 23rd May 2000

[Source: Nick Catford]

Following the outbreak of WW2 and the evaluation of the Chain Home (CH) and Chain Home Low (CHL) radars under operational conditions, a requirement was identified for a control radar to aid the fighter interception of enemy aircraft.

The coastal CH and CHL radars all looked out from the UK, leaving a void inland, this void relied on the Observer Corps (later Royal Observer Corps) for overland reporting, which was not effective in bad weather or at night.

The Ground Control Intercept (GCI) was designed to fill this gap, providing inland coverage to Filter Rooms, Sector Operations Centres (SOC) and Gun Operations Rooms (GOR) in addition to the control of defence fighters combating enemy aircraft by day and night. Sopley was operational 24 hours a day, every day, with a three watch system of staffing.

The GCI system had an extremely convoluted and complex history. The programme was subject to rapid change, some projects being superceded whilst still in the workshop, whilst others were conducted in the field and then retrospectively fitted to other radars. For example, in the spring of 1941 an experimental modification to the height finding at Sopley resulted in the other five original sets being 'Sopleyfied'. Changes in enemy tactics also meant that the siting of GCI's was constantly being subjected to review and revision.

The first GCI's were mobile, at the insistence of the Air Ministry who required maximum flexibility from the equipment and were the first phase of a three part programme, i.e.: Mobile, Intermediate and then Final. The mobile radar convoy was called an AMES (Air Ministry Experimental Station) type 8 and ran to five variants AMES 8a, 8b, 8c, 8e and 8f (8d appears to have been cancelled). The type 8 consisted of 2 trailers each housing a hand turned aerial array for the transmitter and receiver respectively.
The transmitter was housed in a three ton Crossley Tender with the receiver in a second Crossley. The Operations Room was fitted on a four wheeled trailer.

Both Transmitter and Receiver were supplied with power from 2 mobile generators with 2 more in reserve. 2 vehicles containing HF (later vhf) radio transmitter and receiver completed the technical component.
Layout of a mobile GCI operations room

The other type 8 radars were:

  8a Intermediate - mobile GCI with hand turned aerials and hutted operations and transmitter rooms  
  8b Transportable GCI with additional 35' aerial system mounted on a gantry  
  8c Intermediate Transportable GCI - as 8b, but with operations and transmitter rooms in huts  
  8e Mobile GCI Mk IV  
  8f Intermediate Mobile GCI Mk IV - as 8e, but with operations and transmitter room in huts  

The 8e was a Mk IV GCI mobile with only a GCI capability, none of these 8e convoys were used in this manner. They were modified with conversion kits to provide facilities for GCI, COL (Chain Overseas Low) or CH (B) Chain Home, Beam) working and when converted became the AMES type 15 Mk 1 mobile GCI Convoy.

The mobiles were the first of three stages, the Intermediates were the second stage, pending the construction of the permanent, brick built, multifunctional 'Final' GCI with the much delayed Type 7 radar array built over an underground chamber containing the transmitter, and capable of conducting several interceptions simultaneously.

Operational trials started on the first Ground Control Interception radar station at Durrington (near Worthing) on 29th November 1940 and by 3rd December, 36 practice interceptions had been completed. With the success of Durrington, five more GCI stations were proposed with the first mobile installation established at Sopley in December 1940.

On Christmas Day, 1940 a convoy of vehicles arrived at Sopley and set up a mobile GCI installation in a field to the north of the village on land requisitioned from the estate of Lord Manners. This was called an AMES Type 8 which was designed so that the mobile equipment could be driven to the selected site and made operational within twelve hours. Personnel were billeted in the nearby villages of Sopley, Winkton, and Ripley.

The mobile Type 8 radar was designed at the Telecommunications Research Establishment at Worth Matravers in Dorset and built at the War Office/Ministry of Supply Air Defence Research and Development Establishment (ADRDE) in nearby Christchurch. It was adapted from Army Gun Laying trackers and had two manually rotated aerials rotated by airmen pedalling in the cabin mounted on the aerial trailer behind the aerial array.

The aerial's alignment onto a target was originally a manual process with airmen pedalling to operate a mechanical linkage to turn the aerial. The fighter controllers used a bell code and mechanical indicators. The aerial's sweep could also be reversed or concentrated in a defined sector of the sky giving a more frequent update of the track information than a 360 degree scan during the later stages of an intercept.

Photo: Mobile GCI at RAF Sopley on 5th January 1941

The transmitter aerial was fed by a transmitter in an adjacent lorry while the receiver aerial was fed into the operations trailer via a receiver vehicle where a range of displays indicated bearing, range and height and incorporated a Plan Position Indicator [PPI] display with the radar display equipment to drive the PPI's in the Control Van

The ops room had a crew of three, a height finder operator, a fighter controller and a plotter. From the beginning Sopley seems to have been equipped with Identification Friend or Foe (IFF).

IFF allowed a radio operator to identify friendly aircraft. Aircraft were fitted with aerials incorporating motor-driven tuners that caused the reflected signal received by ground radar stations to vary in amplitude.

Later models employed an electronic unit that detected the presence of friendly radar and then transmitted a coded signal causing the ground radar display to indicate a friendly aircraft on the PPI display.

By early 1941, Sopley had become the most effective GCI station with over one hundred successful night interceptions which were achieved by the fighter controllers at Sopley working in conjunction with Bristol Beaufighter night fighter squadrons who destroyed 27 enemy aircraft, more than twice the success rate of any other GCI.

The GCI station operated throughout the second world war using the callsign 'Starlight' and gave radar assistance and control to many squadrons operating initially from RAF Middle Wallop and later from RAF Hurn. Sopley was also responsible for the control of No 456 Squadron RAuxAF from RAF Tangmere, towards the latter part of the war.The early mobile installation was replaced in 1941 by an 'intermediate mobile' with a pair of type 8F radars followed later by an 'intermediate transportable' GCI; utilising a Type 8C radar.

This was an unusual arrangement, the normal course of events was for one or the other but not both.

The intermediate transportable installation at Sopley with a Type 8C radar with transmitter and reciver aerials above and below the gantry
Possibly Sopley, being the first production radar (Durrington was a prototype) was used as a test bed or trials site for new equipment or practices. The equipment was mobile to the extent that it could be easily dismantled and transported to another site. The erection was a fairly lengthy process taking several days to complete. The arrays were mounted above and below a wooden gantry, with operations carried out from wooden hutted control rooms.

Plan of a typical GCI intermediate operations hut

Click here to continue the history of RAF Sopley

[Source: Nick Catford]

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