3. Radio Aids to Aircraft – Gee

Truleigh Hill was given a new role towards the end of the war. Even as its value a CHL radar site declined, a new development in radio navigation was becoming important.

The development of radio beams as a navigation aid to distant aircraft had been used to great advantage by both the Luftwaffe and the RAF. By transmitting signals from two or more ground stations, aircraft receiving the signals could calculate their position with a reasonably high degree of accuracy. The British developed ‘Gee’ was one such system and Truleigh Hill was chosen to be part of a ‘Gee Chain’.

The Gee system is basically quite simple but requires very accurate equipment both on the ground and on board the aircraft. A ‘Master Station’ transmits a fixed-length radio pulse on a frequency that will give good reception at long range. This signal is also received by a second, ‘Slave’, station. The slave station in turn transmits another fixed length pulse a specified time after receipt of the original and another slave station does likewise after yet another specified delay. Most chains used a third slave to give greater accuracy still.

 

An aircraft using the system receives the three signals and they are displayed as ‘blips’ on two traces on a cathode-ray tube. The screen was also marked with a graduated scale and the actual time between the pulses could thus be measured. Since the transmission intervals were known, the position of the aircraft could be calculated from the measurements of the actual delays. This relative timing had to be maintained with great accuracy for the system to work properly. The operators had to concentrate on what must have been a very tedious job with little glory or recognition yet they did so with great dedication and skill throughout the time that Gee was in use. Operators who worked the system took enormous pride in their achievements and many of them spent a great deal of their, otherwise free, time carrying out improvements, re-wiring etc, that was not strictly their responsibility but which led to a more efficient and dependable system.

The set of RANAD Gee Coverage charts of July 1947 (click here to see scans of the originals) show the south of England (‘Area A’) divided into three chains – South Western, Eastern and the one of greatest interest here, the Southern. The three chains were controlled by HQ No 90 Group and overlapped and extended (at 10,000 feet) well into northern France. The following table summarises the three chains. Grade ‘A’ means continuously operational; Grade ‘B’ is operational on stand-by arrays; Grade ‘C’ is on maintenance or test and may transmit for test but must not be used; Grade ‘D’ means forbidden to transmit. The identification was achieved by ‘ghost pulse blinks’ from the master station.

	Southern	South Western	Eastern
Frequency	30.7 Mc/s	25.3 Mc/s	22.9 Mc/s
Identification	3 blinks	Nil	2 blinks
Grade	Grade ‘A’ except grade ‘B’ from 1100 to 1500 GMT each Sunday	Grade ‘A’ except grade ‘C’ from 0200-0300 and 1600-1700 GMT Monday to Saturday and 1500-1700 each Sunday	Grade ‘A’
RF Unit	Type 24	Type 24	Type 24
Aerial Loading Unit	Type 51 Stud 4	Type 51 Stud 2	Type 51 Stud 1
A Master	Bulbarrow 50 50 59 N, 02 18 45 W	Sharpitor 50 30 42 N, 04 02 08 W	Daventry 52 15 45 N, 01 08 06 W
B Slave	Truleigh Hill 50 52 54 N, 00 15 32 W	Worth Matravers 50 35 54 N, 02 02 48 W	Stenigot 53 19 29 N, 00 06 46 W
C Slave	West Prawle 50 13 23 N, 03 43 47 W	Sennen 50 04 01 N, 05 40 08 W	Gibbett Hill 51 06 51 N, 00 42 55 W
D Slave	none	Folly 51 50 05 N, 05 06 24 W	Clee Hill 52 23 15 N, 02 34 05 W
Monitor	Worth Matravers 50 35 47 N, 02 03 06 W	Trefrew 50 22 58 N, 05 04 59 W	Barkway 52 00 30 N, 00 00 37 E

At this time the Southern chain was significantly weakened by the lack of a third slave even though the ‘5 mile accuracy’ line extended well into central France. There were large areas to the east and west of England that lacked sufficient cover but these were covered by the other two chains.

This system remained in action for some time after the war and was operated by ‘National Service’ men and women then later by civilian engineers before finally closing down in 1952.

The Gee site at Truleigh Hill consisted of two receiver sites, at the North-west and North-east corners of the site, and two transmitter sites, at the South-west and South-east corners. Each receiver site consisted of a wooden tower 70 or 80 feet high carrying the aerial and a Nissen hut, housing two receivers, inside blast-walls at ground level. Each transmitter site had a wooden tower, one 90 and the other 120 feet high, carrying the aerial and a Nissen hut housing two transmitters, also inside blast-walls at ground level. Each transmitter site also had a generator shed (Nissen hut inside blast walls) housing two Lister diesel generators. The receivers and their masts were interlinked by underground cables which also carried signals to the transmitters. Similarly, the transmitters and masts were interlinked by ‘box-feeder’ cables carried on wooden posts about 10 feet high.

 The importance of the overall installation may be judged from the built-in redundancy. Not only was each site duplicated, but within each site so was the equipment. In the event, the operators at Truleigh Hill found that they never had to use most of the backups and any one of the duplicated systems was dependable enough, throughout the life of the station.

Today, all that remains are a few disused blast-proof enclosures and mast bases. On the left of the entrance road to the bungalow is a damaged base from the South-west Gee transmitter mast.

 The other transmitter base has been completely removed and the site is now covered by a farm building but the blast wall enclosure for the transmitter hut remains and is used as a stable and vehicle store by the farmer. There are also many fence posts and other timbers in the vicinity that were once spars in the original aerial towers.At the North-west corner of the site is the remains of one of the receiver sites.

 

The blast wall is still intact but evidence of alterations may be seen and parts of the interior structure have not been properly identified yet. There is a brick shelter with a thin concrete roof completely covered by brambles and shrubs but it has proved impossible to get close enough, much less inside it, to see if there are any signs of what exactly it contained.

 

These photographs of the only intact part of the NW receiver site show the standard pattern blast wall with the staggered entrance.