Somerville decided that a chain of 6 Coastal Defence U Boat Radar Sites should be built in the North of Scotland to help protect Scapa Flow and to provide a watch for U Boats/enemy shipping transiting between Shetland and Orkney or passing just north of Unst. The most suitable equipment available at the time was that planned for the RAF Chain Home Low (CHL) & Army Coastal Defence sites. The Naval sites, which were originally developed separately from the CHL sites, were known as Admiralty Experimental Stations (AES) and they were each allocated a number. The site on Saxa Vord was AES 4, sometimes known as Saxavord (all one word) by the Navy. Radar production for all 3 services required a large construction programme, gun-laying radars, ship borne radars, radars against high and low flying aircraft were all needed. Priorities changed as the threat was continually re-evaluated and so, where there were similar roles, radar originally intended for one service, was sometimes diverted to another project belonging to a different service. Some equipment was interchangeable but not necessarily identical. The Naval CDU sites, whilst similar to the RAF CHL sites, sometimes had different equipment.
The map below shows where these Naval units were built. The ones at Dunnet Head and South Ronaldsay were primarily established to protect Scapa Flow. The 2 sites on Fair Isle and the one at Sumburgh were to provide surveillance of the waters between Orkney and Shetland. AES 4 at Saxa was to watch for traffic to the north. (Left click on pictures to nlarge).
The sites were constructed in 2 phases and the programme was carried out swiftly. AES 1, 2 & 3 (Sumburgh and the two sites on Fair Isle) were built in the first phase. The earliest to become operational was at Sumburgh which was opened in Dec 1939 – just over 3 months from conception to completion. The Sumburgh site was later moved nearly a mile northwards to Compass Head, above Grutness. The second phase included AES 4, 5 & 6, with the last one at Dunnet Head (AES 6) becoming operational in Dec 1940.The daring raid of the U47, Captained by Günther Prien, which penetrated Scapa Flow in Oct 39 and sank the Battleship HMS Royal Oak, is told in other places. Would the Coastal Defence Radars have made a difference had they been operational earlier– who knows?
It is tempting to try to recount the history of all 6 AES sites in Scotland but I don’t have the data or time to research the other 5 sites. In passing its worth mentioning that there was a later 7th AES built, but that was in Iceland. From here on this section will be directed towards AES 4 on Unst.
In early 1940 Vice Admiral Somerville and a small party studied a number of locations on Unst (incl Clibberswick – 160m/525’ ASL, an unnamed site about 1 mile west of Outer Skaw 132m/433’ and Libbers Hill 170m/558’) before selecting Saxa Vord as the site of the radar. At 285m/935’ the summit is the highest point on Unst and it was deemed to provide the best coverage for the equipment to be installed. A substantial track constructed in WWI, known locally as Whites Road, led up to near where the later RAF Saxa Vord Mid (or Ops) site would be built. It had been laid to enable a gun to be deployed to cover Burrafirth in case enemy shipping attempted to use the Firth. Between the two wars the local population used the route to access the plentiful peat banks on the hill. This track had to be extended and metalled before construction of AES 4 could commence.
As with the later RAF Station the “top site” was built in 2 parts. The lower part was where the generator and Rest Hut were located and the upper site, where the radar aerials were to be installed. There was no road between the 2 sites as there is now and so a 2ft gauge rail track, approx 120 yds/110m long, was laid between the 2 sites. Heavy material had to be winched to the upper level. Two buildings each 16’ x 16’ with a 10’ high ceiling were constructed as Transmitter and Receiver Blocks. Each of these Blocks had an aerial gantry built over them. The gantries were similar to the one in the photo below, though it is probable that at AES 4 they were initially built of timber:
Inside each of the blocks an operator had to rotate the aerial using apparatus similar to that which drives a bicycle except, in this case it was hand turned. The equipment was designed at Cambridge under the auspices of Dr John Cockcroft . Cockcroft was a very eminent scientist who was knighted after the war (1948) and went on to win the Nobel Prize for Physics in 1951.
At the top of the chain was a gearing system which looked like this:
AES 4 became operational in September 1940 and was manned by Royal Navy personnel, with support from RAF and RCAF mechanics. One of the Canadians, Puss Valeriote, made a nostalgic return visit to Unst in June 2000. The Unit remained on the air for a short while after the war in Europe was over in May 1945. Its work had not been limited to searching for submarines/surface vessels – the radar had quite a useful capability for detecting airborne targets. By the end of 1940/early 1941 the radar was detecting shipping out to the radar horizon, 40 miles or more depending on the size of the ship, though for smaller vessels such as trawlers, ranges of 25 to 30 miles were normal. Aircraft below 10,000 were being seen at around 100 miles but performance of the early CHL was not very good against higher flying targets. By the end of 1941, following updates to the kit, ranges against airborne targets had increased considerably with ranges of 140 miles reported.