History

Initially very little was known about the history or purpose of the chain of sites running along the northern side of Cornwall. They are not mentioned in 1960s plans and did not appear to link into the existing network running between Bristol, Exeter, Plymouth and Goonhilly. The exception was St Just, a long-established radio station which had provided an "Ultra Short Wave" telephony link to St Mary's, Scilly Isles. This scheme was in operation from 1938 using frequencies between 60 and 68 MHz with additional capacity added in subsequent years. Around 1971 this was replaced by an SHF link but this local link would not explain the need for the other sites.

A file of general correspondence with GEC includes a document "GEC: MICROWAVE WORKS" dated 17 November 1976. It lists a number of schemes including "Contract 309727 St Just - Treskinnick" and notes that GEC's quotation for this work had been accepted on 7 September. However the formal contracts for this (and other work) had not been completed. From this we can reasonably assume the new route was not operational until some time in 1977 at the earliest. The contract details also confirm the scope.

The four new sites received tall towers of the "eiffelised" type - square cross-section with a varying taper. The buildings are of the standard type but otherwise nothing is known of the completion date or the band/equipment intially installed. A map of digital links planned for 1988 includes the route with two significant additions: there is a cross-link between Carnemenellis and Goonhilly and an onward link towards Bristol. Also this map shows an interconnection with the Cable Landing Station (CLS) at Widemouth Bay, approximately 4 km from Treskinick Cross.

It seems most likely the St Just - Treskinnick Cross route was originally developed to increase capacity or provide resilience for the "inland" connections between various CLS sites along the coast. Goonhilly was also a CLS and St Just is near to CLS sites at Porthcurno and Sennen Cove. A 1974 diagram shows traffic from Widemouth Bay and Goonhilly carried over three routes to London: the SHF link between Bristol and London, a parallel cable via Reading and another via Exeter and Salisbury. There were a number of cross-connection points between these routes. At this stage the 1963 TAT-3 cable (from New Jersey) carried 138 circuits and the 1974 CANTAT-2 added 1840 circuits from Canada, both landing at Widemouth Bay. The cable section towards Bristol provided 2,700 circuits (45 x 60-circuit "supergroups"). The "external" traffic from Goonhilly is not stated but there were 120 supergroups available by cable and a further 135 on the existing SHF link - a total of 15,300 circuits.

Due to the geography most telephone traffic between North America and Europe passed via the UK with London having a significant role in international switching. A number of relatively short undersea cables ran from the east coast to France, Belgium and The Netherlands but there were also longer connections to Germany, Denmark, Norway and Sweden. The SHF route to France had capacity for 2,160 circuits as at February 1974 with a further 3,600 "planned". Although the 1974 diagram suggests adequate capacity was available within Cornwall there would be rapid growth. In 1983 TAT-7 added 4,000 circuits, landing at Porthcurno, and in 1988 TAT-8 brought a 280 Mbit/s link to Widemouth Bay, requiring a digital link.

The extension towards Bristol required the addition of a new site at Ashbury. Outline planning permission was obtained in July 1975, renewed in April 1981 and further details approved in December 1982. The timing suggests Ashbury was part of a wider plan involving the St Just to Treskinnick Cross route but the main section did not require it to be operational at the same time.