Operation Grapple was a set of four British nuclear weapons tests of early fission bombs and fusion bombs carried out in 1957 and 1958 near the vicinity of Christmas Island., Pacific Ocean .Nine nuclear explosions were initiated, culminating in the United Kingdom becoming the third recognized possessor of thermonuclear weapons.

A Decca Survey  chain  was  installed temporarily in support of the British RAF bomb testing..Donald Edward Bridges of Decca was involved with the installation of these chains.

The late Walter Blanchard offers some additional information on Survey Chains.  "Lambda was an early (1950's) Survey system carried mainly on board Naval Survey vessels operating in remote areas who needed to set up a Decca-like positioning aid quickly. The vessel carried a Master station transmitting a master signal that was re-transmitted by two slave stations the ship had set up on-shore. The vessel could then move about continuously measuring its range to the two slaves and thus obtaining position. Obviously it could only be used by that one vessel. It had a range of about 200 km and an accuracy of 10m.   This was the "towering" mode. By a re-arrangement of frequencies and triggering it could also operate in a hyperbolic mode which had the advantage of being usable by several vessels if the Master vessel anchored and wanted to send out smaller survey launches"

The ID of the Decca Survey chain surrounded  by a dashed oval is not known at this time. It  is being referred as the South Pacific chain temporarily. (Map courtesy Yale University)

Location of Christmas Island.  Courtesy Torpedo Bay Navy Museum)

According to this video. there were  Decca slave stations installed at Penrhyn and Jarvis Islands with the Master being on Christmas Island. (now called Kiritmati)

Here is what Chatgpt had to say:

"There was a Decca Survey chain at Christmas Island in 1958. It was part of the British nuclear test program during the late 1950s and early 1960s, which involved a series of atmospheric nuclear tests conducted by the United Kingdom in the Pacific.In 1957, the UK began conducting nuclear tests at the Montebello Islands off the coast of Australia, but by 1958, the British government was also looking for more remote test locations. Christmas Island (now part of the territory of Australia) was chosen because of its isolation in the central Pacific Ocean

Christmas Island became the site of a major British nuclear testing program between 1957 and 1962. The tests conducted there included both atmospheric and underwater explosions, and the Decca Survey chain would have helped support those efforts by providing accurate positional data for monitoring and assessment purposes. These tests were part of a larger Cold War-era arms race, and the events on Christmas Island are considered a chapter in the history of nuclear testing. The tests and their environmental impacts, including radiation contamination, were controversial and led to significant political and scientific scrutiny in the years that followed".

Sources:

1)  Decca coverage map     https://history.yale.edu/sites/default/files/files/2014%20rankin%20-%20radionavigation%20and%20intangible%20artifacts.pdf
2) https://en.wikipedia.org/wiki/Operation_Grapple
3) Dan Payne [paynepower4u(at)gmail.com]
4) https://film.iwmcollections.org.uk/record/1181/media_id/12615
5) ChatGPT

Nov 11/24