Ranger Sue Loughran, who has returned to St Kilda for the summer, said she had noticed “overwhelming” changes to the island environment – not least to noise levels and the visual impact of the military presence.
The MoD has been on St Kilda – which has the UK’s highest sea cliffs – since 1957 with its radar tracking range forming part of the surveillance of protected airspace around the Outer Hebrides.
The £40m redesign of the site started in 2017 with the project, which was initially due to be completed in 2019, stalled due to weather, archaeological survey work and delays in supplies reaching St Kilda by boat, it is understood.
At points, up to 60 or 70 people could be found on the uninhabited island, the majority of them contractors, as the demolition of old living quarters – including the military pub, the Puff Inn – got underway and construction of new, lower impact buildings began.
Ms Loughran said the redevelopment by security and defence contractors, QinetiQ, had dominated life in Village Bay.
Ms Loughran, writing in an article for National Trust for Scotland, which owns St Kilda, said: “I noticed so many changes to the island when I first arrived; overwhelmingly, the quiet and the lack of visual intrusion.
"Ever since I have worked on St Kilda, the demolition and re-building works being carried out by our neighbours QinetiQ have dominated life in Village Bay. Now that they are complete, there is a drastic reduction in personnel.
"All remnants of machinery and building works have been removed and there is a feeling that the island is taking a step towards looking and sounding more as it may have done before the 1950s.”
New buildings have been designed to sensitively blend into the landscape with a large area between the Manse and the QinetiQ building , which used to house MoD accommodation, now cleared.
Contractors used techniques developed in other challenging environments – such as those used by the British Antarctic Survey – to redevelop the St Kilda site.
Ms Loughran said: “The whole operation has been painstaking due to the complexities of working within a mixed status World Heritage Site. This is the only such site in the UK and one of only 39 worldwide, putting it on a similar level of importance as sites such as Machu Pichu in Peru and Uluru in Australia.
"If any of you who have visited in the last three years return, you will notice a massive improvement.”
The new modernised equipment and iacilities will maximise the MoD’s mission preparedness, a spokesman for QinetiQ said.
St Kilda is part of MoD Hebrides Range, with Hirta, the main island in the remote archipelago, providing deep range tracking during exercises such as US-led NATO training. The other islands are Soay, Boreray and Dun.
Two new BAE Systems tracking radars, worth £16.8m, were installed as part of the upgrade.
The last 36 permanent residents were evacuated from St Kilda in August 1930 as living conditions got too tough.
A small number of military personnel live there, along with seasonal NTS rangers and conservationists who monitor the rare Soay sheep.