ShackEng makes a very valid point, while the obvious threat to the U.K. came from the east and the UK defences were mainly orientated in that direction there was a less obvious but very potent threat lurking in the deep waters of the North Atlantic not so far to the west. It was vitally important that the Soviet nuclear missile submarine fleet was tracked, and occasionally deterred, at all times day and night fair weather and foul. This the Shackleton did for years and years, at low level.