stewyb
Until the advent of GPS based procedures, instrument approach procedures were provided by ground based equipment, either radio navigation aids or radar systems.
If you wanted an instrument approach procedure to make an approach in line with the runway you normally had to locate the radio navigation aid in line with the runway (eg, an ILS localiser), or you could locate a VOR or NDB in line with the runway, the latter is often not practicable due to off airport developments.
SOU has an ILS on one runway, 20. On cost grounds it never installed one on 02. Consequently all approaches to 02 had to be offset using navigation aids that were located either side of the runway (VOR to the east, NDB to the west). The intent is that you approach towards the navigation aid, offset at an angle to the runway centreline which intersects the centreline at around 1000 metres from touchdown, by which time you are hopefully below cloud/in sight of the runway. Then make a turn and land.
VOR and NDB based instrument approaches also do not provide real time vertical guidance information to the pilot.
While better than nothing, the offset approach is far from ideal as it gives you minimal time to spot the runway, sometimes seeing it after you are too close to it and having gone through the runway centreline with no option other than to carry out a missed approach, unless you were a Trislander and were slow enough to make a late turn!
To have the highest possible chance of a successful landing in poor weather the runway needs to be directly in front of you when you break cloud or otherwise see it. Without the expense of ground based navigation aids, a RNAV (GNSS) instrument approach procedure enables you do approach in line with the runway. Advances in GPS based technology can now also provide real time vertical guidance to the pilot, as will be available for the first time ever on runway 02, from 1st March.
VOR and NDB based instrument approach procedures use 1950's and 60's technology, far short of what is, and has been for some time, available in the 21st century.