St Helena navaids relocation
I wonder if anyone knows definitively why the VOR and localizer positions were moved on St Helena? I kind of initially assumed that it was due to terrain reflections, and specifically for the VOR the fact that it was sited metres from the metal buildings at Bradleys construction camp.
However, I saw a comment on facebook from someone in South Africa that the problem was due to reflection off the sea.
I looked up the FAA document "Siting Criteria for VOR etc" (search for FAA doc 6820) and it specifically says that Doppler VORs are not affected by cliff top locations. (This is due to the fact that the circular earth counterpoise tends to cut off radiation below the horizon.) So my guess is the the VOR on St Helena was affected by the close proximity to the buildings.
(I know of DVORs on elevated locations which appear to work normally, e.g. the Praslin VOR in Seychelles - so this tends to support the above analysis.)
However, the localizer looks to me to be a prime candidate for sea reflections. Localizers are usually located at the far end of the runway they serve, so a "cliff top" position is difficult to achieve. However, on St Helena, because of the offset approach, it was located on a raised embankment near runway 20 threshold, so clearly illuminated the sea. The new location is lower down and so I think that it doesn't have such a view of the sea and this probably means there are no significant reflection problems.
Incidentally, in its new location the VOR is adjacent to the middle of the runway, and, unusually, the counterpoise is mounted on the ground (clearly visible in the recent videos of the 737-800 landing). The equipment shelter is located a few metres on the western side so this will have a significant effect on the radiation patterns, I should think. This is besides all the other buildings, terrain and fences etc around. The aerodrome info clearly states that the VOR is unreliable in the western sector so I wonder if it will ever provide a useful guidance aid?