With a rad alt, one needs to know the terrain over which one is going to use it. That is why many aerodromes where precision approaches are provided produce a precision approach terrain chart which allows one to check the relative elevation of the terrain under the approach close to the runway and base any rad alt minima on that.
Over the sea depending on the type of rad alt system there can be some error caused by the movement of the waves and doppler effect.
When it comes to obstacles, all obstacles within the FIR whuch are 100m or more above the surface are required to be included in the AIP ENR 5.4 That includes mobile obstacles.
Any obstacle (including a mobile one) not included in the AIP is NOTAMed and there are a few each year where rigs are known to be moving about the place.
However and perhaps the more maritime among us can comment on this - beyond the 3nm mile limit? (from the coast) as far a vessels are concerned they are on the "high seas" and don't have to tell anyone who they are where they are or what they are up to. This would seem to make it difficult to determine exactly where those obstacles are.
Of course one could be letting down in the middle of the channel and pop out head on to a small naval boat launching a missile at what it sees as a hostile attack!
Taking that last unfortunate point perhaps when "bending the rules" these days we should think of the (unexpected) wider implications. e.g. why does that guy pop out of cloud low over the sea when he could make a normal approach at xyz - might be up to no good!
Regards,
DFC
Last edited by DFC; 27th July 2005 at 10:12.