JimL, it still seems like all that is needed is a better radar - whether it is used for ranging or obstacle avoidance it needs to be accurate, if it is good enough for one then it must be good enough for the other (there is no point in knowing there is an obstacle out there without knowing exactly where it is). Our minimum let down distance is 1/4 nm on the Sea King and that is with a radar that doesn't even face forwards and is very long in the tooth. I suspect that the use of a weather radar for let downs has never been tested in court regarding legality as to my knowledge there hasn't been a crash resulting from its use.
But since the final element of a rig letdown IMC is going to be pointing at or near to it, will using DGPS let the operators use reduced minimas or must they stay the same because the radar is still the limiting factor?
I take your point re separation between IFR helis - a sort of RVSM if you like.