Hi Crab,
A number of States have a generic ARA (JAA, FAA, TC) which permits the approach guidance with a number of aids. These used to be NDB or DECCA but overlaying with the GPS is now more usual.
As was said earlier, DGPS will still rely upon the RADAR for clearing the approach and go-around sector as the obstacle environment is complex in offshore operations. Improving the minima depends upon more accurate positioning and vertical guidance.
Practical considerations will determine the outcome of this research - which is being undertaken mainly to legitimise what it presently being done today. The introduction of EGNOS and Galileo will result in WAAS for offshore operations in Europe within a couple of years.
The gap in RADAR cover will also have to be plugged in the near future -this could be achieved also by using satellite facilities; the GOM appear to be ahead of Europe in this - probably because they have a larger hole to plug.
Jim