Sorry dan I'm going to hijack your thread for a bit, I promise to get back to the original topic in a mo.
Right Miles, just to clarify,
RVR at Lasagne international airport is 600 meters. Atis is saying " Cat 2 operations in progress" I come along in my cat1 aircraft (550 rvr required on the jepps) and carry out an ILS to minima and land all nice and safely. Then the italians do what ? prosecute, complain to JAR? And if this is a state difference why is it not in the jepps if it is in force now. And then what , in my Cat 3b machine we always ask for cat 2/3 if the rvr is even vaguely close to cat 1 limits. Cat2/3 ops gives signal protection for autolands amongst other things.
Back to dans thread
In the old days the final distance checks on an approach would be by 3 marker beacons. Outer, Middle and Inner, all they were/are is a simple vertically orientated radio signal so as you passed through the overhead you would get a pulsed tone in your headset and a little light would come on (er blue, white and orange) Nowadays , middle has gone inner only survives in a few places and outer is rapidly going.
Nowadays the position fix could be an Outer Marker, or an NDB (or a co-located LOM locator outer marker) or a VOR station or a DME range from anywhere or any other fix that is in about the same place as the outer marker used to be before they were all ripped out as too expensive/unreliable etc.
So if you fly over a VOR at 4.5 miles that would be a reasonable equivelent of the old outer marker (inner was usually the threashold anyone remember where middle was?) If there is no equivelent position , however derived then 1000 feet agl (3 and a bit miles) is the check height for your RVR.
sorry for the spelling , its been a long day.