Steve, perhaps not directly relevant, but some history.
As you may recall the backdrop to European AWO was ECAC doc 17; based on these requirements the UK CAA used a linear model to determine operational minima for each aircraft type (circa 1960 / 70s). IRC the model ran in BASIC and matched straight line SVR against RVR for a first contact point against altitude. The main variables were flight-deck cut-off, RVR, and light intensity.
There may have been later versions based on RAE data in actual fog (see below*).
RAE generated and used various fog models using straight line (segmented) interpretations of SVR against RVR; these were used during extensive simulation in 1970s. RAE reports may have been by J Penwill.
A further electronic fog model was developed using a vertical scanning RVR sensor (measurements of actual RVR against altitude up to 100ft?) which predicted SVR. The predictions were validated by inflight reporting, which in my experience were very good. There were several reports in this area including approach lighting by Smith and Puffett.
N.B. The most realistic simulation ‘fog model’ associated with RAE Cat 3 research circa late 1970s consisted of a Perspex disc where segments sanded with different grades of sandpaper obscured the projection of runway lights as a function of altitude!
*Text from a post elsewhere:
The diagrams below are taken from research documents and show the first contact height and visual segment for a range of RVRs against altitude (m)
In (1), the first contact height is at the intersection of fog ‘RVR’ line and cockpit cut-off, i.e. 60M/200ft altitude in 600m RVR.
The visual segment for a particular aircraft type (cockpit cut-off) at lower altitudes, is the horizontal distance from cockpit cut-off line to the fog line, i.e. at 150 ft in 600RVR, the far point is 330m and near point is 110m; vis seg 330-110=220m. The crew would see the centreline and crossbars 2 and 3.
The threshold would be seen in this fog (600m) at an altitude of 35m / 110ft.
For differing fog types/cloud conditions, the lines of RVR will has a different line shape and path.
In the 600m fog shown, the first contact and ‘immediate’ visual segment (just below 200ft) would contain the centreline and crossbar 4. Just enough for Cat 1?
Cat 1 is generally associated with cloud breaks thus a large visual segment would be expected; however in fog, the conditions can have significant variations.
In a shallow fog (2), the threshold might be seen at 200 ft even in an RVR of 300m; hence a min RVR / approach ban.
In more mature fogs (3), nothing will be seen, even though the RVR is 600m.
These diagrams are from the RAE Cat 2 report ‘Manual Landings in Fog’ (Newberry).
It’s on the CD which is in the post
I don’t have any direct information on snow or sand, but from limited experience in Cat2 snow, there is little variation in SVR with altitude once below cloud, which could be very low. However, in blowing snow there can by variations with altitude, but perhaps none at an altitude which includes the flight-deck or is of significance to the decision / landing.
However, again from experience, a very shallow layer of blowing snow is extremely disorienting particularly where the aircraft is pointing into wind and thus appears to drift ‘up-wind’; furthermore, the illusion changes during drift-alignment. Add to that a small touch of wing-down, - then an autoland is recommended even in good Cat 2 RVRs.
I suspect the sand is very similar.
AFAIR (late 1990s) training simulators modelled visibility in similar way to the RAE segments.
The practical difficulty is choosing an appropriate training model particularly where there are many choices.
There are few operations which allow RVR minima to be limiting is a decision (80% success rate, etc), particularly in Cat 3 fog where the fog is more homogeneous with decreasing RVR. The difficult areas are during fog formation (Cat 2) with layered structures (onion rings), or fog dispersal (mini cumulus) which can generate changes in SVR, but again from experience the SVR change is not significant in comparison with change of light levels particularly with the glare of strong sunlight above or runway lights below; I have never seen these effects modelled in simulators.
From the diagrams above, Cat 2 operations have the greater risk of having made a decision to ‘land’ (continue), and the decision subsequently be ‘wrong’.