<P> Today most airports use Instrumented Runway Visual Range or IRVR, which is measured by devices called forward scatter meters which provide simplified installation as they are integrated units and can be installed as single unit (s) at a critical location along the runway or transmissometers which are installed at one side of a runway relatively close to its edge . Normally three transmissometers are provided, one at each end of the runway and one at the midpoint . In the US, Forward Scatter RVRs are replacing transmissometers at most airports . According to the US Federal Aviation Administration: "There are approximately 279 RVR systems in the NAS, of which 242 are forward scatter NG RVR Systems and 34 are older Transmissometer Systems ." </P> <P> Because IRVR is not derived using human observers, the values obtained are not a reliable guide to what a pilot can actually expect to see . This can easily be demonstrated by the fact that when obscuration such as fog is variable, different values can apply simultaneously at the same physical point . </P> <P> For example, a 2000m runway could have reported touchdown, midpoint and rollout IRVR values of 700m, 400m and 900m . If the actual RVR at the touchdown point (300m from the threshold) is 700m as reported, then a pilot could expect to see the light 700m away, at the runway midpoint . If the pilot taxies to the midpoint and looks back through the same air mass, he or she must also be able to see the light at the touchdown point . But since the midpoint RVR is reported as 400m, a light 700m away must be invisible . Similarly, looking forward he or she cannot see the light in the rollout area . But according to the rollout RVR of 900m, the light there is visible, and has already been so for the last 200m . </P> <P> RVR is used as one of the main criteria for minima on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft . The maximum RVR range is 2,000 metres or 6,000 feet, above which it is not significant and thus does not need to be reported . RVRs are provided in METARs and are transmitted by air traffic controllers to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach . </P>

What's the difference between rvr and visibility