What is a visibility sensor?

It measures how much fog, haze, rain, snow or dust weakens a beam of light, and reports the result as the meteorological optical range (MOR), the path length over which a beam falls to 5 % of its original strength. Most sensors also report the extinction coefficient, from which MOR is calculated. It is a measurement of the air, not of what a pilot sees from the cockpit.

A transmitter sends a light beam into a small volume of air. Fog droplets and particles scatter some of that light towards a receiver set at an angle. More scatter means thicker air, and calibration converts the signal into extinction coefficient and then MOR. It needs only one post and no long baseline, but it samples a small volume, so patchy fog can make it read differently from a sensor a few hundred metres away.

A transmitter and a receiver face each other across a fixed baseline, typically tens of metres. The receiver measures how much of the beam gets through, and that transmittance is converted into extinction coefficient. Because it averages over the whole path it has long been the reference instrument for RVR, but it needs two accurately aligned posts, more cable and more civil work than a forward-scatter sensor.

No. Visibility from a sensor is MOR. Runway visual range is the distance a pilot on the runway centreline could see the runway lights or markings, calculated from the extinction coefficient and the runway light intensity (and background light), following ICAO Doc 9328. That is why the lighting intensity step and the sensor are linked, and why RVR is covered in entries 13 and 14 of this division.

Close to the runway: for RVR, ICAO Annex 3 guidance puts them within about 120 m of the centreline and about 2.5 m above the runway, at the touchdown zone and, where needed, the mid-point and stop end. The meteorological authority decides the number and positions, and masts in the protected areas must be frangible.

It is a separate meteorological system and not part of the series circuit, but it shares the airfield. The sensor needs power, earthing and surge protection, and its cables need recording and protection like any other airfield cable. Its readings trigger low-visibility procedures, and at some aerodromes they drive the lighting brilliancy automatically; practice varies, so follow the aerodrome's procedures.

Typical faults are dirty or wet windows, dew, frost or snow on the optics, misalignment in a transmissometer, lamp ageing, calibration drift and insects or debris in the beam. A contaminated window gives false readings. Work near the runway needs the permit-to-work and aircraft-movement controls, and because RVR affects approach minima, tell ATC and the meteorological authority before taking a sensor out of service, particularly in low visibility.