What is an RVR sensor?

RVR is the distance over which a pilot on the runway centreline could see the runway surface markings or the lights that mark the runway or its centreline. The RVR sensor is the instrument at the runway, a transmissometer or a forward-scatter sensor, that measures how much the air weakens light (the extinction coefficient). An RVR computer then turns that reading into a distance.

Calculated. The sensor measures the air, not what a pilot sees. The RVR computer combines the extinction coefficient with the runway light intensity and the background luminance (day or night) and applies the visual-range laws set out in ICAO Doc 9328: Allard's law for lights and Koschmieder's law for markings in daylight. The result is a distance in metres for each sensor position.

In the same fog, brighter lights can be seen from further away, so the calculated RVR is longer at a higher intensity step than at a lower one. The RVR computer therefore needs the intensity of the runway lights as an input, normally the step selected on the lighting control system. If the lights are not emitting what the computer assumes, because of failed lamps, dirty fixtures or a step mismatch, the reported RVR does not match what the pilot can see.

Along the runway, with the touchdown zone first and the mid-point and stop end added where needed, as decided by the meteorological authority. ICAO Annex 3 guidance puts them within about 120 m of the runway centreline and about 2.5 m above the runway. The touchdown-zone sensor is typically about 300 m from the threshold; the other positions are set by the authority. Masts in protected areas must be frangible.

Local reports use a one-minute average. The Annex 3 reporting scale is in steps of 25 m up to 400 m, 50 m from 400 m to 800 m and 100 m above 800 m, and the assessed range is about 50 m to 2 000 m. Values outside the range are reported as below or above the limits. The values are used for approach minima. The ICAO category definitions use, for example, RVR not less than 550 m for CAT I, 300 m for CAT II and 175 m for CAT IIIA, each together with a decision-height limit.

The RVR system is a separate meteorological system and not part of the series circuit, but the two are linked in use. The RVR computer takes the runway light intensity step from the lighting control system, and its output goes to ATC and the AWOS. At some aerodromes RVR drives the lighting brightness automatically; that is the subject of entry 14. The sensors need power, earthing, surge protection and recorded cables like any other airfield equipment.

Typical faults are dirty or wet windows, dew, frost or snow on the optics, misalignment in a transmissometer, lamp ageing, calibration drift and a wrong intensity input. Work near the runway needs the permit-to-work and aircraft-movement controls. Because RVR sets approach minima, tell ATC and the meteorological authority before taking a sensor or the RVR computer out of service, and do not change the intensity feed without agreement.