RVR Sensor

An RVR Sensor (Runway Visual Range sensor) is a ground‑based instrument that measures the horizontal visibility along a runway, specifically the distance at which a pilot can see runway centreline or edge lights. RVR is a critical parameter for low‑visibility operations, as it determines the approach and take‑off minima for CAT I, II, and III operations. The sensor typically uses forward‑scatter or transmissometer technology, combined with ambient light measurements, to calculate the RVR value in metres or feet. RVR data is displayed to air traffic controllers and pilots, and is used to decide whether to continue, hold, or divert an approach.

An RVR sensor exists because the decision to attempt or continue an approach in fog, heavy rain, or blowing snow can't safely be left to a pilot's own estimate of how far ahead they can see — visibility close to the surface changes quickly and unevenly, and the margin for misjudgement narrows sharply as conditions deteriorate toward CAT II/III minima. By measuring visibility directly and continuously at points along the runway, the sensor gives controllers and pilots a shared, objective number to base a go/no-go decision on rather than a subjective impression.

Because RVR is used to set the actual approach and take-off minima for a given category of operation, this single measurement effectively determines whether an approach is legally permitted at all in low visibility — making the sensor's accuracy and reliability directly tied to whether an airport can keep operating safely as weather worsens, rather than shutting down entirely.

Installed at airports that support precision approaches, typically at the touchdown zone (TDZ), mid‑point, and stop end of the runway. For CAT II and III operations, multiple RVR sensors are required along the runway. The sensors are mounted on masts or poles beside the runway, at a height of approximately 2.5 metres above the pavement, and are oriented to measure visibility in the direction of the approach. RVR data is used by ATC, airline operations, and automated weather systems to issue RVR reports for each runway.

Sensors are mounted on masts or poles set beside the runway strip, clear of the obstacle-free zone, at each measurement point required for the runway category, typically the touchdown zone, mid-point, and stop end for CAT II/III operations. Mounting height is fixed at approximately 2.5 m above the pavement so the measurement path represents the pilot's eye height on approach, and each sensor's optical baseline is aligned along the runway so the reading reflects visibility in the direction of travel rather than across it. The mast foundation is set back from the runway edge at the mandated lateral clearance, with power and data cabling run underground in duct back to the AWOS/RVR processing cabinet. Sensor heads are orientated so the optical path avoids facing directly into low sun angles, which can otherwise introduce measurement error.

An RVR sensor combines a visibility sensor (forward‑scatter or transmissometer) with an ambient light sensor (photometer). The visibility sensor measures the Meteorological Optical Range (MOR) by detecting scattered light from a known volume of air. The ambient light sensor measures the background luminance (daylight, twilight, or night). The RVR is then calculated using the Allard‑Koschmieder relationship, which accounts for the contrast threshold of the human eye and the intensity of runway lights. The sensor outputs RVR in metres (or feet) at specified reporting intervals (typically every 1 minute). The system includes self‑diagnostics to ensure data integrity, and is calibrated against reference standards.

A single RVR reading isn't what gets reported — the AWOS/RVR processor aggregates the TDZ, mid-point, and stop-end values (where all three are fitted) and applies the operationally relevant selection logic on top: the value ATC actually reports and pilots are briefed on is typically the controlling RVR, the lowest of the relevant readings for the operation being conducted, since that's the point along the runway where visibility genuinely limits the approach or take-off. Each sensor also runs continuous self-diagnostics — checking optical alignment, window contamination, and internal calibration drift — and reports its health status to the processor separately from its visibility reading, so a fouled or drifting unit can be flagged and excluded from the controlling-value calculation rather than silently corrupting it. From the processor, the controlling RVR and the individual sensor values are distributed in parallel to the ATC tower display, into METAR/ATIS reporting, and — on airports where the two systems are linked — as an input the ALCMS can use to recommend or automatically select the appropriate runway lighting intensity step for the prevailing visibility.

RVR isn't read directly off one instrument — it's calculated. The visibility sensor measures Meteorological Optical Range and the photometer measures background luminance separately; the Allard–Koschmieder relationship combines the two, accounting for the human contrast threshold and runway light intensity, to produce the reported value. CAT II/III runways repeat this whole chain at TDZ, midpoint, and stop end, since visibility can genuinely differ along the runway's length.

An RVR sensor is built around a forward-scatter or transmissometer optical head, with the transmitter and receiver (or the transmitter, receiver, and separate baseline reflector, in transmissometer designs) mounted on rigid arms or towers set a fixed, calibrated distance apart. The optical housings are weatherproof enclosures, typically aluminium or composite, rated to IP66/IP67, with heated windows or hoods over the lenses to prevent condensation, frost, or precipitation from obscuring the measurement path. The mounting structure is a fixed steel or aluminium frame or pole set beside the runway at the standard reference height above the surface, engineered to hold the optical alignment stable against wind loading and vibration since even small angular drift between transmitter and receiver would introduce measurement error. An integrated ambient light (background luminance) sensor is mounted alongside the main optical head to correct the RVR calculation for daylight and lighting conditions. All external cabling to the processing unit is armoured or conduit-protected given the sensor's exposed siting close to the runway, and the frame and fasteners are corrosion-resistant to withstand continuous outdoor exposure and de-icing fluid contact.