Visibility Sensor

A Visibility Sensor is a meteorological instrument used to measure atmospheric visibility, specifically the Meteorological Optical Range (MOR) and Runway Visual Range (RVR). It is a critical component of airfield weather systems, providing data that determines the operational status of the aerodrome, especially for low-visibility procedures (CAT I, II, III). Visibility sensors operate on the principle of light extinction or scattering: either measuring the attenuation of a light beam over a known distance (transmissometer) or measuring the scattering of light from a small volume of air (forward scatter sensor). The sensor outputs a visibility value in metres or kilometres, which is used by ATC, pilots, and automated systems for safe operations.

A visibility sensor exists to give the airfield an objective, continuously updated read on how far a pilot can actually see through the atmosphere, which is the single factor that most directly gates low-visibility operations. Because MOR and RVR values are what set CAT I/II/III minima, an airport's ability to keep operating safely in fog, haze, rain, or snow depends on trusting this number rather than a subjective estimate from a single observer at one point on the field.

The sensor's role becomes most critical exactly when conditions are worst: as visibility drops toward the threshold where an approach can no longer legally continue, accurate, real-time data is what allows ATC and pilots to make that go/no-go call with confidence, rather than discovering the runway environment isn't visible only during the final seconds of an approach.

Installed at airfields as part of the meteorological observation system, typically located near the touchdown zone of each runway, and sometimes at the mid‑point or stop end. Visibility sensors are used for RVR reporting, which is mandatory for precision approach runways. They are also used at remote airstrips, heliports, and offshore platforms where visibility data is essential for flight safety. The sensors are mounted on masts or poles at a height of approximately 2.5 metres above the runway surface, in accordance with ICAO recommendations.

Mounted on a mast or pole beside the runway strip at each required measurement point, typically the touchdown zone, with additional units at the mid-point and stop end on precision-approach runways, set at approximately 2.5 m above the runway surface in line with ICAO guidance. The sensor's transmitter and receiver (or the forward-scatter sample volume) are aligned along the anticipated line of sight so the reading is representative of the visibility a pilot would experience, and the mast is positioned clear of the obstacle-free zone at the mandated lateral offset from the runway edge. Power and data cabling run underground in duct back to the AWOS/RVR processing cabinet, and the optical heads are orientated to minimise direct low-sun glare, which can otherwise skew scatter-based readings.

A forward scatter visibility sensor emits a pulse of infrared or visible light into a small sampling volume of air. A receiver, positioned at a specific scatter angle (typically 35° or 42°), measures the amount of light scattered by atmospheric particles (aerosols, fog, rain, snow). The scattered light intensity is proportional to the extinction coefficient, which is then converted into MOR (Meteorological Optical Range) using the Koschmieder relationship. Transmissometers, on the other hand, use a separate transmitter and receiver separated by a known baseline (typically 30 to 75 metres); the attenuation of the light beam directly yields the visibility. The sensor outputs digital or analogue signals to the weather processing system, which calculates RVR using ambient light and background luminance corrections.

Where a runway has visibility sensors at more than one point, the system's real decision logic sits above the individual sensors, in the AWOS processor that combines them: it selects the controlling value — normally the lowest MOR/RVR among the fitted positions relevant to the operation — rather than reporting each sensor's reading in isolation, since a single obscured section of runway is enough to limit the approach regardless of what the other positions show. Each sensor also continuously self-monitors its own optical path: window contamination, condensation, or heater failure can degrade a reading without an obvious fault, so most units run heated optics and a contamination-check routine, reporting that health status to the processor alongside the raw measurement so a fouled sensor can be down-weighted or excluded rather than silently pulling the controlling value in the wrong direction. From there, the same processor layer that handles RVR calculation distributes the controlling value onward to ATC displays and METAR/ATIS reporting, and — on integrated sites — offers it as an input the ALCMS can use when recommending or auto-selecting a runway lighting intensity step.

A visibility sensor is built around either a forward-scatter optical head, with transmitter and receiver arms angled toward a common sample volume, or a transmissometer configuration with transmitter and receiver units mounted a fixed, calibrated baseline distance apart. The optical housings are weatherproof enclosures, typically aluminium or composite construction rated to IP66/IP67, with heated lens hoods or windows to prevent condensation, frost, or precipitation from degrading the optical path. The sensor arms or transmitter/receiver towers are mounted on a rigid pole or frame set at the standard reference height, engineered to hold precise optical alignment against wind loading and vibration, since drift between the transmitter and receiver directly affects measurement accuracy. Internal electronics for signal processing are housed in a sealed enclosure at the base of the mounting structure or integrated into the optical head itself, connected by shielded, weather-rated cable. Corrosion-resistant materials and fasteners are used throughout given the sensor's typically exposed siting close to the runway or in open terrain.