Airfield Environmental Sensors

Airfield environmental sensors are the suite of instruments that measure meteorological conditions critical to aviation safety — visibility (RVR – Runway Visual Range), wind speed and direction, temperature, humidity, barometric pressure, and precipitation. These sensors are typically installed alongside runways, at thresholds, and at mid‑points, and they feed data to the control tower, MET office, and increasingly to the ALCMS for automated lighting intensity adjustments.

RVR (Runway Visual Range) is a derived value that combines visibility, background luminance, and runway light intensity to estimate the distance a pilot can see runway markings or lights. It is measured using forward-scatter or transmissometer sensors installed at the touchdown, mid, and stop‑end points of each runway.

Wind sensors (anemometers and wind vanes) provide real-time wind speed, gust, and direction, essential for takeoff and landing decisions. Wind data is also used to calculate crosswind and tailwind components.

The safety margins built into aviation operations depend on knowing the actual visibility and wind conditions at the runway right now, not an estimate from a tower window or a forecast from an hour ago. Environmental sensors exist to replace that kind of subjective or stale judgement with a continuous, instrumented measurement pilots, ATC, and airfield systems can rely on. RVR sensors give a specific, defensible distance figure for how far a pilot can actually expect to see runway lights or markings — the number that determines whether an approach or takeoff is permitted to proceed under a given category of operation. Wind sensors give the crosswind and tailwind components that decide which runway is safe to use and whether conditions remain within an aircraft's operating limits. Feeding this same data into the ALCMS closes a further gap: rather than running the runway lighting at a fixed default intensity, the system can match light output to the visibility that has actually been measured, so pilots see the intensity level appropriate to conditions rather than one set for the worst case or guessed by an operator.

Environmental sensors are strategically placed across the airfield to capture representative conditions:

• RVR Sensors (Transmissometers / Forward Scatter): Installed at three positions along each runway — touchdown zone (typically 300m from threshold), mid-point, and stop end — to provide RVR values for the full runway.
• Wind Sensors: Typically mounted on masts near the runway thresholds (both ends) and at mid-field, to capture surface wind conditions.
• Automatic Weather Observing Systems (AWOS): Integrated sensor suites that include temperature, humidity, dew point, barometric pressure, and precipitation detection.
• Ceilometers: Measure cloud base height, important for approach minima.
• Present Weather Sensors: Detect precipitation type (rain, snow, drizzle) and intensity.

Data from these sensors is distributed to ATC, the aerodrome meteorological office, and, in modern systems, directly to the ALCMS for automated intensity step selection based on RVR thresholds.

RVR transmissometers and forward-scatter sensors are mounted on short masts a set offset from the runway centreline at each of the touchdown, mid-point, and stop-end positions, sited so the sensor's optical path is clear of any obstruction and positioned to keep the equipment within the runway strip's obstacle-limitation surface. Wind masts are erected near each runway threshold and at mid-field, at a height and distance from the runway that avoids local turbulence from nearby structures or terrain skewing the reading, with the sensor itself mounted at the standard reference height above ground level used for the airfield's wind reporting. All sensor types are fed by cable — power and signal — run in duct or direct-buried conduit back to a local remote terminal unit or straight to the AWOS processing equipment, with the cable route planned to avoid the runway strip's graded and load-bearing areas. Because these sensors report values pilots and ATC act on directly, foundation and mast installation is carried out under a survey that fixes each sensor's exact position and height, since even a modest siting error can bias the readings the whole safety margin depends on.

Each sensor type uses a distinct measurement principle:

• Transmissometer: Measures the extinction of a light beam over a known baseline (typically 75m). The receiver measures the transmitted light, and the instrument calculates the Meteorological Optical Range (MOR). RVR is then derived using background luminance and runway light intensity.
• Forward Scatter Sensor: Measures the amount of light scattered by particles in a small sample volume. It is more compact than a transmissometer and is widely used as an alternative.
• Anemometer: Uses a three‑cup or ultrasonic design to measure wind speed, and a vane or sonic array for direction. Ultrasonic anemometers are increasingly preferred for their lack of moving parts.
• Temperature / Humidity Sensors: Typically platinum resistance thermometers (PRT) and capacitive humidity sensors housed in a radiation shield.
• Ceilometer: Uses a laser or infrared beam to detect cloud base height by measuring the time of flight of backscattered light.

All sensors are connected to a data acquisition system (often an AWOS) that processes, formats, and transmits data to displays and networked systems. Data is typically updated once per minute or faster.

Individual environmental sensors — transmissometers, forward-scatter sensors, anemometers, temperature/humidity probes, ceilometers — each measure a single physical quantity and have no interconnection with one another; the control architecture lives in the data acquisition and processing layer above them. Each sensor's analogue or digital output is polled by a local data acquisition unit, which timestamps and validates the reading (flagging out-of-range or stuck values) before forwarding it to the central AWOS processor. The AWOS processor combines readings from multiple sensor sites, applies the required algorithms — deriving RVR from MOR and background luminance, for instance — and pushes the resulting product to ATC and pilot-facing displays, typically refreshing at least once a minute; if a sensor drops out or its data fails validation, the processor flags that specific parameter as unavailable rather than silently substituting a stale or estimated value.

Each sensor type within an environmental sensor suite follows construction appropriate to its measurement principle, but shares common design features: a weatherproof sensor head or optical assembly, typically aluminium or corrosion-resistant composite, rated to IP66/IP67, mounted on a rigid pole or mast at a height standardised for that measurement type. Optical sensors such as RVR and visibility instruments house their transmitter and receiver components in sealed enclosures with heated windows to prevent condensation or icing from obscuring the measurement path, while wind and temperature sensors use sealed bearing assemblies or radiation shields appropriate to their mechanism. All sensor heads connect via shielded, weather-rated cable to a common signal processing or data concentrator unit, often housed in a nearby equipment cabinet, which aggregates readings from the full sensor suite before forwarding them to the ALCMS or MET office over the site communication network. Mounting structures are corrosion-resistant steel or aluminium poles set in concrete foundations, positioned to keep each sensor clear of obstructions or turbulence sources that could bias its readings, and all outdoor enclosures and cabling are built to withstand continuous exposure to airfield weather extremes.