Runway Occupancy Sensor

A Runway Occupancy Sensor is a sophisticated detection device that continuously monitors a designated runway zone to determine whether the runway is occupied by an aircraft or vehicle. It is a core component of the Runway Status Light (RWSL) system, providing the real‑time occupancy data required to activate runway entrance lights (RELs), take‑off hold lights (THLs), and runway guard lights. The sensor uses advanced technologies—typically radar, infrared, or inductive loop—to detect targets with high accuracy and reliability, even in adverse weather conditions (fog, rain, snow) and low visibility. Unlike stop bar sensors that detect crossing at a specific point, runway occupancy sensors monitor an entire runway segment (typically several hundred metres) to determine if any target is present on the runway. The sensor's output is fed to the RWSL processing unit, which uses it to determine runway status and control the associated warning lights. By providing precise, continuous occupancy detection, these sensors play a vital role in preventing runway incursions, reducing the risk of runway collisions, and supporting safe, efficient airfield operations.

The runway occupancy sensor's purpose is to give the RWSL system a source of truth about the runway that does not depend on a controller noticing and acting on a specific traffic situation. Because RWSL is designed as an automatic layer that operates alongside, not instead of, standard ATC clearances, the sensor has to detect occupancy continuously across the whole monitored runway segment rather than at a single point, so that any aircraft or vehicle present anywhere in the zone is reflected in the light state without a gap in coverage. This continuous, always-on monitoring is what allows REL and THL lights to illuminate automatically the moment a runway becomes occupied, giving pilots and vehicle operators an independent visual cue that functions even if a clearance was misunderstood or a controller's attention was elsewhere.

Runway Occupancy Sensors are deployed at airports requiring enhanced runway safety surveillance, particularly:

• RWSL (Runway Status Light) Systems: As the primary detection input for runway entrance lights (RELs), take‑off hold lights (THLs), and runway guard lights.
• A‑SMGCS (Advanced Surface Movement Guidance and Control Systems): Integrating with A‑SMGCS for comprehensive surface surveillance and conflict detection.
• ATC Towers: Providing controllers with automated runway occupancy status, enhancing situational awareness.
• Airport Operations Centers: Monitoring runway usage for operational planning and resource allocation.
• Low Visibility Operations (CAT II/III): Enabling safe runway occupancy management when RVR is below 400m.
• Busy Hub Airports: Managing high‑density traffic with multiple simultaneous runway movements to prevent conflicts.
• Runway Crossing Points: Monitoring taxiway/runway intersections to prevent incursions.
• Intersecting Runways: Detecting occupancy on intersecting runways to prevent cross‑runway conflicts.

Radar- and video-based sensors are pole-mounted beside the runway or at intersection entrances, sited and angled to give the required detection zone coverage over the runway surface or the crossing point being monitored, with spacing between poles set by each sensor's field-of-view range. In-pavement variants use inductive loops or magnetometers embedded beneath the surface at the detection points, wired back to a roadside pull box in the same way as other in-ground sensors. Power and data cabling from each sensor or loop is run in duct to a local equipment hut, from which a fibre or copper backbone carries occupancy status to the RWSL logic controller, A-SMGCS processor, or tower display, with sensor siting kept clear of the runway strip's frangibility and obstacle-clearance requirements.

The Runway Occupancy Sensor uses a combination of detection technologies to provide continuous, reliable runway occupancy status. Key functional aspects include:

• Detection Technologies: Sensors typically use one or a combination of:
  • FMCW Radar (24 GHz or 77 GHz): Detects moving and stationary targets across a wide area, providing range, velocity, and position information, immune to weather and lighting conditions.
  • Infrared (thermal or active): Uses thermal signatures or reflected IR to detect targets, effective in darkness, fog, and smoke.
  • Inductive Loop Arrays: A series of buried wire loops that detect metallic targets passing over them, providing precise position and direction information.
  • Fusion Sensors: Combine multiple technologies (e.g., radar + IR) for improved detection reliability and false‑alarm rejection.
• Coverage Zone: The sensor is positioned to cover a specific runway segment, typically 300‑900 metres or the entire runway length (using multiple sensors). The coverage zone is precisely calibrated to the runway geometry.
• Target Tracking: The sensor processor tracks multiple targets simultaneously, maintaining their position, speed, and heading within the coverage zone.
• Occupancy Determination: The system determines runway occupancy status based on the presence of any tracked target within the runway zone, with configurable dwell times and hysteresis to prevent false triggers from transient targets.
• Output Signal: The sensor provides a clean, isolated output—typically a dry contact, transistor output, or digital message (e.g., via Ethernet/Modbus)—that indicates occupied or clear status. The output is connected to the RWSL logic processor.
• Self‑Diagnostics: The sensor continuously monitors its own health (power, communication, signal quality, internal faults) and provides diagnostic outputs to alert maintenance staff of any degradation or failure.
• Integration: The sensor interfaces with the RWSL processor, A‑SMGCS, and ATC display systems via standard protocols (Ethernet, serial, hardwired).

The sensor is typically mounted on a pole adjacent to the runway, on a dedicated mast, or integrated into existing runway infrastructure (e.g., approach lighting masts). Its position and orientation are carefully engineered to provide optimal coverage while minimising false alarms from adjacent taxiways or apron areas.

The occupancy sensor's control logic sits between the sensing element and the airfield lighting/ATC systems it feeds. The sensor (radar, magnetic loop, or video analytics depending on technology) continuously reports a raw occupied/clear state to a local processing unit, which applies a confirmation and debounce logic — requiring the state to hold for a minimum dwell time — to filter out false triggers before declaring a validated occupancy change. The validated state is passed to the ALCMS, which can automatically illuminate runway status lights, or to ATC's surface surveillance display; the sensor also reports its own health continuously, and a dropout or self-test failure is flagged as a distinct sensor fault so controllers don't mistake an offline sensor for a confirmed-clear runway.

The sensor head is housed in a sealed, weatherproof enclosure — typically die-cast aluminium or glass-reinforced polymer — rated IP65/IP67, with a radome or IR-transparent window covering the antenna or optical aperture to protect it from rain, dust, and de-icing overspray while remaining transparent to the sensing wavelength. Pole-mounted units bolt to an adjustable bracket that allows azimuth and elevation aiming to be set and locked during commissioning, with the bracket itself mounted on a frangible or shear-pin base where the pole sits within an obstacle-limitation surface. Internal electronics are conformally coated and built around a sealed connector interface (typically M12 circular connectors) for power and signal cabling, avoiding open terminal strips that could admit moisture. Built-in transient voltage suppression components are mounted close to the cable entry point to absorb lightning-induced surges before they reach the sensing and processing circuitry, and the enclosure includes a grounding lug for bonding to the site's lightning protection system.