Stop Bar Sensor

A Stop Bar Sensor is a compact, high‑reliability detection device installed at runway holding positions (stop bars) to detect the presence of aircraft or vehicles that are approaching, crossing, or occupying the stop bar area. The sensor is a key component of the Runway Status Light (RWSL) system, providing the real‑time occupancy data needed to activate runway entrance lights (RELs), take‑off hold lights (THLs), and runway guard lights. Stop Bar Sensors typically use radar, infrared, or inductive loop technology to detect metallic targets with high accuracy and reliability, even in adverse weather conditions. They are designed for harsh airfield environments, with rugged housings, wide temperature tolerance, and lightning protection. The sensor's output (typically a dry contact or digital signal) is fed to the RWSL processing unit, which uses it to determine runway status and control the associated warning lights. By providing precise occupancy detection, Stop Bar Sensors help prevent runway incursions and support safe, efficient ground operations.

Where a runway occupancy sensor watches an entire runway segment, the stop bar sensor's purpose is narrower and more precise: to detect the exact moment an aircraft or vehicle crosses a specific holding position, the one line on the airfield where a runway incursion actually begins. That point-detection role is what makes it suitable for triggering RELs and THLs at the specific location where entry onto the runway occurs, rather than reporting on runway status more broadly. Its rugged construction and lightning protection reflect the fact that it sits at one of the highest-consequence locations on the airfield, where a missed or false detection has a direct bearing on runway incursion risk rather than being a general monitoring inconvenience.

Stop Bar Sensors are deployed at all runway holding positions where stop bars are installed, and in other critical areas where occupancy detection is required:

• Runway Holding Positions: At taxiway/runway intersections, sensors detect aircraft waiting to enter the runway, triggering RELs and warning lights.
• Runway Entrance Lights (REL) Systems: Sensors provide the detection input that activates the RELs when an aircraft crosses the stop bar.
• Take‑Off Hold Lights (THL) Systems: Sensors detect aircraft at the take‑off position on the runway, activating the THLs to warn other traffic.
• Runway Guard Light Systems: Sensors trigger flashing amber warning lights when a vehicle or aircraft approaches a runway holding position.
• RWSL (Runway Status Light) Systems: As a primary input sensor, providing accurate occupancy data to the RWSL logic processor.
• A‑SMGCS (Advanced Surface Movement Guidance and Control Systems): Integrating with A‑SMGCS for enhanced surface surveillance and conflict detection.
• Apron and Gate Areas: Detecting aircraft or vehicles at gate positions for ground handling coordination.
• Vehicle Access Points: Monitoring vehicle crossings of taxiways or runways to ensure safety.

Sensors are set into or beside the pavement at the runway holding position, either as an inductive loop cut into a saw-cut slot in the taxiway surface or as a surface-mounted microwave/infrared head on a short frangible post clear of wingtip clearance. Loop sensors are sealed into their slot with a bitumen or epoxy compound and home-run in duct back to the local field cabinet, while surface-mounted heads are aligned so their detection zone covers the full stop bar width without triggering on adjacent lanes. Detector output cabling is kept on its own screened pair back to the REL/THL controller or Remote I/O rather than sharing conduit with lighting series-circuit cable, since a nuisance trip on this input directly drives a runway safety light.

The Stop Bar Sensor uses a combination of detection technologies to provide reliable target presence indication. Key functional aspects include:

• Detection Technologies: Sensors typically use one or a combination of:
  • Radar (FMCW or pulsed): Detects moving targets and provides range and velocity information, immune to weather conditions.
  • Infrared (active or passive): Uses thermal or reflected IR to detect targets, effective in darkness and fog.
  • Inductive Loop: Buried wire loops detect changes in inductance when a metallic target passes over; simple and reliable.
• Target Classification: Advanced sensors can differentiate between aircraft and vehicles, or ignore small objects (birds, debris) to reduce false alarms.
• Coverage Zone: The sensor is positioned to cover the stop bar area, typically a zone 5‑10 metres wide and 1‑3 metres deep, ensuring that any aircraft or vehicle crossing the stop bar is detected.
• Output Signal: The sensor provides a clean, isolated output—typically a dry contact (relay) or transistor output—that changes state when a target is detected. The output is connected to the RWSL logic processor or PLC.
• Self‑Diagnostics: The sensor monitors its own health (e.g., power supply, signal quality, internal faults) and provides a diagnostic output (or fault indicator) to alert maintenance staff.
• Environmental Protection: The sensor is housed in a rugged, IP65/67 enclosure, with weatherproof connectors and surge protection for lightning‑prone airfield environments.

The sensor is typically mounted on a small pole (0.5‑1.5 m high) adjacent to the stop bar, or embedded in the pavement (inductive loops). Its position and orientation are carefully calibrated to ensure it only detects targets that are actually within the stop bar zone, avoiding false triggers from adjacent taxiways or apron areas.

The Stop Bar Sensor is a detection input at the edge of the control system rather than a controller itself — it reports a target's presence to the RWSL logic or ALCMS server, typically via a Remote I/O unit or directly wired into the local PLC, and takes no action on the stop bar or runway status lights on its own. That upstream logic combines the sensor's detection with the current runway/taxiway occupancy state and traffic clearances to decide whether the stop bar should be illuminated, and issues the corresponding command back down to the CCR controlling that circuit. Because the sensor's output is only ever an input to that decision, a sensor fault is treated as a loss of detection at that specific location — the system typically fails toward the conservative state (stop bar lit) rather than assuming clear, and the fault is reported to the ALCMS as a sensor-level alarm distinct from a lighting-circuit fault.

The sensor is built into a compact, sealed housing — die-cast aluminium or glass-reinforced polymer — rated IP65/IP67, small enough to mount on a short pole beside the stop bar or flush into the pavement where inductive loop technology is used instead. A radome or optical window covers the radar or IR aperture, transparent to the sensing wavelength while shedding rain, dust, and de-icing fluid, and pole-mounted units include an adjustable bracket for setting detection angle during commissioning. Internal electronics are conformally coated and connected through sealed M12 or similar circular connectors rather than open terminals, keeping moisture out of the detection and output circuitry. Built-in transient voltage suppression is mounted near the cable entry to protect against lightning-induced surges, and the compact housing dimensions keep the unit unobtrusive at the holding position while still meeting the IP and temperature ratings needed for continuous outdoor exposure.