Obstacle Collision Avoidance System
The Obstacle Collision Avoidance System (OCAS) is designed to alert pilots if their aircraft is in immediate danger of flying into an obstacle. OCAS uses a low power ground-based radar to provide detection and tracking of an aircraft's proximity to an obstacle such as high buildings, power line crossings, telecom towers or wind turbines. This capability allows the visual warning lights to remain passive until an aircraft is detected and known to be tracking on an unsafe heading. This leaves the nighttime sky free of unnecessary light pollution thus decreasing public annoyance issues while  improving the environmental habitat. OCAS is currently the only FAA approved Audio Visual Warning System (AVWS) approved in the National Airspace. OCAS is also the only AVWS approved by the International Dark Sky Association. OCAS is operational in the US, Canada, Norway and Germany. Background. A terrain awareness and warning system is crucial in air transportation. The costliest plane collisionthe 1977 collision of two Boeing 747sdid not happen in the air but on the runway at Los Rodeos Airport on Tenerife in the Canary Islands. Since then, accidents often occur on or near the ground involving planes and obstacles. OCAS  has the capability to address the problem through its radar technology that detects obstacles so that an aircraft could be sent away to avoid collision. The system is related to other collision avoidance systems that work through a probability collision calculation, which involves "the relative trajectory between objects and the combined relative position-error probability density." The current OCAS technology still relies on the pilot for overall safety. OCAS does not cover other critical factors such as weather, traffic not online, and air traffic control instructions. Overview. The OCAS system is designed to protect obstacles that are vulnerable to low  flying aircraft. OCAS detects the ground speed, heading and altitude of approaching aircraft and determines whether it will adequately clear the obstacle. A configurable set of rules is applied to define and determine when to warn the aircraft, by which warning device and signal depending on the calculated time to impact with the obstacle. Initial WarningMedium Intensity Lighting: Once the initial threshold is met, the visual warning is activated. This capability allows the lighting system to remain passive (in the off position) the vast majority of the time thus preserving the dark sky environment while adhering to strict safety standards  set out by the FAA. Secondary WarningAudio Broadcast: If the initial warning does not result in the pilot altering the flight path, a programmable VHF radio broadcasts an additional obstruction warning directly to the cockpit. The VHF warning frequencies are adjusted based on local requirements.