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ACAS is being used to describe short-range systems intended to prevent actual metal-on-metal collisions. In contrast, ASAS is being used to describe longer-range systems used to maintain standard en route separation between aircraft (5 nm {9.25 km} horizontal /1000' {305 m} vertical). Obstacle Collision Avoidance System is a ground based system that uses a low powered radar mounted on or near the obstacle. 2  The radar detects aircraft in the proximity of the obstacle and firstly warns aircraft via flashing medium intensity lights and secondly warns aircraft of the obstacle via a VHF broadcast.
For other uses of the acronym WAAS, see WAAS (disambiguation). The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability.