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.
Source: Wikipedia — Obstacle Collision Avoidance System (CC BY-SA 4.0)