The pilots' airborne collision avoidance system is a transponder-based system that communicates with similar devices on aircraft in the vicinity. In the event of a potential conflict, pilots in each aircraft are presented with an audio and visual "resolution advisory" that tells one aircraft to climb and the other to descend by a prescribed amount. The resolution advisory normally occurs between 25s and 1min before the potential collision.
The controllers' short-term conflict alert system causes the on-screen data-blocks of any two conflicting aircraft to flash. The controller normally gets an STCA alert well before the pilots would receive a resolution advisory, but certain types of conflict can cause the airborne and ground-based systems to issue alerts almost simultaneously, which could cause the controller to issue an instruction to a pilot already trying to deal with a resolution advisory.
Source: Flight International
