Mark 4 Air Traffic Control Transponder (ATCRBS/MODE S) describes an Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) airborne transponder with Extended Interface Functions (EIF). The ATC surveillance system is made up of airborne transponders and ground interrogator-receivers, processing equipment, and antenna systems. Mode S is a cooperative surveillance system for air traffic control with ancillary communications capabilities. ARINC 718A supports elementary surveillance. Provisional enhanced surveillance functionality is also defined as a customer option. The Mark 4 transponder, like its predecessor, will support Collision Avoidance System which includes TCAS and ACAS X functions.