Issued in May 2011. This document supersedes ED-73C published in 2008. It provides standards for Mode S airborne equipment (Mode S). Incorporated within these standards are system characteristics that will be useful to users of the system as well as designers, manufacturers and installers. The document considers an equipment configuration consisting of: transponder, control panel, antenna and interconnecting cables. Level 1 through Level 5 transponders are addressed, test procedures are provided as well as information for optional features. It reflects issues identified and discussed by the ICAO Aeronautical Surveillance Panel Technical Subgroup and reported by Civil Aviation Authorities since the publication of ED-73C. The document is a coordinated effort with RTCA.