Description / Abstract: This document supersedes DO-181D published in 2008. It provides standards for Mode S airborne equipment (Mode S implies a combined ATCRBS/Mode S capability). Incorporated within these standards are system characteristics that will be helpful 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 and 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 DO-181D in October 2008. The document is a coordinated effort with EUROCAE WG-49 and includes changes resulting from DO-260B, ADS-B MOPS, published in December 2009. Appendix E details the Changes from DO-181D to DO-181E.