Secondary Surveillance Radar (SSR) Mode Select (Mode S) is a civil aviation initiative to overcome deficiencies associated with SSR Mode A (Military IFF Mode 3) and Mode C, currently in use by Air Traffic Services (ATS) within European States.

SSR Mode S is a co-operative radar surveillance system that employs ground-based interrogators and airborne transponders.  Mode S has been designed as an evolutionary improvement to the existing SSR system operating in Modes 3/A and C.  It will provide the necessary improved surveillance capability required to overcome the limitations of existing SSR and meet future traffic demands.  Ground and airborne Mode S installations are backwards compatible with current 3/A and C ground and airborne installations.

Mode S surveillance affords improved position determination of SSR targets and improved functionality while reducing the number of required replies by transponders.  This reduction of SSR replies is extremely beneficial to the Radio Frequency (RF) environment. This means that the full safety and effective airspace management benefits can be realized in a sustainable environment.

The successful introduction of Mode S will enable more efficient airspace utilization and it will improve safety while coping with increased traffic growth.  The implementation of Mode S will be a phased enhancement to existing radar based surveillance services in Europe over a period of time. 

Within the UK FIR, Mode S Enhanced (EHS) has been mandatory for all aircraft flying IFR as GAT within notified Mode S airspace since 31 Mar 05 (with a 2-year transition period until 31 March 2007), as per AIC 49/2005 (YLO 171).   It is now proposed to mandate carriage of a Mode S Elementary (ELS) transponder within remaining UK airspace by 31 Mar 2008.  It is likely that there will be a 2-year transition period until 31 March 2010, to allow for any implementation difficulties. 

Within other European Mode S implementing states, the target for Mode S (EHS and ELS) implementation is 31 March 2008, with no exemption period.

If you have any queries regarding Mode S equipage, please do not hesitate to contact us at mode.s@dap.caa.co.uk