Mode S transponders are the International Civil Aviation Organisation (ICAO) approved replacement for Mode A and Mode C transponders which use outdated technology. On 31 May 2008 the Civil Aviation Authority (CAA) concluded their consultation on phase 2 of their proposals to extend the use of Mode S transponders. This is part of the CAA’s phased approach to improving the technical interoperability of aircraft in UK airspace. They will consider carefully all the responses they have received before making a decision.
Phase 1, which came into force on 31 March 2008, required the use of Mode S for existing transponders. All aircraft that were required to carry Secondary Surveillance Radar (SSR) Mode A and C transponders had to be upgraded to Mode S capability. A transition period of four years will be provided during which time upgrades of existing transponders can be completed. All new installations of SSR equipment and all new aircraft brought into service with an SSR transponder already installed will now need to be Mode S compliant.