ELEMENTARY SURVEILLANCE: As of March 2003 it is MANDATORY to have Mode S onboard equipments, in accordance with the AICs legislation. Nevertheless, to give the operators enough time to adapt to the requirements, a transition period has been established:
New production aircraft to be compliant by March 31st 2004.
For aircraft flying IFR/ GAT; must be compliant by March 31st 2005.
For aircraft flying VFR; must be compliant by March 31st 2008 .
ENHANCED SURVEILLANCE: As of March, 2005 it is MANDATORY to have Mode S onboard equipments capable of including further aircraft parameters known as Downlink Airborne Parameters (DAPs). France, Germany and the United Kingdom announced their common decision to implement Mode S Enhanced Surveillance as MANDATORY in their respective airspace as of March 2005, allowing a transition period until March 2007.
ICAO Annex 10 Volumes III and IV Amendment 77
ICAO Regional Supplementary Procedures, DOC 7030/4
JAA TGL nº 13 / JAA NPA 20-12ª
More information on Mode S in the following link: http://www.eurocontrol.int/mode_s/Documentation/Doc.htm