I believe that the current plan is for the UK to skip introducing Mode S Elementary Surveillance in 2003, but to progress straight to Enhanced Mode S.
Terminal Control is the first likely candidate for Enhanced Mode S, with a date of 2005 being discussed. Enhanced Mode S will enable a host of Aircraft Derived Data to be downlinked to the ATC system. Only selected items will be displayed to the controller - some items could be used by existing ATC systems, such as STCA, and be transparent to the controller.
One significant Enhanced parameter is Selected Altitude. This will enable controllers to see what altitude the flight crew have inserted in the Selected Altitude window on the Autoflight system, and thus give controllers an early warning of a potential level bust. This feature is seen as being a technical solution to the rising number of Level Busts that are occurring within the UK, despite several awareness campaigns. <img src="frown.gif" border="0">