We also have a proven and operational ADS-B system that provides coverage to close to the surface in the following locations (Source:
High Altitude ADS-B Coverage):
High Altitude ADS-B Coverage
When operating outside of radar coverage, ADS-B derived ATS surveillance services will be provided to operators of authorised aircraft whilst within the coverage volume of commissioned ADS-B ground stations.
ADS-B ground stations are line-of-sight facilities. The ability for a ground station to received ADS-B data from an aircraft depends on altitude, distance from the site and obstructing terrain.
Coverage will exist near the surface within 20 nautical miles of the ground station. High level coverage can exceed 250 nautical miles.
Note: In airspace where ADS-B coverage overlaps radar coverage, the radar derived aircraft position will be displayed to ATC.
The charts below shows the approximate ADS-B coverage provided by currently operating ground stations at FL300.
ADS-B Coverage effective 20 December, 2007
So if you have ADS-B fitted and operational at any of the above places ATC can see you.
Why did they install it rather than SSR? - $$$$, and it works.