What is an Enhanced surveillance system?
Enhanced Surveillance (EHS) is a variant of Mode S transponder protocol that includes Downlink Aircraft Parameters (DAP) in the information returned by the airborne Mode S transponder.

Mode S can be configured as Mode S Elementary Surveillance (ELS) or Enhanced Surveillance. EHS is the concept behind ELS, ELS being a limited implementation. ELS is deemed analog avionics aircraft oriented while aircraft with digital avionics should use EHS.

While ELS allows for selective interrogation of a Mode S transponder to obtain barometric altitude, EHS allows also to request DAP. These data are intended to be used by ATC to face the rapid traffic increase observed in Europe. With no surprise this initiative is driven by Eurocontrol.

The use of DAP is expected to limit voice communication between ATC and crews. DAP includes:


Source: Eurocontrol

In general, ATC automation tends to get all parameters providing information on current position, but also to anticipate short-term position. This is why roll angle or vertical rate are part of the DAP.

Mode S EHS takes benefit of Extended Squitter (1090 ES).

Related:


Transponder
Transponder interrogation modes
Mode S
ELS and EHS