SSR Mode S is a co-operative radar surveillance system which uses ground-based interrogators and airborne transponders. Mode S has been designed as an improvement to the existing SSR system. It will provide the necessary improved surveillance capability required to overcome the limitations of existing SSR and meet future traffic demands.

Elementary surveillance (ELS) is a concept driven largely in part by EuroControl, that takes basic surveillance a step further in adding the features of comm B communications for datalink and identification of the airframe. Elementary surveillance is made up of several new components. These are 25-foot resolution altitude decode, interrogator identification (II) and surveillance identification (SI) verification, flight status, data link capability report, GICB report, aircraft identification, and ACAS active resolution advisor report.

Enhanced surveillance (EHS) is the concept of elementary surveillance but with additional aircraft intent reporting fields. These fields are selected vertical intent, track and turn report and heading and speed report. The selected vertical intent report is used to indicate such items as the barometric altitude of the airframe, the selected altitude of the mode control panel or flight control unit (MCP/FCU) and the flight management system (FMS). The target altitude of the aircraft also reports what altitude the aircraft is intending to use. The track and turn report will indicate the roll angle, true track angle and rate, ground speed and true air speed of the aircraft. The heading and speed report will reflect the indicated air speed and mach, the barometric altitude rate, the magnetic heading and inertial vertical velocity.