The Mode S Elementary Surveillance (ELS) feature includes an eight-character aircraft identification parameter corresponding to that used in voice communications with air traffic services. This aircraft identification parameter, or Flight ID, is entered by a control device such as a radio tuning unit (RTU), control display unit (CDU) or dedicated transponder control unit (CTL). Enhanced surveillance (EHS) builds upon the concept of Elementary Surveillance and consists of the extraction of further aircraft parameters known as Downlink Airborne Parameters (DAPs). This facilitates an increase in the safety and efficiency of Air Traffic Management operations.
As a part of the long-range plan to improve airspace capacity, safety and efficiency, European regulatory authorities have mandated the carriage and operation of Mode S transponders with Elementary and Enhanced Surveillance functionality.
In support of these new requirements, the Rockwell Collins TDR-94D Mode S Transponder, and RTU-870 are available with the Mode S Elementary and Enhanced Surveillance features.