Definition
A type of FAA en route surveillance radar that uses broadband secondary radar technology to track aircraft over long distances by interrogating their transponders. Unlike primary radar (which bounces a signal off the airframe itself), secondary radar relies on a transponder reply from the aircraft to provide identification, altitude, and position data to en route air traffic controllers.
Plain English
A long-range air traffic control radar used between airports that gets its information by talking to the aircraft's transponder rather than by bouncing radio waves off the aircraft.
Context Anchor
Seen in FAA abbreviation lists and may appear in notices or facility-status messages about en route radar equipment.
Derivation
"En route" means the cruise portion of flight between departure and arrival airports. "Secondary" radar is named to distinguish it from "primary" radar — secondary radar requires a cooperative reply from the aircraft, while primary radar works on its own by reflection. "Broadband" refers to the wider range of radio frequencies the system uses, which improves data quality and capacity.
Why Pilots Care
Secondary radar is the reason your transponder must be on and working in controlled airspace — without it, en route controllers cannot see your altitude or identification clearly. If your transponder fails, ATC's picture of you degrades significantly.
Intuition Check
“Secondary” does not mean backup or less important here. It means the radar gets information from the aircraft’s transponder reply, rather than only from a signal reflected off the aircraft. “Broadband” here does not mean internet service for passengers; it describes the radar equipment’s signal-handling design.
Example Sentence 1
ESEC sites across the en route system provide controllers with transponder-based tracking of aircraft in cruise flight.
Example Sentence 2
Technicians verified ESEC coverage before opening the new airway segment.