What Is ATC Radar?

ATC radar (air traffic control radar) is a class of radar systems used to detect, track, and identify aircraft within a defined volume of airspace to support the safe separation and sequencing of air traffic. ATC radar systems provide air traffic controllers with a continuous picture of aircraft positions that enables them to issue clearances, resolve conflicts, and coordinate handoffs between control facilities. The technology encompasses both primary radar, which detects aircraft by reflecting radio waves from their surfaces without any cooperation from the aircraft, and secondary surveillance radar, which interrogates aircraft transponders to obtain identification codes and altitude information. ATC radar draws on radio frequency engineering, signal processing, antenna design, and air traffic management systems.

The FAA in the United States and equivalent civil aviation authorities worldwide operate networks of ATC radar installations covering terminal areas around airports and en-route airspace along airways. International standards for ATC radar performance and interoperability are maintained by the International Civil Aviation Organization (ICAO) and the ITU.

Primary and Secondary Surveillance Radar

Primary surveillance radar (PSR) transmits a pulsed radio signal from a rotating directional antenna and detects aircraft by the energy reflected from their fuselage and wings. The antenna's rotation rate determines the radar's update interval, typically on the order of 4 to 12 seconds for long-range en-route systems and faster for short-range terminal area radars. PSR provides bearing and slant range to a target but not altitude, and returns appear for any sufficiently large reflective object, including weather and terrain. The FAA Aeronautical Information Manual section on surveillance systems describes how PSR and secondary radar are integrated in operational ATC facilities.

Secondary surveillance radar (SSR), also called the Air Traffic Control Radar Beacon System (ATCRBS) in the United States, interrogates aircraft transponders on 1030 MHz and receives replies on 1090 MHz. Mode A replies carry a 4096-code squawk identifier assigned by the controller; Mode C replies add pressure altitude from the aircraft's encoding altimeter. Mode S, the current standard, supports selective interrogation of individual aircraft by unique 24-bit ICAO address and carries extended data including aircraft identity, altitude, and downlinked aircraft parameters. The integrated Airport Surveillance Radar (ASR-11) combines a primary and Mode S secondary radar in a single installation; the FAA has deployed the ASR-11 at terminal radar facilities across the United States.

Ground Mobile Radar Systems

Ground mobile radar systems adapt ATC and surveillance radar technology for deployment by military and civil authorities in locations without fixed infrastructure. These systems are mounted on vehicle trailers or aircraft-transportable platforms and include self-contained power generation, communications, and data processing. Military tactical ATC radars must meet operational requirements for rapid setup time, transportability, and performance in contested electromagnetic environments. Civilian deployable radars serve disaster response, temporary flight restrictions, and remote airfield operations. Ground mobile radar systems span the same primary and secondary radar functions as fixed installations but must achieve them with constrained size, weight, and power budgets. They are evaluated against performance specifications including instrumented range, minimum detectable range, azimuth accuracy, and false alarm rate under the same ICAO and national authority standards that govern fixed systems.

Modern ATC Surveillance Technologies

ATC radar is supplemented by Automatic Dependent Surveillance-Broadcast (ADS-B), in which aircraft derive their own position from GPS and broadcast it on 1090 MHz. ADS-B does not require ground radar infrastructure, provides more frequent position updates (nominally once per second), and includes velocity vectors and intent data. Wide Area Multilateration (WAM) locates aircraft by measuring the time difference of arrival of their 1090 MHz transmissions at multiple ground stations. Both technologies are approved ATC surveillance sources. The FAA surveillance source guidance specifies the conditions under which PSR, SSR, ADS-B, and WAM may each be used as the primary separation source for controlled traffic.

Applications

ATC radar has applications in a range of aviation and defense contexts, including:

• Terminal radar control at commercial airports for approach and departure sequencing
• En-route air traffic control along high-altitude airways
• Military airspace management and tactical air defense
• Search-and-rescue coordination using radar returns for position fixing
• Ground-based deployment at remote or temporary airfields