Is ADS-B replacing traditional ATC radar?

ADS-B (Automatic Dependent Surveillance-Broadcast) supplements but does not replace radar. ADS-B Out, mandatory in most controlled airspace since 2020, broadcasts GPS-derived position on 1090 MHz. It provides better position accuracy (GPS vs radar) and lower infrastructure cost, but it is cooperative only: an aircraft with a failed or disabled transponder is invisible. PSR remains essential for detecting non-cooperative targets, including unauthorized drones and general aviation aircraft with transponder failures.

Radar & Defense

Air Traffic Control Radar

Q: What is the difference between PSR and SSR?

PSR (Primary Surveillance Radar) is a traditional skin-paint radar operating at S-band (2.7-2.9 GHz) or L-band. It detects aircraft by their radar cross-section reflection, requiring no cooperation from the target. SSR (Secondary Surveillance Radar) operates at 1030 MHz (interrogation) and 1090 MHz (reply), querying the aircraft's transponder for identification (squawk code), pressure altitude, and in Mode S, a unique 24-bit ICAO address. PSR provides position only; SSR provides identity and altitude.

Q: How far can ATC radar see?

En-route (ARSR-4) primary radar reaches approximately 250 nmi. Terminal area (ASR-11) primary radar covers 60 nmi, with SSR extending to 120 nmi. Airport surface detection equipment (ASDE-X) covers the runway and taxiway environment out to approximately 3 nmi using X-band radar. Range is ultimately limited by line-of-sight to the radar horizon and target RCS.

/air traf-ik kon-trohl ray-dar/

Air Traffic Control (ATC) Radar is a dual-system surveillance architecture combining Primary Surveillance Radar (PSR) for non-cooperative skin-paint detection with Secondary Surveillance Radar (SSR) for cooperative transponder-based identification. PSR operates at S-band (2.7-2.9 GHz), detecting aircraft by their radar cross-section reflection regardless of transponder status. SSR interrogates at 1030 MHz and receives replies at 1090 MHz, obtaining the aircraft's squawk code, pressure altitude (Mode C), and unique ICAO address (Mode S). Modern ATC facilities fuse PSR, SSR, and ADS-B data into a single composite track picture for controllers.

Category: Radar & Defense

PSR Band: S-Band (2.7-2.9 GHz)

SSR Frequencies: 1030/1090 MHz

Understanding ATC Radar

ATC radar operates in two tiers. Terminal Area Surveillance (ASR-9, ASR-11) covers the approach and departure corridors within approximately 60 nmi of the airport. En-route surveillance (ARSR-4, FPS-117) provides long-range coverage of airway sectors out to 250 nmi. Both tiers co-locate PSR and SSR antennas on the same rotating pedestal, typically at 12-15 RPM for terminal and 5-6 RPM for en-route systems.

The PSR component uses pulse-Doppler processing with MTI to suppress ground clutter from terrain and buildings near the airport. Weather channel processing extracts precipitation intensity data (displayed as color-coded regions on the controller's scope). The SSR component uses monopulse angle estimation on the reply signal for accurate azimuth determination, achieving approximately 0.06-degree accuracy.

ATC Radar Coverage Geometry

Radar horizon (line of sight):
R_horizon = 1.23 × (√h_radar + √h_aircraft) [nmi]

Example: ASR-11 at 30 ft, aircraft at 10,000 ft:
R_horizon = 1.23 × (√30 + √10000) = 1.23 × (5.5 + 100) = 130 nmi

SSR link budget advantage:
SSR range ∝ R² (one-way to transponder + one-way back)
PSR range ∝ R⁴ (round-trip skin paint)
SSR always exceeds PSR range for the same power budget

ATC Radar Systems Comparison

System	Type	Band	Range	Rotation
ASR-11	Terminal PSR+SSR	S-Band	60 nmi (PSR)	12.5 RPM
ARSR-4	En-route PSR	L-Band	250 nmi	5 RPM
Mode S SSR	Secondary only	1030/1090 MHz	120+ nmi	Co-rotates
ASDE-X	Surface detection	X-Band	~3 nmi	Continuous

Common Questions

Frequently Asked Questions

What is the difference between PSR and SSR?

PSR detects aircraft by skin-paint reflection at S-band, requiring no cooperation. SSR queries the transponder at 1030/1090 MHz for squawk code, altitude (Mode C), and ICAO address (Mode S). PSR provides position only; SSR adds identity and altitude.

Is ADS-B replacing ATC radar?

ADS-B supplements but does not replace radar. It broadcasts GPS-derived position on 1090 MHz with better accuracy and lower cost, but is cooperative only. PSR remains essential for detecting non-cooperative targets including unauthorized drones and transponder-failed aircraft.

How far can ATC radar see?

En-route ARSR-4 reaches ~250 nmi. Terminal ASR-11 covers 60 nmi PSR / 120 nmi SSR. Surface ASDE-X covers ~3 nmi. Range is limited by line-of-sight to radar horizon and target RCS.

Related Terms

← Air Surveillance Radar Airborne Early Warning →