PSR

Primary surveillance radar is a ground-based system that detects aircraft by transmitting electromagnetic pulses and measuring the reflections that bounce back from metal fuselages. Unlike secondary radar, which interrogates a transponder aboard the aircraft and relies on that device to reply with identification data, a PSR works passively on any object large enough to return a usable echo. This makes it the foundation of air traffic control and the only radar that can detect aircraft with failed or disabled transponders.

The PSR antenna is typically a large rotating parabolic dish or planar array mounted on a tower or building. It sweeps the sky in a complete circle, usually completing one rotation every 4 to 10 seconds depending on range requirements and antenna design. The radar transmits at frequencies between 1200 and 1400 megahertz. Each pulse travels outward; when it strikes an aircraft, a fraction of that energy scatters back toward the antenna. The receiver measures the round-trip delay to calculate distance, and the antenna's rotational position gives bearing. Modern PSR systems use digital signal processing to filter clutter from weather, terrain, and sea surfaces, which can otherwise blind the radar.

Range and coverage depend on transmitter power, antenna gain, and receiver sensitivity. A typical en-route PSR covers 200 to 250 nautical miles at altitude, though performance degrades as target altitude drops. The radar struggles with small aircraft, helicopters, and very large or very distant targets where the return signal is weak. Terrain masking, sea clutter, and precipitation can create blind zones or false returns. Range resolution, the ability to separate two targets at different distances, is typically 300 to 500 meters.

PSR is always used alongside secondary surveillance radar (SSR) in the same air traffic control facility. Modern control towers display both PSR and SSR data on the same scope, so controllers see a fused picture of position and identification. When a transponder fails or is switched off, the PSR blip remains visible even though the label and altitude data from the transponder disappear. This redundancy is critical to safety.

The term 'primary' reflects the historical development of radar in air traffic control. PSR was the first type deployed for aircraft detection. Secondary radar came later as an add-on, relying on the aircraft's cooperation through its transponder. Today both systems coexist; neither is truly primary, but the name has stuck. PSR hardware is aging in many countries, and upgrades focus on solid-state transmitters, digital beamforming, and improved clutter rejection rather than the old rotating dishes.