What is the primary function of a primary surveillance radar (PSR) in ATC?

• A. It detects and locates aircraft by transmitting pulses and receiving reflections to provide range and bearing information without relying on transponders.
• B. It interrogates aircraft transponders to obtain identity data and altitude data.
• C. It measures weather patterns using Doppler shifts to detect wind.
• D. It provides altitude data by querying barometric sensors.

Answer: (A)

The primary function of a primary surveillance radar in ATC is to detect and locate aircraft without relying on any transponder. It does this by emitting radio pulses and listening for echoes from aircraft. By measuring the time it takes for the pulse to travel to the aircraft and back, the system determines range, and by scanning across the sky it provides bearing information. This means the radar can track aircraft even if they are not transmitting or replying with transponder data.

The other options describe roles that involve transponder interrogation for identity and altitude (which is what secondary surveillance radar handles), or use radar capabilities for weather measurements (Doppler wind detection), or obtaining altitude from non-radar sensors. Those functions are not what a primary surveillance radar provides, which is the pure radar-based detection and position without transponder dependence.