How does Primary Surveillance Radar (PSR) detect and track aircraft targets?

1. By transmitting high-frequency radio pulses that reflect off the physical body of the aircraft, requiring no cooperative equipment on board.
2. By decoding radio signals containing GPS position data transmitted automatically by the aircraft's transponder.
3. By interrogating the aircraft's transponder and measuring the response time.
4. By tracking the heat signature of the aircraft's engines using infrared sensors.