What radar system relies solely on reflected radio signals?

Primary radar is the system that relies solely on reflected radio signals. It operates by transmitting radio waves and then listening for echoes that bounce back from objects, such as aircraft. This technology allows primary radar to determine both the range and bearing of a target by calculating the time it takes for the radio signal to travel to the target and back.

In contrast, ADS-B (Automatic Dependent Surveillance–Broadcast) relies on aircraft broadcasting their position information derived from GPS, making it a dependent system rather than one based on reflected signals. Secondary radar also does not operate solely on reflected signals; instead, it involves a transponder on the aircraft that actively responds to interrogations from the radar system without relying on passive reflections. GPS, while not a radar system at all, uses signals from satellites and does not involve the concept of reflecting radio waves, further distinguishing it from primary radar.