What distinguishes primary radar from secondary radar?

The correct choice highlights that primary radar operates by detecting objects through the use of reflected radio waves. This fundamental principle underpins the functionality of primary radar systems, which transmit radio waves that bounce off objects such as aircraft or weather formations, allowing the radar system to determine the distance and location of those objects based on the time it takes for the transmitted waves to return.

In contrast, secondary radar systems rely on transponders that respond to specific signals sent by the radar, providing more detailed information such as identification and altitude. The nature of primary radar being based on passive reflection rather than active response sets it apart from secondary radar, which does not utilize the object’s reflected waves for detection.

This understanding is crucial for comprehending how different radar technologies are utilized in air traffic control and other surveillance applications. Recognizing the basis of primary radar's operation helps in appreciating its limitations and capabilities compared to secondary radar systems.