Definition
A radar system that detects aircraft and other objects by transmitting radio energy and receiving the energy reflected back from the target. It determines the target's range and bearing from the timing and direction of the returned signal, without requiring any equipment in the aircraft itself.
Plain English
A ground radar that finds aircraft by bouncing radio waves off them and reading the echo. The aircraft does not need to do anything for the radar to see it.
Context Anchor
Pilots encounter this term in radar service, traffic advisories, and discussions of how air traffic controllers see aircraft on their displays.
Derivation
Radar comes from 'radio detection and ranging.' 'Primary' here means the original, self-contained method — the radar does the whole job on its own using reflected energy, in contrast to secondary radar, which depends on a transponder reply from the aircraft.
Why Pilots Care
It provides basic aircraft detection even when onboard equipment is absent or inoperative.
Analogy
Primary radar is like calling out in a canyon and listening for the echo. The echo tells you something is there and roughly where it is, even though the object did not answer you on purpose.
Intuition Check
Primary does not mean “most important radar” in this term. It means the basic radar method that uses a reflected signal rather than an aircraft-generated reply.
Example Sentence 1
After the transponder failed, the controller could still track the aircraft on primary radar but could no longer see its altitude.
Example Sentence 2
Weather can create false returns on primary radar that controllers must interpret carefully.