EASA NAV(A)intermediate
Compare primary radar and secondary surveillance radar (SSR).
Answer
Primary radar sends pulses and detects energy reflected from the aircraft. SSR interrogates an aircraft transponder, which transmits a coded reply. Primary radar can provide distance and direction from timing and antenna orientation; SSR can provide additional coded information.
Explanation
Primary radar relies on reflected radio energy, while SSR relies on an active response from the aircraft's transponder. Radar distance is derived from signal timing, and direction from antenna orientation. SSR replies can carry coded information such as identity or pressure altitude. **Key takeaway:** Primary radar reflects; SSR interrogates and receives a transponder reply.\n\n**Glossary:** **SSR**: secondary surveillance radar; **transponder**: airborne equipment that replies to interrogations.
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