What is the key difference between primary radar and SSR in ATC surveillance?

What is the key difference between primary radar and SSR in ATC surveillance?

• A. Primary radar detects all objects by radar echoes; SSR uses transponder replies from aircraft to identify and provide altitude and flight ID data; SSR requires cooperative transponder.
• B. SSR detects all objects by radar echoes; Primary radar uses transponder replies from aircraft to identify altitude and flight ID data.
• C. Primary radar uses transponder data to identify altitude; SSR relies on weather radar.
• D. SSR does not require a cooperative transponder.

Answer: (A)

The main idea is that primary radar and SSR obtain information in two different ways. Primary radar sends out a pulse and measures the echoes off any object. It can show you where something is in range and bearing, and it can detect non-cooperative targets, but it provides no identity or altitude data because there’s no data coming from the target itself. SSR, on the other hand, is a two-way system. The ground radar interrogates, and the aircraft’s transponder replies with coded information. That reply carries the aircraft’s identity (flight ID) and altitude (Mode C, and more with Mode S). Because it relies on the aircraft’s transponder, SSR data is only available if the aircraft is cooperative and transponding. So the correct statement captures that primary radar detects objects by echoes without identity/altitude data, while SSR uses transponder replies to identify and supply altitude and flight data, and it requires a cooperative transponder. The other choices misstate how SSR or primary radar obtain information or whether SSR can operate without a transponder.

The main idea is that primary radar and SSR obtain information in two different ways. Primary radar sends out a pulse and measures the echoes off any object. It can show you where something is in range and bearing, and it can detect non-cooperative targets, but it provides no identity or altitude data because there’s no data coming from the target itself.

SSR, on the other hand, is a two-way system. The ground radar interrogates, and the aircraft’s transponder replies with coded information. That reply carries the aircraft’s identity (flight ID) and altitude (Mode C, and more with Mode S). Because it relies on the aircraft’s transponder, SSR data is only available if the aircraft is cooperative and transponding.

So the correct statement captures that primary radar detects objects by echoes without identity/altitude data, while SSR uses transponder replies to identify and supply altitude and flight data, and it requires a cooperative transponder. The other choices misstate how SSR or primary radar obtain information or whether SSR can operate without a transponder.