ATCRBS or SSR uses a transponder in the aircraft. What is the basic ground operation in this system?

• A. The ground interrogating unit transmits a coded pulse sequence that actuates the aircraft transponder, which replies with a coded sequence back to the ground, providing positive identification
• B. The ground station provides weather data to the cockpit
• C. The beacon antenna rotates independently of the surveillance antenna
• D. The system primarily provides altitude data to the controller

Answer: (A)

The basic idea behind ATCRBS/SSR is an interrogation-and-reply exchange between the ground station and the aircraft transponder. The ground interrogating unit sends a coded pulse sequence that activates the aircraft transponder. In response, the transponder sends back a coded reply to the ground, providing positive identification (the squawk code). In practice, this interaction gives ATC a distinct identity for each aircraft, and in certain modes the reply can also include altitude data. Weather data, rotating beacons independent of the surveillance system, or altitude data being the sole purpose are not the defining ground operation of SSR.