Which statement correctly describes Modes A, C and S?

Which statement correctly describes Modes A, C and S?

• A. Mode A transmits an identifying code only; Mode C provides altitude automatically; Mode S includes altitude and data exchange
• B. Mode A transmits altitude; Mode C data exchange; Mode S is unknown
• C. Mode A has altitude exchange; Mode C only identifies; Mode S transmits no data
• D. Mode A and S both provide altitude automatically

Answer: (A)

Mode A, Mode C, and Mode S are transponder modes used in secondary radar surveillance, each adding specific capabilities to the basic reply. Mode A simply returns the four‑digit squawk code that identifies the aircraft; it does not include altitude information. Mode C adds altitude reporting by encoding the aircraft’s altitude in the reply when asked, providing altitude automatically in appropriate interrogations. Mode S takes this further by adding selective addressing and two‑way data exchange, so it can include altitude information and additional data (such as aircraft identification and other surveillance data) in its transmissions and respond to data requests. So the statement matches the real differences: Mode A transmits an identifying code only; Mode C provides altitude automatically; Mode S includes altitude and data exchange. The other options mix up which modes carry altitude or data, or treat Mode S as unknown, which isn’t correct.

Mode A, Mode C, and Mode S are transponder modes used in secondary radar surveillance, each adding specific capabilities to the basic reply. Mode A simply returns the four‑digit squawk code that identifies the aircraft; it does not include altitude information. Mode C adds altitude reporting by encoding the aircraft’s altitude in the reply when asked, providing altitude automatically in appropriate interrogations. Mode S takes this further by adding selective addressing and two‑way data exchange, so it can include altitude information and additional data (such as aircraft identification and other surveillance data) in its transmissions and respond to data requests. So the statement matches the real differences: Mode A transmits an identifying code only; Mode C provides altitude automatically; Mode S includes altitude and data exchange. The other options mix up which modes carry altitude or data, or treat Mode S as unknown, which isn’t correct.