TCAS detects and tracks other aircraft by interrogating their transponders and then analyzing the replies to determine range, bearing, and relative __________________________.

• A. Altitude
• B. Heading
• C. Airspeed
• D. Size

Answer: (A)

TCAS uses the data carried in transponder replies to gauge where nearby aircraft are in three dimensions. When it interrogates another aircraft, the transponder responds with information that includes altitude (provided by Mode C or Mode S). This altitude data lets TCAS determine how high or low the other aircraft is compared with your own, giving you the relative altitude. Range and bearing are derived from the timing and processing of the interrogation and response, but the key piece that completes the three-dimensional picture is the altitude. Other attributes like heading, airspeed, or size aren’t provided by the standard transponder replies in this context, so they aren’t used to determine relative position.