Mode C/Mode S altitude information is transmitted by transponders to provide what data for ATC?

• A. Vertical position data
• B. Horizontal velocity
• C. Weather information
• D. Aircraft color

Answer: (A)

Altitude reporting from Mode C/Mode S transponders provides ATC with vertical position data, which is essential for maintaining safe vertical separation and sequencing aircraft in the airspace.

Mode C sends a pressure altitude value—essentially altitude above the standard datum plane in feet—so controllers know how high an airplane is relative to that standard reference. Mode S adds even more capability, delivering altitude information as part of its richer set of data, improving accuracy and enabling more precise tracking.

This isn’t about weather data, which comes from weather radar and other sources, nor about aircraft color or horizontal velocity in this context. The key purpose of the altitude information is to describe how high the airplane is, enabling proper vertical separation and safer air traffic management.