What is the primary function of a transponder in Australian airspace?

• A. An onboard radar beacon that transmits identification and altitude or code; helps ATC identify and separate traffic.
• B. Provides in-flight entertainment.
• C. Measures fuel quantity.
• D. Controls cabin pressurization.

Answer: (C)

The transponder functions as an onboard radar beacon that responds to air traffic control interrogations. When ATC’s radar system asks, the transponder broadcasts a code and, in most modes, altitude information. This lets controllers see who the aircraft is (identity) and how high it’s flying on their radar screens, enabling proper separation and sequencing of traffic. It’s what makes aircraft identifiable beyond just the raw radar return, improving safety and efficiency in busy airspace. The other listed uses—entertainment, fuel quantity measurement, or cabin pressurization—are unrelated to the transponder’s role in air traffic management.