Definition
The letter or number assigned to a specific pulse spacing of the radio signals transmitted or received by ground interrogators or airborne transponders of a Secondary Surveillance Radar (SSR) system. Four modes — A, C, S, and intermode — are specified in ICAO Annex 10 for use in civil aviation.
Plain English
A 'Mode' is the type of conversation a ground radar and an aircraft's transponder are having. The mode tells the equipment what kind of question is being asked and what kind of answer to send back. Mode A sends an identifying code, Mode C adds altitude, and Mode S can exchange detailed data including aircraft identity and other information.
Context Anchor
Seen in air traffic control radar, transponder, and surveillance discussions, especially when referring to Mode A, Mode C, or Mode S.
Derivation
Mode' comes from the Latin 'modus,' meaning 'manner' or 'way.' In SSR, each mode is literally a different 'way' of interrogating and replying — distinguished by the precise spacing between radar pulses. Different spacing = different mode = different kind of information exchanged.
Why Pilots Care
Correct mode selection ensures ATC receives accurate identification and altitude data for safe traffic separation and tracking.
Intuition Check
Do not read mode here as just any general cockpit setting. In this context, it means a defined radar-transponder communication format.
Example Sentence 1
After takeoff, the controller said 'squawk Mode C,' so the pilot confirmed the transponder was set to report altitude.
Example Sentence 2
Mode A provides the aircraft's assigned four-digit code for identification on the radar display.