Definition
The letter or number assigned to a specific pulse spacing of radio signals transmitted or received by ground interrogator or airborne transponder components of the Air Traffic Control Radar Beacon System (ATCRBS). Mode A (military Mode 3) is used for civil and military aircraft identification and for the SSR component of the U.S. National Beacon Code Allocation Plan. Mode C is used for automatic altitude reporting. Mode S is a discrete selective interrogation mode used for both surveillance and data link communications.
Plain English
A 'mode' is just a setting on the radar system that tells the ground station and the aircraft's transponder what kind of information to exchange. Mode A sends the aircraft's identifying squawk code, Mode C adds altitude, and Mode S allows the ground station to talk to one specific aircraft and exchange data with it.
Context Anchor
Seen in radar surveillance and transponder discussions, especially when referring to Mode A, Mode C, or Mode S operation.
Derivation
Mode' comes from the Latin 'modus,' meaning 'manner' or 'way.' Each SSR mode is simply a different way the ground radar and the aircraft transponder can talk to each other.
Why Pilots Care
Using the correct mode ensures controllers receive accurate identification and altitude data needed for safe separation and traffic management.
Intuition Check
Mode does not mean a general style or preference here. In SSR, it means a specific radar signal format that asks the aircraft for a specific kind of reply.
Example Sentence 1
ATC instructed the pilot to confirm the transponder was operating in Mode C so altitude would appear on the controller's display.
Example Sentence 2
Mode S allows the ground system to selectively address individual aircraft and reduce unnecessary replies.