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 identification and reporting. Mode C is used for automatic altitude reporting. Mode S is a discrete addressable mode that supports both surveillance and data link communication between aircraft and ground stations.
Plain English
A setting on the aircraft's transponder that tells ATC's radar what kind of information to send back. Different modes send different things — your identity, your altitude, or both, depending on which mode the controller and the equipment are using.
Context Anchor
Seen in transponder, radar service, and ATC communication discussions, especially in references to Mode A, Mode C, and Mode S.
Derivation
From Latin modus, meaning 'manner' or 'way.' In radar systems, a mode is one specific 'way' the transponder is set to reply — each mode is a different pulse pattern that triggers a different kind of response.
Why Pilots Care
Proper mode selection ensures ATC receives accurate identification and altitude data, which is essential for separation and safety.
Intuition Check
Mode does not just mean a general style or mood here. In this context, it means a specific transponder reply setting that controls what information is sent to ATC radar.
Example Sentence 1
ATC instructed the pilot to squawk 4732 and confirm Mode C was operating before entering Class B airspace.
Example Sentence 2
Mode S provides both identification and altitude plus additional data like flight ID.