2 ATC transponder modes
David Wyatt, Mike Tooley · 2013
Mode A: In this transponder system, the pilot selects the four-digit code on the ATC control panel prior to each flight. The SSR system confirms this aircraft’s azimuth on the controller’s screen with an icon confirming that the aircraft is equipped with a transponder. If the controller needs to distinguish between two aircraft in close proximity an identity code will be requested; the pilot pushes a switch on his ATC control panel, and this highlights the icon on the controller’s screen. Since each aircraft is allocated with a unique code, only one icon per aircraft will be highlighted; this unique identification is referred to as a squawk code.