chapter  21
2 ATC transponder modes
Pages 1

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.