SSR, transponders and Mode S

Navigation · Chapter 43 · DGCA CPL and ATPL

How secondary radar asks and the transponder answers, Modes A, C and S, Mode C pressure altitude, and the codes to know.

Remember

Interrogate 1030 MHz, reply 1090 MHz; P1–P3 8 µs Mode A, 21 µs Mode C.
12 reply pulses: 4 096 codes. Ident: extra pulse ~20 s.
Mode C: pressure altitude (1013.25), 100 ft steps, whatever the subscale.
7700 emergency, 7600 radio failure, 7500 hijack, 2000 no code; in Europe 7000 VFR, 7004 aerobatics.
Mode S: 24-bit address (16.7 million), data link, no garble.

Ask on 1030, answer on 1090

Ask • ssr (secondary surveillance radar): the ground station interrogates, asks a question, on 1 030 MHz with a narrow beam
Answer • A box in the aircraft, the transponder, hears the question and replies on 1 090 MHz, in every direction • The ground gets an answer, not an echo
Why it beats an echo • Each signal travels only one way, so far less power reaches 200–250 NM (a primary echo must go out and back) • The answer does not depend on the aircraft’s size or shape, and there is no clutter from rain or ground
What ATC gets • Identity, pressure altitude, call sign, speed, track history, destination, and emergencies • The SSR aerial sits on top of the primary radar: both pictures on one screen

The controller's screen

Labels • Next to each blip, a label: call sign, flight level, groundspeed, destination • A row of dots behind it shows where it has been: its track
Ident • "Squawk ident": press IDENT and the transponder adds an extra pulse for about 20 s • The blip brightens, so the controller can pick you out
Emergencies • Set an emergency code and your label flashes on every screen