SSRRadio Navigation — DGCA CPL practice questions

Question 1 of 5

SSR operates on frequencies:

A.1030 MHz ground Tx, 1090 MHz airborne Tx

B.1090 MHz ground Tx, 1030 MHz airborne Tx

C.9375 MHz both

D.960 MHz both

All 5 questions — SSR

Radio Navigation · DGCA CPL. The correct option is marked on each.

Q1. SSR operates on frequencies:
- A.1030 MHz ground Tx, 1090 MHz airborne Tx✓
- B.1090 MHz ground Tx, 1030 MHz airborne Tx
- C.9375 MHz both
- D.960 MHz both
Why: Ground transmits at 1030 MHz , airborne transponder replies at 1090 MHz .
Q2. Mode C provides ATC with:
- A.aircraft callsign
- B.airspeed
- C.position
- D.pressure altitude✓
Why: Mode C (P1–P3 spacing = 21 µs) provides pressure altitude referenced to 1013.25 hPa in 100 ft increments.
Q3. Advantage of Mode S over Mode A/C:
- A.no transponder required
- B.not dependent on pressure altimeter
- C.eliminates garbling by selective addressing✓
- D.uses VHF frequencies
Why: Selective addressing. Every Mode S aircraft has its own 24-bit address, so the ground station can interrogate one aircraft at a time and replies from aircraft close together no longer overlap (garble). Mode S does not use VHF: it interrogates on 1030 MHz and replies on 1090 MHz, the same UHF pair as Mode A/C. Reference: ICAO Annex 10 Volume IV, 3.1.1.1.1 and 3.1.1.2.1.
Q4. A transponder code of 7700 indicates:
- A.radio failure
- B.emergency✓
- C.unlawful interference
- D.conspicuity
Why: 7700 = Emergency . 7600 = radio failure. 7500 = hijack (unlawful interference).
Q5. The unique identifier used in Mode S is:
- A.a 4-digit octal squawk
- B.the Mode C altitude code
- C.a 24-bit ICAO aircraft address✓
- D.a 12-bit reply pulse train
Why: Mode S uses a 24-bit ICAO address , providing up to 16.7 million unique aircraft identifiers, enabling selective interrogation and data link capability.