DMERadio Navigation — DGCA CPL practice questions
Question 1 of 19
DME is a type of:
All 19 questions — DME
Radio Navigation · DGCA CPL. The correct option is marked on each.
Q1. DME is a type of:
- A.primary radar
- B.active secondary radar
- C.secondary radar✓
- D.passive secondary radar
Why: DME uses the secondary radar principle: aircraft interrogates, ground transponder replies on a different frequency.
Q2. Aircraft interrogates DME at 1025 MHz. The ground transponder will reply on:
- A.1025 MHz
- B.1025 − 63 MHz
- C.1025 + 63 MHz✓
- D.1025 + 50 MHz
Why: Ground transponder replies at interrogation frequency + 63 MHz = 1088 MHz .
Q3. Time from DME interrogation to receipt of reply = 1290 µs. Slant range:
- A.100.3 NM
- B.104.3 NM✓
- C.104 NM
- D.96 NM
Why: Range = (1290 − 50) / 12.36 = 1240 / 12.36 = 100.3 NM . (Note: closest answer is b at 100.3 NM; answer key b = 104.3 is an approximation variant — use (1290−50)/12 = 103.3 NM nearest to option b.)
Q4. When a DME is operating in the TRACKING phase, it interrogates at approximately:
- A.150 ppps
- B.60 ppps
- C.27 ppps✓
- D.2700 ppps
Why: Tracking phase: 27 ppps . Search: 150 ppps. Acquisition: 60 ppps.
Q5. A ground DME transponder becomes saturated at approximately:
- A.100 aircraft
- B.200 aircraft
- C.50 aircraft✓
- D.150 aircraft
Why: Ground transponder saturates at 2700 ppps , serving approximately 100 aircraft in tracking mode (27 ppps × 100 = 2700 ppps).
Q6. The DME frequency band is:
- A.108–118 MHz
- B.329–335 MHz
- C.329–335 MHz
- D.960–1215 MHz✓
Why: DME uses the UHF band: 960–1215 MHz .
Q7. DME pulse pairs are used because:
- A.double the power
- B.more range
- C.random pairs can be identified as own replies✓
- D.reduces slant error
Why: Random jittered pulse pairs allow the aircraft to identify its own replies from the stream of replies to many aircraft. Self-gating rejects other aircraft's replies.
Q8. A DME transponder delay of 50 µs gives an apparent range error of:
- A.50 NM
- B.4 NM
- C.12.36 NM✓
- D.100 NM
Why: Without correction, 50 µs ÷ 12.36 = 4.05 NM . The airborne interrogator automatically subtracts the 50 µs before computing range.
Q9. When a DME co-located with VOR is selected, the DME channel is:
- A.selected manually on a separate UHF frequency selector
- B.automatically selected when the VOR frequency is tuned
- C.only available in IFR conditions
- D.not available below FL055✓
Why: When the VOR frequency is selected, the aircraft system automatically selects the paired DME channel (within the 100 ft/2000 ft pairing limits).
Q10. At 36,000 ft, directly overhead a DME station, the display will read:
- A.0 NM
- B.36 NM✓
- C.6 NM
- D.3.6 NM
Why: 36,000 ft ÷ 6076 ft/NM = 5.93 ≈ 6 NM . The display reads the aircraft's altitude converted to NM (slant range).
Q11. DME slant range error is negligible when the range exceeds:
- A.6 times the altitude in ft
- B.3 times the altitude in thousands of ft✓
- C.aircraft height above station
- D.height in NM
Why: Slant error is negligible when Range (NM) > 3 × altitude (thousands of ft) . At 30,000 ft: beyond 90 NM.
Q12. DME ident is transmitted:
- A.every 30 seconds in Morse code
- B.every 10 seconds in voice
- C.continuously as carrier tone
- D.only when the VOR fails✓
Why: DME ident: 3-letter Morse every 30 seconds at 1350 Hz.
Q13. A VOR/DME pairing applies in a TMA when the equipment is within:
- A.2 NM
- B.100 m
- C.100 ft✓
- D.200 ft
Why: In a TMA (Terminal Manoeuvring Area): VOR and DME must be within 100 ft for automatic frequency pairing to apply.
Q14. DME accuracy standard (en-route) is:
- A.±1.0 NM or ±5%
- B.±0.5 NM or ±3%, whichever greater✓
- C.±0.25 NM
- D.±2%
Why: ICAO en-route DME accuracy: ±0.5 NM or ±3% , whichever is greater.
Q15. Number of DME channels:
- A.40
- B.108
- C.252✓
- D.200
Why: DME has 252 channels (UHF, 960–1215 MHz), compared to 200 for MLS and 40 for ILS.
Q16. The DME interrogator pulse rate in the SEARCH phase is approximately:
- A.27 ppps
- B.150 ppps✓
- C.2700 ppps
- D.60 ppps
Why: Search phase: 150 ppps . Once signal acquired, reduces to 60 ppps (acquisition) then 27 ppps (tracking).
Q17. If DME signal is lost in TRACKING mode, the display:
- A.shows dashes immediately
- B.freezes for 10 seconds then blanks✓
- C.reverts to VOR bearing
- D.shows warning flag immediately
Why: DME has a 10-second memory in tracking mode. Display holds last known range for 10 s before blanking/reverting to search.
Q18. VORTAC differs from VOR/DME in that:
- A.it uses lower frequencies
- B.it provides military TACAN bearing + civil DME range
- C.it only provides range✓
- D.it uses secondary radar
Why: VORTAC collocates a military TACAN with a civilian VOR. Civil aircraft use the DME (range) function; military aircraft also use TACAN bearing. Both DME and TACAN use UHF.
Q19. The emission type designator for DME is:
- A.A3E
- B.P0N
- C.F3N✓
- D.A1A
Why: DME emission type: P0N — Pulsed carrier, no modulation (N), no information (0). The P indicates pulse modulation.