DGCA CPL / ATPL Study Notes • Radio Navigation • Ch 15

✈ Chapter 15: DME
Distance Measuring Equipment

📋 Contents

1. Introduction 2. Principle of Operation 3. Frequencies & Channels 4. Pulse Pairs & Jitter 5. Airborne Interrogator — Phases 6. Ground Transponder 7. Accuracy & Errors 8. VOR/DME Association 9. Identification 10. Coverage & Uses 11. DME Summary 12. Practice Questions (19 Q)

1. Introduction

DME provides pilots with slant range from a ground transponder. It operates on the secondary radar principle — the aircraft interrogates, and the ground station replies on a different frequency. DME is normally co-located with VOR or ILS (localizer).

📡 DME Classification

DME is a short/medium range secondary radar system (LOS limited)
Provides slant range (NOT ground range)
Typically associated with VOR for ρ–θ (range/bearing) position fixing
DME ground transmitter also called a transponder

2. Principle of Operation

Fig 15.1: DME principle — aircraft interrogates at 1025 MHz, ground replies at 1088 MHz | Fig 15.2: Overall DME system diagram

📡 How DME Works

Aircraft interrogator transmits a pair of pulses at a jittered random rate
Ground transponder receives and re-transmits on a different frequency after a 50 µs delay
Aircraft measures time from transmission to receipt of its own reply; subtracts 50 µs delay
Remaining time ÷ 12.36 = slant range in NM

⚡ DME Range Calculation

Slant Range (NM) = (Echo time µs − 50 µs) / 12.36

Example: echo time = 1,290 µs
Range = (1290 − 50) / 12.36 = 1240 / 12.36 = 100.3 NM

3. Frequencies & Channels

Fig 15.3: DME frequency pairing — X and Y channels, 1 MHz spacing

📡 DME Frequency Plan

Parameter	Detail
Frequency band	UHF: 960–1215 MHz
Total channels	252
Channel types	X channels and Y channels (different internal pulse spacings)
Interrogator (aircraft) frequency	969–1024 MHz (X) or 1025–1150 MHz (Y)
Transponder (ground) offset	±63 MHz from interrogator frequency
Emission type	P0N (pulse pairs, no carrier)
Pulse shape	Gaussian pairs

⚡ Example Frequency Pairing Aircraft interrogates at 1025 MHz (Y channel) → Ground replies at 1025+63 = 1088 MHz
Aircraft interrogates at 1088 MHz → Ground replies at 1088−63 = 1025 MHz

4. Pulse Pairs & Jitter

Fig 15.4: Twin pulse pair structure | Fig 15.5: Jittering — random PRI to distinguish own reply from other aircraft

📡 Why Pulse Pairs?

Single pulse cannot be distinguished from other aircraft's replies or random noise
Twin pulses with specific spacing (X: 12 µs / 36 µs; Y: 30 µs / 30 µs) ensure the receiver recognises only its own pair

📡 Why Jitter?

Many aircraft query the same DME → all get replies in the same pulse stream
Aircraft jitters (randomly varies) its interrogation rate
Only the aircraft's own replies maintain consistent time delay from its transmissions
Other aircraft's replies appear at random intervals and are rejected
Algorithm identifies own replies by their consistent delay — called self-gating

5. Airborne Interrogator — Phases

Fig 15.6: DME phase diagram — search, acquisition, tracking phases and pulse rates

📡 DME Interrogation Phases

Phase	Rate	Purpose
Search	150 ppps	High rate to quickly find replies when first switched on or after loss of signal
Acquisition	60 ppps	Reduces rate once signal found; confirms own replies
Tracking	27 ppps	Locks on to DME; very low rate (memory up to 10s before reverting to search)

⚡ Ground Transponder Saturation

Ground transponder limits total replies to 2700 ppps
Serves approximately 100 aircraft simultaneously
When saturated: system reduces replies to each aircraft proportionally; memory keeps display from blanking immediately
Aircraft 10s memory (tracking mode) before reverting to Search

6. Ground Transponder

Fig 15.7: Ground transponder — located on same mast as VOR or ILS localizer building

📡 Ground Transponder Details

Parameter	Detail
Receive	Aircraft interrogation frequency
Transmit	Interrogation frequency ±63 MHz
Delay	50 µs fixed delay before reply
Max throughput	2700 pulse pairs per second (ppps) — serves ~100 aircraft
Squitter	Transponder transmits 2700 ppps minimum (fills unused capacity with random squitter pulses to prevent search aircraft from missing station)
Ident	3-letter morse ident every 30 seconds; combined with VOR ident if co-located; audio tone 1350 Hz
Memory	Last known DME displayed for 10 seconds if signal lost in tracking mode

7. Accuracy & Errors

Fig 15.8: Slant range error — significant when close to station and at altitude

⚠ DME Accuracy

System accuracy: ±0.25 NM ±1.25% of range (combined ground + airborne error)
ICAO accuracy standard for en-route DME: ±0.5 NM or ±3%, whichever is greater
DME/P (Precision): used with MLS — ±100 ft accuracy for CAT II/III

⚠ Slant Range Error

DME displays slant range (hypotenuse), not horizontal ground distance. Error is significant when:

Slant error becomes negligible when Range (NM) > 3 × Height (thousands of ft)

Example: 30,000 ft altitude → slant error negligible beyond 90 NM.
At overhead the station: DME reads height in NM = altitude in ft ÷ 6076.

⚡ Slant Range Worked Example Aircraft at 36,000 ft (6 NM) directly overhead DME: display reads 6 NM (not 0 NM).

8. VOR/DME Association

Fig 15.9: VOR/DME site — collocated equipment; VOR antenna above DME transponder building

📡 Frequency Pairing Rules

Situation	Pairing Applies When
VOR/DME in TMA	VOR and DME within 100 ft of each other
VOR/DME elsewhere	VOR and DME within 2000 ft of each other
Result	Selecting VOR frequency on nav radio automatically selects paired DME channel

📡 DME/VOR vs. VORTAC

VOR/DME: civilian DME co-located with VOR
VORTAC: military TACAN DME co-located with civilian VOR (TACAN also provides bearing for military aircraft)
TACAN UHF frequency is automatically paired with VOR VHF frequency; civilian aircraft use DME function only

9. Identification

Fig 15.10: DME indicator — display shows distance in NM and groundspeed; time-to-station computed

📡 DME Identification

3-letter morse ident broadcast every 30 seconds
If co-located with VOR: idents interleaved; VOR transmits 3, then DME once → VOR identified first, then DME
Ident tone: 1350 Hz
If DME ident fails: standby mode — memory valid for 10 seconds

10. Coverage & Uses

📡 DME Coverage

LOS limited: Range (NM) = 1.25 × (√H_aircraft + √H_station) (H in feet)
Typical usable range: up to 200 NM
UK UPPER: above FL245 → 200 NM; LOWER: below FL245 → depending on altitude

✓ DME Uses

En-route position fix with VOR/DME (ρ–θ fix)
Approach: often paired with ILS for range check; distance to threshold during ILS approach
Stand-alone DME/DME fix (RNAV): using two or more DMEs to compute position
MLS: DME/P provides precise range for approach (accuracy ±100 ft)
Flight plan fuel calculations: track miles remaining
Groundspeed and time-to-station computed automatically by modern DME indicators

11. DME Summary

Parameter	Detail
Type	Secondary radar; UHF
Frequency band	960–1215 MHz (UHF)
Channels	252
Aircraft-to-ground offset	±63 MHz
Emission	P0N (Gaussian pulse pairs)
Transponder delay	50 µs
Search rate	150 ppps
Acquisition rate	60 ppps
Tracking rate	27 ppps
Saturation	2700 ppps ≈ 100 aircraft
Ident	3-letter Morse, 30 s cycle, 1350 Hz
Memory	10 s in tracking mode
Accuracy	±0.25 NM ±1.25%
Slant error negligible when	Range > 3 × altitude (thousands of ft)
VOR/DME pairing (TMA)	Within 100 ft
VOR/DME pairing (other)	Within 2000 ft

12. Practice Questions

Q1. DME is a type of:

(a) primary radar

(b) active secondary radar

(c) secondary radar

(d) passive secondary radar

Answer: (c)
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

Answer: (c)
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

Answer: (b)
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

Answer: (c)
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

Answer: (c)
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

Answer: (d)
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

Answer: (c)
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

Answer: (c)
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

Answer: (d)
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

Answer: (b)
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

Answer: (b)
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

Answer: (d)
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

Answer: (c)
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%

Answer: (b)
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

Answer: (c)
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

Answer: (b)
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

Answer: (b)
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

Answer: (c)
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

Answer: (c)
DME emission type: P0N — Pulsed carrier, no modulation (N), no information (0). The P indicates pulse modulation.

DGCA CPL/ATPL Radio Navigation Study Notes
Chapter 15 — DME (Distance Measuring Equipment)
Capt Pankaj Pahil | www.ghostaviator.com
For personal study use only.

✈ Chapter 15: DMEDistance Measuring Equipment