1. Distance Measuring Equipment DME

2. DME - Principle of Operation
Secondary Radar
Operates at UHF ( MHz)
Signal travels one way
Requires an interrogator unit in the aircraft and a responder unit on the ground

3. DME - Principle of Operation
The aircraft calculates its distance from the beacon by measuring the time between transmitting a signal and receiving a reply.
There is a 50s delay before the beacon replies

4. DME - Principle of Operation
Unique signal - double pulse transmission (P0N)
2 frequencies: one for transmitting, one for receiving 63MHz apart
UHF channels paired/ganged with VHF frequencies

5. DME Paired Frequencies with VOR/ILS
63MHz

6. DME - Propagation UHF Direct Wave (Line of sight)
“Blade” type antenna on aircraft
Signal may be obscured whilst turning

7. DME - Why two frequencies?
Ground station can send & receive simultaneously
To prevent “re-triggering” of the ground station caused by reflections

8. DME - How is the correct reply identified?
1. By “jittering”(dithering or varying) the pulse interval (PI) between double pulses
2. By locking on to pulses coming back at a constant time interval ()

9. “Jittering” the PRF P1 100s P2 85s P3 110s P4 90s P5
Constant time interval A
“Jittering” the PRF

10. DME - Pulse Repetition Frequency
The initial interrogation rate is 150 ppps
If, after pulse pairs have been transmitted, the DME has not locked on, the rate drops to 60 ppps
However, once locked on and the DME is in “track” mode, the interrogation rate drops to about 5 ppps

11. DME - Ground station capability
The average interrogation rate from all aircraft in view is about 30 ppps
The beacon capability is 3000 ppps
Therefore, the beacon is theoretically capable of handling
3000 30
= 100 aircraft

12. DME Range Distance readout is in nautical miles
Maximum indicated distance is limited by line of sight range and “gate” circuitry in the DME receiver. 50
200 NM
“GATE” 20s wide

13. DME - Interruption of Signal
If the signal from the beacon is lost for more than 10 s the receiver will go into memory mode
There are two types of memory:
Static memory - last measured slant range is retained for 10s before receiver goes into search mode again
Velocity memory - last measured rate of change is retained

14. DME - Failure mode
On modern DME receivers the display goes blank

15. Echo protection
Radio Navigation – Chapter 5

16. The range measured by the DME is the distance along the direct path between the interrogator and the transponder (slant range).
Radio Navigation – Chapter 5

17. The ground distance (plan range) can be calculated from Pythagoras’ theorem:
SR2 = PR2 + height2
PR2 = SR2 - Height2
PR =  SR2 - Height2 2
height2 2
Radio Navigation – Chapter 5

18. Q. Height of aircraft = 18 000 ft, DME range is 5 NM
Q. Height of aircraft = ft, DME range is 5 NM. What is the ground range from the station?
A ft  3 NM, PR =  ( )
=  (25 - 9) =  16 = 4 NM 2
height2 2
Radio Navigation – Chapter 5

19. Accuracy of DME ± 3% of Slant Range or ± 0.5 NM, whichever is greater
E.g. If indicated range is 20 NM, what is the accuracy?
20 x 3% = ± 0.6NM
If indicated range is 10 NM, what is the accuracy?
10 x 3% = ± 0.3NM  Accuracy = ± 0.5 NM

20. Accuracy of DME
Note: If indicated range is NM, what is the accuracy?
16.66 x 3% = ± 0.5NM
If range > 162/3 NM, accuracy = ± 3% of SR
If range < 162/3 NM, accuracy = ± ½ NM

21. Types of ground stations

22. Co-located & Associated DME - AERODROME
Co-located: within 30m/100ft
Call signs the same .
VOR/DME

23. Co-located & Associated DME - AERODROME
Associated: not within 30m/100ft
“WTM” .
VOR
TACAN/DME
“WTZ” (Ch 21X)
Last letter of DME call sign becomes “Z”

24. Co-located & Associated DME - AIRWAY
Co-located: within 600m/2000ft
Call signs the same .
VOR/DME

25. Co-located & Associated DME - AIRWAY
Associated: not within 600m/2000ft .
DME .
Last letter of DME call sign becomes “Z”
VOR

26. Identification DME VOR VOR VOR VOR VOR VOR DME (Tone = 1350Hz) 6045 15
VOR
VOR 30
DME

27. Identification
DME 60
VOR
VOR
VOR
VOR 45 15
VOR
VOR 30
DME

28. Flying the arc.
Radio Navigation – Chapter 5

30. ILS/DME The accuracy of ILS/DME is ± 0.1 NM, provided:
The aircraft is on the centre line of the ILS 27 .
DME
Distance will read ZERO at the THRESHOLD

31. Questions What does DME actually measure? Slant range
What kind of signal is a DME transmission?
Double pulse modulation
In which part of the radio spectrum can DME transmissions be found?
UHF (decimetric)
How do you tune DME?
By selecting a paired VHF frequency
What is the capacity of a DME beacon?
100 aircraft

32. Questions How is an individual aircraft transmission identified?
By “jittering” or variation of the PI/PRF
When does the pulse rate drop to 60 ppps?
After 15,000 pulse pairs if the DME has not locked on
Why do reflections of transmitted signals not re-trigger the DME beacon?
Because the transmit & receive frequencies are different
What is the frequency separation between transmit & receive frequencies?
63 MHz

33. Questions
The reply to an DME transmission takes 124s. Ignoring the ground delay, what is the range of the aircraft from the beacon?
124 s ÷ 12.4 = 10NM
On an aerodrome, a TACAN beacon’s call sign is “BEZ” and the VOR is “BEN”. What is the relationship, if any, between them?
They are associated, not co-located
What is the maximum distance between a VOR & DME to be considered co-located?
30m/100ft on an aerodrome; 600m/2000ft on an airway

34. Questions What is the accuracy of DME/N? 3% of SR or ±0.5NM
How accurate is an ILS/DME (DME/P)?
±0.1NM (on the centreline)