DME (Distance Measuring Equipment) uses radio signals to determine the distance between an aircraft and a ground station. It was developed in Australia in the 1950s and consists of an interrogator in the aircraft and a transponder on the ground. The aircraft sends a pulsed interrogation signal and the ground station responds after a precise time delay. By measuring the elapsed time, the aircraft can calculate its distance from the station. DME provides accuracy up to 25 nautical miles, with errors occurring closer to the station due to slant range differences. It operates using two timing modes, X and Y, which differ in their pulse spacing and ground station delay times.

1. DME IN
AVIATION
BISHOWRAM GAUTAM(KEC-BEX 71020)

2. HISTORY
• Developed in Australia, it was invented by James
"Gerry" Gerrand
• Amalgamated Wireless Australasia Limited
deployed engineered version in the early 1950s
operating in the 200 MHz VHF band.

3. HISTORY
• Australian domestic version was referred by the
Federal Department of Civil Aviation as DME(D)
• Later international version adopted by ICAO as
DME(I)

4. INTRODUCTION
• (DME) -Distance measuring equipment
• Consists of major two equipment
Ground station Transponder
Interrogator in Aircraft
• DME is required for aircraft operating at or
above 24,000 feet

5. OPERATION
• The aircraft sends out a pulsed signal to the
station (interrogation)
• The ground station receives the series of pulses-
pairs
• 100 closest aircraft will receive a good signal.
• “Jitter” makes each interrogation unique to each
aircraft; the station replies in kind.

6. OPERATION
• After a precise time delay (50 µsec), the ground
station replies with an identical sequence of
reply pulse-pairs.
• Aircraft receives the reply and measures the
elapsed time from when it sent the interrogation
• Subtracts the 50 microsecond delay

7. OPERATION
• Calculates its exact distance from the ground
station, given the fact that
Distance = Velocity x Time.

8. OPERATION

9. OPERATION

10. DME ACCURACY
• Measures the straight-line distance from the
aircraft to the ground station.
• Which is called “Slant Range”

11. DME ACCURACY
• Accuracy problem due to the difference between
the actual horizontal distance from aircraft to
station(slant range error)
• Error decreases at longer distances, negligible at
ranges greater than 25NM or at altitudes less
than 5,000ft
• “Slant range error " occurs when the aircraft
passes directly over the station

12. DME ACCURACY
• Instead of reading zero, the DME shows the
altitude of the airplane above the station(in NM)

13. INTERROGATION AND REPLY TIMING
• DME exchanges messages with the ground
stations using one of two different timing modes
X Mode
 YMode

14. .
• X Mode
When the DME is channelled to a frequency ending in
.X0 such as 122.00 or 116.70 MHZ.
The interrogation pulse pairs spacing from aircraft to
ground station is 12µsec.
The reply pulse pair spacing from ground station to
aircraft is 12µsec.
The ground station delay time is 50µsec.

15. .
• Y Mode
When the DME is channelled with the .X5 frequencies,
such as 114.55 or 117.65MHZ.
The interrogation pulse pair spacing from aircraft to
ground station is 36µsec.
The reply pulse pair spacing from ground station to
aircraft is 30µsec.
The ground station delay time is 56µsec.

16. ANTENNA
• Antennas are usually located on the belly of the
aircraft
• Antenna is a little shark fin 1/4 wave antenna

17. DME BLOCK DIAGRAM

18. THANK YOU 