Chapter 13 DME

Chapter 13
DME
Distance Measuring Equipment
DME Overview
DME provides the aircraft with distance to a
station information.
DME is required for aircraft operating at or above

24,000 feet (FL 240).
The aircraft sends out a pulsed signal to the

station (interrogation) and the station replies with
a pulsed signal.
Random spacing, called “jitter” makes each

interrogation unique to each aircraft; the station
replies in kind.
By using the change of distance to station,

ground speed (GS) is calculated.
Using GS and distance, time-to-station (TTS) is
calculated.
Slant range is the distance to the station not just
laterally but vertically as well.
A “scanning” DME receives signals from DME
stations up to 300 miles away and uses
triangulation to compute an accurate fix.
Textbook page 89
DME Ground Station
DME is part of TACAN, a military navigation system which adds DME
to a VOR station.
A VOR with DME is known as a VORTAC station.

DME Tuning
DME is tuned by tuning either a
VOR or ILS station, as applicable.
DME EFIS Display
Here is a DME readout on an EFIS Display.
The characters “NAV1” indicate that the DME being displayed is from the No.
1 VOR receiver.
NAV 1
2.3DME
DME Block Diagram: Aircraft
Textbook page 91
X and Y Channels
Originally there were 100 DME channels, the X channels.

When the system got overloaded, the channels were split to create the Y Channels,
adding 100 more channels for a total of 200 channels.
Airborne pulse spacing on the X channel is 12 microseconds; on the Y channel is 36
microseconds.
Ground Reply pulse spacing on the X channel is 12 microseconds; on the Y channel is
30 microseconds.
Whether the airplane is on the X or Y channel, interrogation goes out on 1143 MHz.
The reply from the ground station comes back at 1143 MHz on the X channel (+63 MHz).
The reply from the ground station comes back at 1080 MHz on the Y channel (-63 MHz).
If the ground station receives more interrogations than it can handle (>100), it will reduce
it’s receiver sensitivity and not reply to the aircraft on the outer edges of it’s receiving
range.
Block Diagram: Ground Station
The ground station receives, decodes
and replies to interrogations from the
airplane.
There is a 50 Microsecond time delay to

avoid interference.
The “squitter” sends out pulses if the

ground station is not receiving any
aircraft interrogations; this causes any
aircraft within range to “wake up” out of
automatic standby mode and start
interrogating again.
Every 30 seconds, the station transmits

a Morse code ID on 1030 Hz
Textbook page 91
What’s
Wrong?
Textbook page 91
FAR Part 91.205
Flight at and above 24,000 feet MSL (FL
240). If VOR navigation equipment is
required under paragraph (d)(2) of this
section, no person may operate a U.S.-
registered civil aircraft within the 50 states
and the District of Columbia at or above FL
240 unless that aircraft is equipped with
approved DME or a suitable RNAV system.
DME Arch
DME Arch
Review Q & A Chapter 13 DME
13.1 An airborne DME sends out a pulse known as an ____.
Answer: Interrogation signal.
13.2 DME is a component of a military system known as ____.
Answer: TACAN.
13.3 A DME station is located as part of a ____ ground station. Together they are
known as a ____.
Answer: VOR, VORTAC.
13.4 In addition to distance-to-station, an airborne DME computes ___and___.
Answer: Ground Speed, Time-to-station
13.5 A distance error in DME is called ____.
Answer: Slant range.
13.6 All aircraft interrogating the same DME ground station are on the same frequency.
How does an aircraft identify its replies from all others?
Answer: “Jitter” or random spacing of pulses.
13.7 How is a DME station tuned in?
Answer: By tuning a VOR or an ILS station.
13.8 What happens when more than about 100 aircraft interrogate the same DME
ground station (overload)?
Answer: The station reduces its receiver sensitivity and does not respond to aircraft at
the edges of its range.
13.9 Why does the DME ground station delay its reply by 50 microseconds?
Answer: To avoid interference.
13.11 Does the DME station transmit an ID?
Answer: Yes.

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Chapter 13

DME is required for aircraft operating at or above

The aircraft sends out a pulsed signal to the

Random spacing, called “jitter” makes each

By using the change of distance to station,

A VOR with DME is known as a VORTAC station.

Originally there were 100 DME channels, the X channels.

There is a 50 Microsecond time delay to

The “squitter” sends out pulses if the

Every 30 seconds, the station transmits

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