Distance measuring

equipment

In aviation, distance measuring equipment

(DME) is a radio navigation technology
that measures the slant range (distance)
between an aircraft and a ground station
by timing the propagation delay of radio
signals in the frequency band between 960
and 1215 megahertz (MHz). Line-of-
visibility between the aircraft and ground
station is required. An interrogator
(airborne) initiates an exchange by
transmitting a pulse pair, on an assigned
'channel', to the transponder ground
station. The channel assignment specifies
the carrier frequency and the spacing
between the pulses. After a known delay,
the transponder replies by transmitting a
pulse pair on a frequency that is offset
from the interrogation frequency by
63 MHz and having specified separation.[1]
D-VOR/DME ground station

DME antenna beside the DME transponder shelter

DME systems are used worldwide, using

standards set by the International Civil
Aviation Organization (ICAO),[1] RTCA,[2]
the European Union Aviation Safety
Agency (EASA)[3] and other bodies. Some
countries require that aircraft operating
under instrument flight rules (IFR) be
equipped with a DME interrogator; in
others, a DME interrogator is only required
for conducting certain operations.

While stand-alone DME transponders are

permitted, DME transponders are usually
paired with an azimuth guidance system
to provide aircraft with a two-dimensional
navigation capability. A common
combination is a DME colocated with a
VHF omnidirectional range (VOR)
transmitter in a single ground station.
When this occurs, the frequencies of the
VOR and DME equipment are paired.[1]
Such a configuration enables an aircraft to
determine its azimuth angle and distance
from the station. A VORTAC (a VOR co-
located with a TACAN) installation
provides the same capabilities to civil
aircraft but also provides 2-D navigation
capabilities to military aircraft.

Low-power DME transponders are also

associated with some instrument landing
system (ILS), ILS localizer and microwave
landing system (MLS) installations. In
those situations, the DME transponder
frequency/pulse spacing is also paired
with the ILS, LOC or MLS frequency.
ICAO characterizes DME transmissions as
ultra high frequency (UHF). The term L-
band is also used.[4]

Developed in Australia, DME was invented

by James "Gerry" Gerrand[5] under the
supervision of Edward George "Taffy"
Bowen while employed as Chief of the
Division of Radiophysics of the
Commonwealth Scientific and Industrial
Research Organisation (CSIRO). Another
engineered version of the system was
deployed by Amalgamated Wireless
Australasia Limited in the early 1950s
operating in the 200 MHz VHF band. This
Australian domestic version was referred
to by the Federal Department of Civil
Aviation as DME(D) (or DME Domestic),
and the later international version adopted
by ICAO as DME(I).

DME is similar in principle to secondary

radar ranging function, except the roles of
the equipment in the aircraft and on the
ground are reversed. DME was a post-war
development based on the identification
friend or foe (IFF) systems of World War II.
To maintain compatibility, DME is
functionally identical to the distance
measuring component of TACAN.
Operation
In its first iteration, a DME-equipped
airplane used the equipment to determine
and display its distance from a land-based
transponder by sending and receiving
pulse pairs. The ground stations are
typically collocated with VORs or
VORTACs. A low-power DME can be
collocated with an ILS or MLS where it
provides an accurate distance to
touchdown, similar to that otherwise
provided by ILS marker beacons (and, in
many instances, permitting removal of the
latter).
A newer role for DMEs is DME/DME area
navigation (RNAV).[6][7] Owing to the
generally superior accuracy of DME
relative to VOR, navigation using two
DMEs (using trilateration/distance)
permits operations that navigating with
VOR/DME (using azimuth/distance) does
not. However, it requires that the aircraft
have RNAV capabilities, and some
operations also require an inertial
reference unit.

A typical DME ground transponder for en-

route or terminal navigation will have a
1 kW peak pulse output on the assigned
UHF channel.
Hardware

DME distance and VOR/ADF cockpit display instruments

The DME system comprises a UHF (L-

band) transmitter/receiver (interrogator) in
the aircraft and a UHF (L-band)
receiver/transmitter (transponder) on the
ground.
Timing

Search mode

150 interrogation pulse-pairs per second.

The aircraft interrogates the ground
transponder with a series of pulse-pairs
(interrogations) and, after a precise time
delay (typically 50 microseconds), the
ground station replies with an identical
sequence of pulse-pairs. The DME receiver
in the aircraft searches for reply pulse-
pairs (X-mode = 12-microsecond spacing)
with the correct interval and reply pattern
to its original interrogation pattern. (Pulse-
pairs that are not coincident with the
individual aircraft's interrogation pattern
e.g. not synchronous, are referred to as
filler pulse-pairs, or squitter. Also, replies
to other aircraft that are therefore non-
synchronous also appear as squitter).

Track mode

Less than 30 interrogation Pulse-pairs per

second, as the average number of pulses
in SEARCH and TRACK is limited to max
30 pulse pairs per second. The aircraft
interrogator locks on to the DME ground
station once it recognizes a particular
reply pulse sequence has the same
spacing as the original interrogation
sequence. Once the receiver is locked on,
it has a narrower window in which to look
for the echoes and can retain lock.

Distance calculation
A radio signal takes approximately 12.36
microseconds to travel 1 nautical mile
(1,852 m) to the target and back. The time
difference between interrogation and reply,
minus the 50 microsecond ground
transponder delay and the pulse width of
the reply pulses (12 microseconds in X
mode and 30 microseconds in Y mode), is
measured by the interrogator's timing
circuitry and converted to a distance
measurement (slant range), in nautical
miles, then displayed on the cockpit DME
display.

The distance formula, distance = rate *

time, is used by the DME receiver to
calculate its distance from the DME
ground station. The rate in the calculation
is the velocity of the radio pulse, which is
the speed of light (roughly
300,000,000 m/s or 186,000 mi/s). The
time in the calculation is ½(total time −
reply delay).
Accuracy

Accuracy of various aviation navigation systems

The accuracy of DME ground stations is

185 m (±0.1 nmi).[8] It's important to
understand that DME provides the physical
distance between the aircraft antenna and
the DME transponder antenna. This
distance is often referred to as 'slant
range' and depends trigonometrically upon
the aircraft altitude above the transponder
as well as the ground distance between
them.

For example, an aircraft directly above the

DME station at 6,076 ft (1 nmi) altitude
would still show 1.0 nmi (1.9 km) on the
DME readout. The aircraft is technically a
mile away, just a mile straight up. Slant
range error is most pronounced at high
altitudes when close to the DME station.

Radio-navigation aids must keep a certain

degree of accuracy, given by international
standards, FAA,[9] EASA, ICAO, etc. To
assure this is the case, flight inspection
organizations check periodically critical
parameters with properly equipped aircraft
to calibrate and certify DME precision.

ICAO recommends accuracy of less than

the sum of 0.25 nmi plus 1.25% of the
distance measured.

Specification
A typical DME ground-based responder
beacon has a limit of 2700 interrogations
per second (pulse pairs per second – pps).
Thus it can provide distance information
for up to 100 aircraft at a time—95% of
transmissions for aircraft in tracking mode
(typically 25 pps) and 5% in search mode
(typically 150 pps). Above this limit the
transponder avoids overload by limiting
the sensitivity (gain) of the receiver.
Replies to weaker (normally the more
distant) interrogations are ignored to lower
the transponder load.

Radio frequency and

modulation data
DME frequencies are paired to VOR
frequencies and a DME interrogator is
designed to automatically tune to the
corresponding DME frequency when the
associated VOR frequency is selected. An
airplane's DME interrogator uses
frequencies from 1025 to 1150 MHz. DME
transponders transmit on a channel in the
962 to 1213 MHz range and receive on a
corresponding channel between 1025 and
1150 MHz. The band is divided into 126
channels for interrogation and 126
channels for reply. The interrogation and
reply frequencies always differ by 63 MHz.
The spacing of all channels is 1 MHz with
a signal spectrum width of 100 kHz.

Technical references to X and Y channels

relate only to the spacing of the individual
pulses in the DME pulse pair, 12
microsecond spacing for X channels and
30 microsecond spacing for Y channels.
DME facilities identify themselves with a
1,350 Hz Morse code three letter identity.
If collocated with a VOR or ILS, it will have
the same identity code as the parent
facility. Additionally, the DME will identify
itself between those of the parent facility.
The DME identity is 1,350 Hz to
differentiate itself from the 1,020 Hz tone
of the VOR or the ILS localizer.

DME transponder types

The U.S. FAA has installed three DME
transponder types (not including those
associated with a landing system):
Terminal transponders (often installed at
an airport) typically provide service to a
minimum height above ground of 12,000
feet (3,700 m) and range of 25 nautical
miles (46 km); Low altitude transponders
typically provide service to a minimum
height of 18,000 feet (5,500 m) and range
of 40 nautical miles (74 km); and High
altitude transponders, which typically
provide service to a minimum height of
45,000 feet (14,000 m) and range of 130
nautical miles (240 km). However, many
have operational restrictions largely based
on line-of-sight blockage, and actual
performance may be different.[10] The U.S.
Aeronautical Information Manual states,
presumably referring to high altitude DME
transponders: "reliable signals may be
received at distances up to 199 nautical
miles [369 km] at line−of−sight altitude".

DME transponders associated with an ILS

or other instrument approach are intended
for use during an approach to a particular
runway, either one or both ends. They are
not authorized for general navigation;
neither a minimum range nor height is
specified.

Frequency
usage/channelization
DME frequency usage, channelization and
pairing with other navaids (VOR, ILS, etc.)
are defined by ICAO.[1] 252 DME channels
are defined by the combination of their
interrogation frequency, interrogation
pulse spacing, reply frequency, and reply
pulse spacing. These channels are labeled
1X, 1Y, 2X, 2Y, ... 126X, 126Y. X channels
(which came first) have both interrogation
and reply pulse pairs spaced by 12
microseconds. Y channels (which were
added to increase capacity) have
interrogation pulse pairs spaced by 36
microseconds and reply pulse pairs
spaced by 30 microseconds.

A total of 252 frequencies are defined (but

not all used) for DME interrogations and
replies—specifically, 962, 963, ... 1213
megahertz. Interrogation frequencies are
1025, 1026, ... 1150 megahertz (126 total),
and are the same for X and Y channels.
For a given channel, the reply frequency is
63 megahertz below or above the
interrogation frequency. The reply
frequency is different for X and Y
channels, and different for channels
numbered 1-63 and 64-126.

Not all defined channels/frequencies are

assigned. There are assignment 'holes'
centered on 1030 and 1090 megahertz to
provide protection for the secondary
surveillance radar (SSR) system. In many
countries, there is also an assignment
'hole' centered on 1176.45 megahertz to
protect the GPS L5 frequency. These three
'holes' remove approximately 60
megahertz from the frequencies available
for use.

Precision DME (DME/P), a component of

the Microwave Landing System, is
assigned to Z channels, which have a third
set of interrogation and reply pulse
spacings. The Z channels are multiplexed
with the Y channels and do not materially
affect the channel plan.
Future
In 2020 one company presented its 'Fifth-
Generation DME'. Although compatible
with existing equipment, this iteration
provides greater accuracy (down to 5
meters using DME/DME triangulation),
with a further reduction to 3 meters using
a further refinement. The 3-meter
equipment is being considered as part of
Europe's SESAR project, with possible
deployment by 2023.

In the twenty-first century, aerial navigation

has become increasingly reliant on
satellite guidance. However, ground-based
navigation will continue, for three reasons:

The satellite signal is extremely weak,

can be spoofed, and is not always
available;
A European Union rule requires member
states to keep and maintain ground-
based navigation aids;
A feeling of sovereignty, or control over
a state's own navigational means.
"Some states want navigation over their
territory to rely on means they control.
And not every country has its
constellation like the U.S.' GPS or
Europe's Galileo."
One advantage of the fifth-generation
equipment proposed in 2020 is its ability
to be function-checked by drone flights,
which will significantly reduce the expense
and delays of previous manned
certification flight tests.[11]

See also
Automatic dependent surveillance –
broadcast (ADS-B)
Gee-H (navigation)
Instrument flight rules (IFR)
Non-directional beacon (NDB)
Squitter
Transponder landing system (TLS)
References
1. Annex 10 to the Convention on
International Civil Aviation, Volume I –
Radio Navigation Aids; International Civil
Aviation Organization; International
Standards and Recommended Practices.
2. Minimum Operational Performance
Standards for Airborne Distance Measuring
Equipment (DME) Operating Within the
Radio Frequency Range of 960-1215
Megahertz; RTCA; DO-189; 20 September
1985.
3. Distance Measuring Equipment
(DME)Operating Within the Radio
Frequency Range of 960-1215 Megahertz;
European Union Aviation Safety Agency;
ETSO-2C66b; 24 October 2003.
4. "Appendix B: IEEE Standard Letter
Designations for Radar Bands". Handbook
of Frequency Allocations and Spectrum
Protection for Scientific Uses (https://www.
nap.edu/read/21774/chapter/10)
(2nd ed.). National Academies of Sciences,
Engineering, and Medicine. 2015.
doi:10.17226/21774 (https://doi.org/10.17
226%2F21774) . ISBN 978-0-309-37659-4.
5. "Engineer exploded myths in many fields" (h
ttp://www.smh.com.au/national/obituaries/
engineer-exploded-myths-in-many-fields-20
130108-2cell.html) . 9 January 2013 – via
The Sydney Morning Herald.
6. U.S. Terminal and En Route Area Navigation
(RNAV) Operations; Federal Aviation
Administration; Advisory Circular AC 90-
100A; 1 March 2007.
7. "DME/DME for Alternate Position,
Navigation, and Timing (APNT)" (https://ww
w.faa.gov/about/office_org/headquarters_o
ffices/ato/service_units/techops/navservic
es/gnss/library/documents/APNT/media/2
0120723APNT_DMEWhitePaper_dc.pdf) ,
Robert W. Lilley and Robert Erikson, Federal
Aviation Administration, White Paper,
undated
8. U.S. Department of Defense and
Department of Transportation (December
2001). "2001 Federal Radionavigation
Systems" (http://www.navcen.uscg.gov/pd
f/frp/frp2001/FRS2001.pdf) (PDF).
Retrieved 5 July 2011.
 9. U.S. Federal Aviation Administration (2
September 1982). "U.S. National Aviation
Standard for the VOR/DME/TACAN
Systems" (http://www.faa.gov) .
10. Aeronautical Information Manual (http://ww
w.faa.gov/atpubs) Archived (https://web.a
rchive.org/web/20080905095605/http://w
ww.faa.gov/atpubs) 5 September 2008 at
the Wayback Machine; Federal Aviation
Administration; 12 October 2017.
11. Thales Introduces Fifth-Generation DME (ht
tps://aviationweek.com/shows-events/worl
d-atm-congress/thales-introduces-fifth-gen
eration-dme?elq2=8e453c953db24c219c9e
f0946749734c) (AW&ST, 11 March 2020)
External links
DME Basics (http://www.avweb.com/ne
ws/avionics/183230-1.html)
UK Navaids Gallery with detailed
Technical Descriptions of their operation
(http://www.trevord.com/navaids/)
Flash based instrument simulator with
DME (http://www.luizmonteiro.com/Lea
rning_VOR_Sim_2.aspx)
U.S. National Aviation Handbook for the
VOR/DME/TACAN Systems (http://www.
faa.gov/regulations_policies/orders_noti
ces/index.cfm/go/document.informatio
n/documentID/12082)
 A free online VOR and ADF simulator
with DME (https://www.fergonez.net/pro
jects/ifrsimulator/)

Retrieved from
"https://en.wikipedia.org/w/index.php?
title=Distance_measuring_equipment&oldid=11504
32347"

This page was last edited on 18 April 2023, at

04:18 (UTC). •
Content is available under CC BY-SA 3.0 unless
otherwise noted.