notes

1. Distance measuring equipment
D-VOR/DME ground station

2. DME antenna beside the DME transponder shelter
Distance measuring equipment (DME) is a transponder-based radio navigation technology that
measures slant range distance by timing the propagation delay of VHF or UHF radio signals.
Developed in Australia, it was invented by James "Gerry" Gerrand[1]
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 to secondary radar, except in reverse. The system was a post-war development of
the IFF (identification friend or foe) systems of World War II. To maintain compatibility, DME is
functionally identical to the distance measuring component of TACAN.

3. Operation
Aircraft use DME to determine their distance from a land-based transponder by sending and
receiving pulse pairs – two pulses of fixed duration and separation. The ground stations are typically
collocated with VORs or ILS systems. A low-power DME can be collocated with an ILS glide slope
antenna installation where it provides an accurate distance to touchdown function, similar to that
otherwise provided by ILS marker beacons.
A typical Distance measuring equipment ground transponder system for en-route or terminal
navigation will have a 1 kW peak pulse output on the assigned UHF channel.
Hardware
The DME system comprises a UHF transmitter/receiver (interrogator) in the aircraft and a UHF
receiver/transmitter (transponder) on the ground.
DME distance and VOR/ADF cockpit display instruments
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.

4. Distance calculation
A radio signal takes approximately 12.36 microseconds to travel 1 nautical mile (1,852 m) to the
target and back—also referred to as a radar-mile. The time difference between interrogation and
reply, minus the 50 microsecond ground transponder delay, 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 (total
time – 50µs)/2.
Accuracy
The accuracy of DME ground stations is 185 m (±0.1 nmi).[2]
It's important to understand that DME
provides the physical distance from the aircraft to the DME transponder. This distance is often
referred to as 'slant range' and depends trigonometrically upon both the altitude above the
transponder and the ground distance from it.
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,[3]
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 transponder can provide distance information to 100 to 200 aircraft at a time. Above
this limit the transponder avoids overload by limiting the sensitivity of the receiver. Replies to weaker
more distant interrogations are ignored to lower the transponder load.
Radio frequency and modulation data
DME frequencies are paired to VHF omnidirectional range (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.

5. Terminal DME
A terminal DME, referred to as a TDME in navigational charts, is a DME that is designed to provide a
0 reading at the threshold point of the runway, regardless of the physical location of the equipment. It
is typically associated with ILS or other instrument approach.
Future
DME operation will continue and possibly expand as an alternate navigation source to space-based
navigational systems such as