Distance measuring equipment 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 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. 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), 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 systems
of World War II. To maintain compatibility, DME is functionally
identical to the distance measuring component of TACAN.
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 co-located with VORs. A typical DME ground transponder system
for en-route or terminal navigation will have a 1 kW peak pulse output on the
assigned UHF channel. A low-power DME can be co-located 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. Hardware
The DME system is composed of a VHF transmitter/receiver in the aircraft and
a VHF receiver/transmitter on the ground.
Timing SEARCH MODE: 150 interrogation
pulse-pairs per second. The aircraft interrogates the ground
transponder with a series of pulse-pairs and, after a precise time delay, the
ground station replies with an identical sequence of pulse-pairs. The DME
receiver in the aircraft searches for reply pulse-pairs with the correct
interval and reply pattern to its original interrogation pattern..
TRACK MODE: less than 30 interrogation Pulse-pairs per second, as the average
amount 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
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, 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. The time in the calculation is/2.
Accuracy The accuracy of DME ground stations is
185 m. 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 altitude
would still show 1.0 nmi 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, 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 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 1,025
to 1,150 MHz. DME transponders transmit on a channel in the 962 to 1,213 MHz
range and receive on a corresponding channel between 1,025 to 1,150 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.
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 GPS and Galileo.
See also Global Positioning Satellite
Instrument flight rules Transponder Landing System
Instrument Landing System Non-directional beacon
Tactical Air Navigation VHF omnidirectional range
Squitter References
External links DME Basics
UK Navaids Gallery with detailed Technical Descriptions of their
operation Flash based instrument simulator with
DME U.S. National Aviation Handbook for the
VORTACAN Systems ICAO Annex 10 Volume 1 International
Standards & Recommended Practices