Distance Measuring Equipment
Overview
 
Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay
Propagation delay
Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...

 of VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...

 or UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...

 radio signals.

Developed in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, it was invented by Edward George "Taffy" Bowen
Edward George Bowen
Edward George 'Taffy' Bowen, CBE, FRS was a British physicist who made a major contribution to the development of radar, and so helped win both the Battle of Britain and the Battle of the Atlantic...

 while employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation
Commonwealth Scientific and Industrial Research Organisation
The Commonwealth Scientific and Industrial Research Organisation is the national government body for scientific research in Australia...

 (CSIRO). Another engineered version of the system was deployed by Amalgamated Wireless Australasia Limited
Amalgamated Wireless Australasia Limited
Amalgamated Wireless Ltd . Throughout most of the 20th century AWA was Australia's largest and most prominent electronics organisation, undertaking development, manufacture and distribution of radio, telecommunications, television and audio equipment as well as broadcasting services.After the...

 in the early 1950s operating in the 200 MHz VHF band.
Encyclopedia
Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay
Propagation delay
Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...

 of VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...

 or UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...

 radio signals.

Developed in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, it was invented by Edward George "Taffy" Bowen
Edward George Bowen
Edward George 'Taffy' Bowen, CBE, FRS was a British physicist who made a major contribution to the development of radar, and so helped win both the Battle of Britain and the Battle of the Atlantic...

 while employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation
Commonwealth Scientific and Industrial Research Organisation
The Commonwealth Scientific and Industrial Research Organisation is the national government body for scientific research in Australia...

 (CSIRO). Another engineered version of the system was deployed by Amalgamated Wireless Australasia Limited
Amalgamated Wireless Australasia Limited
Amalgamated Wireless Ltd . Throughout most of the 20th century AWA was Australia's largest and most prominent electronics organisation, undertaking development, manufacture and distribution of radio, telecommunications, television and audio equipment as well as broadcasting services.After the...

 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
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...

, except in reverse. The system was a post-war development of the IFF (identification friend or foe
Identification friend or foe
In telecommunications, identification, friend or foe is an identification system designed for command and control. It is a system that enables military and national interrogation systems to identify aircraft, vehicles, or forces as friendly and to determine their bearing and range from the...

) systems of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. 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
VHF omnidirectional range
VOR, short for VHF omnidirectional radio range, is a type of radio navigation system for aircraft. A VOR ground station broadcasts a VHF radio composite signal including the station's identifier, voice , and navigation signal. The identifier is typically a two- or three-letter string in Morse code...

.
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 also be co-located with an ILS glide slope
Instrument Landing System
An instrument landing system is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument...

 or localizer where it provides an accurate distance function, similar to that otherwise provided by ILS Marker Beacons.

Hardware

The DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter (transponder
Transponder
In telecommunication, the term transponder has the following meanings:...

) on the ground.

Timing

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 reply pulse-pairs. The DME receiver in the aircraft searches for pulse-pairs (X-mode= 12 microsecond spacing) with the correct time interval between them, which is determined by each individual aircraft's particular interrogation pattern. The aircraft interrogator locks on to the DME ground station once it understands that the particular pulse sequence is the interrogation sequence it sent out originally. 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 pulse takes around 12.36 microseconds to travel 1 nautical miles (1,852 m) to and from; this is also referred to as a radar-mile. The time difference between interrogation and reply 1 nautical miles (1,852 m) minus the 50 microsecond ground transponder delay is measured by the interrogator's timing circuitry and translated into a distance measurement (slant range
Slant range
In radio electronics especially radar terminology, slant range is the line-of-sight distance between two points which are not at the same level relative to a specific datum....

), stated in nautical miles, and 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 300000000 metre per second). The time in the calculation is (total time - 50µs)/2.

Specification

A typical DME transponder can provide distance information to 100 aircraft at a time. Above this limit the transponder avoids overload by limiting the gain of the receiver. Replies to weaker more distant interrogations are ignored to lower the transponder load. The technical term for overload of a DME station caused by large numbers of aircraft is station saturation.

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 1150 MHz range and receive on a corresponding channel between 962 to 1213 MHz.
The band is divided into 126 channels
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...

 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 1350 Hz morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

 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 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer.

Accuracy

The accuracy of DME ground stations is 185 m (±0.1 nmi
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...

). 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 6000 ft (1 nmi) altitude would still show 1 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, EASA
European Aviation Safety Agency
The European Aviation Safety Agency is an agency of the European Union with offices in Cologne, Germany, which has been given regulatory and executive tasks in the field of civilian aviation safety. It was created on 15 July 2002, and it reached full functionality in 2008, taking over functions...

, ICAO
International Civil Aviation Organization
The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...

, etc. To assure this is the case, flight inspection
Flight inspection
The evaluation process, using properly equipped aircraft, regarding continuity, integrity and accuracy of significant parameters from radio navigation aids and procedures, aiming their calibration with international standards....

 organizations check periodically critical parameters with properly equipped aircraft to calibrate and certify DME precision.

ICAO
International Civil Aviation Organization
The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...

 recommends accuracy of 0.25 nmi plus 1.25% of the distance measured.

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
Instrument Landing System
An instrument landing system is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument...

 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
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 and Galileo
Galileo positioning system
Galileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...

.

See also

  • Global Positioning Satellite (GPS)
  • Instrument flight rules
    Instrument flight rules
    Instrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....

     (IFR)
  • Instrument Landing System
    Instrument Landing System
    An instrument landing system is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument...

     (ILS)
  • Non-directional beacon
    Non-directional beacon
    A non-directional beacon is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF...

     (NDB)
  • Tactical Air Navigation
    Tactical Air Navigation
    A tactical air navigation system, commonly referred to by the acronym TACAN, is a navigation system used by military aircraft. It provides the user with bearing and distance to a ground or ship-borne station. It is a more accurate version of the VOR/DME system that provides bearing and range...

     (TACAN)
  • VHF omnidirectional range
    VHF omnidirectional range
    VOR, short for VHF omnidirectional radio range, is a type of radio navigation system for aircraft. A VOR ground station broadcasts a VHF radio composite signal including the station's identifier, voice , and navigation signal. The identifier is typically a two- or three-letter string in Morse code...

    (VOR)

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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