This article is about the aviation equipment. For more general usage, see Distance measurement.
D-VOR/DME ground stationDME antenna beside the DME transponder shelter
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 63MHz and having specified separation.[1]
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 200MHz 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 1kW 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 (1,852m) to the target and back. The time difference between interrogation and reply, minus the 50 microsecond ground transponder delay and the pulse spacing 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 300,000,000m/s or 186,000mi/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 6,076ft (1 nmi) altitude would still show 1.0nmi (1.9km) 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 0.25nmi plus 1.25% of the distance measured.
Specification
A typical DME ground-based transponder 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 1150MHz. DME transponders transmit on a channel in the 962 to 1213MHz range and receive on a corresponding channel between 1025 and 1150MHz. The band is divided into 126 channels for interrogation and 126 channels for reply. The interrogation and reply frequencies always differ by 63MHz. The spacing and bandwidth of each channel is 1MHz and a bandwidth of 1MHz.
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 1,350Hz 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 1,350Hz to differentiate itself from the 1,020Hz 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 (3,700m) and range of 25 nautical miles (46km); Low altitude transponders typically provide service to a minimum height of 18,000 feet (5,500m) and range of 40 nautical miles (74km); and High altitude transponders, which typically provide service to a minimum height of 45,000 feet (14,000m) and range of 130 nautical miles (240km). 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 [369km] 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:[citation needed]
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."[attribution needed]
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]
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
In aviation, the instrument landing system (ILS) is a precision radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a 1⁄2 mile (800 m) of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. Bringing the aircraft this close to the runway dramatically increases the range of weather conditions in which a safe landing can be made. Other versions of the system, or "categories", have further reduced the minimum altitudes, runway visual ranges (RVRs), and transmitter and monitoring configurations designed depending on the normal expected weather patterns and airport safety requirements.
A non-directional beacon (NDB) or non-directional radio beacon is a radio beacon which does not include inherent directional information. Radio beacons are radio transmitters at a known location, used as an aviation or marine navigational aid. NDB are in contrast to directional radio beacons and other navigational aids, such as low-frequency radio range, VHF omnidirectional range (VOR) and tactical air navigation system (TACAN).
Very High Frequency Omnidirectional Range Station (VOR) is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons. It uses frequencies in the very high frequency (VHF) band from 108.00 to 117.95 MHz. Developed in the United States beginning in 1937 and deployed by 1946, VOR became the standard air navigational system in the world, used by both commercial and general aviation, until supplanted by satellite navigation systems such as GPS in the early 21st century. As such, VOR stations are being gradually decommissioned. In 2000 there were about 3,000 VOR stations operating around the world, including 1,033 in the US, but by 2013 the number in the US had been reduced to 967. The United States is decommissioning approximately half of its VOR stations and other legacy navigation aids as part of a move to performance-based navigation, while still retaining a "Minimum Operational Network" of VOR stations as a backup to GPS. In 2015, the UK planned to reduce the number of stations from 44 to 19 by 2020.
The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower end of the microwave range.
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 from an end-user perspective a more accurate version of the VOR/DME system that provides bearing and range information for civil aviation. The DME portion of the TACAN system is available for civil use; at VORTAC facilities where a VOR is combined with a TACAN, civil aircraft can receive VOR/DME readings. Aircraft equipped with TACAN avionics can use this system for enroute navigation as well as non-precision approaches to landing fields.
Gee-H, sometimes written G-H or GEE-H, was a radio navigation system developed by Britain during World War II to aid RAF Bomber Command. The name refers to the system's use of the earlier Gee equipment, as well as its use of the "H principle" or "twin-range principle" of location determination. Its official name was AMES Type 100.
Airband or aircraft band is the name for a group of frequencies in the VHF radio spectrum allocated to radio communication in civil aviation, sometimes also referred to as VHF, or phonetically as "Victor". Different sections of the band are used for radionavigational aids and air traffic control.
In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as "a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or en route obstacle clearance criteria apply."
The microwave landing system (MLS) is an all-weather, precision radio guidance system intended to be installed at large airports to assist aircraft in landing, including 'blind landings'. MLS enables an approaching aircraft to determine when it is aligned with the destination runway and on the correct glidepath for a safe landing. MLS was intended to replace or supplement the instrument landing systems (ILS). MLS has a number of operational advantages over ILS, including a wider selection of channels to avoid interference with nearby installations, excellent performance in all weather, a small "footprint" at the airports, and wide vertical and horizontal "capture" angles that allowed approaches from wider areas around the airport.
Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply to each interrogation signal by transmitting encoded data such as an identity code, the aircraft's altitude and further information depending on its chosen mode. SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II; therefore, the two systems are still compatible. Monopulse secondary surveillance radar (MSSR), Mode S, TCAS and ADS-B are similar modern methods of secondary surveillance.
The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as altitude and the Squawk Code, a four digit code assigned to each aircraft that enters a region. Information about this aircraft is then entered into the system and subsequently added to the controller's screen to display this information when queried. This information can include flight number designation and altitude of the aircraft. ATCRBS assists air traffic control (ATC) surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft and to distinguish those returns from ground clutter.
The local-area augmentation system (LAAS) is an all-weather aircraft landing system based on real-time differential correction of the GPS signal. Local reference receivers located around the airport send data to a central location at the airport. This data is used to formulate a correction message, which is then transmitted to users via a VHF Data Link. A receiver on an aircraft uses this information to correct GPS signals, which then provides a standard instrument landing system (ILS)-style display to use while flying a precision approach. The FAA has stopped using the term LAAS and has transitioned to the International Civil Aviation Organization (ICAO) terminology of ground-based augmentation system (GBAS). While the FAA has indefinitely delayed plans for federal GBAS acquisition, the system can be purchased by airports and installed as a Non-Federal navigation aid.
A transponder landing system (TLS) is an all-weather, precision landing system that uses existing airborne transponder and instrument landing system (ILS) equipment to create a precision approach at a location where an ILS would normally not be available.
An equipment code describes the communication (COM), navigation (NAV), approach aids and surveillance transponder equipment on board an aircraft. These alphabetic codes are used on FAA and ICAO flight plan forms to aid Flight service station (FSS) personnel in their handling of aircraft.
Squitter refers to random pulses, pulse-pairs and other non-solicited messages used in various aviation radio systems' signal maintenance. Squitter pulses were originally, and are still, used in the DME/TACAN air navigation systems. Squitter pulses, because of their randomness and identical appearance to standard reply pulse-pairs, appear the same as unsolicited/unsynchronised replies to other interrogating aircraft.
A portable collision avoidance system (PCAS) is an aircraft collision avoidance system similar in function to traffic collision avoidance system (TCAS). TCAS is the industry standard for commercial collision avoidance systems but PCAS is gaining recognition as an effective means of collision avoidance for general aviation and is in use the world over by independent pilots in personally owned or rented light aircraft as well as by flight schools and flying clubs. Its main competitor is FLARM.
In radio navigation, a VOR/DME is a radio beacon that combines a VHF omnidirectional range (VOR) with a distance-measuring equipment (DME). The VOR allows the receiver to measure its bearing to or from the beacon, while the DME provides the slant distance between the receiver and the station. Together, the two measurements allow the receiver to compute a position fix.
Automatic Dependent Surveillance–Broadcast (ADS-B) is an aviation surveillance technology and form of Electronic Conspicuity in which an aircraft determines its position via satellite navigation or other sensors and periodically broadcasts its position and other related data, enabling it to be tracked. The information can be received by air traffic control ground-based or satellite-based receivers as a replacement for secondary surveillance radar (SSR). Unlike SSR, ADS-B does not require an interrogation signal from the ground or from other aircraft to activate its transmissions. ADS-B can also receive point-to-point by other nearby equipped "ADS-B In" equipped aircraft to provide traffic situational awareness and support self-separation. ADS-B is "automatic" in that it requires no pilot or external input to trigger its transmissions. It is "dependent" in that it depends on data from the aircraft's navigation system to provide the transmitted data.
IFF Mark X was the NATO standard military identification friend or foe transponder system from the early 1950s until it was slowly replaced by the IFF Mark XII in the 1970s. It was also adopted by ICAO, with some modifications, as the civilian air traffic control (ATC) secondary radar (SSR) transponder. The X in the name does not mean "tenth", but "eXperimental". Later IFF models acted as if it was the tenth in the series and used subsequent numbers.
References
1 2 3 4 Annex 10 to the Convention on International Civil Aviation, Volume I – Radio Navigation Aids; International Civil Aviation Organization; International Standards and Recommended Practices.
↑ 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.
↑ Distance Measuring Equipment (DME)Operating Within the Radio Frequency Range of 960-1215 Megahertz; European Union Aviation Safety Agency; ETSO-2C66b; 24 October 2003.
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