“Distance measuring equipment”的意思、由来-开放百科全书

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{{clarification|date=March 2019}} separation.

DME systems are used worldwide; their standards are set forth 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 some other countries, 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 VOR (VHF Omnidirectional Range) 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 (VOR and TACAN) installation provides the same capabilities to civil aircraft but also provides 2-D navigation capabilites to military aircraft.

Low-power DME transponders are also associated with some ILS (Instrument Landing System), ILS localizer and MLS (Microwave Landing System) 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 IFF (identification friend or foe) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.

Operation

In their originally intended use, aircraft employ DME to determine their 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 Instrument Landing System, ILS localizer, or Microwave landing system (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 permits operations that navigating with VOR/DME 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

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

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 {{convert|1|nmi|m|0|lk=in}} 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 {{convert|300000000|m/s|mi/s|-3|abbr=on|disp=or|lk=on}}). The time in the calculation is (total time – 50µs)/2.

Accuracy

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 {{convert|1.0|nmi|km|abbr=on}} 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 interrrogations per second (pulse pairs per second – pps). This means that 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 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.

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 and range of 25 nautical miles; Low altitude transponders typically provide service to a minimum height of 18,000 feet and range of 40 nautical miles; and High altitude transponders, which typically provide service to a minimum height of 45,000 feet and range of 130 nautical miles. However, many have operational restrictions largely based on line-of-sight blockage, and actual performance may be different.^[10] The Aeronautical Information Manual states, presumably referring to high altitude DME transponders: "reliable signals may be received at distances up to 199 nautical miles 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.

Future

DME operation will continue and possibly expand as an alternate navigation source to space-based navigational systems such as GPS and Galileo.^[11]