Distance measuring equipment

thumb|D-VOR/DME ground station

thumb|upright|DME 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-sight 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 separation.

DME systems are used worldwide, using standards set by the International Civil Aviation Organization (ICAO), the European Union Aviation Safety Agency (EASA) 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 co-located with a VHF omnidirectional range (VOR) transmitter in a single ground station, designated as VOR/DME. When this occurs, the frequencies of the VOR and DME equipment are paired. 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.

Developed in Australia, DME was invented by James "Gerry" Gerrand. In 1945, after Edward George Bowen became Chief of the Division of Radiophysics at CSIRO, Brian Cooper developed DME. Based upon the Rebecca/Eureka transponding radar, the 200 MHz system weighed less, and used a dial instead of a screen. In 1946, the Provisional International Civil Aviation Organization adopted the VOR and DME airways model. In 1947, the system was deployed on commercial aircraft, and by 1953, all commercial aircraft in Australia were so equipped.

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 co-located with VORs or VORTACs. A low-power (100 W) DME can be co-located with an ILS or MLS, where it provides an accurate distance to touchdown, similar to that otherwise provided by ILS marker beacons.

A newer role for DMEs is DME/DME area navigation (RNAV). 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 1 kW peak pulse output on the assigned UHF channel.

Hardware

thumb|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 µs), 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 µs spacing) that match 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 that 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 µs to travel to the target and back. The time difference between interrogation and reply minus the 50 µs ground transponder delay, and the pulse spacing of the reply pulses (12 µs in X mode and 30 µs 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 = speed × time, is used by the DME receiver to calculate its distance from the DME ground station. The speed in the calculation is the propagation speed of the radio pulse, which is the speed of light (roughly ). The time in the calculation is ½(total time − reply delay), hence the distance is c × ½(total time − reply delay), where c is the speed of light.

Accuracy

thumb|Accuracy of various aviation navigation systems, of which only GPS, TACAN, inertial, and VOR/DME are still in use.

The accuracy of DME ground stations is 185 m (±0.1 nmi). 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 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 that the error be no larger than (0.25 nmi + 1.25% × actual distance).

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 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 and bandwidth of each channel is 1 MHz and a bandwidth of 1MHz.

thumb | right | DME - pairing between interrogation frequencies (air) and reply frequencies (ground)

Technical references to X and Y channels relate only to the spacing of the individual pulses in the DME pulse pair: 12 µs spacing for X channels and 30 µs spacing for Y channels.

DME facilities identify themselves with a 1,350 Hz Morse code three-letter identity. If co-located 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 and range of ; Low altitude transponders typically provide service to a minimum height of and range of ; and High altitude transponders, which typically provide service to a minimum height of and range of . However, many have operational restrictions largely based on line-of-sight blockage, and actual performance may be different. The U.S. Aeronautical Information Manual states, presumably referring to high altitude DME transponders: "reliable signals may be received at distances up to 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.

References

External links

DME Basics
UK Navaids Gallery with detailed Technical Descriptions of their operation
U.S. National Aviation Handbook for the VOR/DME/TACAN Systems