Distance Measuring Equipment

Distance Measuring Equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of UHF radio signals. It was invented by Edward George "Taffy" Bowen and V.D. Burgmann. DME is similar to secondary radar, except in reverse, and a post-war development of the IFF (Identification friend or foe) systems of World War II. DME is functionally identical to the distance 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 colocated with VORs. A typical DME ground transponder system for enroute or terminal navigation will have a 1 kW peak pulse output on the assigned UHF channel.

A low power DME can also be colocated with an ILS localizer where it provides an accurate distance function, similar to that otherwise provide by ILS Marker Beacons.

The DME system has a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter (transponder) in the ground station. The interrogator transmits interrogation pulses to the transponder, which in reply transmits a sequence of reply pulses with a precise time delay. The DME receiver then searches for two pulses with the correct time interval between them. Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock. The time difference between interrogation and reply is measured by the interrogator and translated into a distance measurement which is displayed in the cockpit.

A typical DME transponder can provide concurrent distance information to about 100 aircraft. 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.

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies. A DME interrogator is designed to automatically tune to the corresponding frequency when the associated VOR 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 for interrogation and 126 channels for transponder replies. 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 36 microsecond spacing for Y channels.

DME facilities identify themselves with a 1350 Hz morse code three letter ident. If collocated with a VOR or ILS it will have the same ident as the parent facility. Additionally, the DME will identify itself between idents of the parent facility. It is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer.

Accuracy

Accuracy of DME is 185 m (±0.1 nm). One important thing to understand is 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 feet altitude would still show one mile on the DME readout. The aircraft technically is a mile away, just a mile straight up. Slant range error is most pronounced at high altitudes when close to the DME station.

Future

It is likely that DME installations will be phased out when space based navigational systems such as GPS and Galileo become widely used for aviation.

Operation

Accuracy

Future

See also

References

External links

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Operation

Accuracy

Future

See also

References

External links