Distance Measuring Equipment (DME) overview & functions

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Distance Measuring Equipment (DME)

 The distance between the aircraft and the DME unit at the ground station is calculated by a DME system and displayed on the flight deck. When the aircraft is flying to the station, it can also show the calculated aircraft speed and elapsed time for arrival.

DME Components

DME ground stations have subsequently been installed at VOR stations as well as in conjunction with ILS localizers. These systems are known as VOR/DME and ILS/DME or LOC/DME. The ILS/DME system used during approach to the runway during landings. The DME system consists of an airborne DME transceiver, display unit, and antenna (in aircraft), as well as the ground based DME unit and its antenna.

How DME works?

The DME is very helpful because a pilot can positively identify the location of the aircraft using the bearing from the VOR and the distance to a known point ( from DME antenna at the VOR to the distance from aircraft antenna). DME (Distance Measuring Equipment) operates in the UHF frequency range from 962 MHz to 1213 MHz. A carrier signal wave transmitted from the aircraft unit is modulated with a string of integration pulses. The ground DME unit receives the pulses and returns a signal to the aircraft. The time that transpires for the signal to be sent and returned between aircraft & ground unit is calculated and converted into nautical miles for display. The time to reach and the aircraft's speed are both computed and displayed on a dedicated DME display, as well as an EHSI, EADI, EFIS, or the primary flight display in a glass cockpit.

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DME Slant & Actual Distance

The co-located VOR or VORTAC frequency is coupled with the DME frequency. The DME is automatically tuned when the VOR signal is tuned to the correct frequency. The VOR station identification tones are broadcast first, followed by the DME tones. While the VOR selector is tuned to a different VOR, the hold selector on a DME panel keeps the DME tuned in. In most situations, a small blade-type antenna mounted to the underside of the fuselage centerline transmits and receives the DME's UHF.

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VOR/ DME Ground Location

The distance between the DME transmitter antenna and the aeroplane is displayed on a typical DME, The slant distance is what it's called. It's quite accurate. The distance to the DME ground antenna from a position directly beneath the aeroplane is shorter since the aircraft is at altitude. Some newer DMEs can calculate and show this ground Distance.

Also Read

Aircraft Flight instruments | Six Basic Flight Instruments

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