Distance Measuring Equipment (DME)
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.
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.
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.
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