﻿ Distance Measuring Equipment

# Distance Measuring Equipment

Distance Measuring Equipment (DME, English for distance measuring device ) is a radio station of the radionavigation service, whose releases are intended to allow an aircraft to determine its distance with respect to the DME. In this case, the slant range of the aircraft is determined by a DME ground station by runtime measurement. The DME is based on the secondary radar technology.

A DME is often used in conjunction with a rotating beacon (VOR). A combined ground station with the VOR transmitter and DME transponder is called VOR / DME.

The technical principle of DME also comes in the TACAN distance measuring portion of the application.

## Hardware

A DME system consists of a UHF-Transmitter/Receiver-Abfragegerät ( interrogator ) in the aircraft and the UHF-Receiver/Transmitter (transponder ) at the bottom.

## Principle of operation

DME operate in the frequency range 962-1213 MHz ( divided into 126 X and 126 Y channels), which is assigned to the aeronautical radionavigation service on primary use.

For determining the distance the on-board device sends pulse pairs with a random repetition rate to the ground station. These will be answered after a predetermined response delay ( 50 microseconds ) and appear next to the answers to the queries from other devices on the board -board receiver. The onboard apparatus filtered from the mixture of all the response signals out, which appear continuously with a ( nearly ) constant time interval for its own query in the receiver and measures the time interval between the transmission of the original signal and the reception of the response. This time interval (less response delay ) is the duration of the radio waves from the on-board unit to the ground station and back. By multiplying by the speed of light it is converted into the distance to the ground station.

Accordingly, the following calculation to calculate the slant range to the DME station:

With

In order to calculate the distance above ground, one applies the Pythagorean theorem:

With

The measured distance is displayed on the aircraft instrument in the cockpit in nautical miles.

Important for the use of DME information is the understanding that not the distance above ground, but resulting from the triangular relationship of altitude and distance on the ground slant range (English slant range) is displayed. Directly above the station so the altitude is displayed as the distance ( 1 → NM ≈ 6,076 ft). The measurement error is approximately 45 to 100 m, with precision equipment 50 m.

## Instrument approach

If DME used in conjunction with a VOR or NDB for instrument approach, the weather conditions have to be better than the minimum conditions of an Instrument Landing System category I.

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