DMEDistance Measuring Equipment

Check the courseguide for references

DME – Distance Measuring Equipment

DME is a RADAR beacon (secondary RADAR) system providing continuous indication of slant range to a selected beacon.

The aircraft carries an “Interrogator”. This device transmits pulse pairs to a ground station, called a “Transponder” or “Ground Beacon” which responds with a reply on another frequency after a short delay.

Distance Measurement

The reply pulses are picked up by the airborne receiver which measures time interval between transmission and reception.

The distance displayed by the DME indicator is the actual distance to the DME ground station, and not the map distance.

DME

Most DME systems also calculate ground speed and time to station.

DME system operates in the frequency range of 962 to 1213 MHz using pulse transmission.

A DME system

Some questions

How can the ground transmitter discriminate between so many aircraft without mixing them up?

How does the airborne equipment distinguish its own message from all the others?

Jitter!

Jitter and DME interrogation

DME signal UHF 962 – 1213MHz

The ground station is capable of transmitting about 2700 pulse pairs per second.

Identification of the ground beacon is by Morse code burst of three letters at an audio tone of 1350 Hz. Once every 30s the beacon transmits its identity.

When there is not enough aircraft to provide the proper number of pulse pairs in the form of replies,the ground station inserts dummy pulses, called squitter.

DME - operation

The airborne receiver receives 2700 pulse pairs of 252 channels, from any ground station, regardless of how many aircraft are using that station.

Automatic Standby: Automatic standby is a mode in the airborne DME where the receiver is operating but the transmitter is inhibited until the received signal has a minimum number of pulse pairs.

Search: is a mode in the DME where the system scans from 0 miles to the outer range for a reply pulse pair after transmitting an interrogation pulse pair.

DME - operation

The pulse pairs received by the airborne interrogator: replies the replies of other aircraftSquitter

How the interrogator can determine which of 2700 pulse pairs that occur at each sec are valid replies to an interrogation?

Distance Measurement

DME – percentage reply

Percentage reply: Only a percentage of the interrogation pulses will be answered by replies.

DME range is:Line of sight function of aircraft altitude.

Memory: facility which allows the distance to be displayed for some seconds even though valid replies may no longer be received.

In standby mode

DME systems are designed to go into automatic standby mode. In this mode the receiver portion of the circuit is waiting for a transmission from a ground station.

The ground station transmits the squitter signal continuously, even when not responding to any aircraft interrogations. The airborne system is simply waiting to receive that signal before proceeding from the automatic standby mode to the search mode.

DME System components

A DME airborne system consists of:Transmitter/receiverControllerIndicatorInterrogator andAntenna

Airbourne components

Transmitter/receiver

DME RT unit contains all the necessary circuits to generate, amplify and transmit the interrogating pulse pairs.

The receiver section contains the circuits required to receive, amplify and decode the received reply pulses.

The computation circuits contained within the DME RT unit determine the validity of the reply pulses and calculate the distance.

Aural station identification outputs are supplied to the aircraft audio system.

Controller

The VHF navigation controller is used to tune the DME receiver/transmitter

The VHF frequency of the X channels is tuned in 100 kHz increments at the 100 kHz VHF spacing

The frequency of the Y channels is tuned in 100 kHz increments at the 50 kHz VHF spacing

This can be summarised as:– VHF frequencies of 108.00, 108.10, 108.20 and

upwards will select DME X channels– VHF frequencies of 108.05, 108.15, 108.25 and

upwards will select DME Y channels.

Indicator

Provides flight crews with a readout of aircraft distance from the tuned DME ground station expressed in nautical miles. The indicator will also display a flag or other warning if the system is malfunctioning or not locked on a reply signal.

Some also indicate ground speed and time to station TTS

Antenna

A single, L-band omnidirectional antenna is used for both transmission and reception of DME signals.

DME RT

transmitter receiver decoder coincidence counter range computer.

Block diagram DME RT

Transmitter

Receiver

Decoder

Coincidence counter

Range computer

DME controls and indicators

DME test equipment

Can be used to test ATC equipment while it is installed in the aircraft.

Questions

Define jitter. What part does jitter play in the operation of a DME system?Why is it necessary to inhibit the receive section of all DME RT units on the aircraft during signal transmission from only one?Describe the interrelationship between the coincidence counter, the range computer and the variable delay circuit.What DME indications would the flight crew see when the system is in the search mode?

List the major components of the DME system.

True or false? The same ramp test set can be used to check ATC transponder & DME systems. Explain your answer

Questions

An aircraft DME receives a return pulse pair signal 445.488 microseconds after transmission. What is the distance displayed on the DME indicator?

If a radio waves take 12.359 microseconds to travel one nautical mile and return.

50 microseconds is the time taken for the ground station to respond to the interrogation.

Questions

Answer

Distance (in nautical miles) = (elapsed time - 50 microseconds)12.359

= (445.488 - 50) 12.359

= 395.488 12 359

= 32The DME distance indicator would read 32 nautical miles.

An aircraft DME receives a return pulse pair signal 445.488 microseconds after transmission. What is the distance displayed on the DME indicator?

Question

Answer

Distance (in nautical miles) = (elapsed time) 12.359

= (445.488) 12.359

= 36.0456348The DME distance indicator would read 36 nautical miles.

I think this is the right answer

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