DME

Question 1

What does a DME do?

Answer 1

Distance Measuring Equipment (DME) provides continuous and accurate indication in the cockpit of the slant range distance between an aircraft and ground station. It is a type of secondary radar.

Question 2

What is slant range

Answer 2

The direct line distance between the aircraft and the DME ground station.

Question 3

What are the different uses for DME? (6)

Answer 3

More accurate flying of holds and the use of DME arcs.

Providing distance and height checks when flying non-precision approach procedures, e.g. VOR/DME approaches.

Indicating accurate distances to the runway threshold, and heights for distance, when flying an ILS/DME procedure, allowing for a glideslope crosscheck.

Aids radar identification when the pilot reports their position in relation to a VOR or NDB.

Facilitates the separation and control of aircraft in non-radar airspace, based upon a VOR/DME fix reported by individual aircraft.

Can be used for RNAV operations (DME/DME) by using multiple DME’s in range to provide a position solution.

Question 4

Two types of DME?

Answer 4

Low power (<250W)
High power (typically 1kW)

Question 5

Basic operation of DME

Answer 5

utilises the UHF Band between 960 and 1215 MHz. Channels are spaced at 1 Mhz, providing 252 potential frequencies or channels.

The aircraft carries the transmitter – the responder unit is the ground station.

The ground station retransmits the signal at either 63MHz above or below the received frequency, so that the aircraft can distinguish between the outgoing and incoming signals.

The DME operates on the principle of measuring the elapsed time that a radio pulse pair takes to make the round trip from the aircraft to the ground station and back to the aircraft.

Question 6

The aircraft based equipment consist of what?

Answer 6

DME transmitter/receiver unit, a Control Display Unit (CDU) and the DME antenna which has vertically polarised waves

Question 7

The ground equipment of a DME consist of what

Answer 7

Each ground station (normally paired with a VOR or NDB) has a transceiver called a transponder, and an omni-directional aerial system.

It responds to all aircraft that maybe interrogating it.

Question 8

What is PRF?

Answer 8

Pulse recurrence frequency - the number of pulses of a repeating signal in a specific time unit

Question 9

What are the characteristics tuning of the DME

Answer 9

Where the DME and VOR are co-located, two IDENT signals with the same coding will be audible. Both can be identified individually.

The DME is identifier is significantly louder than the VOR, NDB or ILS ident (be warned!). The DME ident plays during every 3rd or 4th repeat of the VOR’s ident, and both will be in sync.

Question 10

The DME CDU typically displays what?

Answer 10

Distance to the DME ground station

Groundspeed toward or away from the DME ground station

Time to the DME ground station

Question 11

Explain the operation of a typical DME controller, including:
(a) tuning with a paired VOR or ILS frequency;

Answer 11

Tune to the navigation aid (VOR or ILS) as normal using the nav radio
 Select RMT (remote) on the DME CDU

Distance/Speed/Time information will now be displayed on the CDU.

Question 12

Explain the operation of a typical DME controller, including: tuning directly to a DME frequency and when its used.

Answer 12

Select FREQ (frequency) on the CDU

Use the selector knobs on the CDU to tune the frequency

Select GS/T (Groundspeed/Time), so that the CDU will display the DME information associated with that frequency.

This will be required, for example, when using a DME with a co-located NDB which does not automatically tune the DME.

Question 13

Explain signal loss in the DME

Answer 13

If the transponder signal is lost, the equipment will unlock and start a new search for an acceptable signal. An OFF flag may be displayed.

Question 14

Explain saturation

Answer 14

It is possible to overload the ground-based transponder, but this requires a large number of aircraft requesting interrogated signals at the same time.

Typically the ground station can handle a maximum of 100 aircraft before saturation occurs.

Above this number only aircraft with strong signals will lock on.

Question 15

What is a TACAN?

Answer 15

A TACAN is a Tactical Air Navigation System, used in military applications, a more accurate version of the VOR/DME system, providing the same information (bearing and slant range distance).

The bearing and ranging functions are combined into the one unit, making it much more compact than a VOR/DME

Question 16

Explain the operation of a typical DME controller, including: tuning a VORTAC frequency

Answer 16

You cannot access the TACAN azimuth information on civilian receivers but DME information is available

Question 17

Explain and calculate the following:

(a) design maximum range;

Answer 17

DME is designed with a maximum range of 200 nm at FL300.

It is generally line-of-sight but affected by topography and the same factors as VHF/UHF radio transmitters.

Question 18

Explain and calculate the following: expected maximum range at different altitudes

Answer 18

DME measures slant rant, at altitude, allowances must be made for the difference between slant range and horizontal range

Beyond 10NM it can be said the DME indication is generally accurate at any altitude.

However, at close ranges, altitude plays a larger part in the slant range distance.

For example, when directly overhead the ground station, the DME will indicate the altitude of the aircraft in nautical miles.

Question 19

Explain and calculate the following: DME distance (slant range) versus horizontal range.

Answer 19

DME Distance2 = Horizontal Distance2 + Aircraft Altitude2 (in Nm)

Horizontal Distance2 = DME Distance2 – Aircraft Altitude2 (in Nm)

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