Chapter 13

DME

Distance Measuring Equipment

DME OverviewDME provides the aircraft with distance to a station information.

DME is required for aircraft operating at or above 24,000 feet (FL 240).

The aircraft sends out a pulsed signal to the station (interrogation) and the station replies with a pulsed signal.

Random spacing, called “jitter” makes each interrogation unique to each aircraft; the station replies in kind.

By using the change of distance to station, ground speed (GS) is calculated.Using GS and distance, time-to-station (TTS) is calculated.Slant range is the distance to the station not just laterally but vertically as well.A “scanning” DME receives signals from DME stations up to 300 miles away and uses triangulation to compute an accurate fix.

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DME Ground Station

DME is part of TACAN, a military navigation system which adds DME to a VOR station.

A VOR with DME is known as a VORTAC station.

DME Tuning

DME is tuned by tuning either a VOR or ILS station, as applicable.

DME EFIS DisplayHere is a DME readout on an EFIS Display.

The characters “NAV1” indicate that the DME being displayed is from the No. 1 VOR receiver.

NAV 12.3DME

DME Block Diagram: Aircraft

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X and Y Channels

Originally there were 100 DME channels, the X channels.When the system got overloaded, the channels were split to create the Y Channels, adding 100 more channels for a total of 200 channels.Airborne pulse spacing on the X channel is 12 microseconds; on the Y channel is 36 microseconds.Ground Reply pulse spacing on the X channel is 12 microseconds; on the Y channel is 30 microseconds.Whether the airplane is on the X or Y channel, interrogation goes out on 1143 MHz.The reply from the ground station comes back at 1143 MHz on the X channel (+63 MHz).The reply from the ground station comes back at 1080 MHz on the Y channel (-63 MHz).If the ground station receives more interrogations than it can handle (>100), it will reduce it’s receiver sensitivity and not reply to the aircraft on the outer edges of it’s receiving range.

Block Diagram: Ground Station

The ground station receives, decodes and replies to interrogations from the airplane.

There is a 50 Microsecond time delay to avoid interference.

The “squitter” sends out pulses if the ground station is not receiving any aircraft interrogations; this causes any aircraft within range to “wake up” out of automatic standby mode and start interrogating again.

Every 30 seconds, the station transmits a Morse code ID on 1030 Hz

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What’s Wrong?

Textbook page 91

FAR Part 91.205

Flight at and above 24,000 feet MSL (FL 240). If VOR navigation equipment is required under paragraph (d)(2) of this section, no person may operate a U.S.-registered civil aircraft within the 50 states and the District of Columbia at or above FL 240 unless that aircraft is equipped with approved DME or a suitable RNAV system.

DME Arch

DME Arch

Review Q & A Chapter 13 DME13.1 An airborne DME sends out a pulse known as an ____.Answer: Interrogation signal.13.2 DME is a component of a military system known as ____.Answer: TACAN.13.3 A DME station is located as part of a ____ ground station. Together they are

known as a ____.Answer: VOR, VORTAC.13.4 In addition to distance-to-station, an airborne DME computes ___and___.Answer: Ground Speed, Time-to-station13.5 A distance error in DME is called ____.Answer: Slant range.13.6 All aircraft interrogating the same DME ground station are on the same frequency.

How does an aircraft identify its replies from all others?Answer: “Jitter” or random spacing of pulses.13.7 How is a DME station tuned in?Answer: By tuning a VOR or an ILS station.13.8 What happens when more than about 100 aircraft interrogate the same DME

ground station (overload)?Answer: The station reduces its receiver sensitivity and does not respond to aircraft at

the edges of its range.13.9 Why does the DME ground station delay its reply by 50 microseconds?Answer: To avoid interference.13.11 Does the DME station transmit an ID?Answer: Yes.