Distance Measurement equipment (DME)

Distance Measuring Equipment

DME 415 & 435 General View

Civil Aviation Institute

Ahmad Sajjad SafiCNS Instructor

Reference:

DME Technical Manual

Introduction

• DME (Distance Measuring Equipment) has been standardized by the ICAO as a radio aid for short and medium-distance navigation.

• It is a secondary type of radar, which allows several aircraft to simultaneously measure their distance from a ground reference (DME transponder).

• The distance is determined by measuring the propagation and delay of a RF pulse, which is emitted by the aircraft transmitter and returned at a different frequency by the ground station after reception.

• The DME 415/435 ground equipment, is constructed by THALES Air Systems Division - Milan - Italy.

DME Concept

• DME provides to aircrafts:

- Straight-line distance to the DME ground station.

- Aircraft ground-speed.

- Time to DME ground station.

(If the aircraft is flying straight to the DME ground station)

DME Concept

DME Ground Station

DME Coverage

• The DME coverage is limited by the line of sight, if there isn’t line of sight between the emitter and the receiver there will not be communication link.

• From 0 to 65 NM radius and above 65 NM.

DME Ground Station

DME Association

• Can be used stand-alone or Master.

• It’s always used in association with: TACAN and VORTAC.

• It’s usually used in association with: VOR or ILS.

DME Block Diagram

Ranging CircuitDistance Display

ReceiverTransmitter

Interrogations

Antenna

Aircraft Cabin

Airborne Interrogator

Interrogations

Replies

Replies

Antenna

ReceiverTransmitter

Delay & Process

DME Transponder

Autopilot

Cabin Indicator

DME Identification Tune

Interrogator Antenna: UHF antenna usually placed on the bottom of the aircraft fuselage.

Basic Principle of DME

• When a signal is sent by the aircraft on board DME (interrogator), the on board DME starts counting the time until it gets a reply from the ground station.

• The resulting time depends of the DISTANCE, the propagation speed and the signal reflections.

• The DME ground station transponder generates replies

(artificial echoes) and sends it back to the aircrafts

(“Reverse” Secondary RADAR principle).


• The time interval between interrogation emission and reply reception provides the aircraft with the real distance information from the ground station.

• This information may be read by the pilot or the navigator directly on the airborne indicator.


• The Ground station is identified by a Morse (3 or 4 letters) coded tone modulated at 1350 Hz.

• DME frequency rang is UHF : 960 MHz to 1215 MHz.

• DME have 252 Channels which are separated by 1 MHz .

• 126 X channel and 126 Y channel.

• The ground station signal frequency answer is always:

[interrogator signal frequency] ± 63MHz

Basic Principle of DME• Pulse Coding

The Interrogator sends pulse pairs with a fixed time separation between the 1st and the 2nd pulse: 12μs (Channel code X) or 36μs (Channel code Y)

– The time separation between pulse pair is randomly generated by the on board DME transmitter.

– The pulse time is ~3,5μs.

An electromagnetic wave takes 6.18μs to complete 1NM.


• The ground transponder is able to answer up to about 200 interrogators at a time.

• Search Mode : On board DME will send 150 pp/s until it finds the ground station.

• Track Mode : When On board DME connected with the ground station so it sends 24 pp/s.

• pp/s= pair pulse / seconds


– Compliant with the ICAO specifications in Annex 10, 5th edition.

Basic Principle of DME• The error is near to zero when the aircraft is far from the ground

station and it increases when the aircraft is near from the ground station in the rang of 0,5NM.

• DME performance is not affected by the weather conditions.

• Control by a Personal Computer (PC) at beacon site, which can be duplicated at remote site.

• Used with below software inside computer:

– WINDOWS SUPERVISOR

– WINDOWS ADRACS SUPERVISOR

– EQUIPMENT MANAGER

Current Situation of DME- KAIA

• DME 415 and DME 435

– Traffic Capacity: 200 aircrafts

– Max pp/s: 4800

• Digitally controlled output pulse shape

• Automatically provided ICAO performance checks at programmable intervals

• The equipment is designed to be safe for the user. No dangerous voltages except mains are used.


• DME 415 associated with

ILS 410 Glide Slope 412

• 100 W solid-state DME

• Operate in Approach

DME Antenna


• DME 435 associated with DVOR 432

• 1 kW solid-state DME

• Operate in En-route

DME Antenna

DMEBEACON COMPOSITION

DME 415 (same as DME 435)Cabinet and PC, example of arrangement

It contains an RS232 connector forinterfacing with a computer / Laptop.

DME - COMPOSITION

DME 415/435 equipment divided

into five main modules:

• RF Path

• Interface System

• Transponder and Monitor

• Local I/O

• Power Supply

RF

IF

TRx & Mon

LCSU

PWS

DME - BEACON COMPOSITION

RF Path:

• Duplexer - DPX

• RF COAX Relay - KCX

• Dummy Load -100 W, 50 Ω

• External Filter Antenna KIT


Interface System

• Associated Facility Interface - AFI

• Dummy Interface - TAI

• Modem - MDM


Transponder and Monitor

• Power Supply (DC/DC converter; + 5V, ± 15V ) - PWS

• Monitor - MON

• Receiver - RX

• Digital Processor - DPR

• Digital Modulator - DMD

• Transmitter 100 Wp (also driver for DME 435) - TX 100

• Transmitter Amplifier 1kWp (only for DME 435) - TKW

DME COMPOSITION

Transponder and Monitor

• PWS

• MON

• RX

• DPR

• DMD

• TX 100

• TX Amp


Local I/O:

• Local Control Status Unit - (LCSU)

composed of:

• Control and Status Board - CSB

• Indication and Controls - INC


Power Supply:

• Battery Charger and Power Supply

– BCPS

• AC/DC 600W Module - AC-DC

• Terminal bar- 48Vdc

• Batteries supervisor only for BCPS

DMESimplifiedDiagram

DME - Antenna• The suggested antenna for the DME 415-435 DME

equipment is the omnidirectional DME antenna.

• This antenna is provided with two obstruction lights

which may be turned on and off during the day by an

automatic night switch.

• The antenna for the DME has vertical polarization

9-dB gain.

DME Main Status

End Of DME Lesson

Any question

Technology

Distance Measurement equipment (DME)