Institute of Engineering and Technology

PHILIPPINE STATE COLLEGE OF AERONAUTICS

Piccio Garden, Villamor Air Base, Pasay City 1309

DME

DISTANCE MEASURING EQUIPMENT

In partial fulfilment of the requirements for the subject

AE 514 Aircraft Avionics

Engr. Cynthia V. Dayao

Instructor

DE LEON, Francis Carlo S. DIZON, John Kevin M.

LICO, Bruce Kelly A.

Distance measuring equipment (DME) is a transponder-based radio navigation technology that

measures slant range distance by timing thepropagation delay of VHF or UHF radio signals.

Developed in Australia, it was invented by James Gerry Gerrand under the supervision of Edward George

"Taffy" Bowen while employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and

Industrial Research Organisation (CSIRO). Another engineered version of the system was deployed

by Amalgamated Wireless Australasia Limited in the early 1950s operating in the 200 MHz VHF band. This Australian

domestic version was referred to by the Federal Department of Civil Aviation as DME(D) (or DME Domestic), and

the later international version adopted by ICAO as DME(I). DME is similar to secondary radar, except in reverse.

The system was a post-war development of the IFF (identification friend or foe) systems of World War II. To

maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.

OPERATION

Aircrafts use DME to determine their distance from a land-based transponder by sending and receiving

pulse pairs – two pulses of fixed duration and separation. The ground stations are typically co-located with VORs.

A typical DME ground transponder system for en-route or terminal navigation will have a 1 kW peak pulse output

on the assigned UHF channel.

A low-power DME can be co-located with an ILS glide slope antenna installation where it provides an

accurate distance to touchdown function, similar to that otherwise provided by ILS Marker Beacons.

HARDWARE

The DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF

receiver/transmitter (transponder) on the ground.

SPECIFICATION

A typical DME transponder can provide distance information to 100 aircraft at a time. Above this limit the

transponder avoids overload by limiting the gain of the receiver. Replies to weaker more distant interrogations are

ignored to lower the transponder load. The technical term for overload of a DME station caused by large numbers

of aircraft is station saturation.

ACCURACY

The accuracy of DME ground stations is 185 m (±0.1 nmi). It's important to understand that DME provides

the physical distance from the aircraft to the DME transponder. This distance is often referred to as 'slant range'

and depends trigonometrically upon both the altitude above the transponder and the ground distance from it.

For example, an aircraft directly above the DME station at 6076 ft (1 nmi) altitude would still show 1.0 nmi

(1.9 km) on the DME readout. The aircraft is technically a mile away, just a mile straight up. Slant range error is

most pronounced at high altitudes when close to the DME station.

Radio-navigation aids must keep a certain degree of accuracy, given by international standards,

FAA, EASA, ICAO, etc. To assure this is the case, flight inspection organizations check periodically critical parameters

with properly equipped aircraft to calibrate and certify DME precision. ICAO recommends accuracy of less than the

sum of 0.25 nmi plus 1.25% of the distance measured.