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Radio Nav
NDB Navaid and ADF Avionics
Volume 1 of the Radio Nav series
by Marques Aviation
Edited by
Dr. Pascual Marqués Ph.D.
Published by Marques Aviation Ltd
Southport, United Kingdom TM
Marketing Director: Elena Spiridon
ISBN 978-1-907980-00-8
Copyright © 2010 Marques Aviation Ltd
Copyright policy
This manual remains intellectual property of Marques Aviation Ltd. All rights reserved. No
part of this manual may be reproduced or transmitted in any form, electronically or
photocopying, without permission from the copyright owner in writing, except for brief
quotations embodied in critical articles and reviews. Permissions may be sought directly
from Marques Aviation Ltd by email at admin@marquesaviation.co.uk or via our website.
Disclaimer
The author has exerted every effort to ensure accuracy of the information presented.
Nothing in this manual supersedes any procedures specified in any operational document
issued by the Civil Aviation Authority or aircraft operators. The information presented is
intended for pilot training only. Updated and approved NDB information should be used for
navigational purposes.
Library of Congress / British Library cataloguing-in-publication data
A catalogue record of this book is available from the British Library.
Published by Marques Aviation Ltd
5, Grosvenor Road, Southport, PR8 2HT, United Kingdom
Tel: 01704 569527
sales@marquesaviation.com
www.marquesaviation.com
i
CONTENTS
Page
Introduction 1
Part I – Principles of NDB-ADF operation
Chapter 1: The radio signal 5
Chapter 2: Radio wave propagation 9
Chapter 3: Radio wave modulation 15
Chapter 4: Principles of ADF operation 21
Chapter 5: NDBs around the world 27
Chapter 6: Using the ADF for navigation 33
Chapter 7: Applications - Intercepting track and tracking 43
Chapter 8: Applications – Position fix 53
Chapter 9: The NDB approach and holding pattern 61
Chapter 10: NDB range and ADF accuracy 73
Part II – Multiple-choice questions
Questions 83
Part III – Practical exercises
Exercises 145
2
INTRODUCTION
The NDB-ADF navigation system
Radio direction finding (RDF) is a traditional navigational technique that involves obtaining
the direction of a radio transmitter. The oldest RDF navigational aid (navaid) still in use in
aviation is the non-directional beacon (NDB). The principle of NDB navigation is simple and
consists of two parts. An NDB, or ground station facility, transmits radio signals in all 360°
azimuth directions and an automatic direction finder (ADF) receiver on the aircraft detects
the direction of the incoming signals. The term non-directional is counter-intuitive but, in
truth, no specific direction is defined. The transmitting antenna consists of either a single
mast or a long aerial suspended between two masts. The conventional ADF receiver
comprises a loop plus sense aerial system, a control panel, and an azimuth card and rotating
needle display in the instrument panel (Figure 1). The needle automatically and
continuously points towards the NDB station selected for navigation. By turning to the
direction indicated by the ADF needle the aircraft ‘homes’ to the station and it will
eventually fly over the beacon. Originally, the ADF was known as radio compass because the
needle is superimposed on a compass rose. The compass card is graduated in 5° azimuth
increments from 0° to 355°. In earlier days, the loop antenna was an actual metallic loop
mounted beneath the fuselage and enclosed in an egg-shaped fairing. The loop had to be
rotated manually by the navigator to find the direction of the transmitter. In older aircraft,
the basic design of the sense antenna consists of a long wire mounted from cabin roof to tail
fin. In modern aircraft, both the loop and sense aerials are arranged in a single streamlined
housing mounted near the aircraft centreline under the fuselage. The loop antenna is
coupled with a goniometer that determines the direction of the station. The fundamental
ADF instrument is the relative bearing indicator (RBI). An RBI displays the bearing of the
3
beacon relative to the aircraft’s nose, thus relative to the aircraft’s magnetic heading shown
in the heading indicator. Visualizing the situation with an RBI can be, at first, daunting.
Figure 1: The NDB transmitter and ADF avionics
The NDB is primarily a short-distance navaid. However, in many parts of the world the NDB
provides navigational aid for long-distance en-route tracking. The NDB enables the pilot to
track to or from a ground station, to determine aircraft position (to obtain a fix), to take
delaying action by entering a holding pattern and to perform an NDB letdown in instrument
meteorological conditions (IMC) or at night. IMC conditions involve limited visibility in poor
weather and low ceilings. An NDB letdown is a non-precision instrument approach to an
airfield, thus an approach assisted by lateral course guidance but characterised by the
absence of glideslope vertical guidance. NDB-ADF navigation suffers from the same signal
disturbances common to amplitude modulated (AM) radio broadcasting which influence the
promulgated range of the beacon and the accuracy of ADF indications.
4
To convert magnetic bearings to true bearings use either TB = MB + east magnetic variation
or TB = MB - west magnetic variation. True line of position from the NDB station (coded as
QTE) is used to obtain a position line from the station to the aircraft on an aeronautical
chart based on QDR. QTE is in fact the reciprocal of TB to station. East variation is added to
QDR and west variation is subtracted. In the UK magnetic variation is west, for example QDR
105 - 4° west variation = QTE 101, which is the reciprocal of TB = 281°T (Figure 29).
Figure 29: Determination of true heading, true bearing and QTE
5
Figure 38: Position fix using two NDBs
6
Chapter 3: Radio wave modulation
Q28 The designation for an on-off keyed carrier is ________ and that for a keyed
amplitude modulated carrier is ________.
a) NON, NON A2A c) NON A2A, NON A1A
b) NON A1A, NON A2A d) A1A, A2A
Q29 Beating is caused by the ________ and________ interaction between signals of
________ frequency.
a) constructive, destructive, different
b) constructive, destructive, equal
c) modulated, constructive, different
d) modulated, constructive, higher
Q31 A beat frequency of 1.4 kHz is produced when the BFO is set to 420.5 kHz. What is
the frequency of the NDB?
a) 419.1 kHz b) 1.4 kHz c) 420.5 kHz d) either 419.1 kHz or 421.9 kHz
Q32 An NDB radio wave has a frequency of 630.5 kHz. To obtain a beat frequency of
2.1 kHz, the two possible BFO frequencies are
a) 628.4 kHz and 632.6 kHz c) 630.5 kHz and 632.6 kHz
b) 2.1 kHz and 630.5 kHz d) 628.4 kHz and 630.5 kHz
7
Q102
Q103
8
ANSWER - Exercise 10
Current heading is 280. To intercept track 295 (radial 115) using an intercept angle of 45°
the aircraft alters heading to 250 (295 - 45°; aircraft 2). As the aircraft flies towards the
interception point, the RMI needle falls gradually towards MB 295. Just before reaching MB
295 the pilot turns right on heading 295 and flies inbound (aircraft 3). In a crosswind, due
allowance for expected drift would be required in order to maintain a constant track.
9
GLOSSARY
A
Abeam Direction of the NDB station off one
wing tip at 90° degrees to the aircraft
longitudinal axis.
Aeronautical chart Map designed to assist in
air navigation that includes topographic
features, hazards, obstructions, navaids,
routes, designated airspace and airports.
Aeronautical Information Publication
Manual containing regulations and
procedures of a lasting character essential to
air navigation.
Airway Route in the air laid out between
navigational aids, such as NDBs and VORs.
Alternating current Electric current that
periodically reverses its direction of flow in a
back-and-forth motion.
Amplitude modulation Modification of the
amplitude of the carrier wave to convey audio
information.
Antenna Metallic wire or rod that radiates or
receives electromagnetic energy.
Approach plate Chart that provides a
graphical representation of instrument
approach procedures including radio
frequencies of the constituent navaids, course
headings, waypoints, altitudes, minimum
visibility requirements, communications
information and a sketch of the airport.
Area Navigation Navigation method under IFR
that permits the aircraft to follow any course
within a network of navigation beacons by
removing the need to fly directly to or from a
navaid.
Attenuation Weakening of a radio wave as it
travels over the ground or sea surface
(groundwave) or enters an ionospheric layer
(skywave).
Automatic direction finder Equipment on the
aircraft that detects the direction of the
incoming radio signal transmitted by an NDB.
Azimuth Horizontal angle in degrees
measured clockwise from north as reference
datum.
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