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`FLIR-1006.002
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`Warsash Nautical Books
`6 D.ibles Road
`wa·rsash
`Southampton 5031 9H:Z
`Tel: 01489 572384 Fax: sss1se
`www.nauticalbooks.co.uk
`
`FLIR-1006.003
`
`

`

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`FLIR-1006.004
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`

`

`Electronic Navigation Systems
`3rd edition
`
`FLIR-1006.005
`
`

`

`,
`1
`
`Electronic Navigation
`1.:.1· Systems
`~
`'"I !i
`
`Laurie Tetley 1Eng FIEIE
`Principal Lecturer in Navigation and Communication Systems
`
`;!\
`
`fiJ
`
`~I
`t
`
`!
`
`and
`David Calcutt PhD MSc DipEE CEng MIEE
`Formerly Senior Lecturer, Department of Electrical and Electronic Engineering,
`University of Portsmouth
`
`-119 UTTER_WORTH
`
`E'INEMANN
`
`OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI
`
`FLIR-1006.006
`
`

`

`Butterworth-Heinemann
`Linacre House, Jordan Hill, Oxford OX2 8DP
`225 Wildwood Avenue, Woburn, MA 01801-2041
`A division of Reed Educational and Professional Publishing Ltd
`-@. A member of the Reed Elsevier plc group
`
`First published Electronic Aids to Navigation 1986
`Reprinted 1988
`Second edition published as Electronic Aids to Naviga,tion: Position Fixing 1991
`Third edition 2001
`
`© L. Tetley and D. Calcutt 2001
`
`All rights reserved. No part of this publication may be reproduced in
`a:ny material form (i,ncluding photocopying or storing in any medium by
`electronic means and whether or not transiently or incidentally to some
`other use of this publication) without the written permission of the
`copyright holder except in accordance with the provisions of the Copyright,
`Designs and Pat~nts Act 1988 or under the terms of a licence issued by the
`Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London,
`England WIP OLP. Applications for !Jie copyright holder's written
`permission to reproduce any part of this publication should be addressed
`to the publishers
`
`British Library Cataloguing in Publication Data
`Tetley, L. (Laurence), 1941-
`Electronic navigation systems. - 3rd ed.
`1. Electronics in navigation
`II. Calcutt, D. (David), 1935-
`I. Title
`623.8'504
`
`Library of Congress Cataloguing in Publication Data
`A catalogue record for this book is available from the Library of Congress
`
`ISBN 0 7506 51385
`
`Composition by Genesis Typesetting, Laser Quay, Rochester, Kent
`Printed and bound in Great Britain by MPG Books Ltd, Bodmin, Cornwall
`
`~-POR IM!llY TITLE THAT WB PUBLISH, Btm1!1lWORTIHlBU&ilAIVt
`
`WW.PAYPORD1'.CVTO~-~-~~B.~~
`
`FLIR-1006.007
`
`

`

`f
`
`I -,
`
`'
`
`'
`
`Contents
`
`Preface
`Acknowledgements
`
`Chapter 1 Radio wave propagation and the frequency spectrum
`1.1
`Introduction
`1.2 Maritime navigation systems and their frequencies
`1.3 Radio wave radiation
`1.4 Frequency, wavelength and velocity
`1.5 Radio frequency spectrum
`1.6 Radio frequency bands
`1. 7 Radio wave propagation
`1.8 Signal fading
`1.9 Basic antenna th.eory
`1.10 Glossary
`1.11 Summary
`1.12 Revision questions
`
`Chapter 2 Depth sounding systems
`2.1
`Introduction
`22 The characteristics of sound in seawater
`2.3 Transducers
`2.4 Depth sounding principles
`· 2.5 A generic echo sounding system
`2.6 A digitized echo sounding system
`2. 7 A microcomputer echo sounding system
`2.8 Glossary
`2.9 Summary
`2.10 Revision questions
`
`Chapter 3 Speed measurement
`3.1
`Introduction
`3.2 Speed measurement using water pressure
`3.3 Speed measurement using electromagnetic induction
`3.4 Speed measurement using acoustic correlation techniques
`
`ix
`xi
`
`1
`
`1
`2
`4'
`5
`6
`8
`13
`14
`19
`20
`21
`
`22
`22
`22
`27
`31
`35
`38
`38
`41
`43
`44
`
`45
`45
`45
`52
`57
`
`FLIR-1006.008
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`

`

`vi Contents
`
`3.5 The Doppler principle -
`3.6 Principles of speed measurement using the Doppler effect
`3.7 Doppler speed logging systems
`3.8 Glossary
`3.9 Summary
`3.10 Revision questions
`
`Chapter 4 Loran-C
`4.1
`Introduction
`4.2 System principles
`4.3 Basics of the Loran-C System
`4.4 Loran-C charts
`4.5 Position fixing using the Loran-C System
`4.6 Loran-C coverage
`4.7 Loran-C receivers
`4.8 Glossary
`4.9 Summary
`4.10 Revision questions
`
`Chapter 5 Satellite navigation
`5.1
`Introduction
`5.2 Basic satellite theory
`5.3 The Global Positioning System (GPS)
`5.4 The position fix
`·
`5.5 Dilution of Precision (DOP)
`5.6 Satellite pass predictions
`5.7 System errors
`5.8 Differential GPS (DGPS)
`5.9 GPS antenna systems
`5.10 GPS receiver designation
`5.11 Generic GPS receiver architecture
`5.12 GPS user equipment
`5.13 GPS on the web
`5.14 Global Orbiting Navigation Satellite System (GLONASS)
`5.15 Project Galileo
`-
`5.16 Glossary
`5.17 Summary
`5.18 Revision questions
`
`Chapter 6 Integrated bridge systems
`6.1
`Introduction
`6.2 Design criteria
`6.3 Standards
`6.4 Nautical safety
`6.5 Class notations
`
`60
`63
`72
`85
`85
`86
`
`88
`88
`89
`93
`102
`107
`112
`115
`137
`139
`140
`
`143
`143
`143
`147
`154
`157
`157
`158
`162
`165
`166
`168
`171
`182
`183
`185
`185
`186
`187
`
`189
`189
`190
`193
`194
`195
`
`FLIR-1006.009
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`

`

`6.6 Bridge working environment
`6.7 Ship manoeuvring information
`6.8 Qualifications and operational procedures
`6.9 Bridge equipment tests
`6.10 Examples of integrated bridge systems
`6.11 Glossary
`6.12 Summary
`6.13 Revision questions
`
`Chapter 7 Electronic charts
`7 .1
`Introduction
`7.2 Electronic chart types
`7 .3 Electronic chart systems
`7.4 Chart accur~cy
`7.5 Updating electronic charts
`7.6 Automatic Identification System (AIS)
`7.7
`'Navrn~ster' Electronic Navigation Systern
`7 .8 Glossary
`7.9 Summary
`7 .10 Revision questions
`
`Chapter 8 The ship's master compass
`8.1
`Introduction
`·
`8.2 Gyroscopic principles
`8.3 The controlled gyroscope
`8.4 The north-seeking gyro
`8.5 A practical gyrocompass
`8.6 Follow-up systems
`8.7 Compass errors
`8.8 Top-heavy control master compass
`8.9 A digital controlled top-heavy gyrocompass system
`8.10 A bottom-heavy control gyrocompass
`8.11 Starting a gyrocompass
`8.12 Compass repeaters
`8.13 The magnetic repeating compass
`8.14 Glossary
`8.15 Summary
`8.16 Revision questions
`
`Chapter 9 Automatic steering
`9 .1
`Introduction
`9.2 Automatic steering principles
`9.3 A basic autopilot system
`9.4 M~ual operator controls
`9 .5 Deadband
`
`,,
`
`~-,_
`
`Contents
`
`vii
`
`196
`198
`198
`201
`201
`220
`222
`223
`
`224
`224
`227
`234
`239
`242
`243
`249
`259
`262
`263
`
`264
`264
`264
`271
`271
`275
`281
`281
`287
`292
`299
`306
`307
`310
`317
`318
`319
`
`320
`320
`320
`324
`326
`327
`
`~
`.J
`~,
`~
`~ ...
`
`~~
`' ,.
`~·
`!~
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`
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`...
`
`FLIR-1006.010
`
`

`

`viii Contents
`
`9.6 Phantom rudder
`9.7 An adaptive autopilot
`9.8 An adaptive digital steering control system
`9.9 Glossary
`9.10 Summary
`9 .11 Revision questions
`
`Chapter 10 Radio direction finding
`10.1
`Introduc;!ion
`10.2 Radio waves
`10.3 Receiving antennae
`10.4 A fixed loop antenna system
`10.5 Errors
`10.6 RDF receiving equipment
`10.7 Glossary
`10.8 Summary
`10.9 Revision questions
`
`Chapter 11 Global Maritime Distress and Safety System
`11.1
`Introduction
`11.2 The system
`11.3 The NAVTEX system
`11.4 Glossary
`11.5 Summary
`11.6 Revision questions
`
`Appendices
`Al Computer functions
`A2 Glossary of microprocessor and digital teffilS
`A3 Serial communication
`A.4 United States Coast Guard Navigation Center (NAVCEN)
`
`Index
`
`329
`330
`333
`344
`344
`345
`
`346
`346
`346
`347
`349
`355
`358
`367
`368
`368
`
`369
`369
`369
`380
`388
`388
`389
`
`391
`393
`401
`407
`416
`
`419
`
`FLIR-1006.011
`
`

`

`I
`I
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`·1
`
`Preface
`
`This new edition of Electronic Navigation Systems has been extensively rewritten to provide
`navigators with a detailed manual covering the prineiples and applications of modern systems.
`The past decade has been witness to huge advances in technology and no more so than in maritime
`navigation aI),d position fixing. As you might expect, spearheading this technological advance has been
`the computer. It has become l!.S common on boatd ships as in our normal lives where it now influences
`virtually everything that we do. A new generation of ship's officer has been trained to use computers,
`trained to understand how they work and, more importantly, how they can be made to assist in, the
`business of safe and precise navigation. But it would be a serious error to assume that the technology
`is perfect All the systems currently used for navigation and position fixing are as near perfect as they
`can be, but it would be foolhardy to ignore the human link in the electronic chain of action and
`reaction. In the end, it is a ship's captain who bears the ultimate responsibility and the navigating
`officer who, with pride, safely brings his ship into port.
`Readers will find' that this new expanded edition includes many new systems and techniques
`whereas some older, now obsolete systems have been deleted. The hyperbolic systems, which once
`formed the backbone of global position fi.xing, have been decimated by the continuing expansion of
`the Global Positioning System (GPS).
`The hyperbolic systems Decca and Omega have gone, but Loran-C, the one terrestrial network
`providing extensive coverage, remains as the designated back-up system to the GPS. By Presidential
`order, on 1 May 2000, Selective Availability, the method by which GPS accuracy was downgraded for
`civilian users, was set to zero. This significant event means that submetre accuracy position fixing is
`now available for all users, a factor that will have a major impact on GPS equipment and subsystems
`over the next decade.
`Whilst the GPS is the undisputed king amongst satellite systems, it is by no means the only one.
`GLONASS, created and maintained by the Russian Federation, also provides users with accurate
`position fixes and the European Commlinity is actively considering another system to be totally
`independent of the other tWo.
`Although position fixing by satellite is of paramount importaI).ce there are other systems essential
`to safe navigation. Speed logging, depth sounding, and automatic steering systems are equally as
`important as they were decades ago and even that most traditional of all systems, the gyrocompass,
`has been digitized and refined. But essentially, system parameters remain unchanged; it is the
`collecting, processing and display of data that has been transformed.
`Computerization and continuing development of large"scale integration (LSI) techi1ology have been
`directly responsible for most of the changes. The large-scale manufacture of microchips has enabled
`the production of low-cost equipment with capabilities that could only have been dreamed about a
`decade ago. This reduction in size and cost has also brought sophisticated navigation equipment within
`teach of small-boat owners.
`
`FLIR-1006.012
`
`

`

`x Preface
`
`Electronic Navigation Systems has been written to support the training requirements of STCW-95
`and consequently the book is an invaluable reference source for maritime navigation students. As with
`previous editions, each chapter opei;1s with system principles and then continues with their application
`to modern equipment. Some sections, typically gyrocompass and automatic steering, still contain valid
`descriptions of analogue equipment but these have been further strengthened with the introduction of
`new digital technology. Wherever possible we have described the systems and equipment that you, the
`reader, are likely to meet on board your craft whether it is large or small .
`. The Global Maritime Distress and Safety System (GMDSS) is a subject which no mariner can
`ignore and consequently it has been outlined in this book. For extensive details about the principles
`and applications of this global communicatiops system, see our book Understanding GMDSS.
`Radar and Automatic Radar Plotting Aids (ARPA) are obviously essential to safe navigation and
`indeed are now integrated with other navigation systems. They are discussed in depth in the
`companion volume to this publication, Electronic Aids to Navigation (RADAR and ARPA).
`
`Laurie Tetley and David Calcutt
`2000
`
`FLIR-1006.013
`
`

`

`Acknowledgements
`
`l
`f
`. !~I
`
`A book of this complexity containing leading edge technology must inevitably owe much to the co(cid:173)
`operation of various individuals, equipment manufacturers and organizations. To single out one or
`more organizations is perhaps invidious. In many cases we have had no personal contact with
`individuals but despite this they gave freely of their time when information was requested.
`We are extremely gn1teful for the assistance that the following companies and organizations gave
`during the writing of this book. We are particularly indebted to the organizations that permitted us to
`reproduce copyright material. Our sincere thanks go to the following.
`
`COSPAS-SARSAT Secretariat
`Det Norske Veritas (DNV)
`Furuno Electric Co. Ltd
`Garmin Industries
`ICAN
`The INMARSAT Organization
`The International Maritime Qrganization (IMO)
`Kelvin Hughes Ltd
`Koden Electronics Co. Ltd
`Krupp Atlas Elektronik
`Litton Marine Systems
`The NAVTEX Coordinating Panel
`PC Maritime
`SAL Jungner Marine
`S G Brown Ltd
`Sperry Marine Inc.
`Thomas Walker & Son Ltd
`Trimble Navigation Ltd
`UK Hydrographic Office (UKHO)
`Warsash Maritime Centre
`
`The following figures are from the IMO publications on GMDSS and The Navtex Manual, and are
`reproduced with the kind permission of the International Maritime Organization, London: Figure 11.1,
`page 370; Figure 11.3, page 374; Figure 11.4, page 376; Figure 11.7, page 381; Figure 11.8, page 382;
`Figure 11.10, page 384; Figure 11.11, page 385.
`
`FLIR-1006.014
`
`

`

`Chapter 5
`Satellite navigation
`
`5.1 Introduction
`
`It is surprising that the space technology tha:t we rely on so heavily today had its origins over 50 years
`ago when, in the early 1950s, with the shock launching by the USSR of a rnan-made satellite into low
`orbit, the United States space programme was born. Although a tiny vehicle by present day standards,
`the USSR 's 'Sputnik' had a radio transmitter on boa:rd, the frequency of which exhibited a pronounced
`Doppler shift when observed from any fixed point on the earth's surface. The Doppler phenomenon
`was well documented but this was the first time the effect had been produced by and received from
`a man-made orbiting satellite. Space engineers soon recovered from the initial shock and were quick
`to see that the effect could be expldited to create a truly accurate global positioning system, free from
`many of the constraints of the existing earth-bound hyperbolic navigation systems.
`The first commercially available system to be developed, the Navy Navigation Satellite System
`(NNSS), made good use of the Doppler effect and provided the world's shipping with precise position
`fixing for decades. However, nothing lasts forever. The technology became old and the system was
`dropped on 31 December 1996 in favour of the vastly superior Global Positioning System (GPS).
`Although a number of NNSS Nova satellites a:re still in orbit, the system is no longer used for
`corn.merci~ navigation purposes.
`
`5.2 Basic satellite theory
`
`Whilst it is not essential to understand space technology, it is helpful to consider a few of the basic
`parameters relating to satellite orbits and the specific terminology used when describing them. A
`satellite is placed in a pre-determined orbit, either in the nose of an expen$ble launch vehicle or as
`part of the payload of a space sh~ttle flight. Either way, once the 'bird' has been delivered into the
`correct plane, called the 'inclination', that is the angle formed between the eastern end of the
`equatorial plane and the satellite orbit, it is subject to Kepler's laws of astrophysics.
`Figure 5.1 shows orbits of zero inclination for the equatorial orbit, 45°, and for a polar orbit, 90°.
`The fin!l.l. desired inclination partly determines the laQnching site chosen. In practice it is difficult to
`achieve an inclination which is less than the latitude of the launching site's geographical location. A
`zero inclination orbit is most effectively produced from a launch pad situated on the equator; but this
`is not always possible and a compromise is often made. Laurich normally takes place in an easterly
`dire;ction because that way it is possible to save fuel, and thus weight, by using the earth's rotational
`speed to boost the velocity of the accelerating rocket. For an easterly launch from a site on the equator,
`the velocity needed to escape the pull of gravity, is 6.89 km s-1, whereas for a westerly launch it is
`7 .82 km s-1
`• Lau_nch velocities also vary with latitude and the direction of the flight path.
`
`FLIR-1006.015
`
`

`

`_ 144 Electronic N~vigation Systems
`
`N
`
`!JO degree
`inclination
`
`w
`
`45 degree
`inclination
`
`E
`Zero
`
`Equatorial
`orbit
`
`! !
`'
`I·
`
`Figure 5.1 Illustration of orbital inclination.
`
`s
`
`5.2.1 Kepler's Laws
`
`Essentially, an artificial earth-orbiting satellite obeys three laws that were predicted in the late 16th
`century by Johannes Kepler (1571-1630) who also developed theories to explain the natural orbits of
`the planets in our solar system. When applied to artificial orbiting satellites, Kepler's laws may be
`summarized as follows.
`
`• A satellite orbit, with respect to the earth, is an ellipse.
`• Vectors drawn from the satellite orbit to the earth describe equal areas in equal times.
`• The square of the period of the orbit is equal in ratio to the cube of its mean altitude above the
`earth's surface.
`
`True to Kepler, artificial earth satellites follow elliptical orbits. In some cases the ellipse eccentricity
`is large and is a requirement of the fir:;t stage of a launch to the higher geostationary orbit, but in most
`
`·-·---·-- ~·----------·------.. --~--!!!!!!!! _____
`
`~_!l!!!! __ !i!!l
`
`
`
`___ ~---111!1111!!!!!11!1111'!!!!!!!11!1111'!!!!!!-~~~~--------._..,.,,, ___ ,,,,,,, __ .,..,,, ____ =----=----~-- - . , . . . - - -
`
`FLIR-1006.016
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`

`

`Sateliite navigation 145
`
`$
`Figure 5.2 Illustration of apogee and perigee.
`
`i
`
`•
`
`1
`
`Maximum
`elevation
`
`Surface tangent
`
`Figure 5.3 Showing the changing angle of elevation during a satellite pass. The angle reaches a
`maximum at the closest point of approach to the earth bound observer.
`
`cases it is created because the earth is not a perfect sphere. The closest point of approach to the earth
`of any elliptical orbit is called the 'perigee' and the furthest distance away is the 'apogee', as shown
`in Figure 5.2. The direction vector to the satellite from a fixed point on the earth is called the 'azimuth'
`a:nd is quoted in degrees. The angle between the satellite, at any instant, and the earth's surface tangent
`is the 'elevation' and again is quoted in degrees (see Figure 5.3).
`
`5.2.2 Orbital velocity
`
`A satellite can only remain in orbit if its velocity, for a given altitude, is sufficient to defeat the pull
`of gravity (9.81 ms-1) and less than that required to escape it. The velocity must be absolutely precise
`for the orbital altitude chosen. Eventually, drag will slow the satellite causing it to drop into a lower
`orbit and possibly causing it to re-enter the atmosphere and burn-up. The nominal velocity for a
`satellite at any altitude can be calculated by using the formula:
`
`K
`v =
`I kms-1
`(r + a)Yi
`
`:1,:
`~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~--~~~--~i.~~~----~~~===-=_=====~--
`
`FLIR-1006.017
`
`

`

`146 Electronic Navigation Systems
`
`whery V = orbital velocity in kms- 1
`,
`a = altirude of the satellite above the earth's surface in km.
`r = the mean radiu,s of the earth (approximately 6370 km), and
`K = 630 (a constant derived from a number of parameters).
`
`The earth is not a perfect sphere and therefore its radius with respect to orbital altitude will vary.
`However, to derive an approximate figure for velocity, an earth radius figure of 6370 km is close
`enough. The velocity of a satellite with an altitude of 200 km Would be:
`
`630
`. = 7.77kms-1
`V =
`(6370 + 200)Y,
`
`Orbital paths can be transferred to a Mercator projection chart as shown in Figure 5A. The
`inclination will be the same in both northern and southern hemispheres and corresponds to latitude.
`The six orbits shown are for Navstar (GPS) satellites with an orbital inclination of 55°.
`
`5.2.3 Orbital period
`
`The time period for one complete orbit of a satellite can be readily calculated using the simple formula
`below:
`
`r + a)3/2
`
`P=K - (cid:173)
`(
`r
`
`West 180
`
`120
`
`060
`
`Degrees of longitude
`0
`060
`
`120
`
`180 East
`
`7!\
`-
`Figure 5.4 Mercator presentation of the orbital inclination paths described by satellite orbits.
`
`··-----------
`
`FLIR-1006.018
`
`

`

`I: Ii
`
`' I
`
`Satellite navigation 147
`
`where P = the period of one orbit in min,
`a = the altitude of the orbit above the earth's surface in km,
`r = tbe mean radius of the earth in km, and
`K = 84.49 (a constant derived from a number of parameters).
`
`The orbital period for a satellite at an altitude of 200 km is:
`
`P = 84.49
`
`6371 + 200 ) 3
`12
`· - ·
`6371
`
`(
`
`= 88.45 min
`
`5.3 The Global Positioning System (GPS)
`
`In 1973 a combined US Navy and US Ait force task-force set out to develop a new global satellite
`navigation system to replace the ageing Navy Navigation Satellite System (NNSS).
`The original test space vehicles (SVs) launched in the new programme were called Navigation
`Technology Satellites (NTS) and NTSl went into orbit in 1974 to became the embryo of a system that
`has grown into the Global Positioning System (GPS). GPS was deelared to be fully operational by the
`US Air Force Space Command (USAFSC) on 27April 1995, and brought about the demise of the
`NNSS which finally ceased to provide navigation fixes at midnight on 31 December 1996.
`The GPS, occasionally called NAVSTAR, shares much commonality with the Russian Global
`Navigation System (GLONASS), although the two are in no way COIJlpatible. The GPS consists of
`three segments designated Space, Control and User.
`
`5.3.1 The space segment
`
`Satellite constellation calls for 24 operational SVs, four in each of six orbital planes, although more
`satellites are available to ensure the system remains continuously accessible (see Figure 5.5). SVs
`orbit the earth in near circular orbits at an altitude of 20 200 km (10 900 nautical miles) and possess
`an inclination angle of55°.
`Based on standard time, each SV has an approximate orbital period of 12 h, but when quoted in the
`more correct sidereal time, it is 11 h 58 min, Since the earth is tlirning beneath the SV orbits, all the
`satellites will appear over any fixed point on the earth every 23 h 56 min or, 4 min earlier each day.
`This, totally predictable, time shift is caused because a sidereal day is 4 min shorter than a solar day
`and all SVs complete two orbits in one day. To maintain further orbital accuracy, SVs are attitude(cid:173)
`stabilized to within 1 m by the action of four reaction wheels, and on-board hydrazine thrusters enable
`precision re-alignment of the craft as required.
`This orbital configuration, encompassing 24 SVs, ensures that at least six SVs, with an elevation
`greater than 9.5°, will be in view of a receiving antenna at any point on the earth's surface at any time.
`When one considers the problems of rapidly increasing range error caused by the troposphere at low
`SV elevations, 9.5° has been found to be the IlliI1imum elevation from which to receive data when
`using a simple antenna system.
`The original satellites, numbered 1-11 and designated Block I, have ceased operation. Currently, the
`GPS constellation is based on the next generation of SVs, designated Block II. Block II (numbers
`13-21) and block IIA (nµmbers 22-40) satellites, manufactured by Rockwell International, were
`launched from Cape Canaveral between February 1989 and November 1997. Each SV holds four
`atomic clocks, two rubidium and two caesium, and has selective availability (SA) and anti-spoofing
`(A-S) capabilities, 'although the US Government has now given an assurance that the system
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`55° inclination
`
`60°orbital planes
`
`Figure 5.5 GPS satellite coverage. Twenty-four satellites provide global coverage; four in each of six
`orbital planes.
`
`downgrading functions, SA and A"S, wiH no longer be implemented in the GPS. Block IIR SVs
`(numbers 41-62) are replenishment satellites and have been designed for an operational life of 7 .8
`years.
`All SVs transmit a navigation message comprising orbital data, clock timing characteristics, system
`tiI11e and a status message. they also send an extensive almanac giving the orbital and health d~ta for
`every active SV, to enable a user to locate all SVs once one has been acquired and the data
`downloaded.
`
`5.3.2 The control segment
`
`The GPS is controlled from Scbriever Air Force Base (formerly Falcon AFB) in Colorado. It is from
`there that the SV telemetry and upload functions are commanded. There are five monitor stations (see
`Figure 5.6), which are situated in the Hawaii Islands in the Pacific Ocean, on Ascension Island in the
`Atlantic, on Diego Garcia in the Indian Ocean, on K wajalein Island, again in the Pacific, and at
`Colorado Springs on mainland US territory. SV orbital parameters are constantly monitored by one or
`more of the ground tracking stations, which then pass the measured data on to the Master Control
`Station (MCS) at Schriever. From these figures the MCS predicts the future orbital and operational
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`150
`
`120
`
`90
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`60
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`30
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`30
`
`60
`
`90
`
`120
`
`150
`
`180
`
`Satellite navigation 149
`
`i
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`* MONITORSTATION
`
`Figure 5.6 GPS control segment stations.
`
`+ GROUND ANTENNA
`
`parameters to be fed to the Upload Stations (ULS) on Ascension, Diego Garcia and K wajalein Islands.
`All ground station locations have been precisely surveyed with respect to the World Geodetic System
`1984 (WGS-84). Data are transmitted to each SV from a ULS, to be held in RAM and sequentiaily
`transmitted as a data frl:l1Ile to receiving stations.
`
`Signal parameters
`
`Navigation data are transmitted from the SV on two frequencies in the L band (see Table 5.1). In
`practice the SV clock is slightly offset to a frequency of 10.229 999 995 45 MHz to allow for the
`effects of relativity. SV clock accuracy is maintained at better than one part in 1012 per day. Dual
`frequency transmission from the SV ensures that suitably equipped receivers are able to correct for
`signal delay (range error) caused by the ionosphere. Ionosopheric delays are proportional to 1/f2
`hence the range error produced wil_l be different on each frequency and can be compensated for in the
`receiver.
`The CIA (Coarse and Acquire) code, see Figure 5.7, is a PRN (pseudo random noise) code stream
`operating at 1.023 megabitsls and is generated by a 10-bit register. CIA code epoch is achieved every
`1 ms (1023 bits) and quadrature phase modulates the L 1 carrier only. This code has been designed to
`be easily and rapidly acquired by receivers to enable SPS fixing. Each SV transmits a unique CIA code
`that is matched to the locally generated CIA code in the receiver, A unique PRN is allocated to each
`SV and is selected from a code series called Gold codes. They are specifically designed to minimize
`the possibility that a re(;eiver will mistake one code for another and unknowingly access a wrong
`satellite. Navigati<;m data is modulated onto the L 1 CIA code at a bit rate of 50 Hz.
`
`_... ______ - - - - - - - - -
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`- - - - - - - - · -~---------- . ---------.-.---'-----'--
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`Table 5.1 SV transmission frequencies
`
`Band
`
`Derivation (MHz)
`
`Frequency (MHz)
`
`Wavelength (cm)
`
`Code
`
`154 x 10.23
`120 x 10.23
`
`1575.42
`1227.60
`
`19
`24.5
`
`CIA
`CIA& P
`
`Both carriers are derived from the SV clock frequency 10.2_3 MH_z
`
`L1 CARRIER
`1sis.42Miiz
`
`C/ACODE
`io23MHz
`
`NAVMSG
`5ofiz
`
`P(Y)CODE
`10.23 MHz
`
`I L2CARRIER
`
`f227.80MHz
`
`L1 SIGNAL
`
`L2SIGNAL
`
`Figure 5.7 Schematic diagram of a SV modulation.circuit.
`
`·The P (Precise) code, operating at 10.23 MHz, is a PRN code produced as the modulo 2 sum of two
`24-bit registers, in the SY, tenned XI and X2. This combination creates a PRN code of 248
`- 1 steps
`equating to a complete code cycle (before code repetition occurs) of approximately 267 days. Each SV
`employs a unique and exclusive 7-day long phase segment of this code. At midnight every Saturday,
`GPS time, the Xl and X2 code generators ate reset to their initial state (epoch) to re-initi~te the 7-day
`phase segment at another point along the 267-day PRN code cycle. Without prior knowledge of the
`code progression, it is not possible to lock into it.
`
`The navige1.tion data message
`
`A 50-Hz navigation message is modulated onto both the P code and C/A codes. One data frame is
`1500 bits and takes 30s to complete at the bit rate of 50 bits-1• Navigation data are contained in five
`subframes each of 6 s duration and containing 300 bits. Table 5.2 shows the data format structure.
`
`---- ~- - -- -~----------"----------------------------~
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`Satelilte navigation 151
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`T~ble 5._2 Dat_a format structure
`
`Five words 300 bits each with a total of 6 s
`
`30 bits
`
`30 bits
`
`240 bits
`
`01
`
`02
`
`03
`04
`
`05
`
`TLM
`
`TLM
`
`TLM
`TLM
`
`HOW
`
`HOW
`
`HOW
`HOW
`
`TLM
`
`HOW
`
`Data block 1: Clocl_c cor:rection data. Accuracy and health of the
`signal.
`Data block 2: Ephemeris data. Precise orbital parameters to enable
`a receiver to compute the position of an SY.
`Data block 3: Ephemeris. Continued.
`Data block 4: Almanac. Orbital data, low-precision clock data,
`simple health and configuration status for every SY, user messages,
`ionosopheric model data and UTC calculations.
`Data block 5. Almanac. Continued.
`
`Subframes 4 and 5 hold low precision data, common to all SVs, and less critical for a satellite to acquire quickly.
`
`As shown in Figure 5.8, each of the five subframes commences with a 14-bit TLM word (telemetry)
`containing SV Status and diagnostic data. This is followed by a 17-bit handover word (HOW). HOW
`data enables a receiver, which has knowledge of the code encryption, to acquire the P code. Data
`subframe block 1 contains frequency standard corrective data enabling clock correction to be made in
`the receiver. Data blocks 2 and 3 hold SV orbit ephemeris data. The two blocks contain such data as
`orbit eccentrieity variations and Keplerian parameters. Message block 4 passes alphanumeric data to
`the user and is only used when the ULS has a need to pass specific messages. Block 5 is an extensive·
`almanac that includes data on SV health and identity codes.
`
`ONE DATA
`
`FRAME
`
`SV EPHEMERIS DATA
`
`1500 bits
`
`30 seconds
`
`~ ONE SUBFRAME= 300 bits, 6 seconds ~ T
`jtLM !Howl SV CLOCK CORRECTION
`jrLM !Howl
`jrLM !Howl SV EPHEMERIS DATA
`jrLM !Howl OTHER DATA (SEE TEXT)
`jrLM !Howl
`
`2
`
`3
`
`4
`
`5
`
`I
`
`I
`i
`
`Figure 5.8 Navig_ation data format.
`
`I L_
`
`A~MANAC DATA(ALL SV's)I
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`Table 5.3 Summary of data in a 30-s frame
`
`A
`B
`C
`D
`E
`F
`G
`H
`I
`J
`K
`L
`
`SV orbital parameters.
`SV clock error data
`Sidereal correction figures
`Almanac of all operational SV s
`Polar wander data (Earth axis wander)
`SV pe_rformance status
`Time of last data inject
`Data to enable P code acquisition (HOW)
`Telemetry data (TLM)
`SV number
`Specific messages as required (i.e. an indication that an SV is off station)
`Receiver clock correction data
`
`At the 50-Hz transmission rate, it takes 6 s to download a subframe, 30 s for one data frame (see
`.
`Table 5.3) and a full 12.5 min to access all 25 frames.
`The L1 signal carrier is BPSK-modulated by both the P and CIA PRN codes and the navigation
`message. Modulation possesses both in-phase and quadrature components as shown in Figure
`5.9.
`
`Pcode + D
`
`P code= 10.23 M bits-1
`Data = 50 bit s-1
`
`C/A code+ D
`C/A code= 1.023 M bits-1
`Data= 50 hiti:;-1
`Figure 5.9 Phase relationship between the P and C/A codes.
`
`P code amplitude is -3dB down (half the power level) on the CIA code signal strength, thus the
`slower CIA code provides a better signal-to•noise ratio at the antenna. This makes the CIA code
`easier to access. The L2 carrier is BPSK-modulated by the P code and the navigation message. The
`use of BPSK modulation causes a symmetrical spread of the code bandwidth around the carrier
`frequency. The frequency spectrum produced by both P and CIA codes on the Li carrier is shown
`in Figure 5.10. The bandwidth of the CIA code is 2.046 MHz and that of the P code is
`20.46Mllz. The CIA code component of the L