Radar and ARPA Manual

421 Pages·1990·7.56 MB·English

by  A. G. Bole and W. O. Dineley (Auth.)

Radar and ARPA Manual
A. G. Bole, Extra Master Mariner, FRIN, FNI
W. O. Dineley, Extra Master Mariner, MPhil
Heinemann Newnes
Heinemann Newnes
An imprint of Heinemann Professional Publishing Ltd
Halley Court jordan Hill, Oxford OX2 8EJ
OXFORD LONDON MELBOURNE AUCKLAND SINGAPORE
IBADAN NAIROBI GABORONE KINGSTON
First published 1990
© A. G. Bole and W. O. Dineley 1990
British Library Cataloguing in Publication Data
Bole, A. G.
Radar and ARPA manual.
1. Ships. Automatic radar plotting aids. Use -
Manuals
I. Title II. Dineley, W. O.
623.8933
ISBN 0 434 90118 0
Typeset by August Filmsetting, Haydock, St Helens
Printed in Great Britain by Courier International, Tiptree, Essex
To Keith
Preface
This manual has been planned not only as a comprehen- Throughout the book the operational significance of
sive practical reference for mariners on board ship and the IMO Performance Standards is stressed, as is the
managers ashore, but also to provide all essential infor- role of radar and ARPA in navigation and collision
mation for candidates following ENS, radar observer avoidance.
and professional certificate courses. With the completion of the SOLAS updating and
Over the past decade there have been considerable fitting schedule in view, it has been possible to bring
developments in ARPA design, but perhaps the more together a body of practical information on equipment
significant changes have been seen in the design of basic and techniques which, it is hoped, will both serve the
radar systems: ARPA features are now almost entirely observer using traditional equipment and provide
integrated with the radar display - which means that reliable guidance to the use of newer equipment for
neither can be treated in isolation. Thus this new manual some years to come.
supersedes our earlier ARPA Manual, representing a The companion volume, The Navigation Control
thorough revision and extension to cover the complete Manual, covers the integration of radar and ARPA
radar/ARPA installation. within the total navigation system. Together, the two
The changes in radar displays that are likely to be of books provide a comprehensive treatment of the theor-
greatest significance to the observer are the develop- etical and practical aspects of electronic navigation
ments in signal processing and the advent of raster-scan systems.
displays: these receive exhaustive treatment. The effects
of changes in shipboard operations, such as false echoes A.G.B.
from containers, are also dealt with. W.O.D.
Acknowledgements
The authors wish to express their gratitude to: Mr B. Price of Sandown College, Liverpool for his
helpful comments based on a reading of Chapter 2;
The International Maritime Organization for permis-
Mr Andrew O. Dineley for his assistance in producing
sion to reproduce the various extracts from Resolu-
the computer printout of the manuscript;
tions Adopted by the Assembly;
Families and friends without whose assistance, support
The Controller of Her Majesty's Stationery Office for
and understanding this undertaking would never
permission to reproduce the extracts from M 1158
have been completed.
and Statutory Instrument No. 1203 (1984);
Captain C. E. Nicholls of Liverpool Polytechnic for his
major contribution to Chapter 8;
1 Basic radar principles
exploiting the same fundamental principles unveiled so
1.1 Introduction
long ago. An understanding of such principles is an
essential starting point in any study of marine radar.
The word RADAR is an acronym derived from the
words Radio Detection and Ranging. In the United King-
dom it was initially referred to as radio direction finding
(RDF) in order to preserve the secrecy of its ranging 1.2 Principles of range measurement
capability.
The scientist Heinrich Hertz, after whom the basic
unit of frequency is named, demonstrated in 1886 that 1.2.1 The echo principle
radio waves could be reflected from metallic objects. In
An object (normally referred to as a target) is detected
1903 a German engineer obtained a patent in several
by the transmission of a pulse of radio energy and the
countries for a radio wave device capable of detecting
subsequent reception of a fraction of such energy (the
ships, but it aroused little enthusiasm because of its very
echo) which is reflected by the target in the direction of
limited range. Marconi, delivering a lecture in 1922,
the transmitter. The phenomenon is analogous to the
drew attention to the work of Hertz and proposed in
reflection of sound waves from land formations. If a
principle what we know today as marine radar.
blast is sounded on a ship's whistle, the energy travels
Although radar was used to determine the height of the
outward and some of it may strike, for example, a cliff.
ionosphere in the mid-1920s, it was not until 1935 that
The energy which is intercepted will be reflected by the
radar pulses were successfully used to detect and measure
cliff. If the reflected energy returns in the direction of the
the range of an aircraft. In the 1930s there was much
ship, and is of sufficient strength, it will be heard as an
simultaneous but independent development of radar
audible echo which, in duration and tone, resembles the
techniques in Britain, Germany, France and America.
original blast. In considering the echo principle the fol-
Radar first went to sea in a warship in 1937 and by 1939
lowing points can usefully assist in a preliminary under-
considerable improvement in performance had been
standing of radar detection:
achieved. By 1944 naval radar had made an appearance
on merchant ships and from about the end of the war the (a) The echo is never as loud as the original blast.
growth of civil marine radar began. Progressively it was (b) The chance of detecting an echo depends on the
refined to meet the needs of peacetime navigation and loudness and duration of the original blast.
collision avoidance. (c) Short blasts are required if echoes from close targets
While the civil marine radars of today may, in size, are not to be drowned by the original blast.
appearance and versatility, differ markedly from their (d) A sufficiently long interval between blasts is
ancestors of the 1940s, the basic data that they offer, required to allow time for echoes from distant
namely target range and bearing, are determined by targets to return.
2 RADAR AND ARPA MANUAL
Distance - speed x time / /
!<.. . *\
Range = speed x time '
2
Figure 1.1 The echo principle
While the sound analogy is extremely useful, it must not T = the elapsed time (jis)
be pursued too far as there are a number of ways in S = the speed of radio waves (metres/^s)
which the character and behaviour of radio waves differ
Then D=SxT
from those of sound waves. In particular at this stage it is
and R=(SxT)/2
noteworthy that the speed of radio waves is very much
hence £=(300xT)/2
higher than that of sound waves.
thus R=150T
The application of this relationship can be illustrated by
1.2.2 Range as a function of time
the following example.
It is almost self-evident that the time which elapses Example i.i Calculate the elapsed time for a pulse to
between the transmission of a pulse and the reception of travel to and return from a radar target whose range is
the corresponding echo depends on the speed of the (a) 50 metres (b) 12 nautical miles.
pulse and the distance which it has travelled in making
(a) R=150T
its two-way journey. Thus, if the speed of the pulse is
thus 50=150T
known and the elapsed time can be measured, the range
hence T= 50/150 = 0.33^s
of the target producing the echo can be calculated.
While it is recognised that the velocity of radio waves This value is of particular interest because 50 metres rep-
is dependent on the nature of the medium through resents the minimum detection range that must be
which they travel, for the practical purpose of marine achieved to ensure compliance with the IMO Perform-
radar ranging the speed may be assumed to have a con- ance Standards for Navigational Radar Equipment (see
stant value of 300 000 000 metres per second. Because Sections 10.2.1 and 10.2.2). While this topic will be fully
this number is rather large, it is expedient in practical explored in Section 3.2.4, it is useful at this stage to note
calculations to use the microsecond as the time unit. One the extremely short time interval within which trans-
/is represents one millionth part of a second (i.e. 10" 6 mission and reception must be accomplished.
seconds). The speed can thus be written as 300 metres per (b) R=\50T
//s. Using this value it is possible to produce a simple Since 1 nautical mile= 1852 metres,
general relationship between target range and the 12xl852=150T
elapsed time which separates the transmission of the hence T= 12 x 1852/150= 148.16^s
pulse and the reception of an echo in any particular case
This result is noteworthy as it represents the elapsed time
(see Figure 1.1).
for a commonly used marine radar range scale. Further
Let D = the distance travelled by the pulse (metres) reference will be made to this value in the succeeding
R = the range of the target (metres) section.
BASIC RADAR PRINCIPLES 3
1.2.3 The timebase
It is clear from the values established in the previous
section that the elapsed times are of the order of mil-
lionths of a second and are thus so short as to be beyond
the capability of any conventional time-measuring
device. This difficulty is overcome by using an electro-
nic device known as a cathode ray tube (CRT). The
electronic principles of this important device are dis-
cussed in some detail in Section 2.6.1; it is sufficient at
this stage to appreciate that its display feature is a glass
screen across which travels a very small spot of light.
The speed of this travel can be accurately controlled at
values which allow the spot to transit the screen in as
little as a few microseconds. At such speeds it moves
literally Taster than the eye can see' and hence appears as
a line rather than a spot, but it is important that the
concept of a moving light spot is appreciated as it is
fundamental to an understanding of radar display prin-
ciples.
The CRT can be used to perform an electronic stop-
Figure 1.2 The A-scan display
watch function by arranging that the time taken for the
spot to cross the screen is the same as the time taken for a
radar pulse to make the two-way journey to a target at a instantaneous vertical deflection of the spot which
chosen range. (This can be compared with a mechanical produces a 'spike', known as the transmission mark.
stopwatch in which the second hand transits the circum- (b) The speed of the spot (the sweep rate) is adjusted so
ference of a dial in the same time as the earth rotates that it completes the trace in the same time as a radar
through approximately 15 seconds of arc. It is useful to pulse will take to travel to and from a target located
illustrate the principle with reference to the method at the maximum range which the scaled line is pres-
originally employed in the early radars of the 1940s. ently intended to represent. When the spot has
While this type of display, known as A-scan, is no longer completed the trace, the brilliance is automatically
used in civil marine applications (other than in the form reduced to zero and the spot flies back to the origin
of an oscilloscope such as may be used for servicing) it to await the transmission of the next pulse. At this
demonstrates the principle clearly and is a suitable point event it initiates a further trace along the same path
from which to progress to a description of the ways in as its predecessor.
which the display has been subsequently refined. (c) A returning echo is used to generate an instanta-
neous vertical deflection of the spot, thus producing
1.2.3Λ The A-scan display a further spike. The amplitude of this spike, within
certain limits (see Section 2.5.2.5) is a function of the
In the A-scan display, the spot is used to produce a hori-
strength of the echo.
zontal line (the trace) which originates close to the left-
hand side of the screen of the CRT. The following
Consideration of these three features and Figure 1.2 will
features are of particular importance:
indicate that the horizontal distance between the trans-
(a) The trace commences at the instant each radar pulse mission mark and the echo spike is a measure of the
is transmitted and this event is indicated by an range of the target. Using the result from Example
4 RADAR AND ARPA MANUAL
1.1 (b), it is evident that if the full extent of the trace is to In a radial-scan PPI display, the spot produces the trace
represent a range of 12 miles (the selected range scale) the in the form of a radial line whose origin is (normally)
spot must complete the trace in approximately 148/is. placed at the centre of the circular screen (Figure 1.3).
This quantity is referred to as the timebase and it is on this An echo return from a target is used to produce an
value that the displayed range of all targets is based. If a increase in the brilliance of the moving spot. To this end,
target lay at a range of 6 miles, its echo would return in a competent observer will, in setting up the display,
half the timebase and would be displayed at half the adjust the brilliance of the trace so that it is barely visible
maximum range of the scale in use. Different range (see Section 6.2.3.1), hence maximising the probability
scales are obtained by operating the range scale selector that small increases in brilliance will be detected. Within
which selects the correct timebase for the chosen range certain limits (see Section 2.5.2.5) the brightening of the
scale. For example, on the 3 mile range scale the spot trace by a target return is a function of the strength of the
must travel four times faster than on the 12 mile range received echo. As with the A-scan display, the origin of
scale and hence a timebase of approximately 37)Us would the trace coincides with the instant of transmission of the
be selected. pulse and the duration of the trace is the selected
A limitation of the A-scan display is its inability to timebase. Thus a target which lies at the maximum
display information from any direction other than that range represented by the selected scale will appear at the
in which the antenna (or aerial) is trained at that edge of the screen.
moment. This shortcoming led to the development of When the spot has completed the trace, the brilliance
the plan position indicator (PPI). is automatically reduced to zero and the spot flies back to
In radar terminology the words antenna, aerial, and the origin to await the incidence of the next transmis-
scanner are all used to describe the device which beams sion. At this event it initiates a further line which, in
the transmitted energy into space. Throughout this text contrast with the A-scan case, is drawn along a path
these words may be regarded as synonyms. which is separated from the previous one by a small
angle (about one tenth of a degree). In a PPI system the
1.2.3.2 The radial-scan plan position indicator (PPI) antenna rotates continuously and automatically in a
display clockwise direction, generating approximately some
3600 lines in one complete rotation. The resultant
rotating trace makes it possible to display simultan-
eously targets in all directions in the correct angular rela-
tionship, one to another. Echoes painted by the trace as it
passes will glow for a short period due to a property of
the screen known as persistence or afterglow. Thus, in
general the picture persists until refreshed on the next
revolution of the trace (see Section 1.3.3). The PPI dis-
play is particularly suited to collision avoidance and
navigational applications; the angular build-up of the
picture will be described in Section 1.3.3.
1.2.4 Calibration of the timebase
It has been shown that by making the duration of one
trace equal to the timebase of the selected range scale, the
echo of a target will be displayed at a distance from the
Figure 1.3 The radial-scan PPI display origin which is proportional to its range. Thus, when
BASIC RADAR PRINCIPLES 5
Table 1.1 Timebase and calibration interval values
Range Timebase Calibration interval Remarks
scale duration
(n miles) (/is) (n miles) (/is)
0.75 9.3 0.25 3.1 Normally
only 3 rings
1.5 18.5 0.25 3.1 Ί
3 37.0 0.5 6.2
6 74.1 1.0 12.4 6 rings in
12 148.2 2.0 24.7 each case
24 296.3 4.0 49.4
48 592.6 8.0 98.8 J
the 6 mile calibration pip is the third from the origin, the
mark at the origin counting as zero.) It is evident that the
range scale selector must control not only the timebase
duration but also the interval between calibration
marks. This is illustrated, for some range scales, by the
Figure 1.4 Calibration marks values in Table 1.1.
The timing of the calibration marks can be achieved
with an extremely high degree of accuracy which in
observing the display, it may be possible to estimate the turn becomes implicit in the accuracy with which the
range of a target by mentally dividing the trace into range of a target can be measured provided that its echo
equal sections. However, to measure a range with an coincides with a mark. If the echo lies between two
acceptable degree of accuracy (see Section 6.9.8), the marks, some form of interpolation will be necessary. In
subdivision must be carried out by electronic methods. some circumstances it may be that this can be done by
A device known as an oscillator is used to generate a eye, but in other cases an acceptable degree of accuracy
succession of signals which occur at equal intervals of time. (see Section 6.9.1) will demand electronic assistance.
It is arranged that the first signal of any group coincides Interpolation is facilitated by the generation of a
with the instant of transmission and that the interval single pip, known as a variable range marker, which occurs
between successive pulses is a sub-multiple of the at a selected elapsed time after transmission. The time is
timebase of the selected range scale. Each signal is used to selected by the operation of a manual control which is
produce an instantaneous brightening of the spot, and as also connected to some form of numerical read-out. The
a result bright marks, sometimes called calibration pips or latter will show the number of miles of timebase repre-
calibration marks, will subdivide the trace into equal inter- sented by the selected time. Thus, as illustrated by Figure
vals of time and therefore, by virtue of the timebase 1.5, if the observer adjusts the variable range marker to
value, into equal intervals of range. Hence, on the 12 read 3 miles, the single pip will be produced approxi-
mile range scale, as illustrated by Figure 1.4, the echo mately 37/xs after transmission, i.e. after one quarter of
from a target at a range of 6 miles will arrive after trans- the 12 mile timebase.
mission by an elapsed time equal to half the timebase, In conclusion, it is clear that the precision with which
and therefore will be displayed at the same time, and in the calibration marks are produced is fundamental to the
the same place, as the 6 mile calibration pip. (Notice that accuracy of range measurement. For a detailed treat-