COMPUTERS IN THE CLASSROOM: ANOTHER CASE OF THE

26 Pages·2007·0.56 MB·English

1 Basic radar principles
namely target range and bearing, are determined by
1.1 Introduction
exploiting the same fundamental principles unveiled
solongago.Anunderstandingofsuchprinciplesisan
The word RADAR is an acronym derived from the essential starting point in any study of marine radar.
words Radio Detection and Ranging. In the United
Kingdom it was initially referred to as radio direction
finding (RDF) in order to preserve the secrecy of its
ranging capability. 1.2 Principles of range measurement
The scientist Heinrich Hertz, after whom the basic
unitoffrequencyisnamed,demonstratedin1886that
1.2.1 The echo principle
radio waves could be reflected from metallic objects.
In 1903 a German engineer obtained a patent in Anobject(normallyreferredtoasatarget)isdetected
several countries for a radio wave device capable of bythetransmissionofapulseofradioenergyandthe
detectingships,butitarousedlittleenthusiasmbecause subsequentreceptionofafractionofsuchenergy(the
of its very limited range. Marconi, delivering a lec- echo) which is reflected by the target in the direction
ture in 1922, drew attention to the work of Hertz of the transmitter. The phenomenon is analogous to
and proposed in principle what we know today as thereflectionofsoundwavesfromlandformations.If
marine radar. Although radar was used to determine ablastissoundedonaship’swhistle,theenergytravels
the height of the ionosphere in the mid-1920s, it was outwardandsomeofitmaystrike,forexample,acliff.
notuntil1935thatradarpulsesweresuccessfullyused The energy which is intercepted will be reflected by
to detect and measure the range of an aircraft. In the thecliff.Ifthereflectedenergyreturnsinthedirection
1930s there was much simultaneous but independent of the ship, and is of sufficient strength, it will be
heardasanaudibleechowhich,indurationandtone,
developmentofradartechniquesinBritain,Germany,
resembles the original blast. In considering the echo
FranceandAmerica.Radarfirstwenttoseainawar-
principle the following points can usefully assist in a
ship in 1937 and by 1939 considerable improvement
preliminary understanding of radar detection:
in performance had been achieved. By 1944 naval
radar had made an appearance on merchant ships and (a) The echo is never as loud as the original blast.
from about the end of the war the growth of civil (b) Thechanceofdetectinganechodependsonthe
marine radar began. Progressively it was refined to loudness and duration of the original blast.
meet the needs of peacetime navigation and collision (c) Shortblastsarerequiredifechoesfromclosetar-
avoidance. gets are not to be drowned by the original blast.
Whilethecivilmarineradarsoftodaymay,insize, (d) A sufficiently long interval between blasts is
appearance and versatility, differ markedly from their required to allow time for echoes from distant
ancestors of the 1940s, the basic data that they offer, targets to return.
2 RADAR AND ARPA MANUAL
While the sound analogy is extremely useful, it must T = the elapsed time ((cid:1)s)
not be pursued too far as there are a number of ways S = the speed of radio waves (metres/(cid:1)s)
in which the character and behaviour of radio waves
Then D=S×T
differ from those of sound waves. In particular at this
and R =(cid:2)S×T(cid:3)/2
stage it is noteworthy that the speed of radio waves is
hence R =(cid:2)300×T(cid:3)/2
very much higher than that of sound waves.
thus R =150T
The application of this relationship can be illustrated
1.2.2 Range as a function of time
by the following example.
It is almost self-evident that the time which elapses
Example 1.1 Calculate the elapsed time for a pulse
betweenthetransmissionofapulseandthereception
totraveltoandreturnfromaradartargetwhoserange
of the corresponding echo depends on the speed of
is (a) 40 metres (b) 12 nautical miles.
the pulse and the distance which it has travelled in
makingitstwo-wayjourney.Thus,ifthespeedofthe (a) R = 150T
pulseisknownandtheelapsedtimecanbemeasured, thus 40 = 150T
the range of the target producing the echo can be hence T = 40/150=0(cid:4)27(cid:1)s
calculated.
This value is of particular interest because 40 metres
While it is recognized that the velocity of radio
representstheminimumdetectionrangethatmustbe
waves is dependent on the nature of the medium
achievedtoensurecompliancewiththeIMOPerfor-
through which they travel, for the practical purpose
mance Standards for Navigational Radar Equipment
of marine radar ranging the speed may be assumed
(see Section 11.2.1). While this topic will be fully
to have a constant value of 300000000 metres per
explored in Section 3.2.4, it is useful at this stage to
second.Becausethisnumberisratherlarge,itisexpe-
note the extremely short time interval within which
dient in practical calculations to use the microsecond
transmission and reception must be accomplished.
asthetimeunit.One(cid:1)srepresentsonemillionthpart
ofasecond(i.e.10−6 seconds).Thespeedcanthusbe (b) R = 150T
writtenas300metresper(cid:1)s.Usingthisvalueitispos- Since 1 nautical mile = 1852 metres,
sibletoproduceasimplegeneralrelationshipbetween 12×1852 = 150T
target range and the elapsed time which separates the hence T =12×1852/150 = 148(cid:4)16(cid:1)s
transmissionofthepulseandthereceptionofanecho
This result is noteworthy as it represents the elapsed
in any particular case (see Figure 1.1).
time for a commonly used marine radar range scale.
Let D = the distance travelled by the pulse (metres) Further reference will be made to this value in the
R = the range of the target (metres) succeeding section.
Figure 1.1 Theechoprinciple
BASIC RADAR PRINCIPLES 3
1.2.3 The timebase (a) The trace commences at the instant each radar
pulseistransmittedandthiseventisindicatedbyan
It is clear from the values established in the previ-
instantaneousverticaldeflectionofthespotwhich
ous section that the elapsed times are of the order
producesa‘spike’,knownasthetransmissionmark.
of millionths of a second and are thus so short as to
(b) Thespeedofthespot(thesweeprate)isadjusted
be beyond the capability of any conventional time-
sothatitcompletesthetraceinthesametimeasa
measuringdevice.Thisdifficultyisovercomebyusing
radarpulsewilltaketotraveltoandfromatarget
an electronic device known as a cathode ray tube located at the maximum range which the scaled
(CRT). The electronic principles of this important lineispresentlyintendedtorepresent.Whenthe
devicearediscussedinsomedetailinSection2.6.1;it spot has completed the trace, the brilliance is
is sufficient at this stage to appreciate that its display automatically reduced to zero and the spot flies
feature is a glass screen across which travels a very back to the origin to await the transmission of
small spot of light. The speed of this travel can be the next pulse. At this event it initiates a further
accuratelycontrolledatvalueswhichallowthespotto trace along the same path as its predecessor.
transitthescreeninaslittleasafewmicroseconds.At (c) A returning echo is used to generate an instan-
such speeds it moves literally ‘faster than the eye can taneous vertical deflection of the spot, thus pro-
see’andhenceappearsasalineratherthanaspot,but ducing a further spike. The amplitude of this
itisimportantthattheconceptofamovinglightspot spike, within certain limits (see Section 2.5.2.5)
isappreciatedasitisfundamentaltoanunderstanding is a function of the strength of the echo.
of radar display principles.
Consideration of these three features and Figure 1.2
The CRT can be used to perform an electronic
will indicate that the horizontal distance between the
stopwatch function by arranging that the time taken
transmission mark and the echo spike is a measure of
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 chosen range. This can be
compared with a mechanical stopwatch in which the
secondhandtransitsthecircumferenceofadialinthe
same time as the earth rotates through approximately
15secondsofarc.Itisusefultoillustratetheprinciple
with reference to the method originally employed in
theearlyradarsofthe1940s.Whilethistypeofdisplay,
known as A-scan, is no longer used in civil marine
applications(otherthanintheformofanoscilloscope
suchasmaybeusedforservicing)itdemonstratesthe
principle clearly and is a suitable point from which
to progress to a description of the ways in which the
display has been subsequently refined.
1.2.3.1 The A-scan display
In the A-scan display, the spot is used to produce a
horizontal line (the trace) which originates close to
the left-hand side of the screen of the CRT. The
following features are of particular importance: Figure 1.2 TheA-scandisplay
4 RADAR AND ARPA MANUAL
the range of the target. Using the result from Exam-
ple1.1(b),itisevidentthatifthefullextentofthetrace
is to represent a range of 12 miles (the selected range
scale) the spot must complete the trace in approxi-
mately148(cid:1)s.Thisquantityisreferredtoasthetime-
base and it is on this value that the displayed range of
alltargetsisbased.Ifatargetlayatarangeof6miles,
its echo would return in half the timebase and would
bedisplayedathalfthemaximumrangeofthescalein
use. Different range scales are obtained by operating
therangescaleselectorwhichselectsthecorrecttime-
base for the chosen range scale. For example, on the
3milerangescalethespotmusttravelfourtimesfaster
than on the 12 mile range scale and hence a timebase
of approximately 37(cid:1)s would be selected.
A limitation of the A-scan display is its inability
to display information from any direction other than
that in which the antenna (or aerial) is trained at that Figure 1.3 Theradial-scanPPIdisplay
moment.Thisshortcomingledtothedevelopmentof
the plan position indicator (PPI). When the spot has completed the trace, the bril-
In radar terminology the words antenna, aerial, and liance is automatically reduced to zero and the spot
scannerareallusedtodescribethedevicewhichbeams flies back to the origin to await the incidence of the
the transmitted energy into space. Throughout this next transmission. At this event it initiates a further
text these words may be regarded as synonyms. linewhich,incontrastwiththeA-scancase,isdrawn
alongapathwhichisseparatedfromthepreviousone
by a small angle (about one tenth of a degree). In a
1.2.3.2 The radial-scan plan position indicator
PPIsystemtheantennarotatescontinuouslyandauto-
(PPI) display
maticallyinaclockwisedirection,generatingapprox-
Inaradial-scanPPIdisplay,thespotproducesthetrace imately some 3600 lines in one complete rotation.
in the form of a radial line whose origin is (normally) Theresultantrotatingtracemakesitpossibletodisplay
placedatthecentreofthecircularscreen(Figure1.3). simultaneously targets in all directions in the correct
An echo return from a target is used to produce an angular relationship, one to another. Echoes painted
increase in the brilliance of the moving spot. To this by the trace as it passes will glow for a short period
end, a competent observer will, in setting up the dis- due to a property of the screen known as persistence
play, adjust the brilliance of the trace so that it is orafterglow.Thus,ingeneralthepicturepersistsuntil
barelyvisible(seeSection6.2.3.1),hencemaximizing refreshed on the next revolution of the trace (see
theprobabilitythatsmallincreasesinbrilliancewillbe Section1.3.3).ThePPIdisplayisparticularlysuitedto
detected. Within certain limits (see Section 2.5.2.5) collision avoidance and navigational applications; the
the brightening of the trace by a target return is a angular build-up of the picture will be described in
functionofthestrengthofthereceivedecho.Aswith Section 1.3.3.
the A-scan display, the origin of the trace coincides
with the instant of transmission of the pulse and the
1.2.4 Calibration of the timebase
duration of the trace is the selected timebase. Thus a
targetwhichliesatthemaximumrangerepresentedby Ithasbeenshownthatbymakingthedurationofone
theselectedscalewillappearattheedgeofthescreen. traceequaltothetimebaseoftheselectedrangescale,
BASIC RADAR PRINCIPLES 5
the echo of a target will be displayed at a distance willarriveaftertransmissionbyanelapsedtimeequalto
from the origin which is proportional to its range. halfthetimebase,andthereforewillbedisplayedatthe
Thus, when observing the display, it may be possible sametime,andinthesameplace,asthe6milecalibra-
to estimate the range of a target by mentally dividing tionpip.(Noticethatthe6milecalibrationpipisthe
the trace into equal sections. However, to measure thirdfromtheorigin,themarkattheorigincountingas
a range with an acceptable degree of accuracy (see zero.)Itisevidentthattherangescaleselectormustcon-
Section6.9.8),thesubdivisionmustbecarriedoutby trolnotonlythetimebasedurationbutalsotheinterval
electronic methods. betweencalibrationmarks.Thisisillustrated,forsome
A device known as an oscillator is used to generate rangescales,bythevaluesinTable1.1.
a succession of signals which occur at equal intervals of Thetimingofthecalibrationmarkscanbeachieved
time. It is arranged that the first signal of any group with an extremely high degree of accuracy which in
coincideswiththeinstantoftransmissionandthatthe turnbecomesimplicitintheaccuracywithwhichthe
intervalbetweensuccessivepulsesisasub-multipleof range of a target can be measured provided that its
thetimebaseoftheselectedrangescale.Eachsignalis echocoincideswithamark.Iftheecholiesbetweentwo
used to produce an instantaneous brightening of the marks,someformofinterpolationwillbenecessary.In
spot,andasaresultbrightmarks,sometimescalledcal- somecircumstancesitmaybethatthiscanbedoneby
ibrationpipsorcalibrationmarks,willsubdividethetrace eye,butinothercasesanacceptabledegreeofaccuracy
into equal intervals of time and therefore, by virtue (seeSection6.9.1)willdemandelectronicassistance.
of the timebase value, into equal intervals of range. Interpolation is facilitated by the generation of a
Hence, on the 12 mile range scale, as illustratedby single pip, known as a variable range marker, which
Figure1.4,theechofromatargetatarangeof6miles occursataselectedelapsedtimeaftertransmission.The
time is selected by the operation of a manual control
which is also connected to some form of numerical
read-out. The latter will show the number of miles
of timebase represented by the selected time. Thus,
as illustrated by Figure 1.5, if the observer adjusts the
variable range marker to read 3 miles, the single pip
will be produced approximately 37(cid:1)s after transmis-
sion, i.e. after one quarter of the 12 mile timebase.
Table 1.1 Timebaseandcalibrationintervalvalues
Range Timebase Calibration interval Remarks
scale duration (n miles) ((cid:1)s)
(n miles) ((cid:1)s)
0.75 9(cid:4)3 0(cid:4)25 3(cid:4)1 Normally
 only 3 rings
13.5 1387(cid:4)(cid:4)50 00(cid:4)(cid:4)255 36(cid:4)(cid:4)12
6 74(cid:4)1 1(cid:4)0 12(cid:4)4 6 rings in
1224 124986(cid:4)(cid:4)23 24(cid:4)(cid:4)00 2449(cid:4)(cid:4)74 each case
48 592(cid:4)6 8(cid:4)0 98(cid:4)8
Figure 1.4 Calibrationmarks
6 RADAR AND ARPA MANUAL
to differentiate between the traditional type of dis-
play and the modern, computer controlled ‘Synthetic
Display’.
The microprocessor, a computer device capable
of storing and manipulating data at very high rates,
became available from the early 1970s; as the cost
of computer memory decreased through the 1980s,
a revolution took place in the way in which range
(and other) data was handled. As a result, in modern
radar systems the range data is stored in a computer
memory. The data, once processed, can be used to
synthesize a picture for display on a radial-scan PPI.
Such a picture is sometimes referred to as a re-timed
display. In the mid-eighties it became progressively
more common to produce the picture on a television
screen.Thisisnowuniversallythecase.Suchadisplay
isreferredtoasaraster-scanPPIinordertodistinguish
itfromthetraditionalradial-scanPPI.Detailedconsid-
Figure 1.5 Thevariablerangemarker
erationofraster-scandisplaysisdeferredtoChapter2
(see Section 2.6.7), as an understanding of basic radar
In conclusion, it is clear that the precision with principles is more readily gained from a study of the
which the calibration marks are produced is funda- radial-scan display. However, regardless of whether
mental to the accuracy of range measurement. For a the synthetic picture is finally produced on a raster-
detailed treatment of the correct procedure for their scan or radial-scan PPI, the principle of range mea-
useandtheaccuracywhichcanbeachieved,thereader surement is as described below.
is referred to Section 6.9. For a general introduction to the principle of stor-
age, the computer memory may be likened to a large
number of ‘two-position’ switches each of which can
1.2.5 The synthetic display
be either on or off. Consider a computer that will
The displays produced in the ways so far described sample the received signal 1200 times in the duration
havebecomeknownas‘real-time’displays.Theterm of one complete radial trace. The sampling process
describes the fact that the echoes are written on the commencesattheinstantoftransmissionandtheresult
trace concurrently with their reception, rather in the of each sampling is registered using, consecutively,
way in which one might write down a navigation eachof1200memoryelements.Ifareturningechois
warning, as it is received. It was recognized for many presentatthetimeofsampling,theappropriateswitch
years that if the range data could be stored in such a issetto‘on’,whereasifnoechoispresenttheswitch
way that it could be manipulated, considerable ben- is left in the ‘off’ position. Thus the range of each
efits might accrue, such as the ability to improve the echo will be implicit in the number of the memory
picture quality and to carry out automatic tracking of element in which its presence is recorded.
targets. Pursuing the analogy of the navigation warn- In diagrams, it is usual to depict each element of
ing somewhat further, this may be compared with memory as a box in which the number ‘1’ appears
writing down the warning quickly as it is broadcast if the switch is on and the number ‘0’ appears if the
and subsequently re-writing it slowly and neatly. As switchisoff.Thisformofrepresentationisparticularly
with most analogies, care should be exercised in its suitable because the computer uses a counting system
indefinite pursuit. The term real-time display is used known as binary in which there are only two digits,
BASIC RADAR PRINCIPLES 7
brightening of the trace at a distance from the origin
whichisproportionaltotherangeofthetargetswhich
produced the echoes.
Intheexamplethathasbeenchosen,therangedata
was written in and read out at the same rate. In a
synthetic display this will only be true for one range
scale. This is best illustrated by considering what will
happenifthe6milerangescaleisselected(Figure1.7).
When the 6 mile range scale is selected, the data
mustbewrittenintothememoryoverthedurationof
the6miletimebase(i.e.approximately74(cid:1)s)because
the writing time is determined by the speed of radio
waves. The received signal will be sampled twice as
often so that, if 12 elements are used to represent
6miles,eachelementofmemorywillrepresentarange
intervalof0.5miles.(If1200elementswereused,the
range interval would be 0.005 miles.) If the stored
range data for 6 miles is read out of memory at half
therateatwhichitwaswrittenin(i.e.atthesamerate
Figure 1.6 Storageofrangedata
namely ‘1’ and ‘0’ (see Section 2.6.6.1). Further, it
is practical to show 12 elements, rather than 1200.
Subject to these qualifications, Figure 1.6 illustrates
thegenerationofoneradiallineofasyntheticpicture
when the 12 mile range scale has been selected. In
writing the data into the memory, which will be car-
ried out in the duration of the 12 mile timebase (i.e.
approximately 148(cid:1)s), the sampling process stores a
‘1’ for each received echo and thereby the range of
each target producing a detectable echo is implicit in
theplaceinwhichitisregisteredinthememory.For
example, a target in element 7 lies within the range
limits 7 to 8 nautical miles (using 1200 elements, a
targetinelement700wouldliewithintherangelimits
7.00 to 7.01 nautical miles).
In reading the range data out of the memory, a
standard rectangular pulse is produced for each ‘1’ in
the memory. If the data is read out in the duration of
the 12 mile timebase (i.e. approximately 148(cid:1)s) and
the sweep rate produces a trace whose radial dura-
tion is the same, each of these pulses will produce a Figure 1.7 Fixedsweepratedisplay
8 RADAR AND ARPA MANUAL
as it is read out on the 12 mile range scale) and the means invariably the case. Further, if (as in practice)
sweep rate is maintained at the value for the 12 mile some 1200 memory elements are used, many targets
scale (i.e. giving a timebase of approximately 148(cid:1)s), will register sequentially in more than one element.
a radial trace representing 6 miles will be produced. ThetopicisdiscussedingreaterdetailinSection2.6.6.
Thus,ingeneral,wherecomputerstorageisusedto
developaradialPPIdisplay,therangedataforeachradial
lineiswrittenintomemoryduringonetimebaseofthe
selectedrangescalebutisreadoutinafixedtime,and 1.3 Principles of bearing
displayedatafixedsweepratebothofwhichareinde-
measurement
pendentoftherangescaleselected.Thedurationofthe
fixedread-outtimewillbedeterminedbytheindivid-
ualradardesigner.Commonly,butnotinvariably,itis 1.3.1 Directional transmission
madeequaltothe12miletimebase.Arepresentativeset and reception
ofvaluesisshowninTable1.2.Calibrationmarksand
Toexaminetheprinciplesofbearingmeasurementitis
avariablerangemarkerwillbeavailableonasynthetic
usefultoreturntothesoundanalogyofSection1.2.1.
displaybutitshouldbeborneinmindthattheinterval
If the suggested blast was transmitted in low visibility
betweenthecalibrationpipswilldependonthefixed
it would not be easy to assess the precise direction
sweepratechosenbythedesigner.Additionallyitmay
from which the sound had returned. The difficulty
bepossibletohavethecomputerreadouttherangeof
arises from two principal features:
atargetdirectlyfromtheknowledgeofthenumberof
thememoryelementinwhichitlies. (a) The whistle If it is well placed it will trans-
The above process is sometimes called digital stor- mit sound in all directions and
age,becausethepresenceorabsenceofanechoisreg- thus echoes will return from all
istered by the binary digits ‘1’ or ‘0’ respectively. It is directions.
considerably more complex than the above introduc- (b) The ear Evolutionhasdesignedthehuman
tion might suggest and there is great variation in the ear to be highly receptive over
wayinwhichradardesignershaveexploitedthebasic more than 180 degrees on either
principle. For example, the simple model described side of the head.
producessyntheticechoesallofwhichareidenticalin
Thesetwopropertiesarepreciselythereverseofwhat
brightness. In some systems this is true but it is by no
is required. To determine the bearing of a target,
transmissioninthehorizontalplanemustberestricted
Table 1.2 Range data – typical write and read to one direction at a time, and reception must be
times restricted to that same direction.
In a marine radar system a single antenna (known
Range scale Write time Read-out Sweep time
as the scanner or aerial) is used for both transmis-
(n miles) ((cid:1)s) ((cid:1)s) ((cid:1)s)
sion and reception. It is designed in such a way (see
Section 2.4.2) as to focus the transmitted energy in a
0.75 9(cid:4)3 148 148
beam which is very narrow in the horizontal plane.
1.5 18(cid:4)5 148 148
The angle within which the energy is constrained is
3 37(cid:4)0 148 148
called the horizontal beamwidth (Figure 1.8). It must
6 74(cid:4)1 148 148
have a value of not more than 2.5(cid:1) if it is to com-
12 148(cid:4)2 148 148
plywithIMOPerformanceStandardsforNavigational
24 296(cid:4)3 148 148
Radar Equipment (see Section 11.2.1). There is an
48 592(cid:4)6 148 148
exception for high speed craft in that the horizontal
BASIC RADAR PRINCIPLES 9
convenienttoimaginethattheantennascansthearea
around the ship in about 3600 steps of 0.1(cid:1) and that
each step is represented by a separate trace on the
screen.Thepictureisthus‘builtup’ofapproximately
3600 radial lines.
1.3.3 The build-up of the picture
Figure 1.8 Thehorizontalbeam
The echo of a detectable target will be ‘painted’
once per revolution of the scanner. The inside of the
beamwidth can be up to 4(cid:1) for S-band radar only CRT screen is coated with a luminous material (see
(seeSection11.2.4).However,horizontalbeamwidths Section 2.6.1.2) so that any area, once painted, will
of less than 2(cid:1) are commonplace and shipborne civil glowforatleast2to3seconds.Thustheafterglowcan
marineradarsareavailablewithvaluesaslowas0.75(cid:1). be made sufficiently long to cause the echo paint to
The reception property of the antenna is such that persistuntilitispaintedagainonthenextrevolution.
it will only detect energy which has returned from (There are a few marine systems in which the per-
withintheangularlimitsofthehorizontalbeamwidth. sistence is less than one revolution but they are very
It follows from the directional characteristic of the much the exception.)
antennathatonlythosetargetswhichlieinthedirec- Because the scanner and trace rotate at the same
tion of the beam will appear on the trace. Thus a rate,theradarbeamandthetracedescribeequalarcsin
singletracerepresentstherangesoftargetslyingalong equalintervalsoftime.Thustargetswillbedisplayedin
a specific line of bearing. thecorrectangularrelationship,onetoanother.Iftwo
ships subtend an angle of 50(cid:1) at the observing vessel,
1.3.2 Synchronization of scanner thecentresoftheirechoeswillbeseparatedonthePPI
and trace by a similar angle. As the beam sweeps across a geo-
graphicalfeature,suchasacoastline,echoesonsucces-
Anessentialfeatureofthemodernmarineradardisplay sive traces will add together, side by side, to produce
isthatitshouldprovidecontinouscoverageover360(cid:1) a plan representation of that feature (see Figure 1.9).
of azimuth. To achieve this both the scanner and the The calibration marks, which may be switched on
tracearerotated,atthesamerate,clockwise,continu- or off by the observer, will be painted on each trace
ously and automatically. To comply with the current and thus when switched on will appear as concentric
IMOPerformanceStandardsthedurationofonerev- circles.Thevariablerangemarkerwillsimilarlyappear
olution must not exceed 5 seconds (1.5 seconds for as a single circle of variable radius.
high speed craft); values of about 2 to 3 seconds are
typical for shipborne radar.
The interval between successive transmitted pulses
1.3.4 The heading marker
(and hence between the start of successive traces
on the PPI) may fall within a range of values (see In general, a bearing is the angle between the direc-
Section 2.3.1) but for the purposes of illustration an tion of a chosen reference and that of an object of
average value of 1/1250s (800(cid:1)s) will suffice. If a interest. On a PPI display the fundamental reference
representative time for one revolution of the scanner is the instantaneous direction of the observing ves-
is taken to be 3 seconds, one can deduce that some sel’s head. As the axis of the horizontal beam crosses
3600pulsesaretransmittedduringonerevolutionand the ship’s fore-and-aft line in the forward direction, a
that the scanner rotates through 0.1(cid:1) between pulses. set of contacts is closed, producing a pulse of a few
Thuswhiletherotationiscontinuous,itmaybefound millisecondsduration.Thispulseisusedtoincreasethe
10 RADAR AND ARPA MANUAL
is discussed in Section 6.9.9. Clearly, because both
targets and the heading marker are produced by a
brightening of the spot, there is a danger that a target
maybemaskedifitliesinthedirectionoftheheading
marker. The specification recognizes this danger by
requiring that there is a provision for switching the
heading marker off. However, the provision must be
such that the heading marker cannot be left in the off
position. Normally this requirement is complied with
by the use of a spring-loaded switch which is biased
in the on position. The danger of the heading marker
being left in the off position is that, in the absence of
the correct reference, an erroneous reference (such as
thebearingmarker–seeSection6.9.5)mightbeused
inadvertently.
The appearance of the heading marker confirms
the orientation of the picture (see Section 1.4) and
consequentlywhetherthebearingsarerelativeortrue
(see Figure 1.10).
1.3.5 Bearing measurement
The picture described above offers a plan view rep-
resentation of the area surrounding the ship in such a
form as to make it possible to measure the range and
bearing of any detected target. It is this characteris-
Figure 1.9 Thebuild-upofthepicture
tic that gave rise to the term ‘plan position indicator’
which distinguished the rotating trace display from
brillianceofthespotandhenceproducethecomplete the earlier A-scan display. It is also this characteristic
brighteningupofseveralsuccessivetraces,resultingin which makes the display particularly suitable for use
thedistinctive‘headingmarker’or‘headingindicator’ in collision avoidance and navigation.
(see also Section 2.4.6). Thus all targets are displayed, The IMO Performance Standards (see Section
not only in the correct angular relationship to one 11.2.1) require that provision be made for quickly
another but also in the correct angular relationship to obtaining the bearing of any object whose echo
the observing ship’s heading (see Figure 1.9). appears on the display. Traditionally such provision
The angle between the observed vessel’s heading was fulfilled by a variety of mechanical and electro-
and the direction of the horizontal beam is known mechanicaldeviceswhichenabletheobservertomea-
as the aerial angle. In the case of a synthetic display sure the angle between the heading marker and the
(seeSection1.2.5)theinstantaneousvalueoftheaerial object of interest. In general the target is intersected
angle will be measured and stored in digital form either by an engraved line along the diameter of the
alongside the corresponding set of range data (see rotatable disk of transparent plastic located above the
Section 2.6.6). screenoftheCRT,orbyanelectroniclineemanating
The IMO Performance Standards (see Section from the origin of the display (see Figure 1.10). The
11.2.1) require that the line be able to be aligned to bearingisreadoff,intheformercasefromsomeform
within0–1(cid:1).Theprocedureforcheckingthisaccuracy offixedcircularscalearoundtheedgeofthescreenand