1 Basic radar principles

1 Basic radar principles

1.1 Introduction

The word RADAR is an acronym derived from thewords Radio Detection and Ranging. In the UnitedKingdom it was initially referred to as radio directionfinding (RDF) in order to preserve the secrecy of itsranging capability.

The scientist Heinrich Hertz, after whom the basicunit of frequency is named, demonstrated in 1886 thatradio waves could be reflected from metallic objects.In 1903 a German engineer obtained a patent inseveral countries for a radio wave device capable ofdetecting ships, but it aroused little enthusiasm becauseof its very limited range. Marconi, delivering a lec-ture in 1922, drew attention to the work of Hertzand proposed in principle what we know today asmarine radar. Although radar was used to determinethe height of the ionosphere in the mid-1920s, it wasnot until 1935 that radar pulses were successfully usedto detect and measure the range of an aircraft. In the1930s there was much simultaneous but independentdevelopment of radar techniques in Britain, Germany,France and America. Radar first went to sea in a war-ship in 1937 and by 1939 considerable improvementin performance had been achieved. By 1944 navalradar had made an appearance on merchant ships andfrom about the end of the war the growth of civilmarine radar began. Progressively it was refined tomeet the needs of peacetime navigation and collisionavoidance.

While the civil marine radars of today may, in size,appearance and versatility, differ markedly from theirancestors of the 1940s, the basic data that they offer,

namely target range and bearing, are determined byexploiting the same fundamental principles unveiledso long ago. An understanding of such principles is anessential starting point in any study of marine radar.

1.2 Principles of range measurement

1.2.1 The echo principle

An object (normally referred to as a target) is detectedby the transmission of a pulse of radio energy and thesubsequent reception of a fraction of such energy (theecho) which is reflected by the target in the directionof the transmitter. The phenomenon is analogous tothe reflection of sound waves from land formations. Ifa blast is sounded on a ship’s whistle, the energy travelsoutward and some of it may strike, for example, a cliff.The energy which is intercepted will be reflected bythe cliff. If the reflected energy returns in the directionof the ship, and is of sufficient strength, it will beheard as an audible echo which, in duration and tone,resembles the original blast. In considering the echoprinciple the following points can usefully assist in apreliminary understanding of radar detection:

(a) The echo is never as loud as the original blast.(b) The chance of detecting an echo depends on the

loudness and duration of the original blast.(c) Short blasts are required if echoes from close tar-

gets are not to be drowned by the original blast.(d) A sufficiently long interval between blasts is

required to allow time for echoes from distanttargets to return.

2 R A D A R A N D A R P A M A N U A L

While the sound analogy is extremely useful, it mustnot be pursued too far as there are a number of waysin which the character and behaviour of radio wavesdiffer from those of sound waves. In particular at thisstage it is noteworthy that the speed of radio waves isvery much higher than that of sound waves.

1.2.2 Range as a function of time

It is almost self-evident that the time which elapsesbetween the transmission of a pulse and the receptionof the corresponding echo depends on the speed ofthe pulse and the distance which it has travelled inmaking its two-way journey. Thus, if the speed of thepulse is known and the elapsed time can be measured,the range of the target producing the echo can becalculated.

While it is recognized that the velocity of radiowaves is dependent on the nature of the mediumthrough which they travel, for the practical purposeof marine radar ranging the speed may be assumedto have a constant value of 300 000 000 metres persecond. Because this number is rather large, it is expe-dient in practical calculations to use the microsecondas the time unit. One �s represents one millionth partof a second (i.e. 10−6 seconds). The speed can thus bewritten as 300 metres per �s. Using this value it is pos-sible to produce a simple general relationship betweentarget range and the elapsed time which separates thetransmission of the pulse and the reception of an echoin any particular case (see Figure 1.1).

Let D = the distance travelled by the pulse (metres)R = the range of the target (metres)

T = the elapsed time (�s)S = the speed of radio waves (metres/�s)

Then D = S ×Tand R = �S ×T�/2hence R = �300×T�/2thus R = 150T

The application of this relationship can be illustratedby the following example.

Example 1.1 Calculate the elapsed time for a pulseto travel to and return from a radar target whose rangeis (a) 40 metres (b) 12 nautical miles.

(a) R = 150Tthus 40 = 150Thence T = 40/150 = 0�27�s

This value is of particular interest because 40 metresrepresents the minimum detection range that must beachieved to ensure compliance with the IMO Perfor-mance Standards for Navigational Radar Equipment(see Section 11.2.1). While this topic will be fullyexplored in Section 3.2.4, it is useful at this stage tonote the extremely short time interval within whichtransmission and reception must be accomplished.

(b) R = 150 TSince 1 nautical mile = 1852 metres,

12×1852 = 150Thence T = 12×1852/150 = 148�16�s

This result is noteworthy as it represents the elapsedtime for a commonly used marine radar range scale.Further reference will be made to this value in thesucceeding section.

Figure 1.1 The echo principle

B A S I C R A D A R P R I N C I P L E S 3

1.2.3 The timebase

It is clear from the values established in the previ-ous section that the elapsed times are of the orderof millionths of a second and are thus so short as tobe beyond the capability of any conventional time-measuring device. This difficulty is overcome by usingan electronic device known as a cathode ray tube(CRT). The electronic principles of this importantdevice are discussed in some detail in Section 2.6.1; itis sufficient at this stage to appreciate that its displayfeature is a glass screen across which travels a verysmall spot of light. The speed of this travel can beaccurately controlled at values which allow the spot totransit the screen in as little as a few microseconds. Atsuch speeds it moves literally ‘faster than the eye cansee’ and hence appears as a line rather than a spot, butit is important that the concept of a moving light spotis appreciated as it is fundamental to an understandingof radar display principles.

The CRT can be used to perform an electronicstopwatch function by arranging that the time takenfor the spot to cross the screen is the same as thetime taken for a radar pulse to make the two-wayjourney to a target at a chosen range. This can becompared with a mechanical stopwatch in which thesecond hand transits the circumference of a dial in thesame time as the earth rotates through approximately15 seconds of arc. It is useful to illustrate the principlewith reference to the method originally employed inthe early radars of the 1940s. While this type of display,known as A-scan, is no longer used in civil marineapplications (other than in the form of an oscilloscopesuch as may be used for servicing) it demonstrates theprinciple clearly and is a suitable point from whichto progress to a description of the ways in which thedisplay has been subsequently refined.

1.2.3.1 The A-scan display

In the A-scan display, the spot is used to produce ahorizontal line (the trace) which originates close tothe left-hand side of the screen of the CRT. Thefollowing features are of particular importance:

(a) The trace commences at the instant each radarpulse is transmitted and this event is indicated by aninstantaneous vertical deflection of the spot whichproduces a ‘spike’, knownas the transmissionmark.

(b) The speed of the spot (the sweep rate) is adjustedso that it completes the trace in the same time as aradar pulse will take to travel to and from a targetlocated at the maximum range which the scaledline is presently intended to represent. When thespot has completed the trace, the brilliance isautomatically reduced to zero and the spot fliesback to the origin to await the transmission ofthe next pulse. At this event it initiates a furthertrace along the same path as its predecessor.

(c) A returning echo is used to generate an instan-taneous vertical deflection of the spot, thus pro-ducing a further spike. The amplitude of thisspike, within certain limits (see Section 2.5.2.5)is a function of the strength of the echo.

Consideration of these three features and Figure 1.2will indicate that the horizontal distance between thetransmission mark and the echo spike is a measure of

Figure 1.2 The A-scan display


the range of the target. Using the result from Exam-ple 1.1(b), it is evident that if the full extent of the traceis to represent a range of 12 miles (the selected rangescale) the spot must complete the trace in approxi-mately 148 �s. This quantity is referred to as the time-base and it is on this value that the displayed range ofall targets is based. If a target lay at a range of 6 miles,its echo would return in half the timebase and wouldbe displayed at half the maximum range of the scale inuse. Different range scales are obtained by operatingthe range scale selector which selects the correct time-base for the chosen range scale. For example, on the3 mile range scale the spot must travel four times fasterthan on the 12 mile range scale and hence a timebaseof approximately 37 �s would be selected.

A limitation of the A-scan display is its inabilityto display information from any direction other thanthat in which the antenna (or aerial) is trained at thatmoment. This shortcoming led to the development ofthe plan position indicator (PPI).

In radar terminology the words antenna, aerial, andscanner are all used to describe the device which beamsthe transmitted energy into space. Throughout thistext these words may be regarded as synonyms.

1.2.3.2 The radial-scan plan position indicator(PPI) display

In a radial-scan PPI display, the spot produces the tracein the form of a radial line whose origin is (normally)placed at the centre of the circular screen (Figure 1.3).An echo return from a target is used to produce anincrease in the brilliance of the moving spot. To thisend, a competent observer will, in setting up the dis-play, adjust the brilliance of the trace so that it isbarely visible (see Section 6.2.3.1), hence maximizingthe probability that small increases in brilliance will bedetected. Within certain limits (see Section 2.5.2.5)the brightening of the trace by a target return is afunction of the strength of the received echo. As withthe A-scan display, the origin of the trace coincideswith the instant of transmission of the pulse and theduration of the trace is the selected timebase. Thus atarget which lies at the maximum range represented bythe selected scale will appear at the edge of the screen.

Figure 1.3 The radial-scan PPI display

When the spot has completed the trace, the bril-liance is automatically reduced to zero and the spotflies back to the origin to await the incidence of thenext transmission. At this event it initiates a furtherline which, in contrast with the A-scan case, is drawnalong a path which is separated from the previous oneby a small angle (about one tenth of a degree). In aPPI system the antenna rotates continuously and auto-matically in a clockwise direction, generating approx-imately some 3600 lines in one complete rotation.The resultant rotating trace makes it possible to displaysimultaneously targets in all directions in the correctangular relationship, one to another. Echoes paintedby the trace as it passes will glow for a short perioddue to a property of the screen known as persistenceor afterglow. Thus, in general the picture persists untilrefreshed on the next revolution of the trace (seeSection 1.3.3). The PPI display is particularly suited tocollision avoidance and navigational applications; theangular build-up of the picture will be described inSection 1.3.3.

1.2.4 Calibration of the timebase

It has been shown that by making the duration of onetrace equal to the timebase of the selected range scale,


the echo of a target will be displayed at a distancefrom the origin which is proportional to its range.Thus, when observing the display, it may be possibleto estimate the range of a target by mentally dividingthe trace into equal sections. However, to measurea range with an acceptable degree of accuracy (seeSection 6.9.8), the subdivision must be carried out byelectronic methods.

A device known as an oscillator is used to generatea succession of signals which occur at equal intervals oftime. It is arranged that the first signal of any groupcoincides with the instant of transmission and that theinterval between successive pulses is a sub-multiple ofthe timebase of the selected range scale. Each signal isused to produce an instantaneous brightening of thespot, and as a result bright marks, sometimes called cal-ibration pips or calibration marks, will subdivide the traceinto equal intervals of time and therefore, by virtueof the timebase value, into equal intervals of range.Hence, on the 12 mile range scale, as illustrated byFigure 1.4, the echo from a target at a range of 6 miles

Figure 1.4 Calibration marks

will arrive after transmission by an elapsed time equal tohalf the timebase, and therefore will be displayed at thesame time, and in the same place, as the 6 mile calibra-tion pip. (Notice that the 6 mile calibration pip is thethird from the origin, the mark at the origin counting aszero.) It is evident that the range scale selector must con-trol not only the timebase duration but also the intervalbetween calibration marks. This is illustrated, for somerange scales, by the values in Table 1.1.

The timing of the calibration marks can be achievedwith an extremely high degree of accuracy which inturn becomes implicit in the accuracy with which therange of a target can be measured provided that itsecho coincideswith amark. If the echo lies between twomarks, some form of interpolation will be necessary. Insome circumstances it may be that this can be done byeye, but in other cases an acceptable degree of accuracy(see Section 6.9.1) will demand electronic assistance.

Interpolation is facilitated by the generation of asingle pip, known as a variable range marker, whichoccurs at a selected elapsed time after transmission. Thetime is selected by the operation of a manual controlwhich is also connected to some form of numericalread-out. The latter will show the number of milesof timebase represented by the selected time. Thus,as illustrated by Figure 1.5, if the observer adjusts thevariable range marker to read 3 miles, the single pipwill be produced approximately 37 �s after transmis-sion, i.e. after one quarter of the 12 mile timebase.

Table 1.1 Timebase and calibration interval values

Rangescale

Timebaseduration

Calibration(n miles)

interval(�s)

Remarks

(n miles) (�s)

0.75 9�3 0�25 3�1 Normallyonly 3 rings

1.5 18�5 0�25 3�1

3 37�0 0�5 6�26 74�1 1�0 12�4 6 rings in12 148�2 2�0 24�7 each case24 296�3 4�0 49�448 592�6 8�0 98�8


Figure 1.5 The variable range marker

In conclusion, it is clear that the precision withwhich the calibration marks are produced is funda-mental to the accuracy of range measurement. For adetailed treatment of the correct procedure for theiruse and the accuracy which can be achieved, the readeris referred to Section 6.9.

1.2.5 The synthetic display

The displays produced in the ways so far describedhave become known as ‘real-time’ displays. The termdescribes the fact that the echoes are written on thetrace concurrently with their reception, rather in theway in which one might write down a navigationwarning, as it is received. It was recognized for manyyears that if the range data could be stored in such away that it could be manipulated, considerable ben-efits might accrue, such as the ability to improve thepicture quality and to carry out automatic tracking oftargets. Pursuing the analogy of the navigation warn-ing somewhat further, this may be compared withwriting down the warning quickly as it is broadcastand subsequently re-writing it slowly and neatly. Aswith most analogies, care should be exercised in itsindefinite pursuit. The term real-time display is used

to differentiate between the traditional type of dis-play and the modern, computer controlled ‘SyntheticDisplay’.

The microprocessor, a computer device capableof storing and manipulating data at very high rates,became available from the early 1970s; as the costof 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 modernradar systems the range data is stored in a computermemory. The data, once processed, can be used tosynthesize a picture for display on a radial-scan PPI.Such a picture is sometimes referred to as a re-timeddisplay. In the mid-eighties it became progressivelymore common to produce the picture on a televisionscreen. This is now universally the case. Such a displayis referred to as a raster-scan PPI in order to distinguishit from the traditional radial-scan PPI. Detailed consid-eration of raster-scan displays is deferred to Chapter 2(see Section 2.6.7), as an understanding of basic radarprinciples is more readily gained from a study of theradial-scan display. However, regardless of whetherthe synthetic picture is finally produced on a raster-scan or radial-scan PPI, the principle of range mea-surement is as described below.

For a general introduction to the principle of stor-age, the computer memory may be likened to a largenumber of ‘two-position’ switches each of which canbe either on or off. Consider a computer that willsample the received signal 1200 times in the durationof one complete radial trace. The sampling processcommences at the instant of transmission and the resultof each sampling is registered using, consecutively,each of 1200 memory elements. If a returning echo ispresent at the time of sampling, the appropriate switchis set to ‘on’, whereas if no echo is present the switchis left in the ‘off’ position. Thus the range of eachecho will be implicit in the number of the memoryelement in which its presence is recorded.

In diagrams, it is usual to depict each element ofmemory as a box in which the number ‘1’ appearsif the switch is on and the number ‘0’ appears if theswitch is off. This form of representation is particularlysuitable because the computer uses a counting systemknown as binary in which there are only two digits,


Figure 1.6 Storage of range data

namely ‘1’ and ‘0’ (see Section 2.6.6.1). Further, itis practical to show 12 elements, rather than 1200.Subject to these qualifications, Figure 1.6 illustratesthe generation of one radial line of a synthetic picturewhen the 12 mile range scale has been selected. Inwriting the data into the memory, which will be car-ried out in the duration of the 12 mile timebase (i.e.approximately 148 �s), the sampling process stores a‘1’ for each received echo and thereby the range ofeach target producing a detectable echo is implicit inthe place in which it is registered in the memory. Forexample, a target in element 7 lies within the rangelimits 7 to 8 nautical miles (using 1200 elements, atarget in element 700 would lie within the range limits7.00 to 7.01 nautical miles).

In reading the range data out of the memory, astandard rectangular pulse is produced for each ‘1’ inthe memory. If the data is read out in the duration ofthe 12 mile timebase (i.e. approximately 148 �s) andthe sweep rate produces a trace whose radial dura-tion is the same, each of these pulses will produce a

brightening of the trace at a distance from the originwhich is proportional to the range of the targets whichproduced the echoes.

In the example that has been chosen, the range datawas written in and read out at the same rate. In asynthetic display this will only be true for one rangescale. This is best illustrated by considering what willhappen if the 6 mile range scale is selected (Figure 1.7).

When the 6 mile range scale is selected, the datamust be written into the memory over the duration ofthe 6 mile timebase (i.e. approximately 74 �s) becausethe writing time is determined by the speed of radiowaves. The received signal will be sampled twice asoften so that, if 12 elements are used to represent6 miles, each element of memory will represent a rangeinterval of 0.5 miles. (If 1200 elements were used, therange interval would be 0.005 miles.) If the storedrange data for 6 miles is read out of memory at halfthe rate at which it was written in (i.e. at the same rate

Figure 1.7 Fixed sweep rate display


as it is read out on the 12 mile range scale) and thesweep rate is maintained at the value for the 12 milescale (i.e. giving a timebase of approximately 148 �s),a radial trace representing 6 miles will be produced.

Thus, in general, where computer storage is used todevelop a radial PPIdisplay, the rangedata for each radialline is written into memory during one timebase of theselected range scale but is read out in a fixed time, anddisplayed at a fixed sweep rate both of which are inde-pendent of the range scale selected. The duration of thefixed read-out time will be determined by the individ-ual radar designer. Commonly, but not invariably, it ismade equal to the 12 mile timebase. A representative setof values is shown in Table 1.2. Calibration marks anda variable range marker will be available on a syntheticdisplay but it should be borne in mind that the intervalbetween the calibration pips will depend on the fixedsweep rate chosen by the designer. Additionally it maybe possible to have the computer read out the range ofa target directly from the knowledge of the number ofthe memory element in which it lies.

The above process is sometimes called digital stor-age, because the presence or absence of an echo is reg-istered by the binary digits ‘1’ or ‘0’ respectively. It isconsiderably more complex than the above introduc-tion might suggest and there is great variation in theway in which radar designers have exploited the basicprinciple. For example, the simple model describedproduces synthetic echoes all of which are identical inbrightness. In some systems this is true but it is by no

Table 1.2 Range data – typical write and readtimes

Range scale Write time Read-out Sweep time(n miles) (�s) (�s) (�s)

0.75 9�3 148 1481.5 18�5 148 1483 37�0 148 1486 74�1 148 148

12 148�2 148 14824 296�3 148 14848 592�6 148 148

means invariably the case. Further, if (as in practice)some 1200 memory elements are used, many targetswill register sequentially in more than one element.The topic is discussed in greater detail in Section 2.6.6.

1.3 Principles of bearingmeasurement

1.3.1 Directional transmissionand reception

To examine the principles of bearing measurement it isuseful to return to the sound analogy of Section 1.2.1.If the suggested blast was transmitted in low visibilityit would not be easy to assess the precise directionfrom which the sound had returned. The difficultyarises from two principal features:

(a) The whistle If it is well placed it will trans-mit sound in all directions andthus echoes will return from alldirections.

(b) The ear Evolution has designed the humanear to be highly receptive overmore than 180 degrees on eitherside of the head.

These two properties are precisely the reverse of whatis required. To determine the bearing of a target,transmission in the horizontal plane must be restrictedto one direction at a time, and reception must berestricted to that same direction.

In a marine radar system a single antenna (knownas the scanner or aerial) is used for both transmis-sion and reception. It is designed in such a way (seeSection 2.4.2) as to focus the transmitted energy in abeam which is very narrow in the horizontal plane.The angle within which the energy is constrained iscalled the horizontal beamwidth (Figure 1.8). It musthave a value of not more than 2.5� if it is to com-ply with IMO Performance Standards for NavigationalRadar Equipment (see Section 11.2.1). There is anexception for high speed craft in that the horizontal


Figure 1.8 The horizontal beam

beamwidth can be up to 4� for S-band radar only(see Section 11.2.4). However, horizontal beamwidthsof less than 2� are commonplace and shipborne civilmarine radars are available with values as low as 0.75�.The reception property of the antenna is such thatit will only detect energy which has returned fromwithin the angular limits of the horizontal beamwidth.It follows from the directional characteristic of theantenna that only those targets which lie in the direc-tion of the beam will appear on the trace. Thus asingle trace represents the ranges of targets lying alonga specific line of bearing.

1.3.2 Synchronization of scannerand trace

An essential feature of the modern marine radar displayis that it should provide continous coverage over 360�

of azimuth. To achieve this both the scanner and thetrace are rotated, at the same rate, clockwise, continu-ously and automatically. To comply with the currentIMO Performance Standards the duration of one rev-olution must not exceed 5 seconds (1.5 seconds forhigh speed craft); values of about 2 to 3 seconds aretypical for shipborne radar.

The interval between successive transmitted pulses(and hence between the start of successive traceson the PPI) may fall within a range of values (seeSection 2.3.1) but for the purposes of illustration anaverage value of 1/1250s (800 �s) will suffice. If arepresentative time for one revolution of the scanneris taken to be 3 seconds, one can deduce that some3600 pulses are transmitted during one revolution andthat the scanner rotates through 0.1� between pulses.Thus while the rotation is continuous, it may be found

convenient to imagine that the antenna scans the areaaround the ship in about 3600 steps of 0.1� and thateach step is represented by a separate trace on thescreen. The picture is thus ‘built up’ of approximately3600 radial lines.

1.3.3 The build-up of the picture

The echo of a detectable target will be ‘painted’once per revolution of the scanner. The inside of theCRT screen is coated with a luminous material (seeSection 2.6.1.2) so that any area, once painted, willglow for at least 2 to 3 seconds. Thus the afterglow canbe made sufficiently long to cause the echo paint topersist until it is painted again on the next revolution.(There are a few marine systems in which the per-sistence is less than one revolution but they are verymuch the exception.)

Because the scanner and trace rotate at the samerate, the radar beam and the trace describe equal arcs inequal intervals of time. Thus targets will be displayed inthe correct angular relationship, one to another. If twoships subtend an angle of 50� at the observing vessel,the centres of their echoes will be separated on the PPIby a similar angle. As the beam sweeps across a geo-graphical feature, such as a coastline, echoes on succes-sive traces will add together, side by side, to producea plan representation of that feature (see Figure 1.9).

The calibration marks, which may be switched onor off by the observer, will be painted on each traceand thus when switched on will appear as concentriccircles. The variable range marker will similarly appearas a single circle of variable radius.

1.3.4 The heading marker

In general, a bearing is the angle between the direc-tion of a chosen reference and that of an object ofinterest. On a PPI display the fundamental referenceis the instantaneous direction of the observing ves-sel’s head. As the axis of the horizontal beam crossesthe ship’s fore-and-aft line in the forward direction, aset of contacts is closed, producing a pulse of a fewmilliseconds duration. This pulse is used to increase the


Figure 1.9 The build-up of the picture

brilliance of the spot and hence produce the completebrightening up of several successive traces, resulting inthe distinctive ‘heading marker’ or ‘heading indicator’(see also Section 2.4.6). Thus all targets are displayed,not only in the correct angular relationship to oneanother but also in the correct angular relationship tothe observing ship’s heading (see Figure 1.9).

The angle between the observed vessel’s headingand the direction of the horizontal beam is knownas the aerial angle. In the case of a synthetic display(see Section 1.2.5) the instantaneous value of the aerialangle will be measured and stored in digital formalongside the corresponding set of range data (seeSection 2.6.6).

The IMO Performance Standards (see Section11.2.1) require that the line be able to be aligned towithin 0–1�. The procedure for checking this accuracy

is discussed in Section 6.9.9. Clearly, because bothtargets and the heading marker are produced by abrightening of the spot, there is a danger that a targetmay be masked if it lies in the direction of the headingmarker. The specification recognizes this danger byrequiring that there is a provision for switching theheading marker off. However, the provision must besuch that the heading marker cannot be left in the offposition. Normally this requirement is complied withby the use of a spring-loaded switch which is biasedin the on position. The danger of the heading markerbeing left in the off position is that, in the absence ofthe correct reference, an erroneous reference (such asthe bearing marker – see Section 6.9.5) might be usedinadvertently.

The appearance of the heading marker confirmsthe orientation of the picture (see Section 1.4) andconsequently whether the bearings are relative or true(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 aform as to make it possible to measure the range andbearing of any detected target. It is this characteris-tic that gave rise to the term ‘plan position indicator’which distinguished the rotating trace display fromthe earlier A-scan display. It is also this characteristicwhich makes the display particularly suitable for usein collision avoidance and navigation.

The IMO Performance Standards (see Section11.2.1) require that provision be made for quicklyobtaining the bearing of any object whose echoappears on the display. Traditionally such provisionwas fulfilled by a variety of mechanical and electro-mechanical devices which enable the observer to mea-sure the angle between the heading marker and theobject of interest. In general the target is intersectedeither by an engraved line along the diameter of therotatable disk of transparent plastic located above thescreen of the CRT, or by an electronic line emanatingfrom the origin of the display (see Figure 1.10). Thebearing is read off, in the former case from some formof fixed circular scale around the edge of the screen and


0° (Ship's head)

Target bearing040° (REL)

Headingmarker

Relativebearing Electronic

bearingline

Target

(b)

0°True north

240°

Targetbearing280°(True)

Heading marker

Truebearing

(a)

Target

Figure 1.10 Measurement of bearing: (a) relativebearing, (b) true bearing

in the latter case from an analogue or digital indicatorcoupled to the bearing marker control. In a modernsynthetic display it is usual to have the computer readout the bearing of a target directly from a knowledgeof the radial line on which its echo appears. A vari-ety of bearing measurement facilities and the correctprocedure for their use are discussed in Section 6.9.

Facilities should also be provided to compensate foran offset aerial position (see Section 11.2.1).

1.4 Picture orientation

In marine radar literature, there is frequently an under-standable descriptive overlap between the concepts ofpicture ‘orientation’ and picture ‘presentation’. In thistext orientation is defined as the choice of directionalreference to be represented by the 000� (12 o’clock)graduation on the fixed bearing scale around the CRT(see Figure 1.11). Although the heading marker shouldalways appear at the instant the axis of the beam crossesthe fore-and-aft line in the forward direction, it is intheory possible to rotate the whole picture so thatthe 000� graduation on the fixed bearing scale repre-sents any direction. In practice one of three preferreddirections will be chosen, namely:

1 The ship’s instantaneous heading, in which case theorientation is said to be ship’s-head-up (unstabilized).

2 True north, in which case the orientation is saidto be true-north-up (stabilized).

3 The ship’s true course, in which case the orienta-tion is said to be course-up (stabilized).

Picture presentation (see Section 1.5) will be usedto specify the reference with respect to which echomotion is displayed. In more general terms, the type ofpresentation indicates to the observer that the move-ment of the displayed echoes is shown with respect tohis own ship, or with respect to the water, or withrespect to the ground, i.e. relative motion or true motion(sea-stabilized) or true motion (ground-stabilized).

1.4.1 Ship’s-head-up orientation(unstabilized)

This orientation is so called, because the observer viewsthe picture with the heading marker (and thus the ship’shead) at the ‘top’ of the screen (see Figure 1.11). Whenthe original PPI displays were introduced, it was theonly orientation available. At that time, no provision


Just before alteration –vessel steering 240° (T)

Just after alteration –vessel steering 270° (T)

000 000

270 090 090270

180 180

270° (T) 240° (T)

240° (T)

Lighthouse

270° (T)

Lighthouse

Figure 1.11 Ship’s-head-up orientation (unstabilized)


was made for ‘azimuth stabilization’, by which is meantthe input of compass information to the display (seeSection 2.6.3). In the absence of such an input it isnot possible effectively to maintain either of the othertwo preferred orientations. In modern marine radar sys-tems, almost invariably the ship’s-head-up orientation isunstabilized, i.e. a compass signal is not used in the con-trol of the orientation. Historically there have been twodesigns which did associate the ship’s-head-up orient-ation with azimuth stabilization but they were specialcases in that the stabilization was applied twice. With theadvent of raster-scan displays, the concept may well beexploited again in the future.

The single attractive featureof the ship’s-head-upori-entation(unstabilized) is that it correspondsdirectlywiththe scene as viewed through the wheelhouse window.A well placed display unit will be sited so that the officerviewing the screen faces forward. (UnitedKingdomves-sels are required to have at least one display sited in thisway – see Section 11.4.1.) Thus, irrespective of whetherthe officer is viewing the radar screen or looking for-ward through the wheelhouse window, objects on thestarboard side of the ship will lie on the right and thoseon the port side will lie on the left.

A major shortcoming of the unstabilized orientationis that at every change in the direction of the observingvessel’s heading, the entire picture (apart from theheading marker), will rotate by an equal but oppositeangle (see Figure 1.11). This characteristic limits theusefulness of the orientation in three specific ways:

1 If a large alteration of course is made, any areas ofland echoes are smeared across the screen, making itdifficult to identify specific features. The afterglowcreated during the alteration may obscure isolatedfixed or floating targets for some time after the vesselis steady on her new course (see Figure 1.11).

2 In ideal conditions the ship’s heading would coin-cide at all times with the chosen course; in prac-tice, due to the effect of wind and sea, the shipwill ‘yaw’ about the correct heading. On a displayusing an unstabilized orientation, this superimposesan angular wander on the movement of all targets(see Figure 1.12) which limits the ease and speedwith which bearings can be measured. It becomes

Target wanders–heading marker steady

Target steady–headingmarker wanders

Target steady–headingmarker wanders

Ship’s-head-up(unstabilized)

True-north-up (stabilized)

Course-up (stabilized)

000

270

180

090

180

270

180

000

270

000

090

090

Figure 1.12 The effect of yaw. Observing vessel’scourse 330� (T), yaw ±4�. Target vessel approaching onsteady bearing of 015�(T)


necessary to choose an instant when the target canbe intersected with the cursor or electronic bear-ing line and the vessel’s instantaneous heading readoff simultaneously, or, alternatively, to wait untilthe vessel is right on course. The latter is necessarybecause the bearings read off from this orientationare measured relative to the ship’s head and mustbe converted to true bearings for use in collisionavoidance and navigation. The practical procedurefor obtaining bearings from an unstabilized orient-ation is described in Section 6.9.3.

3 The disruption of the echo movement due to yawlimits the ease and speed with which an observercan plot the movement of targets for collisionavoidance purposes, even if a reflection plotteris used (see Section 7.8.7). Further, an unwaryor untrained observer may be dangerously mis-led by such angular movements. For example, asmall change of course by the observing vesselmay give the impression that the bearing of a tar-get is changing, while in fact the true bearingis remaining constant. The extremely importanttopic of systematic observation of target movementis discussed at length in Chapter 7.

1.4.2 True-north-up orientation(stabilized)

In true-north-up orientation, the heading marker isaligned with that graduation on the fixed bearing scalewhich corresponds with the instantaneous value of theship’s heading. As a result, the 000� graduation repre-sents true north. Thus the observer views the picturewith north at the ‘top’ of the screen and it is for thisreason that the orientation is so named. Compass sta-bilization is essential so as to maintain the orientationtrue-north-up when the observing vessel alters courseor yaws about her chosen course (see Figures 1.12 and1.13). In the absence of stabilization, the picture wouldrotate by an amount equal and opposite to any changein the observing vessel’s heading (see Section 1.4.1).The compass stabilization signal is used to producesimultaneously a commensurate rotation of the picturein the same direction as the change of heading. As a

result there is no nett rotation of the picture, the head-ing marker rotates to the new value of heading, andtrue north remains coincident with the 000� gradua-tion on the fixed bearing scale. The stabilization signalcan be derived from any transmitting compass but inpractice the signal source is almost invariably a gyrocompass. The process whereby the signal is used toeffect stabilization is described in Section 2.6.3.

The addition of compass stabilization overcomes theserious, inherent limitationof the ship’s-head-up(unsta-bilized) orientation by removing the angular smearingwhich is associated with any change in heading. Notonly does this eliminate the masking of targets by theafterglow generated during an alteration of course, butit allows true bearings to be read off directly and quicklyfrom the fixed bearing scale without the need to checkthe direction of the ship’s head at the same instant. Thesefeatures are of particular importance in both collisionavoidance and navigation applications. Further, thereis no angular disruption of the tracks of targets as theirechoesmoveacross the screen.This greatly facilitates thesystematic observation of targets for collision avoidancepurposes and removes a characteristic which, probablymore than any other, has demonstrated over many yearsits potential tomislead theuntrainedorunwaryobserver(see Section 7.12.7).

The orientation compares directly with the chartand very many observers find this agreeable or at leastacceptable. However, despite the advantages of stabi-lization, some officers have a subjective preference forthe ship’s-head-up orientation. They find it awkwardor uncomfortable to view a north-up orientation, par-ticularly when the vessel is on southerly courses. Thispoint of view is advanced by some river and estuarypilots who argue that in locks and narrow channels theangular smearing is secondary when compared withthe importance of port and starboard through the win-dow corresponding with left and right on the radarscreen. It is a view also held by some more senior offi-cers whose initial radar experience was gained beforethe availability of azimuth stabilization and by somewhose prime concern is visual conning of the vessel.Since then there has been a steady increase in the num-ber of observers having a preference for a stabilizedorientation.




000000

270270090

090

180 180

225°

(T)

135° (T)

Lighthouse

225°

(T)

135° (T)

Lighthouse

Figure 1.13 True-north-up orientation (stabilized)




000000

270270090

090

180180

000

225°

(T)

135° (T)

Lighthouse

Northmarker

Reference coursereset to 135° (T)180

270 090

Northmarker

Northmarker

Figure 1.14 Course-up orientation (stabilized) – resetting the reference course


1.4.3 Course-up orientation(stabilized)

In a course-up (stabilized) orientation the headingmarker is aligned to the 000� graduation on the fixedbearing scale at an instant at which the vessel is righton the chosen course. By virtue of the azimuth stabi-lization, changes in the vessel’s instantaneous headingare reflected by sympathetic angular movement of theheading marker, thus maintaining the ship’s course(referred to as the reference course) in alignment withthe zero of the fixed bearing scale. For the same rea-son, the angular wander of echoes associated with anunstabilized display is eliminated. In some modern sys-tems a north marker is displayed at the edge of thescreen (see Figure 1.14).

Provided that the observing vessel does not strayvery far from her chosen course, this orientation effect-ively combines the attractive features of both of theorientations previously described. It eliminates theangular wander of the picture due to yaw, while main-taining the heading marker in a substantially (thoughnot exactly) ship’s-head-up position.

Inevitably a major alteration of course will becomenecessary either due to the requirements of collisionavoidance or to those of general navigation. When the

vessel is steadied on the new course the orientation,although not meaningless, will have lost its property ofbeing substantially ship’s-head-up. The problem is thatthe orientation is still previous-course-up and the pic-ture must be reorientated to align the heading markerwith the zero of the fixed bearing scale at an instantwhen the vessel is right on the new chosen course (seeFigure 1.14). In older systems it was necessary to carryout the realignment by rotating a manual control, butin most modern displays it can be achieved simply bypressing a button.

1.4.4 Choice of orientation

The fundamental function of any civil marine radar isto afford a means of measuring the ranges and bear-ings of echoes and hence to make possible the track-ing of target movements for collision avoidance andthe determination of the observing vessel’s position inorder to ensure safe navigation. The ease with whichthese objectives can be attained is affected by the choiceof orientation. Where the various techniques of colli-sion avoidance and navigation are described in this text,appropriate attention will be given to the influence oforientation. The theory of the production of the ori-entations is described in Section 2.6.3 and the practical

Table 1.3 Picture orientations compared

Feature Orientation

Ship’s-head-up,unstabilized

True-north-up, stabilized Course-up, stabilized

Blurring when observingvessel yaws or alters course

Yes: can produce veryserious masking

None None

Measurement of bearings Awkward and slow Straightforward Straightforward

Angular disruption of targettrails when observing vesselyaws or alters course

Yes: can be dangerouslymisleading

None None

Correspondence withwheel-house window view

Perfect Not obvious Virtually perfect exceptafter large course change

Correspondence with chart Not obvious Perfect Not obvious


setting up procedures in Chapter 6. It is appropriate atthis point to summarize the features of the three orien-tations so far described in the context of the foregoingfundamental requirements (see Table 1.3).

Except in specialized pilotage situations, the ship’s-head-up unstabilized orientation has nothing to offerother than its subjective appeal, because by its verynature it regularly disrupts the steady-state conditionconducive to measurement of bearing and tracking ofecho movement (see Figure 1.12). True-north-up andcourse-up orientations do not exhibit this angular dis-ruption and hence are equally superior in fulfilling thefundamental requirements. Fortunately they are com-plementary in that while one is north-up, the otheris orientated in such a way as not to alienate the userwho has a ship’s-head-up preference (see Figure 1.12).

1.5 Picture presentation

As suggested in Section 1.4, there is frequently someoverlap between the concepts of picture orientationand picture presentation. In this text, the term ‘picturepresentation’ will be used to indicate if the movementof displayed echoes is shown with respect to (a) theobserving vessel, (b) the water, or (c) the ground. In allthree presentations, the observing vessel is representedby the electronic origin of the display, i.e. by thecentre of rotation of the trace or by the own shipmarker on a raster display.

1.5.1 The relative-motionpresentation

In the relative-motion presentation the origin of thedisplay is stationary and the movement of all targetsis shown with respect to the observing vessel. Com-monly the origin is located at the centre of the circu-lar screen (or, in the case of a raster-scan display, atthe centre of the display circle) but this need not bethe case as off-centred relative-motion presentationsare available in many display systems. The essential

feature is that the origin is stationary and as a con-sequence targets exhibit their motion relative to theobserving vessel. This is best illustrated by an example.(The scenario used to illustrate the three cases listed inSection 1.5 is subject to the assumption that the effectof leeway may be neglected.)

For the purpose of illustration it is convenient toconsider the case of an observing vessel on a steadyheading of 000� (T) at a speed of 10 knots throughthe water in a tide (which is uniform throughout thearea) setting 270� (T) at a rate of 4 knots. The chart(see Figure 1.15(a)) also shows four targets:

(i) Vessel A which is located 7 miles due north ofthe observing vessel and is stopped in the waterheading 045� (T).

(ii) Vessel B which is located 8 miles due east ofvessel A and is on a steady heading of 270� (T)at a speed of 10 knots through the water.

(iii) Vessel C which is located 5 miles due north ofvessel A and is on a steady heading of 180� (T)at a speed of 5 knots through the water.

(iv) A large automatic navigational buoy (lanby) L

which is anchored and therefore is stationaryover the ground. Its position is 7 miles due westof vessel A.

To assist in the understanding of relative motion,Figure 1.15(b) represents the observing vessel’s PPIas it would appear at 1000. For comparison,Figure 1.15(c) represents the same PPI showing thepositions of the echoes as they would appear at 1030together with a record of their 1000 positions. It willbe noticed that the shape of the echoes gives no indi-cation of the outline of the targets. This fundamentallimitation of marine radar will be discussed at length inSection 2.6.5. Consider now the movement of each ofthe four echoes in turn, commencing with that of thewater-stationary target A which offers a simple basison which an understanding of all relative motion canbe built. It is important to remember that it is beingassumed that the observing vessel maintains a steadyheading. If the vessel were yawing, azimuth stabiliza-tion would be essential to achieve the continuity ofmovement described below.


Figure 1.15 Relative-motion presentation

In the period 1000 to 1030 the observing vesselwill move north by a distance of 5 miles through thewater. Because the origin remains stationary, and therange of target A decreases at 10 nautical miles perhour, it follows that the echo of A will move down

the heading marker by a distance of 5 miles in the30 minute interval. This reveals the basic propertyof the relative-motion presentation which is that theecho of a target which is stationary in the water willmove across the screen in a direction reciprocal to that


of the observing vessel’s heading, at a rate equal to theobserving vessel’s speed through the water.

Consider now the movement of the echo of vessel B

which at 1000 was 8 miles due east of the stationaryvessel A. As B is heading directly toward A at 10 knots,it follows that its 1030 position will be 3 miles due eastof A. Figure 1.15(c) reveals that the afterglow trail leftby the echo of vessel B offers an indication of howfar off the target will pass if neither vessel manoeu-vres. However, the echo has moved across the screenin a direction and at a rate which is quite differentfrom the target’s course and speed. An appreciation ofthis fact is absolutely essential if the presentation is tobe interpreted correctly and used in assessing collisionavoidance strategy. Further consideration of the figurewill show that the relative motion of echo B is theresultant of that of a water-stationary target (whichis determined by the observing vessel’s course andspeed through the water) and the true motion of thevessel B through the water. The proper use of radarfor collision avoidance is based on systematic observa-tion and analysis of both the relative motion and thetrue motion of the other targets in an encounter (seeChapter 7).

Consider now the movement on the screen of theecho of vessel C. At 1000 its position was 5 milesdue north of the water-stationary vessel A and head-ing directly towards it at 5 knots. It follows that at1030 its position will be 2.5 miles north of vessel A.As shown in Figure 1.15(c), because the echo of ves-sel A has itself moved across the screen by 5 milesin a direction of south, the aggregate movement ofecho C is 7.5 miles in the same direction. Thus, as inthe case of vessel B, the echo has moved across thescreen in a way that is different from the movementof the vessel through the water. However it shouldbe noted that, by coincidence, the track across thescreen of echo C is in the same direction as that ofthe water-stationary target A. This reveals a furtherfeature of the relative-motion presentation, which isthat the echoes of targets which are stopped in thewater, targets which are on a reciprocal course to theobserving vessel and targets which are on the samecourse as the observing vessel, but slower, will all move

across the screen in the same direction (but at differ-ent speeds). This feature has the potential to misleadthe untrained or unwary observer into confusing, forexample, a target that is being overtaken with one thatis on a reciprocal course. This further emphasizes thenecessity of systematic analysis of the information pre-sented, as opposed to inspired guesswork, when usingthe radar for collision avoidance (see Chapter 7).

Initially the east/west distance between the lanbyand the stationary ship was 7 miles. As the tide issetting the stationary vessel down on to the buoy at4 knots, it follows that this distance will have reducedto 5 miles by 1030. A study of Figure 1.15(c) willshow that the echo of the buoy has moved across thescreen in a direction and at a rate which are the result-ant of the motion of a water-stationary target and thereciprocal of the tide. Further consideration of thispoint will reveal that a property of the relative-motionpresentation is that a land-stationary target will moveacross the screen in a direction which is the reciprocalof the observing vessel’s ground track at a speed equalto the speed of the observing vessel over the ground.This property is exploited in the use of radar for navi-gation (as opposed to collision avoidance); the variousprocedures are set out in Chapter 8.

1.5.2 The true-motion presentation

It has been shown that in a relative-motion presenta-tion the movement of all echoes across the screen isaffected by the course and speed of the observing ves-sel. In a correctly adjusted true-motion presentation,the echo movement of all targets is rendered inde-pendent of the motion of the observing vessel. This isachieved by causing the origin of the picture to trackacross the screen in a direction and at a rate whichcorrespond with the motion of the observing vessel.Immediately one must ask the question, ‘Should theobserving vessel’s course and speed be measured withrespect to the water or with respect to the ground?’.The answer will have a fundamental effect on themovement of the displayed echoes. At this juncture,it is important to recognize that either reference canbe used: it is appropriate to consider both possibilitiesand subsequently debate the suitability of each.


If the course and speed through the water areselected, the true-motion presentation is said to besea-stabilized. It was shown that with azimuth stabi-lization, the input of ship’s heading data is used tomaintain true north (or the ship’s chosen course) in afixed direction on the screen. By simultaneously feed-ing in speed data (measured with respect to the water)it is possible to stabilize the picture so that a water-stationary target is maintained in a fixed position onthe screen. This is illustrated by the same scenario aswas used in section 1.5.1.

1.5.2.1 True-motion sea-stabilized presentation

To produce a true-motion sea-stabilized presentation,the origin of the picture must be made to track acrossthe screen in a direction and at a rate that corre-sponds with the observing vessel’s course and speedthrough the water. In the example in the illustration(Figure 1.16) the course is 000� (T) and the speed is10 knots.

Figure 1.16(b) shows the PPI of the observing vesselas it would appear at 1000. The origin of the pictureis offset in such a way as to make optimum use of theavailable screen area (see Section 6.8.2, paragraph 5).Figure 1.16(c) shows the position of the four echoes asthey would appear at 1030 together with an indicationof their 1000 positions for the purpose of comparison.The movement of each of the four echoes will now beconsidered in turn, commencing with target A whichis stopped in the water.

In the interval 1000 to 1030 the origin will movedue north by a scale distance of 5 miles, while inthe same time target A will remain on the headingmarker but its range will decrease by 5 nautical miles.It follows that the nett motion of the echo of targetA will be zero. Consideration of Figure 1.16 revealsthe basic property of a correctly set up true-motionsea-stabilized presentation, which is that the echo ofa target which is stationary in the water will maintainconstant position on the screen.

Figure 1.16 True-motion sea-stabilized presentation


Figure 1.16 (Continued)

At 1000 the moving target B was located 8 milesdue east of vessel A. As it is heading directly towardA its bearing from A will remain steady but the rangewill have decreased to 3 miles by 1030. Figure 1.16(c)shows that the echo of target B will move across thescreen in a direction and at a rate which correspondswith the target vessel’s course and speed through thewater. A similar argument will reveal that the echoof vessel C will move across the screen in a direc-tion of 180� (T) at a scale speed of 5 knots. The

presentation thus has the property that the afterglowtrails offer an indication of the headings of all movingtargets. This feature is complementary to the corre-sponding property of the relative-motion presentation(see Section 1.5.1). It must be stressed that collisionavoidance strategy must be based on systematic analy-sis of the displayed target movements as detailed inChapter 7.

As a result of the tide, the water-stationary vessel Awill be set directly toward the lanby and by 1030 theeast/west distance between the two will have reducedto 5 miles. It has been established that echo A willmaintain its position on the screen and thus it followsthat in the interval from 1000 to 1030 echo L willmove east across the screen by a scale distance of2 miles. Consideration of Figure 1.16(c) will showthat a third property of the true-motion sea-stabilizedpresentation is that land-stationary targets will moveacross the screen at a rate equal to the tide but in theopposite direction to the set.

In considering the properties of the true-motion,sea-stabilized presentation it is essential to appreci-ate that the accuracy with which the displayed targetmovements are presented is completely dependent onthe accuracy with which the direction and rate ofthe movement of the picture origin represents theobserving vessel’s course and speed through the water.The true-motion presentation is only as good as theinput data.

The mechanics whereby the tracking of the ori-gin is achieved, the practical procedure for setting upthe presentation and the effect of errors and inac-curacies are covered in Sections 2.6.4, 6.8 and 7.10respectively.

Because the scenario used a heading of north forthe observing vessel, the question of whether the ori-entation was north-up or course-up did not arise. Itshould be noted that either orientation can be usedwith true motion, irrespective of heading.

1.5.2.2 True-motion, ground-stabilized presentation

To create ground stabilization of a true-motion pre-sentation, the origin of the picture is made to moveacross the screen in a direction and at a rate which


correspond with the observing vessel’s track over theground. To achieve this, the input data must havea component which represents the observing ves-sel’s course and speed through the water plus a fur-ther component which represents the set and rate ofthe tidal stream or current. There are several waysin which the resultant of these components may bededuced and applied as input data. A detailed descrip-tion of the methods is set out in Section 2.6.4.4. Atthis stage it is important to appreciate that the accu-racy of the presentation depends on the accuracy ofboth components. In particular it should be remem-bered that the values of set and drift may have to bebased on past positions. For comparison purposes itis convenient to illustrate the presentation with ref-erence to the same scenario as was used in the twopreceding examples. Figure 1.17(b) shows the PPI ofthe observing vessel as it would appear at 1000, whileFigure 1.17(c) shows the echoes as they would appearat 1030 together with recorded plots of the 1000 posi-

tions. It should be noted that the origin of the picturehas moved in a direction which differs from that of theheading marker. The latter represents the direction inwhich the observing vessel is heading at any instantand is independent of any tidal influence.

In this case it is helpful to start by consider-ing the ground-stationary target L. Reference toFigure 1.17(c) shows that in the period 1000 to 1030the origin of the display will have moved to a positionwhich is a scale distance of 5 miles due north (repre-senting the vessel’s movement through the water) and2 miles due west (representing the set and drift experi-enced) of its 1000 screen location. In the same interval,the north/south distance between the observing ves-sel and the lanby will have decreased by 5 nauticalmiles (the d.lat.) while the east/west distance will havedecreased by 2 miles (the departure). It follows fromthese two statements that the echo of the buoy L willexhibit neither north/south movement nor east/westmovement across the screen. This reveals the key

Figure 1.17 True-motion ground-stabilized presentation


Figure 1.17 (Continued)

property of the correctly set up true-motion ground-stabilized presentation, which is that the echo of a landtarget will remain stationary on the screen.

Consider now vessel A which is stopped in thewater heading 045� (T). As previously established, itwill be set directly toward the buoy by a distance of2 miles in the period 1000–1030. It follows from this,and the fact that echo L is stationary on the screen,that the echo of vessel A will move in a direction of

due west by a scale distance of 2 miles in the intervalconsidered. This reveals a second property of the true-motion ground-stabilized presentation, which is thatthe echo of a target which is stopped in the water willmove across the screen in a direction and at a rate thatcorresponds with the set and rate of the tidal stream.

Having regard to target C, it is clear that at 1030 itsposition will be 2.5 miles north of vessel A. A study ofFigure 1.17(c) will show that, because the movementof echo A represents the tide, the echo of vessel C willtherefore move across the screen in a direction and at arate which is the resultant of the set and drift of the tidalstream, and her course and speed through the water.This shows the general property of the correctly setup true-motion ground-stabilized presentation, whichis that echoes of vessels that are underway will moveacross the screen in a direction and at a rate whichrepresents their track over the ground. The exampleof the vessel A which is underway but stopped is aspecial case of this general rule. The movement of theecho of vessel B also illustrates this feature, but perhapsnot so dramatically because it is heading in the samedirection as the set of the tidal stream.

Consideration of the movements of echoes A andC in particular will emphasize how an untrained orunwary observer might be misled into erroneously con-cluding that vessel A was slow-moving and crossingshowing a red sidelight (when in fact it is stopped inthe path of the observing vessel) and that vessel C wasa passing vessel showing a red sidelight (when in factit is head on).

It is thus essential to appreciate that in this presen-tation the movement of the echoes of vessels that areunderway does not represent their headings. Informa-tion on headings is essential for the proper use of radarin collision avoidance. It follows that, in principle, thispresentation is not appropriate as a basis for planningcollision avoidance strategy and can be dangerouslymisleading if used for this purpose. This extremelyimportant topic is further discussed in Sections 2.6.4.4and 6.8.3.

As noted in the previous section, it should be appre-ciated that either north-up or course-up orientationcan be used with true motion.


1.5.3 Choice of presentation

The choice of presentation made in any given cir-cumstances will be influenced by a number of factors.The available equipment may not offer the choice ofall three presentations described in this chapter. Mate-rial to the decision will be the question of whetherthe radar is being used primarily for collision avoid-ance or for position fixing and progress monitoring. Itmay be that the requirements of both must be satisfiedwith a single presentation. Subjective preferences arelikely to make themselves felt. It is appropriate to defermore detailed discussion of the factors affecting such achoice until after further consideration has been givento the operating principles of the radar system, thepractical procedures for the setting up and maintain-ing of the presentations, and the general philosophyof the use of radar for collision avoidance and naviga-tion (see Chapters 6, 7, and 8). However, at this stageit is useful, for comparison purposes, to summarizethe major features of each presentation and commentbriefly on its suitability for use in collision avoidanceand navigation.

In the relative-motion presentation the echo move-ment of targets which are underway is that of the targetrelative to the observing vessel. Systematic observationof this movement readily offers a forecast of the dis-tance off at which a target will pass (the closest pointof approach or CPA) and the time at which the tar-get will reach its closest point of approach (TCPA).This information is an effective measure of the riskof a close-quarters situation developing. The presenta-tion gives no direct indication of the heading or speedof target vessels. Such information is essential to thechoice of avoiding action in encounters with othervessels and has to be obtained by the resolution ofa vector triangle. This may be done graphically orwith the aid of some semi-automatic plotting device,or automatically by computer. Thus the relative-motion presentation gives direct indictation of someof the information required for collision avoidancebut the remainder must be found by deduction (seeChapter 7).

The echoes of land targets on a relative-motionpresentation trace out a trail which is the reciprocal

of the observing vessel’s track made good over theground. This feature is particularly useful in progressmonitoring and position fixing when the radar is beingused for navigation as opposed to collision avoidance(see Chapter 8).

The true-motion sea-stabilized presentation makesthe headings and speeds of targets available directlybut the observer is required to deduce the CPA andTCPA. In respect of the use of radar for collisionavoidance it can be seen that the relative motion andthe true-motion sea-stabilized presentations are com-plementary. The true-motion does have the addedadvantage that it makes it very much easier to identifytarget manoeuvres and, further, the continuity of tar-get motion is not disrupted when the observing vesselmanoeuvres or yaws.

The true-motion ground-stabilized presentation haslittle to offer other than its ability to maintain landtargets in a fixed position on the screen. No apologyis made for re-stating the fact that it gives no directindication of any collision avoidance information andcan even result in confusion.

Table 1.4 summarizes the features of the presenta-tion described above.

1.5.3.1 Constant display true-motion

Many manufacturers have introduced another varia-tion on the true-motion display presentation to avoidthe need to reset the screen. The variation is nowpractical because of the processing of radar data beforedisplay (see Section 2.5) and use of automatic tech-niques to stabilize the display (see Section 5.3).

The own-ship position is fixed at a display position,either at the centre or offset according to observerpreference, in a similar way to setting up relativemotion. The own ship does not move across the screenand reach a position where it is significantly off-centreand showing a lot of coverage astern and very littlecoverage ahead. Normally in these circumstances areset of the screen is then required.

Effectively all the display data around the shipis artificially moving in a reciprocal direction tothe ship’s course and speed, so that although we have


Table 1.4 Presentations – summary of features

Feature Presentation

Relative motion True-motionsea-stabilized

True-motion ground-stabilized

Ease of assessing target’sCPA/TCPA

Directly available Resolution required Resolution required

Ease of assessing target’scourse, speed and aspect

Resolution required Directly available Resolution required –potentially misleading

Need for additional sensorinputs: course and speed No Yes Yes

Need for data on tide set andrate No No YesDisplayed information relativeto: Observer The water The ground

Particular application forcollision avoidance/navigation

Partial contributionto collisionavoidance data. Idealfor parallel indexing

Partial contributionto collisionavoidance data

Difficult to achievewithout ARPA but ifachieved providesstationary map

Limitations for collisionavoidance

Target heading notdirectly available

CPA not directlyavailable

No collision avoidancedata directly available

Limitations for navigation Movement of landechoes may hindertarget identification

Limited movementof land echoes

None if stabilizationeffective

true-motion, the vessel remains at the same positionon the screen. True trails (afterglow) are shown, butclose inspection will show that both target and after-glow are not advancing on screen in the direction ofthe afterglow. This feature is usually termed constantdisplay true-motion (CDTM) or similar and is veryconvenient.

However, as the own ship no longer moves acrossthe screen, the main danger is that despite the d̋isplaypresentation being indicated in one corner of thescreen, the unobservant navigator may think that thedisplay is in relative-motion when in fact all after-glow and target trails will be as if for true-motion andmistakes could result.

1 Basic radar principles

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