Rad Art Ex

7/31/2019 Rad Art Ex

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RADAR OPERATIONIn the event of formatting errors view thePdfversion of this text.

(Contains extracts & edits of material courtesy of A.N.T.A. publications and Col Tritton, updated version Ranger Hope 2008,)

Section 1: OperationSection 2: The Radar TransmissionSection 3: Radar False InformationSection 4: Using Radar For NavigationCollision AvoidanceAppendix B Deviation card

OperationSection Introduction

1 Radar Controls

1.1 Standby\Transmit

1.2 Brilliance

1.3 Gain

1.4 Tuning

1.5 Range

1.6 Sea Clutter Control

1.7 Rain Clutter Control

2 Setting Up The Display

2.1 Switching Off

2.2 Incorrect Adjustment Of Controls

3 Other Controls

3.1 Interference Rejection

3.2 Echo Enhancement

3.3 Heading Marker And Range Ring Delete

3.4 Pulse Length Control

3.5 Off Centring

4 Radar Presentations

4.1 Relative Motion Unstabilised Display

4.2 Relative Motion Course-Up Stabilised Display

4.3 Relative Motion North-Up Display

4.4 True Motion Display
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Section Introduction

In this section we will examine the procedures necessary to turn on and tune a radar setand the procedures and operations necessary to maintain the display for optimumperformance.

A radar unit can pierce darkness, rain, fog, and weather conditions which the human eye

cannot. It can, when within range, show an observer ships, aircraft, storm clouds, islands,headlands and prominent landmarks.

The name radar was coined from the first letters of the words radio detecting and

ranging. Radar was developed from the work of many scientists. Robert Watson -Watt ofScotland patented a radar system in 1935. British and American scientists, workingtogether, perfected radar during World War II and by 1950 radar had become standardequipment on commercial and military vessels.

The purpose of radar is to enable range and bearings of objects to be obtained for the

safe navigation of your vessel, which includes fixing your position and avoiding collision. Inorder to safely operate a vessel, the master has a responsibility to understand andcorrectly use available instrumentation, including radar.

1 Radar Controls

Unfortunately, with radar sets there is no such thing as a standard radar control panel.Radar control panels and even terminology vary from manufacturer to manufacturer.However, the IMO (International Maritime Organisation) has established a set of standard

symbols to depict the most important controls. Although you wont find these symbolsused on some of the smaller radars.


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Figure 1


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Figure 2: The main control panel of a typical radar.

There are seven main controls that determine the performance of the radar:

standby/transmit

brilliance

gain

tuning

range

anti sea clutter control (STC)

anti rain clutter control (FTC)

1.1 Standby/Transmit

The standby/transmit switch usually has three positions labelled off, standby, andtransmit. Turning the switch to standby will activate the radar set, however it doesntcome on immediately as the magnetron needs a few minutes to warm up before it cantransmit. The radar will have some form of visual signal to indicate when this period isexpired. The radar can then be switched to transmit and on some sets a short or longpulse can be selected at this time, normally long pulse would be selected. A long pulse willbe more likely to show an echo from a weak target or a target at a longer range. A shortpulse will achieve better definition on short ranges.

As well as its main function of giving the magnetron time to warm up, in standby modethe scanner is not rotating (on most sets) and is a way of conserving power andprolonging the life of the magnetron while keeping the set ready for immediate use. It is agood practice at sea to leave the radar on standby at all times as this will preventcondensation forming inside the radar set.


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1.2 Brilliance

The brilliance control on an analogue radar controls the brightness of the rotating traceand will also affects the brightness of the displayed echo so it needs to be adjusted so thatthe trace itself is just visible, to give a good contrast between echo and background.

On a raster scan display the brilliance control regulates the brightness of the picture,making it bright enough for daylight viewing or dim enough so as not to impair theoperators night vision.

1.3 Gain

gain

The gain control may appear to have a similar function as the brilliance control in thatoperating it makes the picture brighter or darker. This similarity however, is only superficialas the gain control has a completely separate function and it is important not to confusethe two.

The gain control affects the receiver and not the display as the brilliance does. Turning upthe gain will increase the amplification of the incoming signal, making weak echoes lookstronger, but confusing the display with background speckle or noise, similar to thebackground crackling of an ordinary radio. Turning down the gain will reduce thesensitivity of the receiver and reduce the speckle but care must be exercised that this isnot overdone as weak or distant echoes may be lost.

1.4 Tuning

The tuning control can be compared to the tuning control of an ordinary radio, in that ittunes the receiver to the frequency of the transmitter. Poor tuning adjustment may not beeasily recognised on the screen. Tuning slightly out will eliminate some very weak echoes,

but still produce a clear picture of the stronger ones. Hence the importance of frequentfine tuning of the set.


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1.5 Range

The range control regulates the range at which the set operates. It simply changes thesize of the area on the display and hence the scale. You would change the range of theradar just as you wouldchange charts for passage planning or close-in piloting. The choice of range woulddepend on what you are using the radar for, and your locality. For coastal navigation youmight select a range of 6 or 12 miles so that appropriate coastal features will be displayed,for collision avoidance a range of 12 miles or 24 miles may be appropriate, for pilotageinto a confined anchorage a range of a mile may be needed.

Changing the range also affects the radars pulse length, PRF (pulse repetitionfrequency), and video presentation.

Note: It is considered good watch keeping practice to vary the range monitored, soas to get best use of radars detection capability.

1.6 Sea Clutter Control (STC)

The radar beam will bounce echoes off the sea around the ship, particularly if the weatheris a little rough. This result will be a bright sunburst pattern in the middle of the screenwhich will be more pronounced in the upwind direction. You could reduce this by turningdown the gain, the down side to that solution however, is that the echoes of more distanttargets will be lost as well.

The solution is the sea clutter control. It works by reducing the receiver gain for a fewmicroseconds after each pulse is transmitted, then gradually restores it to its former level.It works very well, but its use requires care. Too much sea clutter control will result in theloss of close range targets. At sea the sea clutter control must be continually monitoredand adjusted.

1.7 Rain Clutter Control (FTC)


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The rain clutter control will reduce the interference on the screen due to the rain andincrease the chance of seeing targets within rain showers. The effect on returning echoesfrom rain on the screen is usually no more than a transparent smear, looking a little likecotton wool, but it can be dense enough to conceal other echoes within the shower. In atropical downpour however, the rain can completely block out all echoes, at timesrequiring the operator to stop the vessel.

The rain clutter control works by making use of the fact that the returning echo from rain is

different from the returning echo of a solid object. The returning echo from rain is muchlonger and very much less dense than the echo from a solid object. The rain cluttercircuitry works by passing on to the receiver only the leading edge of a returning echo.This does not affect the returning echo from a solid object like a ship, but drawn out, weakreturning echoes from the rain however, will be weakened considerably.

Figure 3

In practice however all returning echoes will be affected, resulting in a reduction instrength from all returning echoes and a reduction in picture quality.

Sometimes the sea clutter control may be used to better effect, to see through the rain.However, if you adopt this approach remember that close-in targets will also be lost, whichmay defeat the purpose.

2 Setting Up The DisplayThe following instructions apply to all radar sets, however the procedure may differ onsome sets that have additional controls.

Check that the scanner is free to rotate so as not to foul rigging and that no crew membersare working in the vicinity of the scanner.

With a raster scan display setting up is a relatively simple matter. Turn it on to standbywait for the set to warm up, then switch to transmit. Most modern raster scan radars willautomatically tune the radar for optimum performance. However, many operators still

prefer to tune the set manually to get that little bit of extra performance from the radar, soall radars have provision for manual tuning.

transmitted pulse

returning echo


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If you have an older radar set with a analogue display, before switching the radar set onyou should first turn the brilliance and gain controls right down. Analogue displays producethe picture by directing a stream of electrons onto a delicate florescent coating inside thescreen. When the set is first turned on, this beam of electrons is directed at the verycentre of the screen, over time this would result in burning out the centre portion of thedisplay. Turning down the brilliance and gain

first will reduce the intensity of the beam and prevent damage to the screen. For the samereason its a good practice at sea, to always have some sea clutter employed so as not toburn out the centre of the screen.

After switching to standby you will then have to wait until the set warms up, this is usually3 minutes. With a raster scan display you can set the brilliance to suit the conditions at thisstage. While you are waiting most sets will give a count down, or an indicator light willcome on after the warm up period is completed. Once warm up is completed the set willbe in standby mode, the transmitter can now be turned on.

As soon as the set is transmitting in a raster scan radar you will get a picture of some sort.

Adjust the brilliance again at this point if necessary. With a analogue display the brillianceand gain should have been set to zero, so you will need to turn the brilliance up first sothat the time-base trace is just visible.

Next, with an analogue display adjust the gain up to about 70% or until a light backgroundspeckle can just be seen, this will ensure that weak echoes will be seen. With a rasterscan display adjust the gain up to about 70% or until a light background speckle can justbe seen, then turn it down until the speckle just disappears.

Next select a suitable range for tuning, usually this will be one of the radars middleranges, for a 72 nm radar the 12 mile range would be selected and at the same time

check that other radar controls such as, rain clutter and interference rejection (IR) areturned off.

The last step is to tune the radar. To do this you have to be receiving something, even ifits only sea clutter. Preferably choose a distant weak target, as the effect of tuning will bemore obvious. Then adjust the tuning knob for the clearest and brightest picture. If you areat sea, with no targets visible, adjust the set for maximum sea clutter.

Tuning the set for optimum performance will take a few minutes. Adjust the tuning controlslightly then wait for a few sweeps to see the result. Continue this process until you findthat particular setting that results in the clearest, brightest picture with the most targetsdisplayed.

All modern radars will have some form of tuning indicator to assist you with the process.Tune for the maximum number of tuning lights or highest deflection of a meter or someother indicator, but dont totally rely on the tuning meter however, as your eye is thesuperior indicator. After tuning readjust the gain for a lightly speckled background.

Next, switch to the desired range scale and adjust the anti-clutter controls. Then check theVRM against the range rings and the alignment of the heading marker against the shipshead. If a performance monitor is fitted, check that performance is satisfactory. The radar

is now ready for operation.


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After 10 minutes or so recheck the gain control and retune the set, because as the setwarms up the transmitter frequency is likely to have changed slightly. Check gain, cluttercontrols and tuning again after every hour or so of operation as the set may gradually driftout of tune.

To assist you to remember the start up sequence, after you have switched on, adjust theBrilliance, Gain, Range and Tuning in alphabetical order.

2.1 Switching Off

Before shutting down the radar set turn the brilliance and gain, to the minimum and turnoff the anti-clutter controls. This will extend the life of the display and allow the nextoperator to set up the radar using standard procedure in the minimum time.

2.2 Incorrect Adjustment Of Controls

If the radar controls are incorrectly adjusted the performance of the radar will be adverselyaffected. This could result in small and weak targets remaining undetected and largertargets being detected at a reduced range. Good watchkeeping procedure should ensurethat the radar is carefully monitored and always tuned for optimum performance.

3 Other Controls

3.1 Interference Rejection (IR)

Figure 4

VRM

2.5

EBL

270


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A third type of clutter on the radar screen, known as Mutual Radar Interference is causedby other radars in the locality which operate on a similar frequency to your ships radar.The interference shows up as bright spots scattered over the screen randomly, or as adistinctive pattern of dotted lines curving outwards from the centre of the screen. It is morecommon on longer range scales as on shorter range scales only a few of the interfering

pulses will be displayed.

If only one other radar is involved this is not too serious, but in busy traffic areas the cluttercan be dense enough to cause confusion. An interference rejection circuit can minimisethis problem. It works by rejecting any echo which does not return from any twosuccessive pulses. While large targets will not be effected by IR, some small echoes maybe lost. There is no IMO symbol for IR.

3.2 Echo Enhancement

To assist the operator to spot small targets, most modern radars have the ability toexpand them. Usually named echo stretch or expansion, its sole object is to make smalltargets look bigger. This can at times be a great benefit, but it also tends to distort thepicture and reduces range and bearing discrimination. Expansion may be useful at timesbut should be switched off when not required. There is no IMO symbol for echoexpansion.

3.3 Heading Marker And Range Ring Delete

The heading marker and the range rings can obscure small targets. So its a good idea tohave the range rings turned off when they are not in use and to delete the heading markerevery few minutes to see if it is masking a small target, dead ahead. The heading markerdelete control is usually self cancelling; that is the heading marker will reappear as soonas the button is released. There is no IMO symbol for these controls.

3.4 Pulse Length Control

The pulse length is normally selected automatically when you select the range scale. Onmost sets however, in the mid ranges (6 mile to 48 mile on a 72 mile radar), it is possibleto manually select the pulse length, which will have a significant effect on the performanceof the radar. A long pulse length increases the chance of detecting targets at long range.Selecting a short pulse length will increase range discrimination, making it possible to

distinguish between a tug and its tow for instance.


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In the figure on the left it can be seen thatwhen long pulse is selected the small targetalmost dead ahead is visible, but the tug andtow are merged and shown as a singlecontact. Also, on the left the land mass has

merged with the islands.

When short pulse is selected the small targetahead is lost, but the tug and tow are shownas separate and the islands have separatedfrom the land mass.

Figure 5

3.5 Off-Centring

By using the off-centring control the centre of the picture can be moved downwards orupwards and on some newer sets it can also be moved sideways. Moving the centre ofthe picture downwards expands the effective range of the radar forwards at the expenseof range astern for instance on a six mile range, off-centring would enable you to see ninemiles ahead but only three miles astern. This could be an advantage when piloting but

may be a disadvantage when using radar for collision avoidance as a faster vessel cancatch up to your ship very quickly and you may be unaware of its presence until you see itovertaking through the wheelhouse window.

HULP12 nm

HUSP12 nm


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4 Radar Presentations

4.1 Relative Motion Unstabilised Display

Most small vessels are equipped with a relative motion unstabilised display. A relativemotion display shows the picture and not the vessel in motion. Your ship is at the centre ofthe display and the heading marker is aligned with 000, on the azimuth ring head-up.

Having your own vessel at the centre of the display has the advantage of making it easy torelate the real world to the radar image. Everything on the right hand side of the screenwill be to starboard of your vessel and everything on the left hand side will be on the portside of your vessel.

When the vessels course is altered, the heading marker remains upward at 000, whilethe whole picture rotates in the opposite direction by the amount of the course alteration.

All bearings are therefore relative to the vessels fore and aft line. During the time thevessel is altering course, the whole radar picture is rotating, making the radar useless untilthe vessel settles on its new heading. This is a severe disadvantage during pilotage.


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The figure on the left shows the sequence ofevents that occur when a vessel alters coursewith a relative motion unstabilised display.The top figure shows the vessel on a course is270T.

The true bearing of the target is:270 + 330 = 600 - 360 = 240T.

The vessel then alters course by 30 tostarboard. The middle diagram shows the displayafter the vessel has altered course by 15.

The bottom figure shows the display after thevessel has settled on the new heading of 300T.The targets true bearing is now 300 + 300 =600 - 360 = 240T.

Figure 6

000

090

180

270

EBL330

HULP

VRM2.6

000

090

180

270

EBL315

HULP

VRM2.6

000

090

180

270

EBL300

HULP

VRM2.6


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4.2 Relative Motion Course-Up Stabilised Display

With the addition of heading information from the vessels electronic compass. This couldbe a magnetic fluxgate compass or a gyro compass, generally a gyro compass would beemployed for this purpose as all courses and bearings will then be true bearings and

courses. The heading marker will appear at the top of the screen and will indicate thevessels true heading. The true bearings of targets can be read from the azimuth ring. Theradar is then said to be stabilised.

As it is still the picture that moves and not the vessel, the display is still ships head-up,which means that the whole picture will move when the vessel alters course. However, thedisplay will not be affected by yaw which makes the course-up stabilised display ideal forcollision avoidance purposes.


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The figure on the left shows the same sequenceof events this time with a ships course-upstabilised display. The vessels true course isshown at the top of the display. Therefore the

bearing of the target will be a true bearing.

Note: that it is still the picture that moves.

Figure 7

300

000

180

270

EBL240

CULP

VRM2.6

090

285

000

090

180

EBL

240

VRM

2.6

270CULP

270

000

090

180

EBL240

VRM2.6

CULP


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4.3 Relative Motion North-Up Display

The navigator can also choose to operate the stabilised display in the north-up mode. Innorth-up mode the radar picture is stabilised so that north is at the top of the screen, whenthe vessel alters course it is only the heading marker that moves. This display makes

comparison with the chart easier and allows continuous observation of targets, even whilealtering course which makes this display ideal for pilotage purposes.


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The figure on the left shows the same sequenceof events on a relative motion north-up display.

Note: that only the heading marker moves.

Figure 8

270

000

090

180

EBL240

VRM2.6

NULP

270

000

090

180

EBL

240

VRM

2.6

NULP

300

270

000

090

180

EBL240

VRM2.6

NULP

300


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4.4 True Motion Display

With the addition of information from the vessels log as well as the gyro or fluxgatecompass, most modern radars can be switched to show true motion. In a true motiondisplay the centre of the picture moves across the screen in time with the actual

movement of the vessel. The vessel is then seen to pass the coastline rather than thecoastline pass the vessel, stationary objects like buoys will appear stationary on thedisplay.

True motion is the preferred choice of many operators for pilotage operations as you canwatch your vessels progress on the display in the same manner as you would with a GPSplotter. True motion also has the advantage that echoes of vessels under way can bedistinguished at once by their trails, indicating their true courses, while the echoes ofstationary objects can be readily identified by the absence of echo trails. To achieve thisthe radar must be ground stabilised.

True motion displays can be either sea stabilised, or ground stabilised. With a seastabilised display, information concerning the vessels heading and speed through thewater are fed to the radar. In a ground stabilised display additional information concerningthe set and rate of the tide is also applied.

Your ships cursor can be made to start at any point on the screen and then movesoutwards towards the edge. It will automatically reset itself when your ships cursor hastraversed two thirds of the radius of the screen, or it can be reset manually.

For collision avoidance purposes however, a relative motion display is preferred ascalculating the closest point of approach (CPA) in true motion is a more complicated

process.


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The figure on the left shows again the samesequences of events this time with a true motiondisplay.

Note: that moving objects display echo trails.

Figure 9

270

000

090

180

300

TMLP

own shiptarget

270

000

090

180VRM2.6

300

TMLP

own shiptarget

270

000

090

180

TMLP

own ship

target


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The Radar Transmission

Section Contents


1 The Radar Wave

1.1 Characteristics Of A Radar Wave Written Activety 1

2 Performance

2.1 Echo Strength

2.2 Horizontal Beam

2.3 Vertical Beam

2.4 Maximum Range

2.5 Minimum Range

2.6 Range Accuracy

2.7 Bearing Accuracy

2.8 Bearing Error

3 Major Components

3.1 Power Supply

3.2 Transmitter3.3 Waveguide

3.4 Scanner

3.5 Time Base Unit

3.6 Receiver

3.7 Display Unit

3.8 Colour Displays

3.9 LCD Display

3.10 Interfacing

4 Principles Of Operation

4.1 Range Measurement

4.2 Pulse Length

4.3 Pulse Repetition Frequency

4.4 Bearing Measurement

4.5 Wavelength And Frequency

4.6 Marine Radars4.7 Refraction

4.8 Radar Horizon

5 Effects Of Weather


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5.1 Super-Refraction

5.2 Ducting

5.3 Sub-Refraction

5.4 Attenuation

5.5 Rain Clutter

5.6 Other Causes Of Attenuation

6 Echo Strength

6.1 Target Size

6.2 Target Material

6.3 Target Aspect

6.4 Target Shape

6.5 Surface Texture

7 Practical Targets

8 Radar Reflectors8.1 Passive Radar Reflectors

8.2 Active Radar Reflectors

8.3 Search And Rescue Transponders

8.4 Anti-Collision Radar Transponders

9 Performance Monitor

10 The Radar Logbook

11 Performance Standards

12 Installation, Maintenance And Safety Precautions

12.1 Maintenance And Troubleshooting

12.2 Safety Precautions

13 Siting Of A Marine Radar

13.1 Siting Of The Scanner

13.2 Siting Of The Display


In this section we will examine the basic principles of radar, the main components of aradar unit and also the nature of the radar transmission. It is very important that youunderstand how a radar operates and what information you can obtain from it and moreimportantly, how radar can deceive you.


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1 The Radar Wave

Radar uses electromagnetic energy in the form of radio waves. In marine radars, thewaves are not transmitted continuously but in pulses. The pulses travel outwards in adirectional beam at a constant speed of 300 million metres per second, or 162 nauticalmiles. When the radar pulse strikes a reflective surface, part of the wave bounces backfrom the object, the way sound waves bounce back from an object, and produces anecho. We can calculate the range of the target with the formula:

2

SxTRange

where:T = Elapsed timeS = Speed of radar wave

Figure 2: A 96 mile 25 kW colour radar.

1.1 Characteristics Of A Radio Wave

Figure 3

Volts

+

-

3

3

2

2

1

1

00.5 1 1.5

Time(secs)

One Cycle

Wave Length

Crest

Amplitude


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cycle One complete oscillation

frequency The number of cycles passing a given point per second. The unit offrequency is the Hertz (Hz)

wavelength The distance between successive crests

amplitude The vertical height of the wave from crest to centreline

2 Performance

2.1 Echo Strength

Figure 4

As the radar beam spreads out with increasing range, due to the wide vertical beamwidth,power decreases rapidly. The radars energy is inversely proportional to the fourth power

of the distance from the scanner.

2.2 Horizontal Beam

Figure 5

In the horizontal plane the radar beam consists of a strong narrow main lobe and smallerside lobes. The width of the horizontal beam (measured between the half power points of

the main lobe) is equal to 70 x wavelength divided by scanner width.

Half Power Points

Main Lobe

Side Lobes

Horizontal View


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d

x70

where:

= width of horizontal beam

= wavelengthd = scanner width

2.3 Vertical Beam

Figure 6

The width of the vertical beam is determined by the width of the scanner. It must be wideenough to give good target echoes when the vessel is rolling or pitching heavily, and isusually between 20 and 30.

2.4 Maximum Range

The maximum range of the radar depends on various factors including wavelength, PRF(pulse repetition frequency) power output, beamwidth, receiver sensitivity and scannerheight. Experience and the radar log will tell you typical detection ranges for various typesof targets.

The theoretical maximum range can be calculated by using the following formula found innautical tables.

h2.21H2.21milesnauticalinRange

where:H = height of the scanner in metresH = height of the target in metres

2.5 Minimum Range

The minimum range of a radar depends mainly on the pulse length, and is slightly morethan half the shortest pulse length, a typical figure would be 25 metres.


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2.6 Range Accuracy

The range accuracy of an analogue radar depends mainly on the accuracy of thetimebase and the range markers. It should not exceed 1.5% of the maximum range of therange scale in use, or 70 metres, whichever is greater. It should be checked by radarcalibration at suitable opportunities.

2.7 Bearing Accuracy

The bearing accuracy of a radar depends mainly on horizontal beamwidth. The narrowerthe beam, the better the bearing accuracy. The bearing accuracy will be quoted in themanufacturers manual, but should not exceed 1.

2.8 Bearing Errors

Misalignment of the heading marker contacts will cause bearing errors and thesealignments should be frequently checked. On most sets with the radar in ships head -uppresentation, the heading marker can be adjusted so that it indicates 000 on the bearingscale by an adjusting screw in the scanner unit. This is best done by bringing a targetdead ahead visually and checking to see that the same target is bisected by the headingmarker on the display.

On an unstabilised display, yaw error will result, unless the ships head is noted at theinstant of taking a bearing.

3 Major Components

Figure 7: Major components of a typical radar set

Receiver

PowerSupply

Scanner

Bearing Data

T/R

Waveguide

Transceiver

Video

TimingTrigger

Transmitter

Modulator

Oscillator

Magnetron

Timebaseunit

Display


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3.1 Power Supply

Supplies the power to the radar. It is a small solid state power pack or motor alternator.

3.2 TransmitterSuper high frequencies of electromagnetic energy waves (3000 to 10000 MHz, these arewavelengths of 10 and 3 cm, respectively) are produced in the oscillator. The operatingcycle of the oscillator is initiated by the trigger, which determines the pulse repetitionfrequency (PRF). The pulses are shaped in the modulator, which determines the pulselength, then passed to the magnetron, which converts the energy into radio waves anddetermines the radar frequency. They are then passed to the transmitter, which in modernradars is usually combined with the receiver and called the transceiver. The transceiver isusually located within the scanner unit.

3.3 Waveguide

The pulses are transmitted to the scanner unit by the waveguide. A waveguide is hollowcopper tubing, usually rectangular in cross section, having dimensions according to thewavelength of the carrier frequency. An electronic switch in the waveguide, called thetransmit/receive cell (T/R) isolates the receiver during transmission to protect it from thehigh power of the transmission. In modern radars the waveguide and the T/R switch areusually located within the scanner unit.

3.4 ScannerThe figure below shows the most common type of radar aerial, the open array scannerunit.

Figure 8

The figure below is a radome. Radomes are particularly suited to yachts as the rotatingscanner is enclosed within the dome, allowing it to turn without fouling sails and rigging.

Figure 9


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The scanner unit radiates the radar pulses and passes returning target echoes to thereceiver. The scanner also focuses the outgoing microwaves into a tight beam in much thesame way as a torch reflector focuses the light from a bulb. This is done in most marineradars by feeding the microwaves into a hollow tube inside the rotating scanner, which isalso called a waveguide. The waveguide is sealed at both ends but has a series of smallslots in one side. Each slot acts like a small aerial but their combined effect is to focus themicrowaves into a narrow beam. This type of scanner is called a horizontal slotted

waveguide.

The length of the scanner will determine the horizontal beamwidth of the radar .A widerscanner will result in a narrower beamwidth which will produce better bearingdiscrimination.

In every revolution of the scanner, radar pulses hit the target not once but many timesduring the time it is aimed at the target. The effect on the screen is cumulative, the morehits the brighter the target appears on the screen. Should insufficient pulses hit the targetonly a weak echo will be displayed and it will disappear quickly. The number of scannerrevolutions should be between 20 and 30 revolutions per minute in order to both display

the target brightly and prevent the disappearance of the picture between scannerrevolutions. Most scanners operate at 24 rpm.

Figure 10: The inside of a horizontal slotted radar scanner

A more recent innovation is the patch aerial. This aerial uses a printed circuit consisting ofan array of copper pads, to focus the beam.

3.5 Time Base Unit

With an analogue radar, a motor rotates the scanner at approximately 24 rpm and a signal

from the time base unit to the display unit causes the trace to rotate in synchronisationwith the scanner. As the scanner passes the fore-and-aft line, a heading marker willappear on the display.

Digital radars use microprocessors to add heading and timebase information to thedisplay.


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3.6 Receiver

The receiver unit detects incoming echoes which are at more or less the same frequencyas the outgoing frequency and mixes them with a signal from the local oscillator to reducethem to an intermediate frequency, usually about 60MHz. The signal is then amplified by

the IF amplifier, weaker distant targets are amplified more than stronger/closer targets,then passed to the video amplifier, which converts the signal to a suitable form for videodisplay. The signals are then passed on to the display unit, with the addition of range andbearing marker signals. The receiver in modern radars is combined with the transmitterand called the transceiver. It is usually located within the scanner unit.

3.7 Display Unit

Analogue type radars consist essentially of a cathode ray tube (CRT), the face or screenof which is commonly referred to as the scope, and various timing circuits and controls. Inthe scope a stream of electrons is directed towards a fluorescent screen. The phosphorusglows when illuminated by the electrons, while internal circuitry forms the trace or sweep.A beam begins at the centre and sweeps out again and again, each sweep correspondingto the progress of a microwave pulse going out and back, and each successive sweepmoving a little further around the screen in time with the rotating scanner. A returning echois added to the sweep signal so that the screen is more brightly illuminated at a pointcorresponding to the bearing and range of the targets echo. This produces a very clearbut very dim picture and the scope is fitted with a cowling to allow daylight viewing. Thispresentation is called a Plan Position Indicator (PPI).

Figure 11

The above diagram shows a cathode ray tube (CRT) using magnetic deflection obtainedfrom coils placed around the neck of the CRT. Other CRTs use electrostatic deflection, byuse of deflection plates.

Intensifier Anode

Deflector Coil

Cathode

Grid

Focusing Anode

Accelerating Anode

Filament

Electron BeamScreen


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Modern presentations, called rasterscan radars use microprocessors to analyse the rangeand bearing information and present it in a more TV-like style. The rasterscan radars alsouse a CRT however, the end of the tube where the picture is displayed is rectangularrather than circular as in PPI display. In a rasterscan display there is no rotating traceinstead the incoming signal is digitalised and the display is drawn by a liner scan, line byline across the screen in much the same way as a television is produced. As the sweep

moves across the screen the electron beam from the CRT illuminates the pixels (picturecells) on the screen.

Figure 12: A small rasterscan CRT radar

To obtain a sharp image a high resolution is required. Resolution is measured by the

density of the pixels, the more pixels per square inch the higher the resolution. A commonradar display resolution of 640480 means that the screen consists of 640 columns by 480rows of dots, that is, 640 x 480 = 307,200 pixels.

Pixels are electronically refreshed or updated so they will remain bright, at a rate of about30 or more times each second. Displays may be interlaced or non-interlaced. Interlaceddisplays do not draw the entire picture in one pass. On the first pass, it draws every otherline and draws the remaining lines on the second pass. Non-interlaced displays willtherefore be more stable.

On early model rasterscan radars piccolos were either on or off, which meant that therewas no visible difference between a weak echo and a strong one. This could result inweak echoes like rain clouds completely blocking out the display and masking targetswithin the blocked out display. Later models have multi-level quantisation displays, inwhich each pixel can operate at several levels of monochrome, which allows the operatorto distinguish between a weak and a strong echo. On modern rasterscan radars remotedisplays or full dual displays units are available.

Most radars produced for small craft today have a green and black monochromequantised display. However, some of the larger more expensive radars feature full colourdisplays.


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3.8 Colour Displays

Colour radars offer several different formats, with up to four different colours. Multi-levelmonochrome displays can display 16 levels of quantisation in yellow or green. Ormulticoloured displays can identify the echos relative strength and display the echoes as

different colours (green, yellow and red) on the display. A drawback with this type ofdisplay is that the operators attention is drawn to the red echoes while weaker lessintense echoes, but possibly more significant echoes, are ignored. Colour technologyprovides valuable information such as filtering out fog, rain, etc, and are preferred bymany operators.

Figure 13: A 72 mile colour radar

Other colour displays use colour to differentiate between genuine targets and display

information, EBLs and VRMs, etc. Still others apply colour to targets to increase theircontrast, usually a yellow-orange colour for targets and a different colour for displayinformation.

Probably the best feature of colour radars is their ability to select the background colour.In daylight a blue background would usually be selected as the display is easy to read inbright sunlight. While at night most operators would select a black background as itreduces glare, preserving night vision.

The figure below shows a Furuno 72 mile analogue radar. The scope radars arecharacterised by their circular display and daylight viewing hood.

Figure 14


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3.9 LCD DisplayAnother type of modern presentation is the LCD (liquid crystal display). LCD displays userasterscan technology to create the picture. But, instead of using a CRT, LCD radars usean array of liquid crystal diodes, sandwiched between panel of glass to produce thepicture. LCD displays are lightweight and require little power. However, they produce less

contrast, produce more glare and have poorer screen resolution than CRTs. To overcomethe contrast problem most are back lit, allowing daylight viewing. On the other hand,CRTs are bulky, fragile, and consume a lot of power.

Figure 15: A small LCD radar

3.10 Interfacing

Most radars today are capable of interfacing with other navigational equipment: such asGPS navigators, electronic compasses, logs, sounders, weather instruments, engineinstruments and autopilots and are capable of showing the information from each or incombination on the radar display. For example, when interfaced with a GPS navigator,electronic compass and log, the position and vector of any radar target on the screen thathas been acquired by the ARPA (Automatic Radar Plotting Aids) can be digitally displayedon the chart.In order to interface the instruments must be able to communicate with each other in thesame language. To allow this most manufacturers have adopted a standard code definedby the National Marine Electronics Association (of America). The latest version of thisstandard is NMEA-0183.

Figure 16

000

CUR17 40.56146 35 43

NUSP

LL

17 44.67146 34 87

WP Dive

07634 .6 nm

CRS 089

SMG 18.8

Temp 29

Baro 1026Sea 23

ETA 1024

6.00 nm1.00 nm

VRM1.45

TUNING

IR


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The above figure shows a north-up radar display interfaced with the GPS and otherinstruments to give own ships position, position of target acquired by the ARPA, SMG,CMG, waypoint information, air temperature, sea temperature and barometer reading.

4 Principles Of Operation

4.1 Range Measurement

In analogue radars, target range is measured electronically by a beam of light that iscontinually moving across the screen from the centre to the edge at a speed equivalent tohalf that of the radar wave. The light beam forms a line called the timebase or trace onwhich the target echoes appear.

In more modern rasterscan radars range is measured electronically. When the pulse istransmitted a very precise clocklike timebase signal is produced in the switch register,corresponding to the speed of the transmitted pulse. If a returning echo is received at

precisely the same time as the timebase signal is produced a switch is turned on and thereturning echo displayed. The range is measured by either fixed range rings or a variablerange marker (VRM).

4.2 Pulse Length

As mentioned earlier, radar does not transmit continually, because weak echoes andclose targets would be masked by the noise of transmission, but transmits in very shortbursts called pulses. The pulse is transmitted and then the receiver listens for a returning

echo from the pulse before another pulse is transmitted.

The beginning of the pulse is called the leading edge and the end of the pulse is called thetrailing edge. For a pulse of one second duration the leading edge would travel a distanceof 161,829 nautical miles before the trailing edge leaves the transmitter, or travels adistance of 80,914.5 nm bounces off an object and returns to the transmitter. If there weretargets closer than 80,914.5 nm the trailing edge of the transmission would not havecleared the transmitter preventing the weaker leading edge being received. Had the targetbeing further away than 80,914.5 nm the trailing edge would have cleared the transmitterleaving it ready to receive. So a radar with a pulse length of one second would have aminimum range of 80,914.5 nm. Of course there are no radars with a pulse length of one

second and a minimum range of 80,914.5 nm. Most marine radars operate between 0.8sand 1.2s


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Example

What would be the minimum range of a radar with a pulse length of one s (one millionthsof a second). First divide 161,829 by 1,000,000:

1000000

161829Range

= 0.161829nm

to find minimum range divide by 2:

2

0.161829rangeMinimum

= 0.0809nm

What is the minimum range of a radar with a pulse length of 0.8s?

Minimum range = 0.8 x 0.0809= 0.0647nm

or as 1 nm. equals 1852 metres.Minimum range = 0.0647 x 1852

= 120 metres

A long radar pulse uses more energy than a short pulse of the same power. It will alsotravel further and the returning echo will produce a more conspicuous echo on the radarscreen. However a short pulse will achieve better range discrimination and definition

resulting in a clearer radar picture.

So a long pulse is required to detect targets at long range and a short pulse is required fordefinition and good range discrimination. To overcome this contradiction most radars areequipped with typically 3 to 5 different pulse lengths, from 0.08 to 1.2 s (millionths of asecond). The pulse length on most radars is a function of range. If you select a shortrange, a short pulse length will be selected automatically, if you select a long range, a longpulse will be selected automatically.

4.3 Pulse Repetition Frequency (PRF)

Pulse repetition frequency is the number of pulses transmitted per second. Whileminimum range is affected by the duration of the pulse the maximum range is affected bythe pulse repetition frequency (PRF) as there must be a silence period between thetransmission of each pulse to allow the weaker returning pulse to return without collidingwith and cancelling out the next outgoing pulse. Most radars will have a different PRFcorresponding to each pulse length. A typical pulse repetition frequency is 500 pulses persecond for long pulse and 3000 pulses per second for short pulse.


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To obtain an accurate bearing, the radar pulse is concentrated in a narrow beam in thehorizontal plane, and is rotated continuously through 360 in synchronisation with thetrace in an analogue radar. The radar assumes that if it receives an echo, its beam mustbe pointing straight at the target in question. A line on the screen called the headingmarker provides a bearing reference, and target bearings are measured by an electronicbearing line (EBL) or in older radars a bearing cursor.

4.5 Wavelength And Frequency

The wavelength () and frequency (f) of the radar wave are inversely proportional to eachother, and are related by the formula:

c = x for:

cf

where:c = The constant speed of the wave

Example

With a wavelength of 3 cm at a constant speed of 300 m/s, what is the frequency?

03.0

300000000

cf

= 10000000000 Hz= 10000 MHz

4.6 Marine Radars

4.6.1 X Band Or 3 cm Radar

Radar wavelengths are expressed in centimetres, the commonest commercial marineradar being 3 cm (actually 3.1 to 3.2 cm). This corresponds to a frequency of 9300 to 9500megahertz (millions of cycles per second) and is known as X-band radar.

A 3 cm radar requires a smaller scanner to achieve the desired beamwidth and definitionas compared with a 10 cm radar. X-band radars are particularly suited to coastalnavigation and pilotage due to their high picture definition and quality. On the negativeside a 3 cm radar produces more side lobes than a 10 cm radar and the radio waves are

not as refracted to the same extent. Therefore they will not achieve the same range as a10 cm radar.


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4.6.2 S Band Or 10 cm Radar

Another common marine radar wavelength is 10 cm (actually 9.2 to 10 cm) with afrequency of 3000 to 3246 megahertz. This is known as S-band radar. A 10 cm radar willrequire a scanner up to 5 metres long to focus the beam. However, once produced thebeam will produce fewer side lobes and travel further along the surface, resulting in agreater maximum range. Another advantage of S-band radars because of their longer

pulse length and greater power is that they will be less affected by attenuation than X-band radars.

S-band radars are suited to long range landfall navigation and early detection of targetsfor collision avoidance, due to their superior range and the cohesive quality of their pulses.On the negative side 10 cm radars require a larger scanner, greater power and often alonger pulse length resulting in a loss of definition at shorter ranges.

A 3 cm radar is nearly always the preferred choice for small craft due to the greater powerand scanner size requirements of 10 cm radar.

4.7 Refraction

If the radar wave travelled in straight lines, the distance to the radar horizon would only bedependent on the radars power output and the height of the scanner. In other words, thedistance to the radar horizon would be the same as that of the geometrical horizon for thescanner height. However, both light and radar waves are subject to downward bending asthey pass through the atmosphere, the amount of bending being greater for longerwavelengths. This bending is known as refraction.

Figure 17

4.8 Radar Horizon

Radar waves are longer than light waves, so bending due to refraction will be greater,resulting in the radar horizon lying beyond the visible horizon. The radar horizon undernormal atmospheric conditions is about 6% greater than the visual horizon.

TangentScanner

GeometricalHorizon

RadarHorizon

VisibleHorizon


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5 Effects Of Weather

Weather conditions affect radar performance, in three ways:

non-standard atmospheric conditions

attenuation

unwanted echoes

The radar horizon assumes standard atmospheric conditions. Which are:

pressure = 1013 hpa decreasing at 36 hpa / 1000 feet of height

temperature = 15C decreasing at 2C / 1000 feet of height

relative humidity = 60% and constant with height.

In the standard atmosphere the temperature and moisture content decrease relativelyslowly with height and the radars range will be normal. In non -standard atmosphericconditions the radars range will differ from normal.

5.1 Super-Refraction

Super-refraction will occur when a warm air layer overlies a cooler sea surface (i.e.temperature inversion). The radar beam is refracted more than normal causingconsiderably increased target detection ranges, typically up to 25%.

Figure 18

Scanner

NormalRefraction

SuperRefraction


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5.2 Ducting

An extreme form of super-refraction is known as ducting, when the radar beam isconducted for long periods within a duct formed by air layers. When this occurs, unusuallylong detection ranges of targets may be experienced. It is during periods of ducting that

second trace echoes may appear.

Figure 19

5.3 Sub-refraction

When a cold layer of air overlies a warmer sea surface. The radar beam is refracted atless than normal, causing reduced target detection range.

Figure 20

ScannerCold air

Warm dry air

Beam trapped within duct

Scanner

NormalRefraction

Sub-Refraction


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5.4 Attenuation

Attenuation occurs when part of the radar energy is absorbed by water vapour in theatmosphere. It is greatest in heavy rain and can cause severe reduction in detectionranges but may also occur in hail, snow, sleet and, to a lesser extent, in fog. A 10 cm

radar because of its greater power will be less affected by attenuation than a 3 cm radar.

5.5 Rain Clutter

Echoes from precipitation have much the same effect as sea clutter. It looks like cottonwool and the strength of the echo depends on the amount of precipitation. Heavy rain willcompletely mask out target echo, especially on older style colour radars where everythingturns red.

5.6 Other Causes Of Attenuation

Clouds may cause echoes similar to rain clutter if they are low and have precipitationwithin them. Other conditions such as sand and dust storms, smoke and haze do notusually cause echoes.

6 Echo Strength

The ability of a radar to detect a target depends on two factors, the peak power of thetransmitted pulse and the characteristics of the target.

The maximum range of a radar set will depend on the peak power of the radar, scannerheight, weather conditions and the reflective properties of the target. Peak power for a x-band radar varies from about 1.5 kW for a 12 nm radar to 25 kW for a 120 nm radar. Aminimum of 3 kW is required to pierce fog, 4 kW will be more effective. Considerablygreater power is required to transmit in the S-band.

The reflective properties of the target include its:

size

material

aspect

surface texture


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6.1 Target Size

Figure 21

All other things being equal, a large target will reflect more of microwave energy than asmall one. So it will produce a stronger echo, and show up on the screen as a brighterimage at a longer range.

6.2 Target Material

Targets of hard, dense material with good conducting properties give the strongestechoes. Rock and concrete are good reflectors but the best reflector is metal. Wood isalmost invisible to radar, because wood tends to absorb the microwaves instead ofreflecting them. Sand and mud will also produce a very weak echo. GRP and otherplastics are very poor reflectors because the microwaves will go straight through them.

6.3 Target Aspect

own ship


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A target at 90 aspect to the radar beam will reflect most of the microwaves back to thescanner and produce the strongest echo.

Figure 22

Targets at oblique angles will reflect most

of the energy in other directions and returna weak echo.

Figure 23

If the targets surface is uneven at leastsome of the microwave energy will bereflected back.

Figure 24


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6.4 Target Shape

Generally speaking flat surfaces are better radar reflectors than curved or pointed ones.So the best reflector will be a perpendicular plane surface, like the side of a ship.

A sphere is a poor reflector as it will only return an echo from that part which is at rightangles to the beam, which is a very small reflecting point in the centre. The rest of themicrowaves will be reflected outwards and lost.

A cylinder is a moderate radar reflector. Like a sphere it will only reflect from the partwhich is end on to the radar beam.


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A cone is a very poor reflector. It reflects all the energy upward and away.

6.5 Surface Texture

A perfect, mirror like smooth surface would produce a very weak echo unless it wasperfectly aligned at right angles to the radar pulse. In practice this rarely occurs, almosteverything includes surface irregularities, which scatter the pulse, so that some of it willreturn to the scanner. Therefore, a target with a rough surface scatters energy evenly andgives a fair echo at any aspect.

7 Practical Targets

The echo strength and detection range of practical targets such as coastlines, beaconsand other vessels depends on a combination of all previously mentioned characteristics.The most important factor being aspect. A large tanker, at an acute angle to you may notgive as a strong return as a small fishing vessel beam on to the radar pulse. A cylindricalobject like a lighthouse (unless fitted with a radar reflector), may give a very poor echo,despite the fact that it can easily be seen with the naked eye. Mountains and coastlineswill show up because of their shape rather than their height. You can never be sure whichpart of the landmass is reflecting the best and, therefore, you cannot be sure of exactlywhat you are seeing on the display.

Experience will tell you what detection range to expect for each type of target. Rememberthe weakest echoes, or those not seen at all may be the most dangerous.

Note: that from a ship underway, a target presentation on the screen can alter asthe aspect changes.


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8 Radar Reflectors

Radar reflectors are fitted to vessels or navigational buoys or other charted features toimprove their reflective properties so as to make them visible to searching radar. Thereare two types of radar reflectors:

passive

active

8.1 Passive Radar Reflectors

Vessels or other objects constructed of wood or GRP are almost invisible to radar. Ifthese craft are to show up on another vessels radar they must be equipped with a radarreflector. All radar reflectors work on the principle of the corner reflector.

When two flat plane plates form a corner, the corner has the ability to reflect directly backto wherever the signal is coming from. Figure 29.

Most radar reflectors are improvements on this principle, the basis of which is that threeplates at right angles to each other will give the reflector the ability to reflect a strongsignal over a wide angle, both horizontally and vertically. In order to cover all angles, radar

reflectors are usually arranged into clusters. The most common type being the octahedral(8 corner reflectors).


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Octahedral clusters work well if they are mounted correctly. It should never be attached toa halyard by any of its points. This destroys a large part of the reflecting capability. It isbest rigidly mounted with one of its open faces uppermost, so that the device could retainwater in the cup. This is known as the rain catch position.

The figure on the left shows an octahedral radar reflector. The radar reflector would

perform better if it were rigidly mounted in the rain catch position.

The newer encapsulated type of radar reflector can be hauled up the mast on a halyard.

The figure on the left shows a well equipped yacht with a radar and an encapsulated radarreflector.

8.2 Active Radar Reflectors

There are two types of active radar reflectors or radar transponders which are used toassist the navigator to identify charted objects:

racons

ramarks


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8.2.1 Racons

A small pulse transmitter is mounted on the beacon, which when triggered by the radarpulse of a vessel radiates an identifying signal in all directions. The ships scanner canonly receive this signal when the scanner is pointed in the direction of the beacon. So thepulse will be displayed on the screen at the correct bearing, but because of the slightdelay in transmission, the beginning of the pulse will be displayed at a greater range than

the true echo.

Due to the system of scanning used, the mark will be displayed on the screen onlyintermittently or every few sweeps. The true echo of the beacon will appear on the screenin front of the identification signal, when it is in range.

Figure 33

The above figure shows the Eden Reef Racon (morse code O). Racons are particularlyuseful for showing up objects which otherwise may be difficult to identify. In this example,an important navigation mark in a busy traffic area or it could be an isolated point on afeatureless coastline, or it could mark the entrance to an important navigational channel.

True target

Raconpaint

VRM0.5


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8.2.2 Ramarks

Figure 34

The above figure shows a ramark (radar marker). Ramarks transmit pulses continuouslyor to special time schedules, rather than on receipt of a vessels radar pulse. They appearon the display as a bright radial line from the centre of the display to the edge and are

identified by breaking up the radial line into a series of dots or dashes. Their majordisadvantage is that they may mask other important echoes on the screen. Ramarks arenow rare, there are none in Australian waters.

VRM4.5


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8.3 Search And Rescue Transponders (SARTs)

Figure 35

A SART is a portable battery powered radar transponder which operates on x-band or 3cm radar. In an emergency situation when the SART detects the incoming radar pulsefrom a searching aircraft or ship it responds by transmitting a distinctive signal which

shows up on the radar screen of the searching vessel as a series of 12 blips extendingapproximately 8 nautical miles outward from the SARTs position along its line of bearing.Figure 35.

When a SART is not being interrogated by searching radar, the SARTs receiver is rapidlysweeping the radar band, searching for radar signals. As all marine radars do not operateon exactly the same frequency within the x-band, there may be a small delay in SARTresponse as the SART locks on to the searching radar signal. When the SART receiverhas locked on to the searching radar there will be a delay as the SART switches fromreceive to transmit mode.

These delays will result in a slight range error, and therefore the first blip of the SARTresponse may be some distance outside the position corresponding to the actual locationof the SART. At medium ranges of about 6 nm the range delay may be between 0.6 nm to150 metres. As the SART is approached by the searching radar the range delay should beno more than 150 metres.

When searching for a SART the IMO recommends that a range scale of 6 or 12 miles(with short pulse selected) be used because the spacing between the SART responses isabout 0.6 nm or 1125 metres and it is necessary to see a number of responses todistinguish the SART from other responses.

VRM2.5

EBL090


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Figure 19: A search and rescue transponder or SART.

8.4 Anti-Collision Radar Transponders

Anti-collision radar transponders will produce a line of five blips over 1 nm on aninterrogating vessels radar screen. A vessel carrying the transponder will also be alertedto the other vessels presence by a visual or audible warning signal when it is beinginterrogated by the other vessels radar signal.


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9 Performance Monitor

A performance monitor informs the radar operator whether the radar is operating at fullefficiency or not. It should be used when the set is first turned on and at frequent intervalsthere after when no targets are present on the screen. A lack of targets may be due to a

lack of targets but may also be due to poor radar performance.

There are two types of performance monitors, external and built-in. Both types both utilisea metal echo box mounted behind the scanner and resonant to the radars frequency tocapture some of the radars transmitted energy.

The external type makes a long plume like line on the screen in the direction of the echobox. The length of the plume depends upon the following:

transmitted power

receiver sensitivity

correct tuning

Therefore the monitor gives an indication of the overall efficiency of the set. The length ofthe plume is measured when the set is installed and known to be performing well. If at anytime the plume is shorter, one or more of the above factors may have deteriorated andallowance for this made, or the fault rectified. The built-in type is used in the samemanner, but produces a circular pattern the radius of which is measured.

The figure on the left shows the plume from an

external performance monitor. The length of whichaccording to the radars logbook should be 1 nm.Some readjustment of the radar is required.

Figure 10

VRM1.0

Davis Mark 2


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10 The Radar Logbook

The radar logbook serves two functions one technical and the other operational. On thetechnical side, information recorded in the logbook would include: maintenance andrepairs carried out, spares on board and their part numbers, technical and performance

details and information concerning arcs of shadow and blind sectors. On the operationalside, information recorded in the logbook would include: details of detection ranges forvarious types of targets and information concerning detection ranges of conspicuousobjects.

11 Performance Standards

Recommendations on performance standards for radar equipment can be found in MarineOrder No. 16. Appendix 3 of Marine Order 16 contains the requirements and performancestandards for radar equipment installed on or after 1 September 1984. The appendix isreproduced in Appendix A of this guide.

12 Installation, Maintenance And Safety Precautions

Today, a technician is no longer required to install a radar. Its just a matter of securing theradar and scanner with a few bolts, plugging the cables together, earthing the set andconnecting up to the power supply. The cable connecting the display unit to the scannercan be provided in custom lengths to fit the specific installation. The cable should never becut.

When installing a radar, the scanner unit should be positioned as high as practical to givethe best possible range and clear of obstructions to avoid shadow sectors. It should alsonot be so close to other electromagnetic equipment as to cause interference and shouldbe away from excessive heat or vibration, but accessible for maintenance. Once installedthe range and heading marker accuracy should be checked by competent personnel andthe limits of any shadow sectors established and recorded in the radar logbook anddisplayed on a card near the radar.

The display unit should be positioned so that it is in easy reach of the navigator and facingforward to facilitate lookout. It should be at a convenient level and position for observing

and servicing, away from the magnetic compass, other electromagnetic equipment, andexposure to weather. If the radar is flush mounted ensure that there is good ventilation tothe rear of the set.


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12.1 Maintenance And Troubleshooting

Maintenance of modern radars is a relatively simple affair. Firstly, read and follow theinstructions in the manufacturers operators guide. Keep a radar log, and record in itmaintenance, service, faults and repairs, as these could be invaluable to assist the radar

technician to diagnose a problem.

Radars are generally very reliable and need only to be protected from water, heat andphysical damage. A regular maintenance program would consist of periodically checkingmounting bolts and brackets, keeping wiring connections clean, tight and external wiringconnections smeared with a thin coating of petroleum jelly. The set itself should be keepclean and free of salt spray.

Troubleshooting would first involve checking that the unit is receiving power and that thecorrect start-up procedure has been followed. If the power supply is OK, all connectionsare clean and tight, the main circuit breaker or fuse is OK, the internal fuses are all right

and the scanner is free to rotate and the set still does not work, then it is probably time tocall a technician.

12.2 Safety Precautions

Personnel should avoid microwave radiation hazard by keeping clear of an operatingscanner. If working aloft on the scanner unit or other equipment near the scanner unit.Ensure that:

the radar unit is turned off, the power disconnected and a sign placed on the displayinforming others that you are working aloft

clip on a safety harness

tie on a tool bag that wont spill if inverted

dont raise or lower power tools by their cord

if at all possible have someone assist you

The high voltage circuits inside the display unit can cause electrocution and must not betouched except by qualified personnel with the radar switched off. The display unit has

potentially lethal voltages inside the unit even after it has been turned off.

A good rule to remember is that: if the back has to be removed, leave it to theexperts.


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13 Siting Of A Marine Radar

13.1 Siting Of The Scanner

Siting of the scanner unit requires careful consideration so that a compromise can be

reached between the effect of height on range performance and sea clutter and the needto minimise shadow sectors and false echoes. The higher the scanner is placed the morepronounced the sea clutter return.

The scanner should be mounted on a structure which will not twist and cause bearingerrors. To ensure that the heading marker indicates the true fore and aft line of the vesselthe scanner should be sited as near as possible to the centre line of the vessel.

13.2 Siting Of The Display

The following should be considered when siting the display unit:

under Department of Transport regulations, installation instructions include theminimum safe distance that the display should be mounted from the magneticcompass

light issuing from the display unit should not interfere with visual lookout when thebridge is in darkness

it should be possible for two observers to view the display simultaneously with anyfitted display visor removed

the display should be mounted so that an observer faces forward when viewing it soas a visual lookout ahead can be maintained

the display should be mounted sufficiently rigidly so as not be adversely affected byvibration and away from excessive heat and fumes

when compass adjustments are being made, the radar set should be in operation.


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False Radar Information

Section Contents


1 False Echoes

1.1 Side Echoes

1.2 Indirect Echoes

1.3 Multiple Echoes

1.4 Interference1.5 Shadow And Blind Sectors

1.6 Second Trace Echoes

1.7 Other Causes Of False Echoes

2 Beamwidth Distortion

3 Range Discrimination

4 Bearing Discrimination


In this section we will discuss the causes and effects of false radar information. We willlearn how to interpret the display so as to be able to obtain information from the radar andeven more importantly we will learn how the radar can deceive us by displaying targetsthat may not exist at all.

1 False Echoes

1.1 Side Echoes

Real Target

False Targets

Side lobe

Side lobe

Main lobe


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As mentioned previously, not all of the radar wave can be focused by the radar aerial intothe main lobe. Some escapes into what are known as side lobes. Side echoes are causedby reflections from the side lobes of the radar beam. They are likely to appear when atarget is a good radar reflector and in range of the radars weaker side lobes.

The true target will always be the stronger echo in the centre of the pattern. Side echoes

can be removed by reducing the gain, or by using the sea clutter control.

1.2 Indirect Echoes

Indirect echoes are caused when some of the outgoing radar energy is reflected from anobject close to the scanner such as a funnel or mast. The echo may return by the samepath or directly to the scanner. The false echo will appear (usually intermittently) on thedisplay at the correct range because the additional distance between the scanner and thereflecting object will be negligible, but on the bearing of the obstruction. The true target willalso appear on the display at the correct range and bearing.

Indirect echoes usually occur in shadow sectors however, they can appear on bearingswhere there are no shadow sectors. Indirect echoes are usually associated with funnelsand other large objects close to the scanner.

Although the bearing of the real target may change, the bearing of the deflected target willremain constant and may if the range is decreasing, appear to be on a collision course. Todetermine if the target is an indirect echo or not, alter course about 10, if the relativebearing of the echo remains constant, then the echo is a false one. Alternatively the gaincan be reduced, or if the echo appears in a known blind sector it can be ignored.

The other form of reflected or indirect echoes uses strong targets such as buildings,bridges or large ships usually when navigating in rivers or harbours. While some of theradar beam is returned to the scanner, much of it is deflected in other directions but atclose range it can produce a beam which will be reflected from the secondary target. Thiswill show on the display on the same bearing as the strong target but at an increasedrange. It will also show up in its correct position so it should not be too much of a problem.

Indirect echo

True echo


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1.3 Multiple Echoes

Multiple echoes are caused when a strong echo arrives back at your vessel and bouncesoff it, effectively retransmitting the signal. For this to occur the other target must be largeand close, and both the target, which may be a land target such as a bridge or headlandor another vessel, and your ship must be good radar reflectors. The false echoes (whichmay be any number) will appear at multiples of the true range on the same bearing.Multiple echoes can be removed by reducing the gain.

1.4 Interference

Echo is partly reflected fromown vessel

True echo

False echo

VRM2.5

EBL270


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Radar transmissions from another vessel in the locality on a similar frequency to yourvessels radar can cause interference similar to the figure above. In most radars thisinterference can be eliminated by switching on the IR control.

1.5 Shadow And Blind Sectors

Obstructions such as funnels, masts, or trawl gear in the path of the scanner can causeshadow or blind sectors on the display, in which targets may be lost or only detected atreduced range or not detected at all. As the radar wave will bend to some extent bendaround obstructions by diffraction, some targets in shadow sectors will be displayed attimes, while targets in blind sectors will not be detected at all.

The radar aerial should always be placed to minimise shadow and blind sectors.

A good method to check for blind or shadow sectors is to find a section of slightly choppywater and turn off the sea clutter control. If no blind or shadow sectors are present thedisplay will fill over an arc of 360 with a mass of small contacts, caused by the echoesreturned from the waves. Any dark streaks radiating outwards from the centre representblind or shadow sectors.

If blind sectors or shadow sectors are present their limits must be determined andrecorded in the vessels radar log. Sea clutter is one way to determine these limits, or asextant could be used. The best method however, is to observe a small target vessel atthe edge of a short range, as your vessels course is altered. Note: the bearing when thetarget disappears and then reappears.

Blind sector

Shadow sector

Shadow sector

Funnel

Shadowsector

CHART

Ship


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1.6 Second Trace Echoes

Second trace echoes will appear at times during periods of severe super-refraction orducting. Targets at very long range will appear at a false range on the correct bearing, onthe second sweep of the timebase. In other words, an echo may return from a distant

target after a second pulse has been transmitted and the receiver is open.

The higher the PRF, the more likely that second trace echoes will occur as more pulsesare transmitted and the corresponding silence period is reduced.

Second trace returns can usually be made to disappear by changing the range scale inuse.

1.7 Other Causes Of False Echoes

When navigating in rivers and harbours overhead power cables at close quarters canreturn an echo from the direction where the cable is at right angles to the radar beam, theeffect on the display is a small echo which is closing on a collision course. The echodisappears when the vessel is close to the overhead cable as the radar beam is no longerin contact with the target.

2 Beamwidth Distortion

Beamwidth distortion is directly related to the radars beamwidth which is largelydetermined by the length of the scanner. A small 2 foot radome will produce a beamwidth

of about 7 while an 8 foot scanner will produce a beamwidth of about 1.2

A target echo will be painted on the display the whole time the radar beam takes to sweepthe target. This will have the effect of enlarging all targets, extending each side by anangle equal to half the beamwidth of the radar. Therefore radar with a beamwidth of 2 willshow a small target such as a buoy to be as an arc 2 wide. An island 10 wide will appearto be 12 wide and a headland will appear to extend 1 further to sea than it really does.

In Figure 6 the small buoy may only be wide but as the radar beam rotates it will paintthe buoy from the moment the leading edge of the beam first touches the buoy and duringthe whole time it sweeps across the buoy until the trailing edge is clear of the buoy.Therefore on the radar display the buoy will appear to be 2 wide.

Rotation

Buoy

2 degrees

12 degrees

Rotation10 degrees


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The advantage of larger beamwidths is that smaller weaker targets will be painted on thescreen larger, therefore they will have a better chance of being detected from a smallvessel as it moves around in a seaway. The down side is less bearing resolution and poorbearing discrimination.

Beamwidth is very important when you are looking for a narrow passage between islands,a narrow harbour entrance or a river entrance. Because of beamwidth distortion, a radarwith a large beamwidth will not be able to see the gap until very close.

In the above figure the yacht with a narrow beamwidth radar will be able to see theentrance to the harbour on the radar display as the beam can fit entirely between the

headlands. However, to the yacht with the wide beam width radar the entrance will remainhidden and the coast appear continuous until the yacht is much closer. The effect ofbeamwidth distortion can be improved somewhat by reducing the gain but, remember toturn it up again afterwards.

The range at which it would be possible for a narrow gap to be visible on radar can befound by the formula:

BWsin

distanceRange

where:distance = size of the gap

BW = beamwidth

Example

You are approaching Mourilyan harbour from seaward, the entrance of which is 0.07 nmwide, your radar has a beamwidth of 6. At what range would you expect to see theentrance on radar?


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nm0.67

0.104528

0.07

BWsin

distanceRange

3 Range Discrimination

Range discrimination is significant, as it can have an effect on the useful operating rangeof the radar. A long pulse uses greater power and will detect a target at a longer range.However, a long pulse has a down side too. In Figure 8 a radar transmitting on long pulsemay have a pulse length of 300 metres, after it travels outwards for a distance it comesupon two vessels 150 metres apart. Part of the leading edge of the pulse will be reflectedoff the nearer ship while part will carry on to be reflected off the second ship. Before the

trailing edge of the pulse has reached the first ship, the leading edge will be already on itsway back, so that the echoes from the two ships will merge. Instead of seeing the twoships separately on the display they will appear as a single target.

Figure 8

If the pulse length is more than twice the distance between two targets, the echoes willmerge.

A shorter pulse length, of say 100 metres, will keep the echoes separate.

Outgoing pulse

Returning pulses

Outgoing pulse

Returning pulses


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So a short pulse is required for good range discrimination, and a long pulse length isrequired to detect weak targets at long range.

To overcome this contradiction most radars are capable of operating at different pulselengths in the mid ranges. For example; you may be having difficulty distinguishing aisolated rock just off the coast from the coastline itself (as the two objects have merged)on long pulse at a range of 24 miles. By switching to short pulse the rock may now appear

on the display as a separate object.

4 Bearing Discrimination

Bearing discrimination is similar to range discrimination in that it governs the radars abilityto discriminate between two close targets, but this time to distinguish between two targetsclose together at the same range. The radar beam can be likened to a rotating pie slice, asmall target will be painted on the screen the whole time it takes the beam to pass over it.So that if two targets close together can both be covered by the beam at the same time,they will appear on the display as one target. Bearing discrimination depends on

horizontal beamwidth; the narrower the beam the better the bearing discrimination.

A wide-beamwidth radar will receive echoes from both targets at once as both are withinthe beam. They will appear on the screen as one target.

A narrower beamwidth radar will pass between the targets and therefore the echoes willappear separately.

Beam width 6

Beam width 2


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Horizontal beamwidth is dependent on the scanner size, a small 2 foot scanner willproduce a beamwidth of about 7, while a 6 foot open array scanner will produce abeamwidth of 2. Bearing discrimination can be improved somewhat by reducing gain.


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Using Radar For Navigation

Section ContentsSection Introduction

1 Measuring Radar Bearings


1.2 Using The EBL

1.3 Radar Bearing Errors

2 Measuring Radar Range

2.1 Using The VRM

2.2 Floating EBL And VRM

3 Distance To The Radar Horizon

4 What The Radar Sees

5 Using Radar For Navigation

6 Fixing By Radar Bearings

7 Fixing By Radar Ranges

8 Long Range Position Fixing

9 Using Radar For Coastal Navigation

10 Blind Pilotage Or Parallel Indexing

11 Anchoring By Radar

Section Summary


In this section we will learn to use radar to assist in the safe navigation of a vessel using avariety of techniques, including blind pilotage or parallel indexing.

1 Measuring Radar Bearings

The crucial factor in radar navigation is to correctly identify radar conspicuous featuresthat are recognisable on the chart as well as on the radar screen. Towers, monuments,buildings and other prominent features that are favourites with compass navigation will notbe visible on radar. However, objects such as headlands, islands, buoys and beacons will,with the added advantage that they will be equally useable by day or night.


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Your ship is always at the centre of the display (except in a true motion display) andbearings are read off from the graduation scale around the circumference of the display.

With a North-up or Course-up stabilised display, where the radar is interfaced with a gyrocompass the heading marker will always point to the true course being steered. Therefore,

all bearings of targets read from the graduation scale around the circumference of thedisplay will be True Bearings.

In a Ships Head-up display where the heading marker always points straight up to the000 on the graduation scale. All bearings will be Relative Bearings, that is, relative tothe ships head or the fore and aft line of the vessel.

Relative bearings can be represented by two methods:

Red 0 to 180 or Green 0 to 180 depending whether they are to Port or Starboardof the heading marker. Echoes to Port would be read as Red 0 to 180 and echoesto Starboard would be read as Green 0 to 180.

Circular configuration 000 to 360. Measured from ships head (000) in a clockwisedirection. An echo bearing 30 to Starboard would be read as 030 Rel. and an echo30 to Port would be read as 330 Rel.

Note: to avoid confusion all bearings are identified as follows. Colours alwaysproceed the degrees:

G90 = Green 90

R90 = Red 90

For relative bearings R or Rel will follow the degrees and all relative bearings will bein three figure configuration:

090R = 090R or Rel.


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Figure 1

In the figure above:

Target A would be called; R40 or 320 Rel.

Target B would be called; G38 or 038 Rel.

What would targets C and D be called?

Note 2: all radars that have EBLs use the circular configuration 000 to 360 relativemethod.

Before relative bearing can be used for radar navigation or collision avoidance thevessels true course must be known. The true course being steered is found by applyingthe sum of deviation and variation (compass error) to the compass course being steered.

A

B

C

D

Ship's Head


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Next the relative bearing of the target must be converted to a true bearing. This is done byadding the relative bearing to the vessel's true course. If this value exceeds 360 whichmeans that you have passed through north, then 360 must be subtracted from theanswer.

When using Red or Green relative bearings Green bearings will be added to the true

course and Red bearings will be subtracted. So:

True course + Rel. bearing = True bearing

True course + Green bearing = True bearing

True course - Red bearing = True bearing

Example

Your ship steering 035 compass: Variation 7 East, Deviation 4 West.

Target bearing 030 Rel. Find the true bearing of the target?

Your ship 035 compass

Deviation 4 West Dev. 4 West

Magnetic 031 Var. 7 East

Variation 7 East CE. 3 East

True course 038T

True bearing of target.

Ships head true + Relative bearing = True bearing

038 + 030 = 068

True bearing of target = 068T


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1.2 Using The Electronic Bearing Marker (EBL)

The EBL is a straight line from the centre of the display to the edge of the display which isused to measure bearings of objects from your ship, in much the same way as a compasswould. In a stabilised display bearings taken with the EBL will be true bearings and with an

unstabilised display the bearings will be relative to the ships fore and aft line.

To take the bearing of an object simply adjust the EBL until it cuts through the centre ofthe object and read off the bearing from the EBL readout on the display.

Modern radars incorporate an improvement on the EBL by using a electronic cursor. Thecursor is usually a cross which can be moved around the screen by using a trackball orother pointing device. Simply place

Rad Art Ex

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