BE & BRM - 1 – UNIT 2

2.1 – Fundamental Principles of Radar – General Description

Marine Radar consists mainly of 4 units – 1) Transmitter, 2) Scanner, 3) Receiver,

and 4) the Display Unit.

Transmitter – Sends out short powerful bursts of EM energy called pulses,

through the Scanner at a specific no. of times per second, called PRF or PRR.
These pulses travel at the speed of light. These pulses strike a target in their path
and are reflected back to the scanner as echoes.

The Receiver processes these echoes and shows it as a blip on the screen of the
Display Unit,

The display unit shows a circular area around the ship according to scale and is
called as a PPI. The distance from any blip can be measured by a variable scale to
measure range (distance off from ship).

Range Determination –

There is 1 trace created for each pulse transmitted. The tracing spot leaves the
centre of the PPI at the same time that the pulse leaves the scanner. On the
display unit, the tracing spot travels at a speed of half of the speed of light. So if a
target is located at 3 miles from own ship, while the radio waves travel 3 up and 3
back miles, the tracing spot moves 3 miles on the PPI and it is this range that will
be measured. The range of a target can be measured by 2 methods – Range Rings
and Variable Range Marker(VRM).

Range Rings – These are a series of concentric, equidistant rings which show up
on the display when enabled. They roughly indicate the range of a target when
viewed w.r.t targets around the ship.

1
VRM – Variable Range Marker is a variable indicator of range which can be
positioned on a target and show the accurate range on a display box.

Bearing Discrimination –

The scanner and the Trace on the display unit are coordinated such that while the
scanner rotates about 20-30 times a minute, so does the Trace. Note that while
the PRF is about 500-4000, the RPM of scanner/ trace is 20-30. So the time taken
by the tracing spot to travel from the centre to the edge of the display is
negligible compared to any angular movement of the trace, considering the huge
difference in PRF and RPM.

The display is marked with the circular scale of 0 deg. – 359 deg. Around the
display unit so that the bearing of any target can be read off with the Electronic
Bearing Line or the relative bearing can be read off with the Heading Marker
pointing to the 0 deg. Mark from the centre of the display.

Electronic Bearing Marker –

This line can be brightened and can be used to indicate both the relative and true
bearing from head up or north up displays. The true bearing is indicated in a
display box.

BLOCK DIAGRAM OF A MARINE RADAR – Page 23 of book.

As already mentioned marine radar consists of 4 parts – transmitter, scanner,

receiver and display unit.

Power Source – The AC input, depending on the make and model of the set is
usually directly from ship’s mains, if suitable; through a transformer or through a
motor alternator.

Trigger unit – Sends spike wave signals to the modulator and Display processor
unit. The no. of spikes per second equals the PRF.

2
Modulator – Is a device which switches the magnetron on or off as required. Each
spike wave from the trigger causes the modulator to release one powerful DC
pulse of a high voltage from the delay unit to the magnetron. The duration of
each pulse is the PL and the no. of pulses per sec. is the PRF.

Magnetron – It is a high power RF oscillator capable of being switched on and off

For short durations (equal to the PL) at the desired PRF, by the pulses from the
modulator. The output of the magnetron consists of RF pulses of e.m. energy that
are sent to the scanner through a hollow, metal tube called a waveguide.

Scanner : Sends the pulses out and receives the echoes, one direction at a time.
since it rotates at a constant speed, the entire area around it gets scanned
regularly.

TR Cell – The full name is transmit/ receive switch. It blocks the receiver branch of
the waveguide during transmission so that the high power transmission does not
enter the receiver and damage it. Soon after transmission, the TR cell switches to
reception and receives the incoming echo signals into the receiver.

Local Oscillator – Oscillates at a constant low power RF of about 30-60MHz below

or above the magnetron frequency, the difference being called the intermediate
frequency(IF).

Mixer – mixes the echoes with the local oscillations and makes available to the IF
amplifier, the echoes reduced from RF to IF.

IF amplifier – amplifies the IF signals several million times and passés them on to
the video amplifier.

Video amplifier – controls the amplification of signals fed to the display processor
unit.

Display processor unit – It is the brain of the radar set. It has several functions
which include :

1) Creation of a rotating trace within the circular part of the rectangular screen.
3
2) making available all other functions and controls needed to operate the radar
set.

3) display of symbols and words inside and outside the circular part of the screen.

Display unit – Consists of a rectangular screen within which a circular part

provides a visual display of all targets in the vicinity. Because it gives a birds eye
view or plan, the circular area within the screen is called the plan position
indicator (PPI). All controls needed to operate the radar set are provided on this
unit.

2.2 - MORE ABOUT MAGNETRON, WAVEGUIDE AND SCANNER

Magnetron – This is a resonant cavity oscillator that converts the electrical pulses
received from the modulator into em pulses. The input is electrical, through HT
leads, while its output is electro-magnetic, through a waveguide. The frequency
range of X Band radar (also called 3 cm radar) is between 9200 & 9500 MHz and
that of S band radar (also called 10 cm radar) is between 2900 & 3100 MHz. The
exact value of each individual marine radar set is that of its magnetron. Each
magnetron has its own fixed frequency. It oscillates only as and when it receives
an EHT ( Electric High Tension ) square wave. In between pulses the magnetron is
idle.

Waveguide – this is a tube of uniform cross-sectional area, usually rectangular,

which carries the RF pulses from the magnetron to the scanner and also the RF
echoes from the scanner to the mixer (through the TR Cell). A small length of
waveguide also connects the LO and the mixer. The waveguide is made of
corrosion resistant material such as copper or suitable alloys. Its area of cross –
section depends on the wavelength – the larger the wavelength, the greater the
area of cross-section and vice versa. The length of the waveguide, the number of
bends that it takes, damage to it, water or dirt inside it, all cause severe
attenuation in the waveguide ( considerable loss of transmitted power and also of

4
echo strength ) and consequent decrease in the range of first detection of all
targets.

Scanner – This is a unidirectional aerial that beams the energy, and receives the
echoes, one direction at a time. The size and type of scanner determines the HBW
and VBW of the set and hence its aerial gain. (Aerial gain = If an omni-directional
aerial and a unidirectional aerial were to transmit signals of the same power, the
former would send out the energy equally in all directions whereas the latter
would concentrate it as a beam in a one direction). It is therefore obvious that at
a given distance inside the beam, the field strength of a unidirectional aerial will
be a number of times stronger that the field strength of an omni-directional
aerial.

As per performance standards for navigational radar(IMO), the scanner must

rotate at a constant RPM of not less than 20, and also start and stop in relative
wind speeds upto 100 knots. The scanner motor situated just under the scanner
is, therefore, very powerful.

The scanner should never be painted, as ordinary paints would alter the surface
characteristics and cause severe attenuation at the scanner.

Radar scanners are of five types – parabolic plate, parabolic mesh, cheese, double
cheese and slotted waveguide. On modern merchant ships, only slotted
waveguide scanners are used.

USE OF VARIOUS BASIC CONTROLS :

Anti-clutter Sea & Rain – This is also called clutter suppression or swept gain
control or STC (sensitivity – time control).

In a slight sea, there will be indications of sea-echoes on the PPI, around the
centre spot, up to a range of about 3 to 4 miles. Paint on the PPI caused by sea-
echoes is called clutter. The clutter area will be roughly oval in shape, symmetrical
about the wind direction, with the greater part towards windward. This is because

5
a wave presents, in the radar pulses, a near- vertical front profile (when
approaching the ship), and a smooth and sloping rear profile (when going away).

The clutter echoes will change positions with every rotation of the scanner. As the
sea gets more and more rough, the clutter echoes will increase in density. Ina very
rough sea, the clutter echoes may saturate the central part of the PPI (join
together and form a bright patch around the centre spot, in which area even
echoes of large targets cannot be distinguished). Even under moderate
conditions, echoes of small targets such as buoys, boats, etc tend to get drowned
or swamped by clutter (become indistinguishable against so many clutter echoes).
If the clutter echoes were suitably reduced in number and brightness, the smaller
targets would become distinguishable on the PPI. This is the object of the anti-
clutter circuit.

Modern radar sets are provided with automatic clutter control (ACC). This is
based on the fact that clutter echoes from sea and rain are random echoes
whereas target echoes are systematic. When the ACC is switched on, the echoes
received from one pulse are compared with the echoes received from the earlier
pulse. If they are inconsistent (do not agree in echo strength and time of arrival),
they are not fed to the CRT. Hence most clutter echoes are rejected, without loss
of target echoes. ACC is superior to manual clutter control because it provides the
correct level of gain for nearby targets, regardless of the varying clutter density to
windward and leeward. Furthermore, the ACC automatically adapts to changes in
sea conditions unlike the manual clutter control which has to be frequently
adjusted.

When entering harbor or when sailing very close to land, it is advisable to switch
over from ACC to manual clutter control as strong echoes from targets ashore
may result in over suppression and consequent loss of small targets.

Range Rings – These are also called calibration rings when required, equidistant
blips appear on the trace. Since these blips occur on every trace, they join up in
azimuth and appear as concentric equidistant circles, called range rings, each

6
representing a definite value of range, depending on the range scale in use. The
brightness of these rings can be varied, as desired, by the observer.

Variable Range Marker (VRM) – A single blip is painted on the trace. The blips of
each successive trace join together in azimuth and form a circle,. The radius of
this circle can be varied, at will, by the observer. The value of the radius of the
VRM, in nautical miles and decimal of a mile, is indicated by a digital display. The
brightness of the VRM can be adjusted, at will, by the observer.

Electronic Bearing Line (EBL) – This is also called the electronic bearing marker
(EBM) or electronic cursor. It consists of a radial line that is made to appear on
the PPI when desired. The EBL does not flash, like the heading marker, when the
rotating trace passes over it. The EBL can be rotated by a hand control and made
to pass through any target on the PPI. The angle between the EBL and the heading
marker (the relative bearing) can be read off a digital display. When the display is
gyro-stabilised, the digital display of the EBL gives gyro bearings.

The EBL control consists of 3 parts :

1. a Brilliance control for the EBL,

2. a rotary knob to rotate the EBL in azimuth as desired,

3. a digital readout which indicates the bearing – relative bearing in the case of an
unstabilised display and gives gyro bearing in the case of a gyro – stabilized
display.

Electronic range and bearing line (ERBL) – ERBL is a line whose direction and
length correspond to the EBL and VRM. Two ERBLs are provided on modern ships.

The ERBL may be used in several modes –

1. Attached to own ship mode,

2. Detached from own ship static mode,

3. Detached from own ship dynamic mode.

7
Tuning – This control is provided to manually alter the frequency of the LO so as
to bring the frequency of the IF signals to the correct value required by the IF
amplifier.

The fine tuning control is fitted on the display unit, for use by the radar observer
as and when necessary. Where an AFC (automatic frequency control) circuit is
provided, fine tuning by the observer would not be necessary.

A meter or a magic eye gives indication of correct tuning. If the tuning indicator is
defective, tuning may be carried out while watching the PPI. Correct tuning is
reached when maximum target echoes or clutter echoes are seen, or when the
length of the performance monitor signal is greatest.

Gain – Gain controls the amplification of all echoes received. If gain is set very
low, no targets would show up on the PPI owing to insufficient amplification. If set
slightly low, targets that return weak echoes (such as buoys 2M away or a cluster
of high rise buildings 30 to 40M away) would not show up on the PPI.

If gain is set slightly high, ‘receiver noise’ (amplifier-created disturbances that

resemble clutter) would be visible in all parts of the PPI, resulting in less contrast
between targets and background. However, this is safer than setting gain low. If
set too high, receiver noise would saturate the screen.

Correct setting of gain is achieved by increasing it until a speckled background of

noise is just visible while on a medium or long range scale.

Brilliance – This is also called Brightness. The setting of brilliance depends on the
amount of external light falling on the PPI. For correct setting, ensure that the
gain, anti-clutter and anti-rain-clutter controls are at zero (anti-clockwise).Turn up
the brilliance until the rotating trace just appears. Then turn down the brilliance
until the rotating trace just disappears.

Range Selector – This switch gives the observer the choice of range scale. The
range scales to be variable, as per IMO Marine radar Performance Specs. Are
0.25, 0.5, 0.75, 1.5, 3, 6, 12 and 24 N.miles.

8
Standby – When the radar observer desires to switch off the set temporarily in
order to prolong its working life, and yet have the facility of obtaining a picture at
an instant’s notice, he may put the set on standby, leaving all the basic controls in
their correct settings.. When a picture is required the standby switch is put ON (or
to TRANSMIT, as the case may be ) and a picture appears instantaneously, already
adjusted and ready for use. If manual control of anti-clutter is in use, it may be
necessary to re-adjust its setting, in case the weather situation has altered during
the period that the set was on standby.

Pulse Length Selector – it is a switch marked ‘short/long’ and gives the observer a
choice of pulse length. Longer pulses mean greater detection ranges of all targets
and it is especially useful when making land fall after a long ocean passage. In
some radar sets, the PL selector switch is incorporated in the function switch that
is marked ‘off/standby/short pulse /long pulse’.

Heading Marker On/ Off – This switch is provided such that the HM, which is
always showing on the PPI when the radar is operational, can be momentarily
switched off because of inability to track some weak echo on the HM line. So
when this spring loaded switch is switched on the HM goes off and when left, the
HM is shown back on the screen.

Performance Monitor – This switch is used for checking the overall efficiency of
the radar set. When switched on and at a low range scale of say 1.5 NM, with
Gain set normal and both anti-clutter and differentiator set low, the length of a
Plume or Radius of the Sun (either of which may show on that radar) can be
measured and this with respect to earlier measurements of maximum lengths can
be used to check the Relative Efficiency of the set.

Racon & SART – Refer Pg. 49 & 50 of Capt. Subra’s Book.

2.3 - THE CHARACTERISTICS OF RADAR SETS :

Bearing/ Range accuracy – Bearing accuracy as per Performance Standards for

Marine Radar (IMO) should not be more than +/- 1 deg.

9
Range accuracy as Per the IMO standards measured by Range rings or VRM
should not exceed 1% of the max. range of the range scale in use or 30mtr

Whichever is greater.

HBW – When the trace is rotating around the PPI, the half power points start
before and after, between the leading and trailing edges of the trace. This is half
the beam width from when the trace begins striking the target to half the beam
width when the trailing edge of the trace leaves the target. So, if the beam width
is AB in reality, it will show half ahead and half behind of AB’s width too.

VBW – VBW is the vertical angle at the scanner contained between the upper and
lower edges of the radar beam. The upper and lower edges of the beam are taken
to be the lines joining the half-power points above and below centre of the beam.
If the VBW was too small, targets would be missed due to rolling and pitching. If
the VBW was too large, the radar energy sent out through the scanner would be
spread out over a large vertical angle which means a decrease in the intensity of
the beam.

Pulse Length (PL) – PL is the time taken for a pulse to leave the scanner , i.e., the
interval between the instant the leading edge of the pulse leaves the scanner and
the instant the trailing edge does so. PL is therefore, usually expressed in micro-
seconds but, the speed of radio waves being taken to be 300 mtrs/micro second.
PL may also be expressed in metre, if and when required to do so. PL, also
referred to as PW(pulse width), is controlled by the transmitter.

When an echo returns from a target, it will be the same length as the pulse. When
the leading edge of the echo enters the receiver, the tracing spot on the screen
becomes fat and bright and remains so until the entire echo comes in. When the
trailing edge has come in, the tracing spot reduces to its original size.

To ensure range accuracy, the tracing spot is synchronised with the leading edge
of the pulse. Hence the correct range of a target is the range of the nearest edge
of its paint on the screen.

10
Pulse Repetition frequency (PRF) – PRF is the number of pulses sent out through
the scanner in one second. The unit, if used is Hertz. Commercial marine radar
sets usually have two or three values of PRF, between 500 and 4000. PRF is also
referred to as Pulse Recurrence Rate (PRR).

A high value of PRF is preferable for a clear and detailed picture (good picture
resolution). On longer range scales, this is not possible because a greater interval
between pulses is required, for each pulse to go long distance and come back,
necessitating a low PRF. Longer range scales therefore have a low PRF while the
shorter range scales have a high PRF.

Wavelength – WL of a commercial marine radar set may be about 3 cm (9200 to

9500 MHz called the X Band) or about 10 cm (2900 to 3100 MHz called the S
band). Radar pulses of different wavelengths are influenced differently by
external factors. Hence the wavelength of a radar set directly affects its
performance. After a radar pulse has left the scanner, its path of travel and
energy content are influenced by two main factors –attenuation and diffraction.

Thus, when using the S band, attenuation in the atmosphere is less and diffraction
is more than when using the X band. That is why a cliff 60 miles off may be
detected by radar using S band but may not be detected by X band of same
power. S band is thus good for long range detection.

2.4 : SOME LIMITATIONS OF A RADAR :

Range Discrimination – It is the ability of a radar set to clearly distinguish two

small targets, on the same bearing and slightly different ranges, as two separate
targets on the PPI. The factor that governs this is the PL which causes all the paint
to expand radially outwards by ½ PL in meters.

Considering 2 small targets that are on the same bearing and close to each other,
the paint of the nearer target would expand towards the other , on the PPI, by ½
PL in meters. If the distance between the two targets is equal to or less than ½ PL,
their paints would merge on the PPI and show as if they were one target. If the

11
targets are further apart than ½ PL, they would paint as two separate targets on
the PPI.

Bearing Discrimination – It is the ability of a radar set to clearly distinguish two

targets, on the same range and slightly different bearings, as two separate targets
on the PPI. The governing factor is the HBW of the set.

HBW causes all targets on the PPI to expand in azimuth by ½ HBW on either side.
The paints of the two targets, on the same range and slightly different bearings
would, therefore, expand towards each other by a total of one HBW. If the angle
subtended at the scanner, by the closer edges of the two targets, is equal or less
than HBW their paints would merge on the PPI and they would appear as one big
target. If the angle so subtended is more than HBW, they would paint as two
separate targets. Bearing Discrimination is thus usually expressed in degrees and,
as per Performance Standards for Navigational Radar (IMO), it should not exceed
2.5 degrees.

Minimum Range – The minimum detection range of radar set depends on –

a) The pulse length

b) The de-ionisation delay

c) The VBW and the height of the scanner

d) The wavelength

As per Performance standards, the minimum detection range, with a scanner 15m
high , shall not exceed 40 mtrs.

Maximum Range – The maximum range of a radar set depends on the following
factors :

a) Height of scanner

b) Power of the set

c) Wavelength
12
d) Pulse repetition frequency

e) Pulse length

f) VBW and HBW

g) Receiver sensitivity

h) Nature of target

i) Weather effects

j) Anomalous propagation

k) Sea and swell.

Range Accuracy – As per Performance Standards the error in range obtained by

using Range rings or VRM should not exceed 1% of the max. range scale in use or
30 mtrs. , whichever is greater. Range accuracy depends on :

a) Correct syncronisation

b) Uniformity and rectilinearity of the time base

c) The scale / size of the tracing spot

d) Height of scanner

Bearing Accuracy – As per Performance standards, the radar bearing of an object,

whose echo appears on the edge of the display, should be capable of being
measured with an accuracy equal to, or better than +/- 1 degree.

2.5 : SITING OF COMPONENTS AND SAFE DISTANCES –

Location of Scanner and its effect : The Radar Scanner on a ship may be
considered under 4 headings – Vertical positioning, transverse positioning,
Longitudinal positioning and other factors.

a) Vertical positioning – This refers to height of the scanner above sea level and
also height of scanner with respect to other shipboard objects. It has been
13
observed that a scanner height between 12mtrs and 18mtrs above sea level gives
best all –round radar performance.

b) Transverse positioning – The Scanner and the Transceiver unit should be as

vertically aligned as possible to keep the length of the waveguide, and the
number of bends in it to a minimum so as to avoid undue attenuation in the
waveguide.

c) Longitudinal positioning – On very long ships subject to unusually great trim by

the stern, the foc’sle may tend to obstruct the radar beam, especially during
pitching. In such cases, it may be advisable to fit the scanner on top of the
foremast and the transceiver may also be positioned close to the scanner in order
to keep the waveguide in line with the scanner.

LOCATION OF DISPLAY UNIT : The concerned issues are –

a) Magnetic safe distances – The concerned safe distances from Magnetic

compasses should be maintained.

b) Lighting – At night, the light emanating from the display unit may cause a
viewer to have difficulty in adjusting to the dark view outside. So appropriate
quick and easy adjustment should be possible.

c) Viewing ability from site – While views from screen to the external view should
be simple, it should also be ensured that direct sunlight does not fall on the
display.

d) It should be possible for atleast 2 officers to be able to view the screen at one
time.

e) The display should be so located that clear view screens, electric telegraph,
fans etc. should not interfere with it in case of sparking issues.

LOCATION OF TRANSCEIVER UNIT : It should be bulkhead mounted and at

eyelevel for easy maintenance. It should be as directly located below the scanner
as possible. It should be at safe distance from the magnetic compasses.

14
SPARE PARTS LOCATION : Spare parts especially spare Magnetrons, should be
safely located away from magnetic compasses and in a specially designed box.

2.6 – CHARACTERISTICS OF A TARGET WHICH INFLUENCE ITS RANGE –

a) Height above sealevel – Other things being equal, higher objects are detected
further away than lower objects.

b) Horizontal size –The larger the horizontal size of the target, the greater the
echoing surface and better the detection range, other things being equal.

c) Composition – Hard substances are better reflectors than soft substances.

d) Nature of the surface – Smooth and rough surfaces can have varying degrees of
reflectivity to radar waves.

e) Aspect – The Aspect is the angle from which an object is viewed. For plotting
purposes, it is the angle between the target ship’s head and the theoretical line of
sight, expressed from 0 deg. To 180 deg. ‘Red or Green’, i.e. Own ship’s Relative
bearing as seen from the target expressed Red or Green from 0 deg. To 180 deg.

When the aspect is 90 deg. R or G, the echo is much improved than at 0 deg. Or
180 deg.For other ships around own ship. The superstructure also plays a large
role.

For land targets, the word ‘aspect’ is viewed in the literal sense,i.e. angle of view.
A land target may be steep on one side and gently sloping on the other. When the
steep side is viewed by radar, good detection range is obtained while when the
aspect is changed and now the other side is viewed, the detection range is
considerably less.

15
2.7 : WEATHER EFFECTS ON RADAR PERFORMANCE :

When radio waves pass through the atmosphere, some of the energy is lost due
to absorption, scattering, diffraction, etc. Such loss of energy is termed
attenuation in the atmosphere. Weather phenomena such as drizzle, rain, hail ,
snow, fog, etc., cause varying amounts of attenuation. Attenuation due to
weather effects causes loss of echo strength and consequent decrease in
detection ranges of targets.

a) Drizzle – Drizzle means small droplets of water, less than 0.5 m.m.diameter
which falls towards the ground. Detection ranges of targets within or beyond the
drizzle area are not much affected. Targets within the drizzle area generally show
up clearly.

b) Rain –Drops of falling water, larger than 0.5 m.m. diameter, is called rain.
Rainfall areas show up clearly on the PPI. Targets inside the rainfall area may be
distinguishable by use of the differentiator.

In a heavy tropical downpour, the rainfall area appears as a bright solid block
inside which targets cannot be distinguished. They may easily be mistaken for
land echoes because of their large size, bright appearance, clearly defined edges
and regularity in painting.

c) Hail – Hail stones give echoes on the PPI, large stones being stronger than small
stones. Rain in the hail being less stronger, attenuation due to hail is much lesser
than due to rain.

d) Snow – If snow falls in single crystals, such echoes are not troublesome on the
PPI as against snow crystals joining together and falling in large flakes, in which
case the echoes show up on the PPI as rain. As snow precipitation is lesser than
that of rain, attenuation due to snowfall is much lesser.

e) Fog – Though echoes from fog particles are negligible, attenuation may be
severe. In cold climates, dense fog will appreciably decrease the detection ranges

16
of all targets. In warm climates however, the detection ranges are not much
affected.

f) Sand storms – These are common in the Red Sea, Persian Gulf, etc. Though they
greatly reduce optical visibility, no adverse effect on radar performance has been
noticed.

2.8 : SHADOW AREAS, SHADOW SECTORS, BLIND SECTORS AND THEIR

MEASUREMENT OF LIMITS –

a) Shadow areas – When the transmitted radar pulses strike a large target, they
are reflected – some of the energy comes back to the scanner, most of the energy
gets reflected elsewhere. A very limited amount of energy may go beyond the
large target because of diffraction. Targets lying in such areas, directly behind
large targets are said to lie in ‘ Shadow areas ‘. Only very radar conspicuous
objects, in Shadow areas show up on the PPI, that too with very reduced echo
strength.

b) Shadow Sectors –Shipboard structures such as masts, Samson posts, etc. partly
obstruct the radar beam. Because of diffraction, targets directly beyond them do
appear on the PPI, but their detection ranges are considerably reduced. Such
spaces are hence known as ‘Shadow sectors’ and they are recorded, in the radar
log, by the relative bearings of their extremities, as seen on the PPI.

c) Blind Sectors –Some shipboard structures such as the funnel may completely
obstruct the radar beam such that targets beyond them are not detected at all.
Such targets are said to be in the ‘blind sector’ of the scanner because no echoes
are received from them. Bad siting of the scanner causes this.

d) MEASUREMENT OF LIMITS OF SHADOW AND BLIND SECTORS :

Shadow and Blind sectors can be measured in two ways –

1. When there is a fair amount of wind, if the gain is reduced such that clutter
echoes are only just visible, shadow sectors ( and blind sectors ) will be

17
conspicuous by the total absence of clutter in them. If the gain is increased,
clutter will appear in shadow sectors also, but not in blind sectors.

2. By observing a small target such as a buoy, using very little gain while swinging
the ship around. The paint will disappear when it enters a shadow sector ( also a
blind sector ) and reappear when it leaves such a sector. If the gain is kept normal,
the target would show up in shadow sectors also but not in blind sectors.

The radar logbook should contain the relative bearings of the extremities of all
shadow and blind sectors.

18