radar arpa (midterm) Flashcards
may occur in the vicinity of another shipborne radar
operating in the same frequency band. It is seen on the screen as a number of
bright
spikes either in irregular patterns or in the form of usually curved spoke-like dotted
lines extending from the center to the edge of the picture.
Mutual radar interference
magnifies not only small target pips but also returns (clutter) from sea surface, rain and radar interference
echo stretch
track both the closet and most distant edges of the echo. Clutter can be acquired and tracked as targets. Adjust the [A/C RAIN] control. If it is heavy rain, switch to S-band if provided, or switch on the interference rejector on the radar.
Electronic circuits
If interference is extreme due to another radar operating at close range, _ may appear momentarily.
spiral “dotting” and/or false targets
Targets detected by the side-lobes of the antenna beam pattern are called
side-lobe echoes.
are caused by reflections from the side lobes of the radar beam. They are likely to appear when a target is a good radar reflector and in range of the weaker side lobes.
Side echoes
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.
are caused when some of the outgoing radar energy is reflected from
an object close to the scanner such as a funnel or mast. The echo may return by the
same path or directly to the scanner. The false echo will appear (usually intermittently)
on the display at the correct range because the additional distance between the
scanner and the reflecting object will be negligible, but on the bearing of the
obstruction. The true target will also appear on the display at the correct range and
bearing.
Indirect echoes
usually occur in shadow sectors however, they can appear on bearings
where there are no shadow sectors. Indirect echoes are usually associated with
funnels and other large objects close to the scanner
Indirect echoes
are caused when a strong echo arrives back at your vessel and
bounces off it, effectively retransmitting the signal. For this to occur the other target
must be large and close, and both the target, which may be a land target such as a
bridge or headland or another vessel, and your ship must be good radar reflectors. The
false echoes (which may be any number) will appear at multiples of the true range on
the same bearing.
Multiple echoes
Multiple echoes can be removed by
reducing the gain
In certain situations, echoes from very distant targets may appear as _ on the screen. This occurs when the return echo is received
one transmission cycle later, that is, after a next radar pulse has been transmitted
false echoes
(second trace echoes)
will appear at times during periods of severe super-refraction
or ducting. Targets at very long range will appear at a false range on the correct
bearing, on the second sweep of the time base.
Second trace echoes
the more likely that second trace echoes will occur as
more pulses are transmitted and the corresponding silence period is reduced.
Second trace returns can usually be made to disappear by changing the range
scale in use.
The higher the PRF,
rotates to scan the entire
surrounding area. Bearings to the target are determined by the orientation of the
antenna at the moment when the reflected echo returns
The radar antenna (called the scanner)
Radars transmit directional microwave radio pulses with a _ . It detects the bearing and range of echoing pulse
returns from significant surrounding targets to produce a map like display.
rotating ariel (the scanner)
in a 360o circle around the machine
The radar will have some form of
visual signal to count down this wait period, the approved best standard being _ . The radar can then be switched to ‘transmit’ and on some sets a short or long pulse
can be selected at this time, normally long pulse would be selected. A long pulse will be more
likely to show an echo from a weak target or a target at a longer range. A short pulse will
achieve better definition on short ranges.
within 120
seconds
power/standby/transmit switch usually has three positions.
radar on, radar stand-by, aerial rotating
corresponds to the number of transmitted pulses in
one second. The unit is in Hz or PPS
The Pulse Repetition Frequency
two other facets related to PRF that the designer must weigh very carefully;
the beamwidth characteristics of the antenna, and the required periodicity with which the radar
must sweep the field of view.
radar with a 1° horizontal beamwidth that sweeps the entire
360° horizon every 2 seconds with a PRF of
1080 Hz will radiate 6 pulses over each 1-degree arc.
can be compared to the tuning control of an ordinary radio, in that
it tunes the receiver to the frequency of the transmitter. Poor tuning adjustment may
not be easily recognized 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 frequent fine tuning of the set. Not all sets have a tuning control.
tuning control
may appear to function like the brilliance control in that it makes the
picture brighter or darker, but it is completely different so it is vital not to confuse the
two. Gain affects the receiver and not the display as the brilliance does. Turning up the
gain increases the amplification of the incoming signal, making weak echoes look
stronger, but confusing the display with background speckle or noise, similar to the
background crackling of an ordinary radio. Turning down the gain will reduce the
sensitivity of the receiver and reduce the noise, but care must be taken not to overdo
as weak or distant echoes can be lost.
gain control
is a non-saturating amplifier that does not ordinarily use any
special gain-control circuits. The output voltage of the logarithmic amplifier is a linear
function of the input voltage for low-amplitude signals. It is a logarithmic function for
high-amplitude signals. In other words, the range of linear amplification does not end at a
definite saturation point, as is the case in normal IF amplifiers. Therefore, a large signal
does not saturate the logarithmic amplifier; rather, it merely reduces the amplification of a
simultaneously applied small signal.
logarithmic amplifier
The radar beam will bounce echoes off the sea around the ship, particularly if the
weather is a little rough. This result will be a bright sunburst pattern in the middle of
the screen which will be more pronounced in the upwind direction. You could reduce
this by turning down the gain, the down side to that solution however, is that the
echoes of more distant targets will be lost as well.
The solution is the
sea clutter control.
will reduce the interference on the screen due to the rain and
increase the chance of seeing targets within rain showers. The effect on returning
echoes from rain on the screen is usually no more than a transparent smear, looking a
little like cotton wool, but it can be dense enough to conceal other echoes within the
shower. In a tropical downpour however, the rain can completely block out all echoes,
at times requiring the operator to stop the vessel.
rain clutter control
on an analogue radar (old style) controls the brightness of the
rotating trace and will also affects the brightness of the displayed echo so it needs to
be adjusted so that the trace itself is just visible, to give a good contrast between echo
and background.
brilliance control
On a raster scan display (new style) the brilliance control regulates the brightness of
the picture _ , making it bright enough for daylight viewing or dim
enough so as not to impair the operators night vision.
(scale illumination)
On the radar screen, the circles that mark fixed ranges. On the 6-mile range, for
example
range rings
An electronic means of measuring the radar range of a target with an expanding range
ring circle
variable range marker
regulates the range at which the set operates by changing the size or
scale of the area on the display.
range control
An electronic means of measuring the radar bearing of a target with a rotating radial
line on the radar screen.
electronic bearing line
can obscure small targets
heading marker and the range rings
Your radars
brilliance should be adjusted until the _ and the screen
can be easily viewed. The examples below show too much, too little and just the right
amount of brilliance.
rotating tracing is barely visible
Gain should be set until _ is observed on the radar screen.
lightly speckled background
use of tuning
tuning indicator, sea echo, or target on extremity of display
rain clutter control can also be known as the _ A heavy rainstorm can complete block out radar targets. The rain
clutter setting solves this issue by ‘thinning out’ the returned targets on the screen. In
effect it only shows the targets with a signal strength above a threshold limit which is
adjusted by altering the rain clutter control. The rain echo will be much weaker than
those returned from a solid object such as another vessel so you can easily remove the
interfering echoes from your radar screen. Be careful when adjusting this clutter
control. If set to high you can also thin out real targets such as other small vessels.
A good use of this control is to reduce the rain clutter just enough to observe vessels in
the area of rain.
Differentiator or the Fast Time
Constant (F.T.C).
This is also known as _. Like rain the returned echoes from the sea are
much weaker than those returned by solid objects or vessels. Sea clutter works by
reducing the gain applied to returned echoes. This reduction is applied from the centre
of the screen outwards. As the sea clutter control is turned up the amount of gain is
reduced gradually outwards from the centre of the screen. Care should be taken when
adjusting sea clutter that you don’t obscure small targets.
swept gain clutter
increased the amplification of the returned signals. It can make a weaker echo appear
much stronger. Care must be taken when setting gain. If set too low you will not see
some targets on the radar screen such as small fishing vessels. This clearly has serious
implications for keeping a lookout and collision avoidance. Conversely setting the gain
to high can result in significant clutter which results in the screen becoming
unreadable and targets becoming lost within the clutter.
increased the amplification of the returned signals. It can make a weaker echo appear
much stronger. Care must be taken when setting gain. If set too low you will not see
some targets on the radar screen such as small fishing vessels. This clearly has serious
implications for keeping a lookout and collision avoidance. Conversely setting the gain
to high can result in significant clutter which results in the screen becoming
unreadable and targets becoming lost within the clutter.
Gain
This relates to the brightness of your radar screen
brilliance
correct order of making adjustments and states the criteria for
optimum setting of the controls
initial setup, brilliance, gain, tuning, rain clutter, sea clutter
display controls
(brilliance, illumination, focus, shift, range selector, range
rings, VRM, EBM, mechanical cursor, heading marker, clear scan, anti-clutter)
receiver controls
(tuning, gain,
linear/logarithmic gain, sensitivity time control, fast time control)
transmitter controls
(standby,/transmit, pulse length, PRF
main controls
(power, antenna)
is random, it is not three bang correlated,
and it is filtered out and not classified as an echo
receiver noise
is sensitive to the range of frequencies being transmitted and
provides amplification of the returned signal. In order to provide the greatest range, the
receiver must be very sensitive without introducing excessive noise. The ability to
discern a received signal from background noise depends on the signal-to-noise ratio
(S/N).
receiver
is displayed on modern indicators through the use of a microprocessor
computing target true motion rather than depending on an extremely long persistence
phosphor to leave “trails
True motion
radar displays own ship and moving objects in their true motion
relative motion radar, own ship’s position is not fixed on the PPI. Own ship and other
moving objects move on the PPI in accordance with their true courses and speeds. Also
unlike relative motion radar, fixed objects such as landmasses are stationary, or nearly so,
on the PPI. Thus, one observes own ship and other ships moving with respect to
landmasses
True motion
true motion radar display is _ With this
stabilization, the display is similar to a plot on the navigational chart. On some models
the display orientation is Heading-Upward. Because the true motion display must be
stabilized to an unchanging reference, the cathode-ray tube must be rotated to place the
heading at the top or upward.
stabilized with North-Upward.
displays the motion of a target relative to the motion of the observing (own)
ship. With own ship and the target in motion, the successive pips of the target
do not indicate the actual or true movement of the target. A graphical
solution is required in order to determine the rate and direction of the actual
movement of the target.
If own ship is in motion, the pips of fixed objects, such as landmasses,
move on the PPI at a rate equal to and in a direction opposite to the motion of
own ship. If own ship is stopped or motionless, target pips move on the PPI
in accordance with their true motion.
relative motion radar
_ display, the target pips are painted at their measured distances in
direction relative to own ship’s heading. In the
HEADING-UPWARD
_ display, target pips are
painted at their measured distances in true directions from own ship
NORTH-UPWARD
North-Upward display in which the orientation of the display is fixed to an
unchanging reference (north) is called a
STABILIZED display
Heading-Upward display
in which the orientation changes with changes in own ship’s heading is called an
UNSTABILIZED display
different types of display mode
- CHOICE OF RANGE SCALE,
- RANGE MEASUREMENT,
-BEARING MEASUREMENT,
-GAIN,
-REDUCING SEA CLUTTER / RAIN,
-OF CENTRE DISPLAY,
-TARGET TRAILS,
-PI (PARALLEL INDEX) LINES,
-HEADING/SPEED/COURSE,
-BRILLIANCE,
-WATCH ALARM,
-VECTOR MODE,
-PAST POSITION,
-MARK,
-TARGET TRACKING/ AIS DATA BOX, - PRESENTATION MODES
RANGE MEASUREMENT: Measurement of range to a target can be achieved either by
the
fixed range rings or the Variable Range Marker (VRM)
CHOICE OF RANGE SCALE: Appropriate range scales should be used depending on the
prevailing circumstances and conditions of the environment the ship is in. Where two
radars are used,
one radar can be kept on a longer range scale to obtain advance warning
of the approach of other vessels, changes in traffic density, or proximity to the coastline.
BEARING MEASUREMENT: _ is used to take the bearing of
targets.
Electronic Bearing Lines
is used to adjust the sensitivity of the radar. It
should be so adjusted that the background noise is just visible on the screen. In simple
words, if the _ is set too low, weak echoes may be missed while excessive sensitivity
yields too much background noise.
gain control
If _ is set too low, targets will be
hidden in the clutter whereas if set too high can cause targets to disappear from the
radar screen.
rain or sea clutter
: Own ship position can be displaced to expand the view field
without switching to a large range scale.
OF CENTRE DISPLAY
can be of great assistance to the radar observer in making
an early assessment of the situation
Target trails
This is a useful method of monitoring cross track tendency.
It helps us to assess the distance at which the ship will pass a fixed object on a particular
course. The index line is drawn parallel to the planned ground track and should touch the
edge of a radar echo of a fixed object, at a range equal to the desired passing distance
PI (PARALLEL INDEX) LINES:
HEADING/SPEED/COURSE: The top right corner of the radar screen display shows the
heading, speed, course, and speed over the ground, own ship position, and the source.
Speed can be entered from a log(STW) or GPS(SOG) or manually
the function of the _ is quite similar to that of BNWAS.
The watch alarm sounds the audio alarm at selected time intervals to help keep regular
watch of the radar picture
watch alarm
enables the officer to mark any prominent target or a point
of particular interest
MARK menu
Radar users must clearly understand what they are seeing. North up
relative motion is the normal default radar display format. Within that relative and true vector and
trails can be selected. T
PRESENTATION MODES:
During a turn when _ will be greater and when speed
estimation is more difficult, the radar observer should recognize that the accuracy of the
ground stabilization may be degraded appreciably
compass errors
uses the principle of Doppler shift to calculate the speed through water.
A wave transmitter is installed at the bottom of the ship which transmits waves at an angle
(usually 60 degrees) to the ship’s keel
Doppler log
Traffic police measures the speed of the moving vehicles using the equipment that works on
Doppler shift. And these measurements are quite accurate. But ship is a different place
altogether. We do not have ideal situations to have the equipment measure as accurately as on
land
Traffic police
