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8/14/2019 Radar Presentation 01
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RADAR OBSERVING AND PLOTTING
ASST. PROF. DR. CPT. ENDER ASYALI
-01-
Fundamentals of RADAR
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The word RADAR is an acronomy from the words:
RAdio Dedection And Ranging.
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-The scientist Heinrich Hertz demonstrated in 1886 that radio wawes could be
reflected from metallic objects.
-In 1903 a German engineer obtained a patent in several countries for a radio
wave device capable of dedecting ships,but it had very limited range.
-Marconi, delivering a lecture in 1922, drew attention to the work of Hertz and
proposed in principle what we know today as marine radar.
-Although the radar was used to determine the height of the ionosphere in
the mid-1920s, it was not until 1935 that radar pulses were succesfully used to
dedect and measure the range of an aircraft.
-In the 1930s there was much simultaneous but independent development ofradar techniques in Britain, Germany, France and America.
-Radar first went to sea in a war ship in 1937.
-Radar first used in a merchant ship in 1944
HISTORY OF RADAR
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The requirement to carry RADARExtract from regulation 12, chapter V of the IMO -SOLAS (1974)
Convention as amanded to 1983
1- ships of 500 tons gross tonnage and upwards constructed on
or after 1 september 1984 and ships 0f 1600 ton gross tonnage
and upwards constructed before 1 september 1984 shall be fitted
with a radar installation.
2-ships of 10000 tons gross tonnage and upwards shall be fitted
with two radar installation, each capable of being operated
independently of the other.
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The echo principle
-The echo is never as loud as the original blast.
-The chance of detecting an echo depends on the loudness
and duration of the original blast.
-Short blasts are required if echoes from close targets are not to
be drownedby the original blast.
-A sufficient long interval between blasts is required to allowtime for echoes from distant targets to return.
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PRINCIPLES OF RADAR OPERATION
Introduction
Radar determines distance to an object by measuring
the time required for a radio signal to travel from a transmitter
to an object and return. Since most radars use directionalantennae, they can also determine an objects bearing.
However, a radars bearing measurement will be less accurate
than its distance measurement.
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1.2.2 the range as a function of time:
The speed of radio waves is 300.000.000. metres Per second.(161,830 nm per sec)Or 300 metres Per microsecond (metres/s). In one second radar pulse will
travel around the world 7 times.
Let D= the distance travelled by the pulse (metres)
R= the range of the target (metres)
T= the elapsed time (s)
S= the speed of the radio waves(metres/s)
D= S x T
R= (S x T)/2
R= (300x T)/2
R= 150T
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Question:calculate the elapsed time for a pulse to travel to andreturn from a radar target whose range is
a)-50 metres
b)-12 nm
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Answer:
a)-0.33 microsec
b)-148.16 micro sec
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The Timebase
* the elapsed times are of the order of millionth of a second
* beyond the capability of any conventional time measuring device
* so an electronic device known as Cathode ray tube (CRT) is used
*inventors.about.com/library/inventors/ blcathoderaytube.htm
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RANGE
SCALE
(Nm)
TIME BASE
DURATION
(micro sec)
0.75 9.3
1.5 18.5
3 37.0
6 74.1
12 148.224 296.3
48 592.6
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Cathode-ray tube
Special-purpose electron tube in which electrons are
accelerated by high-voltage anodes, formed into a beam by
focusing electrodes, and projected toward a phosphorescent
screen that forms one face of the tube.
The beam of electrons leaves a bright spot wherever it strikes
the phosphor screen. To form a display, or image, on thescreen, the electron beam is deflected in the vertical and
horizontal directions either by the electrostatic effect o
electrodes within the tube or by magnetic fields produced by
coils located around the neck of the tube.
Cathode-ray tubes are used in television sets, computers,and radar displays.
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Signal CharacteristicsIn most marine navigation applications, the radar signal
is pulse modulated. Signals are generated by a timing
circuit so that energy leaves the antenna in very short pulses.
When transmitting, the antenna is connected to the
transmitter but not the receiver. As soon as the pulse leaves,
an electronic switch disconnects the antenna from the transmitterand connects it to the receiver. Another pulse is not
transmitted until after the preceding one has had time to
travel to the most distant target within range and return.
Since the interval between pulses is long compared with thelength of a pulse, strong signals can be provided with low
average power.
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Pulse Length, Pulse Duration, or Pulse Width
The duration or length of a single pulse is called pulse length,
pulse duration, or pulse width.
This pulse emission sequence repeats a great many times, perhaps
1,000 per second. This rate defines the pulse
repetition rate (PRR). The returned pulses are displayed
on an indicator screen.
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The Display
The most common type of radar display usedis the plan position indicator (PPI). On a PPI, the
sweep starts at the center of the display and moves
outward along a radial line rotating in synchronization
with the antenna.
A detection is indicated by a brightening of the
display screen at the bearing and range of the return.
Because of a luminescent tube face coating, the glow
continues after the trace rotates past the target.On a PPI, a targets actual range is proportional to its
echos distance from the scopes center.
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The Radar Beam
The pulses of energy comprising the radar beam would
form a single lobe-shaped pattern of radiation if emitted in
free space. Figure 1303a. shows this free space radiation
pattern, including the undesirable minor lobes or side lobesassociated with practical antenna design.
The beam width depends upon
*the frequency or wavelength of the transmitted energy,*antenna design, and
*the dimensions of the antenna.
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Although the radiated energy is concentrated into a
relatively narrow main beam by the antenna, there is no
clearly defined envelope of the energy radiated. The energy
is concentrated along the axis of the beam.
The most common convention defines beam width as theangular width between half power points.
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0.6-2 DEGRE
30-40 DEGREE
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*For a given antenna size (antenna aperture), narrower beam
widths result from using shorter wavelengths.
*For a given wavelength, narrower beam widths result from
using larger antennas.
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Diffraction And Attenuation
Diffraction is the bending of a wave as it passes an obstruction.
Because of diffraction there is some illumination
of the region behind an obstruction or target by the radar
beam. Diffraction effects are greater at the lower frequencies.
Thus, the radar beam of a lower frequency radar tendsto illuminate more of the shadow region behind an obstruction
than the beam of a radar of higher frequency or shorter
wavelength.
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Attenuation is the scattering and absorption of the energy
in the radar beam as it passes through the atmosphere.
It causes a decrease in echo strength. Attenuation is greater
at the higher frequencies or shorter wavelengths.
While reflected echoes are much weaker than the transmittedpulses, the characteristics of their return to the
source are similar to the characteristics of propagation. The
strengths of these echoes are dependent upon the amount of:
*transmitted energy striking the targets and
* the size and reflecting properties of the targets.
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Refraction
If the radar waves traveled in straight lines, the distanceto the radar horizon would be dependent only on the
power output of the transmitter and the height of the antenna.
In other words, the distance to the radar horizon would
be the same as that of the geometrical horizon for the antenna
height.
However, atmospheric density gradients bend radar rays as
they travel to and from a target. This bending
is called refraction.
Types of refraction:SUB-REFRACTION
SUPER REFRACTION
EXTRA SUPER REFRACTION
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The following formula, where h is the height of the antenna
in feet, gives the distance to the radar horizon in nautical miles:
d =1.22 h
The distance to the radar horizon does not limit the distancefrom which echoes may be received from targets. Assuming
that adequate power is transmitted, echoes may be
received from targets beyond the radar horizon if their reflecting
surfaces extend above it. Note that the distance to the radar
horizon is the distance at which the radar rays pass tangent to
the surface of the earth.
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Factors Affecting Radar Interpretation
Radars value as a navigational aid depends on :*the navigators understanding
*its characteristics and
*limitations.
Some of the factors to be considered in interpretation
are discussed below:
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1-Resolution in Range.The ability of a radar to separate targets close together on
the same bearing is called resolution in range. It is related
primarily to pulse length.
The minimum distance between targets that can be
distinguished as separate is half the pulse length.
Thus, several ships close together may appear as an island.
Echoes from a number of small boats, piles, breakers, or
evenlarge ships close to the shore may blend with echoes
from the shore, resulting in an incorrect indication ofthe position and shape of the shoreline.
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2-Resolution in Bearing.
Echoes from two or more targets
close together at the same range may merge to form
a single, wider echo. The ability to separate targets is
called resolution in bearing. Bearing resolution is afunction of two variables:
1-beam width and
2-range between targets.
A narrower beam and a shorter distance
between objects both increase bearing resolution.
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3-Height of Antenna and Target.
If the radar horizon is between the transmitting vessel
and the target, the lower part of the target will not be visible.
A large vessel may appear as a small craft, or a shorelinemay appear at some distance inland.
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4-Reflecting Quality and Aspect of Target.
Echoes from several targets of the same size may be quite different
in appearance. A metal surface reflects radio
waves more strongly than a wooden surface. A surface
perpendicular to the beam returns a stronger echo thana non perpendicular one. For this reason, a gently sloping
beach may not be visible. A vessel encountered
broadside returns a stronger echo than one heading directly
toward or away.
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5-Frequency.
As frequency increases, reflections occur
from smaller targets.