radar presentation 01

8/14/2019 Radar Presentation 01

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RADAR OBSERVING AND PLOTTING

ASST. PROF. DR. CPT. ENDER ASYALI

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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.

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