radar presentation 02

8/14/2019 Radar Presentation 02

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Simple block diagram of a radar system


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1-TRANSMITTER FUNCTION

The fuction of transmitter is to generate pulses of

electromagnetic energy having the correct

repetion frequency,

length,

shape,

power and

radio frequency.

WAVEGUIDEThe pulses normaly travel to the aerial by way of

hollow cooper tubing which has precisely machined

rectangular or circular cross-section and is known

as waveguide


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Range measurement:Timing must commence at theinstant of transmision. This is achieved by using the

same pulse to inititate transmission as is used to start

the corresponding trace on the CRT display.

This pulse is known as synchronizing pulseortrigger.

The line connecting the transmitter to the receiver

also carries the trigger Pulse to inititate sea cluttersuppression.


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Range scale

and pulse lenght

selector

Modulator and

pulse formingnetwork

Trigger generator magnetronModulator

pulse

Trigger

Pulse length

selectionPRFselection

RF pulse

to T/R cell

The transmitter a block diagram

Transmitter


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1- The trigger generator: controls the repetion

frequency of the transmitted pulses

2- The modulator and associated pulseforming

Network : defines the length, shape and powerof the transmitted pulse.

3- Magnetron: determines the radio frequency of the

pulse which travels up the waveguide to the areial


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The pulse repetion frequency:

PRF is the number of the radar pulses transmitted

in one second. Trigger generator controls the PRF

Typical values. 3400, 1700 and 850 pulses per second

Long ranges..........Low PRF

Short ranges..........High PRFSelection is automatic


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The pulse length

Pulse length is the duration of the transmitted

radar pulse and is usually measured in microseconds.

--Receiver s can amplify long pulses more effectively

than short pulses.

--- shorter the pulse length better the detail

Short pulse ---short ranges

Long pulse ---long distance


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PRF nad Pulse Length, some representative values


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features

Short pulseLong pulse

Long range target

detection

Poor. Use when short

range scales are selected

Good.Use when long


and for poor responsetargets at short range

Minimum range Good.use when short


Poor. Use when long


and min range is not a

mjor consideration

Range discrimination good Poor

Effect on echo paint Produces a well defined

picture when short range

scales are selected

Effect on sea clutter Reduces the probabality

of the masking of targets

due to saturation

Increase the probabilty

of the masking targets

due to saturation.

Effect in

precipitation

Reduces the probability

of the masking of targets

due to saturation

Increase the probabilty

of the masking targets

due to saturation.also

long pulse will increase

the probability of

detecting targets which

lie beyond rain


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The power of the transmitted pulse

The range at which a target can be dedected is

dependent on the power of the transmitted

pulse.

The theoretical max. Radar dedection range is

limited by the tranmitter power.

For small craft radar systems a

transmitterpower of 3 kw and on merchant

vessels 10 to 60 kW is used


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The radio frequency of the transmitted pulse

Two groups of radio frequencies are allocated for

use by civil marine radar systems.

One group lies in the X-band and include

frequencies which lie between 9300 and 9500

MHz. Wavelength is app. 3 cm.

Second group lies in the S-band and includefrequencies which lie between 2900 and 3100

MHz. Wavelength is app. 10 cm


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X-band and S-band compared


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2- The Aerial functionAerial, scanner and antenna are all same names

commanly used to describe the device which radiates

the radio energy into space and intercepts the returning

echoes.

Its construction defines the power distribution of the

radar beam in both the horizontal and the vertical

planes.

Horizontal limits must be narrow also the vertical beam

must be Wide.


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To achieve 360of azimuth coverage the scanner is

rotated continously and automatically in a

clockwise direction.

A signal representing this rotation is relayed to the

PPI display.


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3- the receiver function:the fuction of the receiver is to amplify the very weak

echoes intercepted by the aerial so as to generate

pulses whose form and power will produce a visible

response on the screen of a cathode ray tube.( orprovide a suitable input for digital storage)

A single aerial is used for transmission and

reception. Also the waveguide is common to both

trnasmitter and receiver


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The powerful pulses generated by the transmittermight be able to pass directly into the

receiver???????

The receiver is protected from this by T/R cell.

T/R cell blocks the input to the receiver during

transmission.


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Inputs to the receiver:

1-received signals from the aerial and

2-the trigger pulse to initiate sea clutter

suppression

output from receiver: amplified signals to the

display


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4-The display function

The prime function of the display is to indicate the

presence of dedectable objects by generating, on the

screen of the cathode ray tube, a visible response whoseangular and radial position with respect to the heading

line and the origin of the trace are representative of the

bearing and range, respectively, at which the

corresponding target lies.


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Display generates the radar picture


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Trigger/

Echo signal

Heading marker

Aerial rotation

transmitter

receiverAerial

Aerial


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1-the trigger: Ensures that each trace commences

at the instant of a transmision.

2- The amplified echoes:Each echo on any giventrace produces a brightening of the spot on the

CRT at an elpased time after the transmission

which initiate the trace. The lenght of the elapsed

time is proportional to the range of the target.


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3- The rotation signal: This signal causes the trace to

rotate at the same angular rate as the aerial.

4- The heading marker signal: This generates the

brightened radial indicator corresponding with theforward direction of the ships fore and aft line , thus

providing a refernce from which bearings can be

measured


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Time relation between signals are very important.

1 and 2 recur several thousand times per second

3 and 4 have a period of afew seconds


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The radar picture is being presented on the screen of a

cathode ray tube .


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*Electrons are emitted at the narrow end of the tube

*formed into a very narrow beam which strikes the inside

of the screen at the other end and

*there produces a spot of light whose brightness, size and

position may be controlled. The generation and control of

the spot is best described by considering the CRT tocomprise three major elements:


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1- The electron gun which emits the electron stream.

2- The coated screen which reacts to produce light when

struck by the electrons.

3 -The deflection system which controls the location at

which the beam strikes the screen.

Each of these elements will now be considered in turn.


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The electron gun

An electron is the fundamental particle which carries

unit negative electrical charge. The electron gun is an

arrangement of cylindrical metal components which areknown as electrodes.

The electrodes are located along the axis of the tube and

their function is to liberate, accelerate and concentrate the

electrons into a narrow beam which is fired toward the

screen.


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The electrode which emits the electrons is called the

cathode and it is from this that the tube takes its name.

The various electrodes which attract the electrons towards

the screen are called anodes. These are maintained at very

high positive potentials with respect to the cathode in

order to accelerate the electrons to speeds of tens of

thousands of kilometres per second.

The strength of the electron stream is controlled by an

electrode known as the control grid, normally referred to

merely as the grid.


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The cathode is a metal cylinder closed at the end nearer

the screen and is coated on the outside with an oxide

having an atomic structure such that electrons are emitted

from the surface at a relatively low level of heat. The

emission is promoted by heating the cathode indirectly as a

result of passing electric current through a wire filament,

known as the heater, which is located inside the cathodecylinder.


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The electrons emitted by the cathode form a cloud in its

vicinity known as the space charge. As the cloud builds

up, the loss of negative charge to the cloud renders the

cathode positive with respect to the cloud; hence someelectrons tend to be recaptured. However, at any given

temperature a state of dynamic equilibrium will be

reached in which as many electrons are leaving the cloud

as are joining it and thus a space charge of constantpopulation will surround the end of the cathode.


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Electrons are drawn from this cloud and accelerated

toward the screen to form the electron beam.

Returning echoes, after being amplified by the receiver,are applied to the cathode as a pulse of negative polarity

which will produce an increase in the strength of the beam

and hence a brightening of the spot.


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The grid is cylindrical in shape with one end closed except fora small hole which constrains electrons travelling along or

close to the tube axis to pass through and emerge in the form

of a convergent beam. It is located between the cathode and

the first anode but closer to the cathode . The grid potentialcan be adjusted by the observer, using the brilliance control,

and will be a few tens of volts negative with respect to the

cathode.

radar presentation 02

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