Radar Presentation 02

2. THE RADAR SYSTEM OPERATIONAL PRINCIPLES -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. WAVEGUIDE The pulses normaly travel to the aerial by way of hollow cooper tubing which has precisely machined rectangular or circular cross-section and is known 3 as waveguide

Range measurement:Timing must commence at the instant 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 pulse”or “trigger”. The line connecting the transmitter to the receiver also carries the trigger Pulse to inititate sea clutter suppression. 4

Trigger generator

Trigger

PRF selection

Modulator and pulse forming network

Modulator pulse

magnetron RF pulse to T/R cell

Pulse length selection

Range scale and pulse lenght selector

Transmitter

The transmitter – a block diagram 5

1- The trigger generator: controls the repetion frequency of the transmitted pulses 2- The modulator and associated pulse–forming Network : defines the length, shape and power of 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 PRF

Selection 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 8

PRF nad Pulse Length, some representative values

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features

Short pulse

Long pulse

Long range target detection

Poor. Use when short range scales are selected

Good.Use when long range scales are selected and for poor response targets at short range

Minimum range

Good.use when short range scales are selected

Poor. Use when long range scales are selected 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 11

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 include frequencies which lie between 2900 and 3100 MHz. Wavelength is app. 10 cm 12

X-band and S-band compared

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2- The Aerial function Aerial, 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. 15

To achieve 360°of 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.( or provide 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 18

The powerful pulses generated by the transmitter might 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 whose angular 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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Aerial Heading marker

Aerial rotation Aerial

receiver Echo signal

Trigger/ transmitter

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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 given trace 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 the forward direction of the ship’s 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 to comprise three major elements: 31

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. 32

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 are known 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. 33

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. 34

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 cathode cylinder.

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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 some electrons 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 constant population will surround the end of the cathode. 36

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 for a 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 potential can be adjusted by the observer, using the brilliance control, and will be a few tens of volts negative with respect to the cathode.

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