comprehensive writen report radar

8/10/2019 Comprehensive Writen Report RADAR

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Fundamentals of RADARS Communications


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RADAR

An object-detection system that usesradio waves to determine the range, altitude, direction,

or speed of objects. It can be used to detectaircraft,ships,spacecraft,guided missiles,motor

vehicles,weather formations,and terrain. A radar system usually operates in the ultra-high-

frequency (UHF) or microwave part of the radio-frequency (RF) spectrum

The radar dish or antenna transmits pulses of radio waves ormicrowaves that bounce off any

object in their path. The object returns a tiny part of the wave's energy to a dish or antenna

that is usually located at the same site as the transmitter.

The range of the object is determined by measuring the time it takes for the radar signal to

reach the object and return. The object's location with respect to the radar unit is determined

from the direction in which the pulse was received. In most radar units the beam of pulses is

continuously rotated at a constant speed, or it is scanned (swung back and forth) over a sector,

also at a constant rate. The velocity of the object is measured by applying the Doppler principle:if the object is approaching the radar unit, the frequency of the returned signal is greater than

the frequency of the transmitted signal; if the object is receding from the radar unit, the

returned frequency is less; and if the object is not moving relative to the radar unit, the return

signal will have the same frequency as the transmitted signal.

Radar was secretly developed by several nations before and duringWorld War II.The

term RADAR was coined in 1940 by theUnited States Navy as anacronym for RAdio Detection

And Ranging.

Most radar systems determine position in two dimensions: azimuth (compass bearing) and

radius (distance). The display is in polar coordinates. A rotating antenna transmits RF pulses at

defined intervals. The delay between a transmitted pulse and the echo, or return pulse,

determines the radial position of the plotted point(s) for each azimuth direction on the display.

The greater the echo delay from a particular object in space, the farther from the display center

its point appears. The maximum range of a UHF or microwave radar system depends on the

height of the antenna above average terrain, the topography of the surface in the region, the

atmospheric conditions in the region, and in some cases the level of radio background noise.
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In Air Traffic Management (ATM) long-range surveillance radars like the Air Route Surveillance

Radar (ARSR) works in this frequency band. Coupled with a Monopulse Secondary Surveillance

Radar (MSSR) they use a relatively large, but slower rotating antenna. The designator L-Band is

good as mnemonic rhyme as large antenna or long range.

E/F-Band (S-Band Radar)

The atmospheric attenuation is higher than in D-Band. Radar sets need a considerably higher

transmitting power than in lower frequency ranges to achieve a good maximum range. As

example given theMedium Power Radar (MPR) with a pulse power of up to 20 MW. In this

frequency range the influence of weather conditions is higher than in D-band. Therefore a

couple of weather radars work in E/F-Band, but more in subtropic and tropic climatic

conditions, because here the radar can see beyond a severe storm.

Special Airport Surveillance Radars (ASR) are used at airports to detect and display the position

of aircraft in the terminal area with a medium range up to 5060NM (100km). An ASR

detects aircraft position and weather conditions in the vicinity of civilian and military airfields.

The designator S-Band (contrary to L-Band) is good as mnemonic rhyme as smaller

antenna or shorter range.

G- Band (C-Band Radar)

In G- Band there are many mobile military battlefield surveillance, missile-control and ground

surveillance radar sets with short or medium range. The size of the antennas provides an

excellent accuracy and resolution, but the relatively small-sized antennas don't bother a fast

relocation. The influence of bad weather conditions is very high. Therefore air-surveillance

radars use an antenna feed with circular polarization often. This frequency band is

predetermined for most types of weather radar used to locate precipitation in temperate zone

like Europe.

I/J- Band (X- and Ku- Band Radars)

In this frequency-band (8 to 12 GHz) the relationship between used wave length and size of the

antenna is considerably better than in lower frequency-bands. The I/J- Band is a relatively

popular radar band for military applications like airborne radars for performing the roles of

interceptor, fighter, and attack of enemy fighters and of ground targets. A very small antenna

size provides a good performance. Missile guidance systems at I/J- band are of a convenient

size and are, therefore, of interest for applications where mobility and light weight are

important and very long range is not a major requirement.
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This frequency band is wide used for maritime civil and military navigation radars. Very small

and cheap antennas with a high rotation speed are adequate for a fair maximum range and a

good accuracy. Slotted waveguide and small patch antennas are used as radar antenna, under a

protective radome mostly.

This frequency band is also popular for space borne or airborne imaging radars based

onSynthetic Aperture Radar (SAR) both for military electronic intelligence and civil geographic

mapping. A special Inverse Synthetic Aperture Radar (ISAR) is in use as a maritime airborne

instrument of pollution control.

K- Band (K- and Ka- Band Radars)

The higher the frequency, the higher is theatmospheric absorption and attenuation of the

waves. Otherwise the achievable accuracy and therange resolution rise too. Radar applications

in this frequency band provide short range, very high resolution and high data renewing rate. In

ATM these radar sets are calledSurface Movement Radar (SMR) or (as p. o.) Airport Surface

Detection Equipment (ASDE). Using of very short transmitting pulses of a few nanoseconds

affords a range resolution, that outline of the aircraft can be seen on the radars display.

V-Band

By the molecular dispersion (here this is the influence of the air humidity), this frequency band

stay for a high attenuation. Radar applications are limited for a short range of a couple of

meters here.

W-Band

Here are two phenomena visible: a maximum of attenuation at about 75 GHz and a relative

minimum at about 96 GHz. Both frequency ranges are in use practically. In automotive

engineering small built in radar sets operate at 7576GHz for parking assistants, blind spot and

brake assists. The high attenuation (here the influence of the oxygen molecules O2) enhances

the immunity to interference of these radar sets.

There are radar sets operating at 96 to 98 GHz as laboratory equipments yet. These applications

give a preview for a use of radar in extremely higher frequencies as 100 GHz.
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B.

Navigation

Navigation is a field of study that focuses on the process of monitoring and controlling the

movement of a craft or vehicle from one place to another.[1]

The field of navigation includes

four general categories: land navigation, marine navigation, aeronautic navigation, and spacenavigation.

a. Fundamental principle of navigation

The basic principle of operation of primary radar is simple to understand. However, the theory

can be quite complex. An understanding of the theory is essential in order to be able to specify

and operate primary radar systems correctly. The implementation and operation of primary

radars systems involve a wide range of disciplines such as building works, heavy mechanical and

electrical engineering, high power microwave engineering, and advanced high speed signal and

data processing techniques. Some laws of nature have a greater importance here. Radarmeasurement of range, or distance, is made possible because of the properties of radiated

electromagnetic energy. Reflection of electromagnetic waves

1. Reflection of electromagnetic waves

The electromagnetic waves are reflected if they meet an electrically leading surface. If these

reflected waves are received again at the place of their origin, then that means an obstacle is in

the propagation direction.

2. Electromagnetic energy travels through air at a constant speed, at approximately thespeed of light,

300,000 kilometers per second or

186,000 statute miles per second or

162,000 nautical miles per second.

This constant speed allows the determination of the distance between the reflecting objects

(airplanes, ships or cars) and the radar site by measuring the running time of the transmitted

pulses.

3. This energy normally travels through space in a straight line, and will vary only slightly

because of atmospheric and weather conditions. By using of special radar antennas this

energy can be focused into a desired direction. Thus the direction (in azimuth and

elevation of the reflecting objects can be measured.
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These principles can basically be implemented in a radar system, and allow the determination

of the distance, the direction and the height of the reflecting object.

(The effects atmosphere and weather have on the transmitted energy will be discussed later;

however, for this discussion on determining range and direction, these effects will be

temporarily ignored.)

b. Modes of navigation

Terrain avoidance

A mode in which the radar is set at a fixed depression angle and short range to continuously

sweep the ground area directly in front of the aircraft in order to avoid mountains. This is

particularly useful during flight into unfamiliar territory when clouds, haze, or darkness obscure

visibility.

Ground mapping

A mode in which the radar uses a variety of techniques to enhance ground features, such as

rivers, mountains and roads. The mode is unlike air-to-air modes where ground return is

rejected from the display.

Precision velocity update / Doppler navigation

A mode in which the radar again tracks ground features, using Doppler techniques, in order to

precisely predict aircraft ground speed and direction of motion. Wind influences are taken into

account, such that the radar can also be used to update the aircraft inertial navigation system.

C.Basic Principles of Radars

The electronic principle on which radar operates is very similar to the principle of sound-wave

reflection. If you shout in the direction of a sound-reflecting object (like a rocky canyon or

cave), you will hear an echo. If you know the speed of sound in air, you can then estimate the

distance and general direction of the object. The time required for an echo to return can be

roughly converted to distance if the speed of sound is known.

Radar uses electromagnetic energy pulses. The radio-frequency (rf) energy is transmitted to

and reflected from the reflecting object. A small portion of the reflected energy returns to the

radar set. This returned energy is called an ECHO, just as it is in sound terminology. Radar sets

use the echo to determine thedirection anddistance of the reflecting object.
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a.

Radar system

i.

Average power and Duty Cycle

Average power


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Duty Cycle


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ii.

The Duplexer

a switch which alternately connects the transmitter or receiver to the antenna. Its

purpose is to protect the receiver from the high power output of the transmitter.

During the transmission of an outgoing pulse, the duplexer will be aligned to the

transmitter for the duration of the pulse, PW. After the pulse has been sent, theduplexer will align the antenna to the receiver. When the next pulse is sent, the

duplexer will shift back to the transmitter. A duplexer is not required if the

transmitted power is low.

No practical mechanical switches are available that can open and close in a few microseconds.

Therefore, electronic switches must be used.


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b.

The radar receiver

i.

Circuits of the indicator

A radar indicator, sometimes called a radar repeater, acts as the master timing device in

analyzing the return of the video in a radar system. It also provides that capability to various

other locations physically remote from the radar system. Each indicator should have the

ability to select the outputs from any desired radar system aboard the ship.

The three most common types of displays, called scopes, are the A-scope, the RANGE-HEIGHT

INDICATOR (RHI) SCOPE, and the PLAN POSITION INDICATOR (PPI) SCOPE.

PLAN POSITION INDICATOR (PPI).

The ppi scope is by far the most used radar display. It is a polar coordinate display of the area

surrounding the radar platform. Own ship is represented as the origin of the sweep, which is

normally located in the center of the scope, but may be offset from the center on some sets.

The ppi uses a radial sweep pivoting about the center of the presentation. This results in a

map-like picture of the area covered by the radar beam. A long-persistence screen is used so

that the display remains visible until the sweep passes again.

Bearing to the target is indicated by the target's angular position in relation to an imaginary

line extending vertically from the sweep origin to the top of the scope. The top of the scope is

either true north (when the indicator is operated in the true bearing mode) or ship's heading

(when the indicator is operated in the relative bearing mode).

PPI Block Diagram

The basic block diagram, figure 3-4, illustrates the major units of a plan position indicator.

Synchronization of events is particularly important in the presentation system. At the instant

a radar transmitter fires (or at some predetermined time thereafter), circuits which controlthe presentation on the indicator must be activated. These events must be performed to a

high degree of accuracy to ensure accurate range determination. The synchronization of

these events is provided by the gate circuit.


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GATE CIRCUIT - The gate circuit develops pulses which synchronize the indicator with the

transmitter. The gate circuit itself is synchronized by trigger pulses from the synchronizer. It

then provides timing for the intensity gate generator, sweep generator circuit, and the sweep

control circuit.

SWEEP CONTROL CIRCUIT - The sweep control circuit converts mechanical bearing

information from the antenna into voltages which control sweep circuit azimuth.

SWEEP GENERATOR CIRCUIT - The sweep generator circuit produces currents which deflect anelectron beam across the crt. Varying voltages from the sweep control circuit are applied to

deflection coils. Gate voltages determine sweep rate, and therefore, the effective distance

(range) covered by each sweep. Sweep potentials consist of separate north-south and east-

west voltages; the amplitudes of these voltages determine sweep azimuth. The sweep

generator is synchronized by an input from the gate circuit.

INTENSITY GATE GENERATOR - The intensity gate generator provides a gate which unblanks

the crt during sweep periods. The intensity of the trace appearing on the crt is determined by

the dc level of this gate. This circuit is also synchronized by the gate circuit.


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ii.

Antenna Synchronization


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iii.

Heading Flash


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iv.

Echo Box

The ECHO BOX is an important test instrument for indicating the overall radar systemperformance. The echo-box test results reflect the combined relative effectiveness of thetransmitter as a transmitter of energy and the receiver as a receiver of energy.

The echo box, or RESONANCE CHAMBER, basically consists of a resonant cavity, as shown inview A of figure 4-4. You adjust the resonant frequency of the cavity by varying the size of thecavity (the larger the cavity the lower the frequency). A calibrated tuning mechanism controlsthe position of a plunger and, therefore, the size of the cavity. The tuning mechanism is adjustedfor maximum meter deflection, which indicates that the echo box is tuned to the precisetransmitted frequency. The tuning mechanism also indicates on a dial (figure 4-5, view A) boththe coarse transmitted frequency and a numerical reading. This reading permits the technicianto determine the transmitted frequency with greater accuracy by referring to a calibration curveon a chart (figure 4-5, view B).


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Energy is coupled into the cavity from the radar by means of an rf cable connected to the inputloop. Energy is coupled out of the cavity to the rectifier and meter by means of the output loop.You can vary the amount of coupling between the echo box and the crystal rectifier by changingthe position of the output loop. A schematic diagram of the output circuit is shown in figure 4-4,view B. The energy picked up by the loop is rectified, filtered, and applied to the meter. Themethod of connecting the echo box in a radar system is shown in figure 4-4, view C.


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c.

Operating the radar set

RADAR PRINCIPLES OF OPERATION

Radar systems, like other complex electronics systems, are composed of several

major subsystems and many individual circuits. This section will introduce you to

the major subsystems common to most radar sets. A brief functional description

of subsystem principles of operation will be provided. A much more detailed

explanation of radar subsystems will be given in chapters 2 and 3. Since

most radar systems in use today are some variation of the pulse radar system, the

units discussed in this section will be those used in pulse radar. All other types of

radar use some variation of these units, and these variations will be explained as

necessary.

RADAR COMPONENTS

Pulse radar systems can be functionally divided into the six essential components

shown in figure 1-16. These components are briefly described in the following

paragraphs and will be explained in detail after that:

Figure 1-16. - Functional block diagram of a basic radar system.


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The SYNCHRONIZER (also referred to as the TIMER or KEYER) supplies the

synchronizing signals that time the transmitted pulses, the indicator, and other

associated circuits. The TRANSMITTER generates electromagnetic energy in the

form of short, powerful pulses. The DUPLEXER allows the same antenna to be

used for transmitting and receiving. The ANTENNA SYSTEM routes theelectromagnetic energy from the transmitter, radiates it in a highly directional

beam, receives any returning echoes, and routes those echoes to the receiver.

The RECEIVER amplifies the weak, electromagnetic pulses returned from the

reflecting object and reproduces them as video pulses that are sent to the

indicator. The INDICATOR produces a visual indication of the echo pulses in a

manner that, at a minimum, furnishes range and bearing information.

While the physical configurations of radar systems differ, any radar system can be

represented by the functional block diagram in figure 1-16. An actual radar setmay have several of these functional components within one physical unit, or a

single one of these functions may require several physical units. However, the

functional block diagram of a basic radar set may be used to analyze the

operation of almost any radar set.

In the following paragraphs, a brief description of the operation of each of the

major components is given.

Synchronizer (Timer)

The synchronizer ensures that all circuits connected with the radar

system operate in a definite timed relationship. It also times the interval between

transmitted pulses to ensure that the interval is of the proper length. Timing

pulses are used to ensure synchronous circuit operation and are related to the

prf. The prf can be set by any stable oscillator, such as a sine-wave oscillator,

multivibrator, or a blocking oscillator. That output is then applied to pulse-shaping

circuits to produce timing pulses. Associated components can be timed by the

output of the synchronizer or by a timing signal from the transmitter as it is

turned on.


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Transmitter

The transmitter generates powerful pulses of electromagnetic energy at precise

intervals. The required power is obtained by using a high-power microwave

oscillator, such as a magnetron, or a microwave amplifier, such as a klystron, thatis supplied by a low-power rf source. (The construction and operation of

microwave components can be reviewed in NEETS, Module 11, Microwave

Principles.) The high-power generator, whether an oscillator or amplifier, requires

operating power in the form of a properly-timed, high-amplitude, rectangular

pulse. This pulse is supplied by a transmitter unit called the MODULATOR. When a

high-power oscillator is used, the modulator high-voltage pulse switches the

oscillator on and off to supply high-power electromagnetic energy. When a

microwave power amplifier is used, the modulator pulse activates the amplifier

just before the arrival of an electromagnetic pulse from a preceding stage or afrequency-generation source. Normally, because of the extremely high voltage

involved, the modulator pulse is supplied to the cathode of the power tube and

the plate is at ground potential to shield personnel from shock hazards. The

modulator pulse may be more than 100,000 volts in high-power radar

transmitters. In any case, radar transmitters produce voltages, currents, and

radiation hazards that are extremely dangerous to personnel. Safety precautions

must always be strictly observed when working in or around a radar transmitter.

Duplexer

A duplexer is essentially an electronic switch that permits a radar system to use a

single antenna to both transmit and receive. The duplexer must connect the

antenna to the transmitter and disconnect the antenna from the receiver for the

duration of the transmitted pulse. The receiver must be completely isolated from

the transmitted pulse to avoid damage to the extremely sensitive receiver input

circuitry. After the transmitter pulse has ended, the duplexer must rapidly

disconnect the transmitter and connect the receiver to the antenna. As previously

mentioned, the switching time is called receiver recovery time, and must be very

fast if close-in targets are to be detected. Additionally, the duplexer should absorb

very little power during either phase of operation. Low-loss characteristics are

particularly important during the receive period of duplexer operation. This is

because the received signals are of extremely low amplitude.


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Antenna System

The antenna system routes the pulse from the transmitter, radiates it in a

directional beam, picks up the returning echo, and passes it to the receiver with a

minimum of loss. The antenna system includes the antenna, transmission linesand waveguide from the transmitter to the antenna, and the transmission line

and waveguide from the antenna to the receiver. In some publications the

duplexer is included as a component of the antenna system.

Receiver

The receiver accepts the weak echo signals from the antenna system, amplifies

them, detects the pulse envelope, amplifies the pulses, and then routes them to

the indicator. One of the primary functions of the radar receiver is to convert the

frequency of the received echo signal to a lower frequency that is easier to

amplify. This is because radar frequencies are very high and difficult to amplify.

This lower frequency is called the INTERMEDIATE FREQUENCY (IF). The type of

receiver that uses this frequency conversion technique is the SUPER HETERODYNE

RECEIVER. Superheterodyne receivers used in radar systems must have good

stability and extreme sensitivity. Stability is ensured by careful design and the

overall sensitivity is greatly increased by the use of many IF stages.

Indicator

The indicator uses the received signals routed from the radar receiver to produce

a visual indication of target information. The cathode-ray oscilloscope is an ideal

instrument for the presentation of radar data. This is because it not only shows a

variation of a single quantity, such as voltage, but also gives an indication of the

relative values of two or more quantities. The sweep frequency of the radar

indicator is determined by the pulse-repetition frequency of the radar system.

Sweep duration is determined by the setting of the range-selector switch. Since

the indicator is so similar to an oscilloscope, the term RADAR SCOPE is commonly

used when referring to radar indicators.


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e.

Basic radar maintenance

Maintenance and Troubleshooting

Radars are generally very reliable and need only to be protected from water, heat

and physical damage. A regular maintenance program would consist of

periodically checking mounting bolts and brackets, keeping wiring connections

clean, tight and external wiring connections smeared with a thin coating of

petroleum jelly. The set itself should be keep clean and free of salt spray.

Troubleshooting would first involve checking that the unit is receiving power and

that the correct start-up procedure has been followed. If the power supply is OK,

all connections are clean and tight, the main circuit breaker or fuse is OK, the

internal fuses are all right and the scanner is free to rotate and the set still does

not work, then it is probably time to call a technician.

Safety Precautions

Personnel should avoid microwave radiation hazard by keeping clear of an

operating scanner. If working aloft on the scanner unit or other equipment near

the scanner unit. Ensure that:

The radar unit is turned off, the power disconnected and a sign placed on

the display informing others that you are working aloft

Clip on a safety harness

Tie on a tool bag that wont spill if inverted

dont raise or lower power tools by their cord

If at all possible have someone assist you


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The high voltage circuits inside the display unit can cause electrocution and must

not be touched except by qualified personnel with the radar switched off. The

display unit has potentially lethal voltages inside the unit even after it has beenturned off.

A good rule to remember is that: if the back has to be removed, leave it to the

experts.


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D.

Air Traffic Control Radar Beacon System (ATCRBS)

A system used inair traffic control (ATC) to enhance surveillanceradar monitoring and

separation of air traffic. ATCRBS assists ATC surveillance radars by acquiring information about

theaircraft being monitored, and providing this information to the radar controllers.

ATCRBS, the Air Traffic Control Radar Beacon System, is a secondary surveillance radar system

developed for use within the air traffic control system for more precise position reporting of

planes. It is used in conjunction with the primary radar, which is used to determine the

presence of planes in the airspace. ATCRBS supplements this positional information with

positive identification and altitude information, allowing controllers to track each plane more

precisely and efficiently.
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E.

Small boat radar

It wasn't just the cost - although that did come into it - but the thought of all the

battery juice that the system would suck up.

But in the process of designing a new gantry for her stern, it seemed wise to build

in the facility for one in the future, along with the stern light, solar panels, wind

charger, Wi-Fi enhancer, GPS and NAVTEX antennas.

It wasn't long before the future caught up with me, and a radar scanner appeared

in its allotted space.

Radio Detection and Ranging (RADAR) is used at sea detect the presence of

objects ('targets') at a distance, and to detect their speed if they are moving. Itworks by transmitting radio pulses, then detecting the echoes of these pulses

from objects within the range of the pulse, and displaying them graphically as

targets on the display.

Being a line-of-sight device, maximum range is limited by the curvature of the

earth and depends on the height of the scanner and the height of the target, as

shown here.

Functionality of a Typical Small Boat Radar System

Its primary function a small boat radar system is as an aid to avoiding collision,

but here's what else the display can do...


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Zoom - A typical sailboat radar unit will have a maximum range of 24nm

and will zoom down to 1/8nm. Larger, open array scanners will have a 48nm

range. Short range scales are best suited as you approach coastlines and

anchorages, providing greater detail of echoes close to your boat. Long range

scales provide the best overview of your boat's position relative to land masses,

large ships and squalls.

Mini Automatic Radar Plotting Aid (MARPA) - For this collision avoidance

system to be effective, accurate data of your boat's heading, speed over the

ground (SOG) and course over the ground (COG) must be inputted to the radar.

MARPA will track up to 10 targets and will activate an alarm if any one of them is

considered to represent a collision risk.


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Range Rings - Range rings are the concentric circles displayed on screen at

pre-set distances, and centered at your boats position. They're used to estimate

distance between points on the display. The distance between them changes with

the zoom level, typically 1/16nm at 1/8nm range and 4nm at 24nm range.

Electronic Bearing Lines (EBLs) - This is a line drawn from your boat to the

edge of the display. When rotated to align with a target it will indicate the target's

relative bearing from your position.

Variable Range Markers (VRMs) - Very much like an adjustable range ring,

this will indicate the target's range. Used together with the EBL, range and

bearing of a target will be confirmed.

Guard Zones - These are areas relative to your boats heading that can be

set manually. If a target enters the zone an alarm rings. The areas set can be

either a sector or a circle, as shown in the sketch.

Radar Scanners for Small Boat Radar Systems
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There are two types of scanners:

Radomes which are best suited for sailboats, RIBs and small power boats;

Open Arrays which have a higher power consumption and are most

frequently seen on larger power boats.

Typically, Radomes will be either 18" or 24" across, inside of which will be a 4kW

transmitter giving the radar the capability to detect targets out to a maximum

range of 48 nautical miles.

Coupled to a compatible multi-function display unit, the radar screen can be

viewed independently or overlaid on the chart plotter screen.

On a sailboat, the scanner would normally be mounted either on the mast at a

height of around 15 feet above the deck.

With the boat heeled, the range of the scanner - particularly on the leeward side -

will be adversely affected.

However, a self-leveling device like the one shown here will ensure that the unit's

performance remains unaffected by the angle of heel.
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An Unexpected Benefit of a Small Boat Radar System

As a result of fitting a radar set, midway across the heavily trafficked English

Channel with visibility closing in weren't quite as buttock clenching as they had

been previously.

But far offshore and away from shipping lanes, you could be forgiven for thinking

that small boat radar is largely superfluous.

Not so. It was on an Atlantic crossing we discovered that during the hours of

darkness, the radar will detect approaching squalls from long range (providing

they have rain in them), often giving you time to adjust our course to avoid them

completely - or, at worst, to just suffer a glancing blow.

F.

Collision Avoidance Radar

A safety system designed to reduce the severity of an accident. Also known

as precrash system, forward collision warning system or collision mitigating

system, it usesradar and sometimeslaser and camera sensors to detect an

imminent crash. Once the detection is done, these systems either provide a

warning to the driver when there is an imminent collision or take action

autonomously without any driver input
http://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Radar


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G.

Classification of Radars

Imaging Radar / Non-Imaging Radar

An Imaging Radar forms a picture of the observed object or area. Imaging radars have been

used to map the Earth, other planets, asteroids, other celestial objects and to categorize targets

for military systems.

Typically implementations of a Non-Imaging Radar system are speed gauges and radar

altimeters. These are also called scatterometers since they measure the scattering properties ofthe object or region being observed. Non-Imaging Secondary Radar applications are

immobilizer systems in some recent private cars.

Primary Radar

APrimary Radar transmits high-frequency signals which are reflected at targets. The arisen

echoes are received and evaluated. This means, unlikesecondary radar sets a primary radar set

receive it's own emitted signals as an echo again.

Secondary Radar

At these radar sets the airplane must have atransponder (transmitting responder) on board

and this transponder responds to interrogation by transmitting a coded reply signal. This

response can contain much more information, than a primary radar set is able to acquire (E.g.

analtitude,an identification code or also any technical problems on board such as a

radiocontact loss ...).
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Pulsed Radars

Pulse radar sets transmit a high-frequency impulse signal of high power. After this impulse

signal, a longer break follows in which the echoes can be received, before a new transmitted

signal is sent out.Direction,distance and sometimes if necessary theheight oraltitude of the

target can be determined from the measured antenna position and propagation time of the

pulse-signal.

Continuous- Wave Radar

CW radar sets transmit a high-frequency signal continuously. The echo signal is received and

processed. The receiver need not to be mounted at the same place as the transmitter. Every

firm civil radio transmitter can work as a radar transmitter at the same time, if a remote

receiver compares the propagation times of the direct signal with the reflected one. Tests are

known that the correct location of an airplane can be calculated from the evaluation of the

signals by three different television stations.

Unmodulated CW- Radar

The transmitted signal of these equipments is constant in amplitude and frequency. These

equipment is specialized in speed measurings. Distances cannot be measured. E.g. they are

used asspeed gauges for police. Newest equipments (LIDAR)work in the laser frequency range

and measure not only the speed.

Modulated CW- Radar

The transmitted signal is constant in the amplitude butmodulated in the frequency.This one

gets possible after the principle of thepropagation time measurement with that again. It is an

advantage of this equipment that an evaluation is carried out without reception break and the

measurement result is therefore continuously available. These radar sets are used where the

measuring distance isn't too large and it's necessary a continuous measuring (e.g. an altitude

measuring in airplanes or as weather radar/windprofiler).

A similar principle is also used by radar sets whose transmitting impulse is too long to get a well

distance resolution. Often this equipment modulate its transmitting pulse to obtain a distance

resolution within the transmitting pulse with the help of thepulse compression.
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H.

Kinds of Radars

a. Active Radar

Active radar is the type of radar most of us are familiar with. Its principle of operation is simple:

a radio wave is emitted from an antenna and reflects off objects the wave encounters. Thesignal is reflected back to the emitter location, where a receiving antenna picks up the echoed

signal. When the transmitter and the receiver of a radar system are collocated, the radar is said

to be monostatic.

Once the echo is received, the distance between the radar system and the object can be

determined with a simple time-of-flight calculation. Since the speed of an RF wave in the air is

the speed of light (3x108m/s), and since the time between the emission of the wave and its

reception takes into account a round trip to the target and back, the distance to the object can

be calculated by the simple formula D = t * c / 2, where:

D is the distance in meter

t is the time delay between the emission of the signal and its reception

c is the speed of light (~3x108m/s)

The following figure explains the basic mechanism of an active radar system. In the figure, the

variable t (time delay) equals the total time for the signal to be transmitted to the object and

reflected back: ttransmitted + techo.


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b.

Passive Radar

Instead of using collocated transmission and reception antennas, a passive radar system relies

on a signal transmitted from a different location. This type of radar system is called bistatic.

The ranging of this type of radar is done by calculating the delay between the signal receiveddirectly from the transmitter and the signal received after being reflected off a target.

Since only the time delay can be calculated from this technique with one transmitter and one

receiver, the single conclusion that can be drawn is that the detected object is located

somewhere on an ellipse whose foci are the transmitter and the receiver.

The figure below illustrates this concept.

In the figure, t1+t2 = t3+t4. This holds true for every object that is located on the ellipse of the

figure. For both of the objects in the diagram, the time delay between the original signal and

the reflected signal as seen by the receptor is exactly the same. Only by using multiple

transmitters and receivers can this type of radar system precisely locate an object. The

performance of the system is highly dependent on the number of transmitters and receivers

and their geometry.

comprehensive writen report radar

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