Radar Presentation 03

8/14/2019 Radar Presentation 03

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TARGET DEDECTION

-03-


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The ability of the radar system to detect anddisplay a given target depends on a large numberof factors some of which are constant and otherswhich may vary in quite a complex manner.

The radar range equation is an expression whichattempts to formalize the relationship between the

maximum range at which a target can be detectedand the parameters on which that range depends.


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Transmitter characteristics

As might be expected, the ability to detect distanttargets can be improved by using a more powerfultransmitter .

The maximum detection range varies as the fourthroot of transmitter power.

The power of transmitters designed for fitting tolarge vessels varies with manufacturer, but 10kWand 50kW are representative of low and high values.


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Antenna characteristics

The maximum detection range is a function ofantenna gain and aperture area. Detection rangevaries as the square root of antenna gain. Thus aerialgain has a greater influence than transmitter power

on long range performance.

It may seem strange that wavelength does not appearin the equation. It is in fact implicit in the aerial gainand the target radar crosssection, both of which arefunctions of wavelength


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Receiver characteristics

Smin, the minimum detectable signal, is a function ofreceiver sensitivity.

The receiver sensitivity, and hence the maximum rangeat Which targets can be dedected, is thus a function ofthe pulse lenght selected by the observer


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Target characteristics

Energy in the pulse which is intercepted by the targetis then available for return towards the antenna andhence to the receiver which is now in a receptivestate. The amount of energy which is returnedtoward the antenna, as opposed to that energy whichis absorbed and scattered by the target, is dependent

upon the following five prime characteristics of thetarget.


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1- Aspect

Aspect is the angle which the radar rays make with the

plane of the mirror and, the response will be good whenthe aspect is 90 and poor at virtually all other angles.


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2-Surface textureThe extent to which reflection is specular is dependentupon the surface texture of the target, i.e. whether the

surface is 'rough' or 'smooth'.


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3-Material

In general, materials which are good conductors ofelectricity also return good radar responses. This occurs asa result of absorption and re-radiation of the waves at thesame wavelength as those received


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Some bodies absorb radiation but, when they reradiate,the wavelength is different from that at which it was

received; still other bodies absorb radiation and re-radiatevery little of the energy (this results in the temperature othe body rising, i.e. the received radiation is coverted toheat). Some materials are simply transparent to radarenergy. GRP behaves to large extent in this way, steel willreturn good responses, while wooden boats generallyproduce poor responses.


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4. Shape


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5- Size

The response is related to the area of the target irradiatedby the beam (at any instant). This is not necessarily thesame as the intrinsic size of the target.

Since the radar beam is angularly wider in the verticalplane than in the horizontal, tall targets will in generalproduce stronger responses (all other factors being equal).


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Consider two targets presenting the same area to theradar; if the linear width of the horizontal beam at therange of the targets is equal to the linear width of target Athen, in the case of B, only the small irradiated portion of

the target will contribute to echo strength, while, in thecase of A, virtually the total area will be irradiated.


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Responses from specific targets


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1 -Ice

Large icebergs, such as those which are formed on the eastcoast of Greenland and drift down toward the NorthAtlantic shipping routes, have been found to give greatlyvarying radar responses.

Detection ranges as great as 11 nautical miles have beenexperienced while, on the other hand, quite large icebergshave approached to within 2 n mile without being detected.

Even the same iceberg may give greatly differing responseswhen viewed from different directions.


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Strength of echoes returned from icebergs are only about1/60 th of the strength of echoes which would be returned

from a steel ship of equivalent size .


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2-Radar-conspicuous targets

Targets which are designated radar-conspicuous

should be those which are known to provide goodradar responses and are readily identifiable.


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3 -Ships

The structure of ships is such that there are many natural'corner reflectors' and hence, when a target vessel isrolling and pitching in a seaway, its echo strength does notvary quite as much as might be expected.

Long vessels may appear as two or three individual echoes(each of which when tracked by ARPA might appear to begoing in a slightly different direction). They may also beconfused for a tug-and-tow or vice versa. Supertankers,because of their low freeboard, may not be dedectedinordinately great ranges.


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Target Enhancement-PassiveIt is essential that some targets which would normallyprovide poor radar responses, e.g. buoys, glass fibre and

wooden boats etc, are detected at an adequate range byradar.


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1- Corner reflectors

A corner reflector was seen as a simple device which wouldreturn virtually all of the energy which entered it, i.e , theenergy would be returned in the direction from which it

had come almost irrespective of the angle at which it hadentered the corner


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Target enhancement Active

1- The raconThe ships radar pulse triggers the racon transmitter on

the navigation mark, which then responds by transmittinga pulse (virtually instantaneously)


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2-The RamarkThis is a radar beacon in which the frequency is swept

continously at such high speed that the transmission at

each frequency is in effect continuous (not triggered)


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Sources of radar beacon information

The Admiralty List of Radio Signals Volume 2 contains

information ralating to racons working in both the S-bandand the X-band as well as ramarks.

ALRS VOLUME 2


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ALRS VOLUME 2


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Racons for survival craftTo assist in the detection and identification of survivalcraft and other small craft in distress, the IMO havespecified a Search and Rescue Transponder (SART) whichis a coded racon and which will appear on the radar screenas a series of twenty dots. Since the racon flash will always

appear as a radial signal beyondthe target, it willimmediately indicate the course to steer to the target.


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It is intended that the beacons should respond to radarequipment working in the standard marine X-band. Afeature of the beacon is an audible or visible signal that letsthe survivors know there is a craft in the area withoperational radar. The increasing strength of the signal

would indicate that the vessel using the radar isapproaching and this is seen to provide a valuablepsychological stimulus for survival.

It is mandatory for certain classes of craft with theimplementation of the Global Maritime Distress andSafety Service (GMDSS).


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The radar horizonAt marine radar transmission frequencies (nominally

10000 and 3000 MHz) , the paths followed by the signalsnay be considered as 'line of sight', This means that eventhough the radar is delivering a powerful pulse and thetarget is capable, if irradiated, of returning a detectable

response, the target will not be detected if it is below theradar horizon. This is analogous to the visual observationof objects in the vicinity of the horizon,The effect of the atmosphere on the horizon is a further

factor which must be taken into account when assessingthe likelihood of detecting a particular target andespecially when considering the expected appearance ofcoastlines,


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1 -The effect of standard atmospheric conditions

Under standard atmospheric conditions, the radar beamtends to bend slightly downward, the distance to the radarhorizon being given by the formula

dnmile=1.22 h ft or dnmile=2.21 hm

where h is the height of the antenna in feet or metres


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The possibility of dedecting targets beyond radar horizonwill, in addition to all the ther factors that will be

discussed in this chapter ,depend upon the height of thetarget.Thus the teorical dedection range based purely onthe antenna and target heights is given by the formula.

Rd = d+DRd = 1.22 h ft +1.22 H ft orRd = 2.21 h m+2.21 H m

Where h and H are heights of antenna and targetrespectively in feet or metresIn both cases , Rd is thetheorical dedection range in nautical miles.Rd is the theoretical detection range in nautical miles.


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This relationship is of course theoretical since it assumesthat:(a) Standard atmospheric conditions prevail. .(b) The radar pulses are sufficiently powerful.

(c) The target response characteristics are such as toreturn detectable responses.(d) The weather conditions, such as precipitation etc.,through which the pulses have to travel, will not unduly

attenuate the signals.


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Standard atmospheric conditions'Standard' conditions are precisely defined as:Pressure=1013mb decreasing at 118mb/1000 m of height

Temperature = 15C decreasing at 6.5C/1000 m of heightRelative humidity = 60% and constant with height.


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Sub-refraction

1 The effects of sub-refraction on detection ranges (Figure3.49)Sub-refraction Occurs when the refractive index of theatmosphere decreases less rapidly with height than under

standard conditions. As a result, the radar beam is bentdownward slightly less than under standard conditions.This means that, with an other factors constant, the sametarget will be detected at a slightly reduced range. In

practice, this is likely to mean something of the order of80% of the detection range under standard conditions butwill obviously depend on the severity of the conditionsprevailing at the time.


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Super refraction:

Super refraction occurs when the rate of decrease inrefractive index with height is greater than understandart conditions.When sub refraction occurs .the

radar beam tends to be down slightly more and sotargets may be dedected at ranges which are slightlygreater than standart.Increase of some 40% are notuncommon.

Atmopheric conditions associated with superrefractionare :


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Under these conditions , the radar energy is, in effect,trapped in a duct formed by the Earths surface and ahighly refractive layer which may be as litle as 100 ft

above the ground.The effect is concentration of energy.This increase energy will follows the Earth surface , thusreducing contraint of the radar horizon and considerablyextending the dedection ranges of targets.

Extra super refraction or ducting


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The radarflare

This is a rocket which is fired from a pistol. At some 400metres altitude, the rocket ejects a quantity of dipoles

which respond strongly to 3 cm radar waves and at thesame time the rocket gives out a very bright white light.


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Radar Presentation 03

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