Unit 9 Radar Clutters

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RADAR CLUTTERS

UNIT 9


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RADAR CLUTTERS

SURFACE CLUTTER RADAR EQUATION

SEA CLUTTER

LAND CLUTTER EFFECTS OF WEATHER ON RADAR

ANGLES ECHOES


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Introduction to Radar clutter

Clutter is the term used to denote unwantedechoesfrom the natural environment.

These unwanted echoes clutter the radar andmake difficult the detection of wanted

targets.

There are also point, or discrete, clutterechoes, like TV and water towers, buildings ,

and other similar structures that produce

large backscatter.


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Clutters

Large clutter echoes can mask echoes fromdesired targets and limit radar capability.

When clutter is much larger than receivernoise, the optimum radar waveform and signal

processing can be quite different from that

employed when only receiver noiseis thedominant limitation on sensitivity.


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Radar echoes from the environment are not always

undesired.

Reflections from storm clouds, can be a trouble to aradar that must detect aircraft; but storm clouds

containing rain are what the radar meteorologist

wants to detect in order to measure rain fall rate over

a large area.

The backscatter echoes form land can interfere with

many applications of radar, but they are the target ofinterest for ground mapping radar, synthetic

aperture (space) radars, and radars that observe

earth resources.


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Thus the same environmental echo might be

the desired signal in one applicationand the

undesired clutter echo in another.


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Remote sensing of the environment

The observation of land, sea, weather and othernatural phenomena by radar and other sensors

for the purpose of determining something about

the environment is known as Remote sensing

of the environment or Remote Sensing.

Eg. Doppler weather Radar is used forRemote sensing.


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SURFACE CLUTTER: Echoes from land or sea.

VOLUME CLUTTER: Echoes from rain and chaff.

The magnitude of the echo from distributed surface

clutter is proportional to the area illuminated.

In order to have a measure of the clutter echo that is

independent of the illuminated area, the clutter cross

section per unit area, denoted by the symbol 0

,is used to describe surface clutter.


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CHAFFS USED IN RADARS

Strips of metal, foil, or glass fiber with a

metal content, cut into various lengths and

having varying frequency responses, that

are used to reflect electromagnetic energyas a radar countermeasure.

These materials, usually dropped from

aircraft, also can be deployed from shellsor rockets.


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Sigma Zero 0

It is also called as

Scattering coefficient

Differential scattering cross section Normalized radar reflectivity

Backscattering coefficient

Normalized radar cross section (NRCS)


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The clutter cross section per unit area,

( Sigma Zero) 0

= c

(7.1)Ac

Where

c

= radar cross section of the clutteroccupying an area Ac.


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The Zero is a super script since the subscript is

reserved for the polarization employed.

Sigma Zero is a dimensionless quantityand

is expressed in decibels with a reference value

of one m2/m2.

Similarly a cross section per unit volumeis

used to characterize volume clutter. It is

defined as

= cV c


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= cV c

Where c in this case is the radar cross

section of the clutterthat occupies a

volume V c.

Clutter cross section per unit volume , is

called the Reflectivity.

Eqn (7.2)


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Multipathreduces the energy

propagating at low anglesbecause of cancellation of the direct

energy by the out of phase surface

reflected energy.


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raz ng ang e The grazing angleis used to describe the

aspect at which clutter is viewed.

INCIDENCE ANGLE

It is defined with respect to the normal to thesurface

The Grazing angle is defined with respect to the

tangent to the surface. DEPRESSION ANGLE

It is defined with respect to the local horizontal

at the radar.


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Normal to the

surface

(Complement

of Grazing

angle)

Tangent to

the surface(Aspect at

which

clutter is

viewed)

local horizontal at the

radar used for

Rough or varying

earths surface

Backscatter can

be quite large at

high grazing

angles

Th i id l i th l t f th


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The incidence angleis the complement of the

grazing angle.

When the earths surface can be consideredsmooth and flat, the depression angle and the

grazing angle are the same.

When the earths curvature must be taken into

account as in space borne radars, the depression

anglecan be quite different from the grazing angle.

The incidence angle is usually used when

considering earth backscatter at near perpendicular

incidence, as in the altimeter and the scatterometer.


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Some engineers prefer to use the

depression anglewhen a rough or

varying earths surfaceis viewed at low

grazing angles since it might be easier todetermine than the grazing angle when the

earth is not a flat surface.

Variation of surface clutter with


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Variation of surface clutter with

grazing angle

There are three different scattering regions. At high grazing angles, the radar echo is due

to mainly reflections from clutter that can be

represented as a number of individual planarfacets oriented so that the incident energy is

directed back to the radar.

The backscatter (is the reflection of waves,particles, or signals back to the direction from

which they came)can be quite large at high

grazing angles.

At th i t di t i l b k
http://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Wave


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At the intermediate grazing angles, back

scattering is influenced by shadowing (masking)

and by multipath propagation.

Shadowing of the trough regions by the crest of

waves prevents low lying scatters form being

illuminated.

Multipathreduces the energy propagating at

low angles because of cancellation of the

direct energy by the out of phase surface

reflected energy.

Th d i fi 7 3 i d i ti f th


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The curve drawn in fig. 7.3 is descriptive of the

general character of both land and sea

scattering; but there are significant differences

in the details depending on the particular typeof clutter.

The difference between the maximum clutter

at perpendicular incidence and the minimum

clutter at grazing incidencecan be many tens

of dB.


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Surface Clutter Radar Equation

LOW GRAZING ANGLE: Consider the geometry of fig. which depicts a

radar illuminating the surface at a grazing angle

.

Assume the grazing angle is small. A small

grazing angle usually implies that the extent ofthe resolution cell in the range dimension is

determined by the radar pulse width rather

than the elevation beam width.


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Th idth f th ll i th


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The width of the cell in the cross range

dimension is determined by the azimuth beam

width Band the range R.

The power C received from the clut ter is

Cl tt ti i i


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Clutter cross section is given as :

With this substitution the radar equation forsurface clutter is

Where C is the velocity of propagation

(13-5)


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Thus the echo from surface clutter varies inversely as

the cube of the range rather than inversely as thefourth power as is the case for point targets.

The signal power S returned from a target with

cross section t.

(13-6)

Combining Eqs (13 5) d (13 6) th i l


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Combining Eqs. (13.5) and (13.6), the signal-

to-clu t ter rat io fo r a target in a backg round

of surface clutter at low grazing angle is

If the maximum range R max, corresponds to

the minimum discernible signal-to-clutter

ratio ( S/C)min

then the radar equation can be

written

In this equation the clutter power C is assumed large


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In this equation, the clutter power C is assumed large

compared to receiver noise power.

This is an entirely different form of the radar equation than

when the target detection is dominated by receiver noise alone.

The range in Eq. (13.8) appears as the first power rather

than as the fourth power in the usual radar equation of Eq.

(13.6).

This means there is l ikely to be greater variation in the

maximum range of a clutter-dominated radar than a noise-

dominated radar.


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For example, if the target cross section inEq. (13.8) were to vary by a facto r o f

two, the maximum range would also vary

by a factor of two.

However, the same variation in target

cross section would only cause a variationin range of a factor of 1.2 when the radar

per formance is determined by receiver

noise.


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The transmitter power does not appearexplicitly.

Increasing the transmitter power will indeedincrease the target signal, but it will also cause

a corresponding increase in clutter.

Thus there is no net gain in the delectability of

desired targets.


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The only demand on the transmitter power

is that it be great enough to cause the

clutter power at the radar receiver to be

large compared to receiver noise.

If otherwise, Eq. (13.8) wou ld no t app ly.

The antenna gain does not enter except as it is


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The antenna gain does not enter, except as it is

affected by the azimuth beam width B .

The narrower the pulse width the greater the

range.

This is just opposite to the case of conventional

radar detection of targets in noise.


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A long pulse is desired when the radar

is limited by noise in order to increase the

signal-to-noise ratio.

When clutter dominates noise, a long

pulse decreases the signal-to-clutter ratio.

When pulse compression is used the pulse width in


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When pulse compression is used, the pulse width ,in

eqn (13.8 ) is that of the compressed pulse.

If the statistics of the clutter echoes are similar to thestatistics of receiver noise, then the signal to clutter

ratio in eqn. (13.8) can be selected similar to that for

signal to noise ratio as described in simple eqn.

The improvement in range due to the integration of n

pulses is not indicated in this eqn.

There can be a considerable difference in the

integration when clutter limited from when noise

limited.


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Clutter echoes , unlike receiver noise, might

be correlated pulse to pulse , especially if the

clutter is stationary relative to the radar.

Radar noise is usually de correlated in a time

equal to 1/B, where B = receiver (IF)bandwidth.

The de correlation time of clutter is usuallymuch greater than this.


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SEA CLUTTER


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SEA CLUTTER Sea-clutter are disturbing radar-echoes of sea wave crests.

This clutter gets also a Doppler- speed by the wind. This

means, the scenario moves away, i.e. changes with time,

while for ground clutter it stays the same. Therefore, in practice,

Sea-clutter is very difficult to control without some loss in

detection. Sea-Clutter can be seen here in the picture. The wind comes

either from about 310(NO) or from the opposite direction.

(Unfortunately, whether the Doppler frequency is positive or

negative cannot be recognized on the PPI-Scope.) But this region, in which the radial speedof the waves is very

small, is cleaned by the MTI system very clearly.

The radar echo from the sea when viewed at
http://www.radartutorial.eu/18.explanations/ex18.en.htmlhttp://www.radartutorial.eu/18.explanations/ex18.en.html


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The radar echo from the sea when viewed at

low grazing angles is generally smaller than the

echo from land.

The nature of the radar echo from the sea

depends upon the shape of the sea surface.

Echoes are obtained form those parts of the

sea whose scale sizes (roughness) are

comparable in dimension to the radar

wavelength.

The shape or roughness of the sea depends


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The shape , or roughness of the sea depends

on the wind.

Sea clutter also depends on the pointing

direction of the radar antenna beam relative to

the direction of the wind.

Sea clutter can be affected by contaminants

that change the water surface tension.

The temperature of the water relative to that of

the air is also thought to have an effect on sea

LAND CLUTTER


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LAND CLUTTER

General nature of land clutter is determined at

low, medium and high grazing angle.


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Land clutter at low Grazing angle

An extensive multiple frequency database ofland clutter at low angels was acquired by the

MIT Lincoln Laboratory.

This is one of the few collections of land clutter

data that have been obtained over a long period

of time.


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Measurements were made at five frequencies:

VHF(167 MHz)

UHF (435 MHz)

L ( 1.23 MHz)

S (3.24 GHz)

X ( 9.2 GHz) bands.


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The rms accuracy of the clutter echo


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The rms accuracy of the clutter echo

measurements over all sites was said to be 2

dB, a very good value for field operations.

The radar measured 0 F4called the clutter

strength ,

Where 0 is the clutter cross section per unit

area and

F is the propagation factorused in radar

equation to account for effects such as

multipath reflections, diffraction, and

attenuation.


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Propagation factor F

Defined as: Ratio of the incident field that

actually exists at the clutter cell being

measured to the incident field that would

exist there if the clutter cell existed by itselfin free space.

Clutter observations were made at lowdepression angles, at ranges from 1 to 25

or 50 km or more.

The depression angle was used rather than the


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The depression angle was used rather than the

grazing angle since it was difficult to define the

grazing angle over a non flat surface such as

natural terrain.

The depression angle is the complement of the

incidence angle at the backscattering terrainpoint under consideration.

This definition includes the effect of earth

curvature on the angle of illumination but not

the effect of the local terrain slope.

Clutter strength is given as the median value of


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Clutter strength is given as the median value of

the measured means by terrain type and

frequency.

The values in the figure and table were

averaged over both vertical and horizontal

polarizations, and with both 150 m and 15 or 36m range resolution.

This averaging was done since the variations of

the mean clutter echo with both polarization and

resolution were small, generally about 1 or 2

dB.

A radar which must detect targets over land has a more


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difficult task than one which must detect targets over the

sea.

Even though a radar at sea might not be bothered by

sea clutter, nearby land clutter can be so large that it can

enter the radar via the antenna side lobes and degrade

performance.

At vertical incidence there is less backscatter from land than

from sea, but this is usually undesirable since it reduces the

range of radar altimeters over land.

Land clutter is difficult to quantify and classify.


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Land clutter is difficult to quantify and classify.

The Radar echo from land depends on the type

of terrain as described by its roughness and

dielectric properties.

Desert , forest , vegetable, bare soil, cultivated

fields, mountains , swamps, cities , roads and

lakes all have different scattering

characteristics.

The radar echo will depend on the moisture

content of the surface scatterers, snow cover,

and the stage of growth of any vegetation.

Building , towers, and other structures give


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Building , towers, and other structures give

more intense echo signals than forest or

vegetation because of the presence of flat

reflecting surfaces and Corner Reflectors.

Bodies of water, roads and airport runways

backscatter little energy but are recognizable onradar PPI displays as black areas amid the

brightness of the surrounding ground echoes.

A hill will appear to stand out in high relief on a

PPI.

The near side of the hill will give a large return,


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e ea s de o t e g e a a ge etu ,

while the far side, which is relatively hidden

from the view of the radar, will give little of no

return.

The radar cross section of a farmers field will

differ before and after ploughing, as well asbefore and after harvesting.

It will also depend on the direction of the radar

beam relative to the direction of the ploughed

furrows.

The echo from forest differs depending on the


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p g

season.

Sea echo is more uniform over the oceans of

the world, providing the wind conditions are the

same.


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Information about the radar backscatter from land is

required for several different applications, each of whichhas its own special needs.

These applications include:

e e ec on o a rcra over an ,50 to 60 dB greater than aircraft echoes. MTI or pulse-doppler radar is commonly


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used for this application to remove the background clutter.

The detection of surface targets over land, where moving vehicles or personnel

can be separated from clutter by means of MTI. Fixed targets require high resolution

for their detection.

Altimeters which measure the height of aircraft or spacecraft. Large clutter

energy is desired since the "clutter" is the target.

The detection of terrain features such as hills and mountains ahead of an aircraft towarn of approaching high ground (terrain avoidance) or t o al low the aircraf t to

fo l low the contou r of the land( ter rain fo l low ing)

Mapping or imaging radarsthat utilize high resolution. Ground objects are

recognized by their shape and contrast with surroundings.

Remote sensing with imaging radars. altimeters, or scatterometers to obtain

specific information about the nature of the surface characteristics.

The data for land clutter is usually reported in


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y p

terms of 0, the cross section per unit area.

It is sometimes given by a parameter which

equals 0/sin ,

Where is the grazing angle. For ideal rough

terrain, is approximately independent of the

angle , except at low grazing angles and

near perpendicular incidence.

An example of clutter 0for several broad


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This applies to X-band clutter. The boundaries of the various

i id t i di t th id i ti f th d t ithi


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regions are wide to indicate the wide variation of the data within

the classes of terrrain.

Figure 13.9 illustrates airborne data at X and L bands. The

azimuth beam width was 50and the pulse width was 0.5 s at

each frequency.

The lack of smoothness of the data is due in part, to the fact

that the data was not all taken at the same time.

For a paticular grazing angle the two frequencies had to be

obtained by reflying the aircraft along the same flight path.

Different grazing angles also required reflying the aircraft over

the same area.

Each point on the curve us an average over 1 to 2 miles of

ground track.

EFFECTS OF WEATHER ON


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EFFECTS OF WEATHER ON

RADAR ANGLES ECHOES Radar could see through weather effects such

as fog, rain, or snow.

Performance of some radars can be strongly

affected by the presence of meteorologicalparticles (hydrometeors).

In general, radars at the lower frequencies are

not bothered by meteorological or weather

effects, but at the higher frequencies, weatherechoes may be quite strong and mask the

desired target signals just as any other

unwanted clutter signal.


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Whether the radar detection of meteorological


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particles such as rain, snow, or hail is a blessing or a

curse depends upon one's point of view.

Weather echoes are a nuisance to the radar operatorwhose job is to detect aircraft or ship targets.

Echoes from a storm, for example, might mask or

confuse the echoes from targets located at the same

range and azimuth.

Radar return from rain, snow, or hail is of considerable

importance in meteorological research and weatherprediction.

Radar may be it used to give an up-to date pattern of

precipitation in the area around the radar.

It is a simple and inexpensive gauge for


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measuring the precipitation over relatively large

expanses.

As a rain gauge it is quite useful to thehydrologist in determining the amount of water

falling into a watershed during a given period of

time. Radar has been used extensively for the study

of thunderstorms, squall lines, tornadoes,

hurricanes, and in cloud-physics research.

Not only is radar useful as a means of studying the


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basic properties of these phenomena, but it may also

be used for gathering the information needed for

predicting the course of the weather. Hurricane tracking and tornado warning are examples

of applications in which radar has proved its worth in

the saving of life and property.

Another important application of radar designed for thedetection of weather echoes is in airborne weather-

avoidance radars, whose function is to indicate to the

aircraft pilot the dangerous storm areas to be avoided.

Scattering from water-coated


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Scattering from water-coated

ice spheres Moisture in the atmosphere at altitudes where

the temperature is below freezing takes the form

of ice crystals, snow, or hail.

As these particles tall to the ground they meltand change to rain in the warmer environment of

the lower altitudes.

When this occurs, there is an increase in the

radar backscatter since water particles reflectmore strongly than ice.

As the ice particles, snow, or hail begin to melt,

they first become water-coated ice spheroids.

At radar wavelengths, scattering and


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attenuation by water-coated ice spheroids the

size of wet snowflakes is similar in magnitude to

that of spheroidal water drops of the same sizeand shape.

Even for comparatively thin coatings of water,the composite particle scatters nearly as well as

a similar all-water particle.

Radar observations of light precipitation show a

h i t l" b i ht b d" t ltii d t hi h th


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horizontal" bright band" at an altii ude at which the

temperature is just above Oc.

The measured reflectivity in the center of the brightband is typically about 12 to 15 dB greater. than the

reflectivity from the snow above it and about 6 to 10

dB greater than the rain below.

The center of the bright band is generally from about100 to 400 m below the OC is isotherm.

Although the bright band is relatively thin,

considerable attenuation can occur' when radarobservations are made through it at low elevations.

The bright band is due to changes in snow falling

th h th f i l l


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through the freezing level.

At the onset of melting the snow changes from flat or

needle-shaped particles which scatter feebly tosimilarly shaped particles which, owing to a water

coating, scatter relatively strongly.

As melting progresses, the particles lose their extreme

shapes, and their velocity of fall increases causing adecrease in the number of particles per unit volume

and a reduction In the backscatter.


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Since the diameter of could droplets is about


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one- hundredth the diameter of rain drops , the

echoes from fair weather clouds are usually of

little concern.

It is also possible to obtain weak echoes from a

deep, intense fog at millimeter, wavelengths butat wavelengths of 3 cm and longer , echoes due

to fog may generally be regarded as

insignificant.


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Attenuation in Precipitation When precipitation (rain /Rainfall) particles are

small compared to the radar wavelength (

Rayleigh Region) , the attenuation due to

absorption is small.

This is the case for frequencies below S band.

Since rain attenuation is usually small and

unimportant at the longer wavelengths, the

relative simplicity of the Rayleigh scattering

approximation is of limited use for predicting

attenuation through rain.


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Effect of weather on Radar Because the echo from precipitation varies as

f 4, where f = frequency, UHF radars ( 420

450 MHz) are seldom bothered by weather

effects.

At L band weather echoes can be a problem

and some method for seeing aircraft targets in

weather is usually needed.

A radar at S band will have its range

considerably reduced in modest rainfall if

Radars at higher frequencies are even further

d d d b i


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degraded by rain.

Airborne weather avoidance radars at X band ,

for eg. Can be severely degraded by heavy rain

and prevent the radar from seeing hazardous

weather.

A typical specification for an air surveillance

radar might be that it has to detect its targetwhen rainfall in the vicinity of the target is at the

rate of 4 mm/h.

This is called a moderate rain.


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Attenuation is not a problem at frequencies

below X band, unless the precipitation is very

heavy.

Unit 9 Radar Clutters

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