unit 9 radar clutters
TRANSCRIPT
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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
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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
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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.