lecture session1
TRANSCRIPT
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Course on
Dual Polarization Weather Radar
and Applications
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Instructor:
Professor V. ChandrasekarColorado State University and
University of Helsinki
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3 Dual Polarization Weather Radar and Appli cation s
Dual Polarization Weather Radarand Applications
V. Chandrasekar
Colorado State University and
University of Helsinki
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4 Dual Polarization Weather Radar and Appli cation s
Overview of Dual Polarization WeatherRadar and Applications
Fundamentals of Weather Radar
Concept of Electromagnetic Polarization
Introduction to Dual Polarization Weather Radar
Doppler Weather Radar Theory
Dual Polarized Weather Radar Fundamentals
Rainfall
Precipitation Microphysics
Hydrometeor Classification
Attenuation and Attenuation Correction
Examples of Dual-Polarization Weather Radar Observations
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5 Dual Polarization Weather Radar and Appli cation s
Fundamentals of weather radar
Radar equation for
Precipitation
Reflectivity factormeasurements Context for Dual
polarization
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6 Dual Polarization Weather Radar and Appli cation s
The characteristics of a radar is essentially determined by theproperties of how electromagnetic waves interact with physicalobject such as airplanes or precipitation particles.
Radar
Radar a contraction of termsRadio detection and ranging
http://upload.wikimedia.org/wikipedia/commons/0/07/Radarops.gif -
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Basic Principle of a Radar
The basic principle of a radar can be described by the followingdiagram.
The radar consists of a transmitter connected to the transmittingantenna, propagating electromagnetic wave outward from thetransmitter and a receiver connected to a receiving antenna forthe reception of any scattered wave.
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8 Dual Polarization Weather Radar and Appli cation s
The portion scattered in the direction of the receiver travels at
the speed of light to the receiver located at a distance R2 awayand is received by the receiving antenna, which converts thewave to a received signal s(t).
The information about the scatterer is contained in the receivedwaveform.
Continued
The scatterer or target exists in the mediumbetween the transmitter and receiverlocations.
The transmitter waveform u(t)that is produced by thetransmitter is converted to radiating electromagnetic wave thattravels at the speed of light.
This wave encounters a scatter at a range R1.
The incident electromagnetic wave is scattered in all directions.
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9 Dual Polarization Weather Radar and Appli cation s
Propagation Medium
The simplest propagation medium is free space.
However in the context of weather radar, the medium throughwhich the electromagnetic wave propagates can beprecipitation.
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10 Dual Polarization Weather Radar and Appli cation s
Function of a Weather Radar
The detection function consists of detecting features of
weather/precipitation characteristics from the received
signal.
2) Measurement
1) Detection
Measurement of the scattering volume characteristicsinvolve measuring scatterer location in three dimensionalspace (range, azimuth, elevation), scatterer velocity as wellas the strength or type of scatterers.
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11 Dual Polarization Weather Radar and Appli cation s
Key Variables in Observation
Range: measure the distance of the target from radar
Direction: the angular position of the target
The radiation is directional,focused by the antenna.
Intensity: the size and dielectric properties of thetarget
Compare the power changes between the radiation and the echoes
Velocity: the relative movement of the target
Compare the frequency change between the radiationand the echoes
Doppler effect: where you get tickets for speeding
2
cr
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12 Dual Polarization Weather Radar and Appli cation s
Radar As a System
Radar is a good example of a system composed of many types
of subsystems or elements. Specially weather radars aredesigned to observe what is termed as volume targets. Thefollowing shows a simple block diagram of a pulsed radar.
Antenna
IF Matched
Amp Filter
Second
demodulator
VideoAmp
MixerLow-noise
Amp
Duplexer
Local OSC
Output
PowerAmp
Pulse
Modulator
WaveformGenerator
Block diagram of pulsed radar
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13 Dual Polarization Weather Radar and Appli cation s
In a Doppler radar the received signal is compared against a copyof the transmitted signal. In coherent systems where this is
done, the transmitter waveform is compared against receivewaveform. The transmitter waveform is a high frequency sinusoidmodulated by a train of pulses. The information carrying portion ofthe transmitter is the rectangular waveform. The high frequencysinusoid is called the carrier.
The transmitter waveform is generated by a combination of twooscillators, stable local oscillator (STALO) and coherent localoscillator (COHO)
Radar As a System (Continued)
The transmitted waveform is generated by the oscillators termedCOHO, STALO and amplified and fed to the antenna. The simplesttransmitter waveform is a train of pulses.
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14 Dual Polarization Weather Radar and Appli cation s
DigitalModulator
GPSReference
PowerAmplifierMixer
STALO
sT
0T
Low NoiseAmplifier (LNA)
From STALO
ADCDigital Quadrature COHO
Low PassFilter
Low PassFilter
I
Q
IF Amplifier
Duplexer
Transmitter
Receiver
Radar As a System (Continued)
CSU-CHILL Radar System
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15 Dual Polarization Weather Radar and Appli cation s
The scattered signal will be a replica of the transmitted signalexcept for a range time delay and Doppler frequency shift.
The STALO is used to bring down the carrier frequency toproduce the intermediate frequency signal. Subsequently these
are demodulated to obtain the information carrying part of thesignal for analysis.
In the process of describing the block diagram of the radar manyterms are introduced.
Waveforms: In a pulsed radar the transmitted waveform isdescribed as a train of rectangular pulses denoted by u(t).However this pulse modulates a high frequency sinusoid requiredfor radiation.
Radar As a System (Continued)
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Radar As a System (Continued)
The mathematical form of such a waveform is
is the carrier frequency.
)](2cos[)()( ttftuts o of
Radar transmitted signalshown as a sinusoid offrequency (f0) modulatedby a rectangular pulse ofduration T0
Magnitude ofthe spectrum ofthe transmittedsignal
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17 Dual Polarization Weather Radar and Appli cation s
The location of precipitation to be measured by radar needs
definition of coordinate system.
Radar Coordinates
The natural coordinatesystem for a radar is
spherical polar coordinatesystem. The angles usedare azimuth angle,elevation angleand thedistance reference is
range. The azimuth angleis measured with respectto north.The elevationangle is measured withrespect to horizontal.
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18 Dual Polarization Weather Radar and Appli cation s
Radar signal return intensities (or powers) are expressedin 2D image plots called Plan Position Indicator (PPI) or
Range Height Indicator (RHI).
Radar Coordinates (Continued)
PPI
RHI
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Range to a Target
The common radar waveform is a train of rectangular
pulses modulating a sine wave carrier. The range to
the target is determined by the time tR.
Electromagnetic waves travel at speed of light C.
The time for the signal to travel to range R andreturn is
C
R2
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20 Dual Polarization Weather Radar and Appli cation s
Therefore, the range to the target is
IfR is in micro sec then
}{)(081.0)(15.0
)(150
2
1)(103
2
10)( 86
milesNauticalnmiskms
ms
ssmm
sCR
R
R
R
RR
2RCR
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Maximum Unambiguous Range
Once a signal is radiated into space by a radar,
sufficient time must elapse to allow for all the echo
signals to return to radar before the next pulse is
transmitted.
This elapsed time is determined by the largest range at
which targets are expected. If the time between the
pulses or sampling time TS is too short, then the echofrom this pulse arriving after the next pulse
transmission will be associated with that pulse.
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sT
oT sT sT2 sT2 sT3
)( tVr
)( sr TtV
)2( sr TtV
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PRF
CCTR S
22max
Echoes that arrive after the transmission ofnext pulse are called second trip echoes.
PRF = Pulse Repetition
Frequency
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24 Dual Polarization Weather Radar and Appli cation s
1000
100
10 1000100PRF
104
10000
unR
nauticalmiles
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25 Dual Polarization Weather Radar and Appli cation s
Radar Frequencies
Radars in the past have been built from 100 MHz to 36GHz. These are not hard limits.
Over the horizon radars operate at several MHzwhereas mm wave radars have been built at 240 GHz.
During world war II codes such as S, X and L were
used to designate frequency bands.
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26 Dual Polarization Weather Radar and Appli cation s
IEEE standard radar-frequency
letter-band nomenclature
BandDesignation
NominalFrequency
Range
Specific Frequency Ranges forRadar based on ITU Assignments in
Region 2
HF
VHF
UHF
L
S
C
X
3-30 MHz
30-300 MHz
300-1000 MHz
1-2 GHz
2-4 GHz
4-8 GHz
8-12 GHz
138-144 MHz216-225 MHz
420-450 MHz
850-942 MHz
1215-1400 MHz
2300-2500 MHz;2700-3700 MHz
5250-5925 MHz
8500-10,680 MHz
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BandDesignation
NominalFrequency Range
Specific Frequency Ranges forRadar based on ITU
Assignments in Region 2
Ku
K
KaV
W
mm
12-18 GHz
18-27 GHz
27-40 GHz
40-75 GHz
75-110 GHz
110-300 GHz
13.4-14 GHz
15.7-17.7 GHz24.05-24.25 GHz
33.4-36 GHz
59-64 GHz
76-81 GHz
92-100 GHz
126-142 GHz ;144-149 GHz
231-235 GHz ;238-248 GHz
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Applications of Radars
Military : Radar is an important part of air-defense
systems as well as operation of offensive
missiles.
Remote sensing :1) Atmospheric observations.
2) Planetary observation.
3) Below ground probing.
4) Sea ice mapping.
Air traffic control (ATC) :
ASR(Air Surveillance Radar)-TDWR(Terminal DopplerWeather Radar)
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29 Dual Polarization Weather Radar and Appli cation s
Law Enforcement / Highway safety :
- Police radar, collision avoidance systems.
- Aircraft safety/ Navigation/ Radar altimeter.
Space:
- Space vehicles use radar for rendezvous anddocking, Radio astronomy.
History :
- Basic concept of electromagnetic radiation shown
by Heinrich Hertz during 1885-1888.- He experimentally demonstrated James clerkMaxwells theory (published in 1864).
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- Early development , Hulsmeyer in early 1900sdeveloped to sell ship collision avoidance system.
- Marconi promoted the usage and presented tothe Institute of Radio Engineers (now the IEEE).
- Then came the heavy military bomber aircraft in1930s, which suddenly needed a device.
USA :
- The Naval Research Laboratory developedhundreds of radars by 1940.
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UK :
- Deployed radars by 1938.- The radars primarily were credited for turningthe momentum in the war.
- The invention of the magnetron revolutionizedthe development of high frequency radar.
- The cavity magnetron was delivered to MITRadiation Lab to accelerate the development.
Magnetron :
- Invented in Birmingham, UK.- Allowed development of small radars to becarried on ships and aircraft.
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After WW-II
- Doppler radars.
- High power amplifiers, Klystron, travelingwave tubes solid state transmitters.
- Monopulse radar.
- Pulse compression.
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Summary of Weather Radar Principles
Weather radar consists of a transmitter, the propagationmedium, precipitation scatters, and receiver.
TransmitterPropagation
Medium
Scatterer
ReceiverPropagation
Medium
Transmitter determines the frequency and radiated power andthe antenna determines the direction of transmission andpolarization.
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The receiver/processor determines how weak a signalcan be measured, and processed.
The receiver/processor module also generates userdefined products.
The transmitter receiver and processor are designed tomake the desired observation about the scatterers(which for weather radars is precipitation)
Summary (Continued)
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35 Dual Polarization Weather Radar and Appli cation s
Fundamentals of weather radar
Radar equation for
Precipitation
Reflectivitymeasurements Context for Dual
polarization
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Weather Radar Equation for a Single Raindrop
Consider a pulsed Doppler radar transmitting a pulse of high frequencysinusoid (most weather radars)
Radar equation relates the range of a radar to the characteristics ofthe transmitter, receiver, antenna, target, and environment. It serves
as a means for understanding the factors affecting radar performance.
If the transmitter power Pt is radiated by an isotropic antenna, thenthe power density at a distance R from the radar is given by,
Power density
where 4R2is the surface area of a sphere
)(4
2
2mw
R
Pt
24 R
Pt
Raindrop
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Weather Radar Equation for a Single Raindrop (Continued)
Radars use directive antennas to concentrate the radiated
power, Pt in a particular direction. The gain of the antenna is ameasure of the increased power density radiated in somedirection as compared to that of an isotropic antenna.
Power density of a directive antenna =
antennaisotropicanbyradiateddensitypower
antennadirectiveabyradiateddensitypowerG
24 R
GPt
The scatterer intercepts a portion of the incident energy and scatters
it in various directions.
Only the power radiated in the direction of the radar is of interest.
The radar cross section () of a scatterer determines the amount of
scattered power.
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38 Dual Polarization Weather Radar and Appli cation s
Concept of Radar Cross Section
When the object is large compared to the wavelength of theincident electromagnetic wave (such as large metal sphere)the scattering cross section of the radar is the same as thegeometric cross section.
2
D Whenever an electric field is incident on a dielectric sphere
such as a raindrop, then the dipoles in the water moleculesget aligned parallel to the electric field, and the net effect isan electric field with in the sphere. This dipole radiates
back, whose strength is proportional to the volume of thedrop.
Therefore the scattered field is proportional to volume of thesphere or V and the power is proportional to or
3D 6D2V
Weather Radar Equation for a Single Raindrop (Continued)
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The factor comes from the fact that the electric fieldwithin the sphere is less than what is applied due to depolarizingfactor. The factor comes from plane wave scattering. Scatteringfor such objects is called Rayleigh scattering.
)2/()1(3 rr
4
0k
Concept of Radar Cross Section (Continued)
If the scatterer is small compared to the wavelength , then forobjects such as raindrops (that are dielectric objects) the radar cross
section is given by
where Vis the volume of the raindrop.
2
24
0
)2(
)1(3
4V
k
r
r
Substituting and3
6DV
20 k
;62
4
5
DK
2
2
2
1
r
rK
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40 Dual Polarization Weather Radar and Appli cation s
The total RCS is given by,
RCS of simple objects(i) Sphere
2
0 0
sin),(4
1
S S
SSSSSt dd
Direction to receiving
radar
x
y
z
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41 Dual Polarization Weather Radar and Appli cation s
Due to symmetry waves scattered from a perfectlyconducting sphere are co-polarized with incident waves.
The normalized cross section for a perfectly conductingsphere in a Mie series is given by,
where, is the radius of the sphere.
is spherical Bessel function of the first kind of ordern.
1
2)12()1(
n
n n
kr
j
r
)(
)(
)()(
)()()1()1()1(
1
1
krH
krJ
krnHkrkrH
krnJkrkrJ
n
n
nn
nn
;2
k
r
nJ
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42 Dual Polarization Weather Radar and Appli cation s
is the Hankel function of ordern
Given by,is the spherical Bessel function of 2nd kind.
)1(
nH
)()()()1( krjYkrJkrHnnn
nY
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43 Dual Polarization Weather Radar and Appli cation s
For optical region
Rayleigh region (small sphere)
r2r
42 )(9 krr r
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The back scatter matrix for a dielectric sphere is,
where,
i
v
i
h
FSA
ssss
ss
ss
rik
s
v
s
h
E
E
Sk
iSk
i
Sk
iS
k
i
r
e
E
E
)(cos
)(sin
)(sin
)(cos
2
0
2
0
1
0
1
00
S
Snne
S
Snno
n
S
P
d
dP
nn
nS
sin
)(cos)(cos
)1(
12)(
1
1
1
1
1
1
S
Snne
S
Snno
n
Sd
dPP
nn
nS
)(cos
sin
)(cos
)1(
12)(
1
1
1
1
1
2
Radar cross section of a dielectric sphere
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45 Dual Polarization Weather Radar and Appli cation s
)()()()(
)()()()(
0)2(00)2(0
0000
1
nnnnr
nnrnn
no
hjjh
jjjj
)()()()(
)()()()(
)2()2(1
nonoononr
ononrnono
ne
jhhj
jjjj
ak00 ; r 0
arChandrasekandBringiofaeqasSameno )(104.2.1
arChandrasekandBringiofbeqasSameno )(104.2.1
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Extending and computing the radar scattering cross
section
where,
2
),(4),( iifiib
2
1
112
0))(12()1(
nneno
n
nk
2
12
0
))(12()1(
n
S
n
S
n
n bank
;1neSna no
S
nb 1
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47 Dual Polarization Weather Radar and Appli cation s
The total scattering cross section,
The extinction cross section
Fast numerical methods are available in, Barber and
Hill 1990.
The number of terms in series summations you need
is,
1
22
2
0
)12(2
n
S
n
S
nS bank
1
2
0
Re)12(2
n
S
n
S
next bank
2)(05.4 3/1max akakn oo
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Weather Radar Equation for a Single Raindrop (Continued)
The radar cross section is defined as the area , such that the
incident power density intercepted by this area , radiating as
an isotropic radiator yields the same power density as received
by the radar.
The cross section is dependent
on target shape and size.
2244 RR
GPt
Power density backscattered towards the radar
W th R d E ti f Si l R i d (C ti d )
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The radar antenna receives the energy incident on it. The Powerreceived by the radar is the product of the incident power density and
the effective area.
Weather Radar Equation for a Single Raindrop (Continued)
Effective area (Ae) of the receiving antenna is related to the physical
area by the aperture efficiency .a
e
tt
r ARR
GP
wattsP 22 44)(
42)4( R
AGP ett
,AAae
43
22
)4( R
GPt
The term comes from a two-way propagating wave, becausethe roundtrip distance is 2r.
kRje 2
kRjt e
GP2
234
The corresponding voltage signal is
Weather Radar Equation for Volume Targets
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If there is a large continuum ofprecipitation particles in range such as
raindrops and ice particles Then the backscattered signals from
all the hydrometeors in the range
to arrive at the same
time such that
is the range resolution
R RR
oT
c
RR
c
R
22
2
ocTR
mR 150 For a of 1 microsec. (typical)
o
T
The phase term of the received voltage
runs through several cycles of360o over 150m (because it cycles 360o
over one wavelength typical 10 cm)
Rkj oe2
oT
c
r2
R
RR
time
Weather Radar Equation for Volume Targets
W th R d E ti f V l T t (C ti d )
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Thus if we observe the received signal at time t, it comes froma volume of scatters between the ranges to . Because
of the random phase, the average power received is sum ofthe powers from individual precipitation particle (calledincoherent addition). Thus in radial extent, the contribution islimited to , but in angular extent (azimuth or elevation)the contribution is limited by the narrow antenna beam.
RR R
RR R
R
43
22
4 R
GP iit
The total power received from all the precipitation particles asseen by a single power sample is given by
Weather Radar Equation for Volume Targets (Continued)
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62
4
5
DKb
It was shown earlier that for a single precipitation particle isb
2
c
1
1
Weather Radar Equation for Volume Targets (Continued)
If is the volumetric reflectivity per unit volume, then the term
can be written as
where the integral for a Gaussian beam is reduced
to
432
4 R
Gii
dV
R
fGo
43
22
4
,
2
11
3
22
22ln84 R
rcTGP ooot
,2f2ln8
11
),(
),(
f
GGo where
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In radar meteorology it is conventional to express in terms of
equivalent reflectivity as
where is computed for water
25
4
w
e
kZ
2
wk 93.0
The equivalent reflectivity factor is in the radar volumeexpressed as reflectivity in units of mm6/m3. The above equationis called meteorological radar equation.
6
iD
2
25
11
32
2
2ln842 r
rZkGPcTrP
ewotor
Therefore when the receiver sees power Prat a time tafter thepulse transmission, r is obtained as , and the aboveequation is used to compute reflectivity. 2
tcr
Reflectivity measurements
Sensitivity/Minimum Detectable Signal
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The equivalent reflectivity factor iscomputed from the received power
as r
t
e PrGPcTK
Z 22
1120
3
025
2ln8421
For correct detection, the Signal-to-Noise Ratio (SNR) needs to exceed
some threshold. The noise power canbe expressed through system noisefigure:
Assuming SNR=1, the minimumdetectable reflectivity at range r0 is
kTBr
GPcTKZ
t
e2
02
1120
3
025
min 2ln8421
Sensitivity/Minimum Detectable Signal
kTBPn
System Sensitivity of the CSU-CHILL Radar
Frequency = 2.725 GHz
Peak Transmit Power = 800 kW
Antenna Gain = 42
Receiver Noise Level = -113 dBm
Pulse Width = 1 us
Beamwidth = 1 deg
Receiver Loss = 1.5 dB
The sensitivity of CSU-CHILL is -8 dBZ at 50 km
(-7.5 dB at 50 km for NEXRAD)
Resolution
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Resolution
Resolution is another figure of merits for a radar system. Highresolution observations can reveal fine weather features andimprove the accuracy of quantitative retrievals.
The sensitivity implies a preference of long pulse and wide antenna
beam, in addition to higher transmit power and larger antenna gain.
However, short pulse and narrow antenna beam are essential to
obtain high resolution measurement.
kTBr
GPcTKZ
t
e2
02
1120
3
025
min 2ln8421
Resolution (continued)
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56 Dual Polarization Weather Radar and Appli cation s
Resolution (continued)
The measured reflectivity is the mean value for the 3-D resolutionvolume.
dVr
R
fGo ),,(
4
,43
22
- The range resolution depends on the pulse width
-The cross-beam resolution depends on the beamwidth and range
- Resolution becomes poorer at far ranges
- Sidelobe can couple in unwanted echoesin other direction
2
ocTR
Sidelobe
Reason for Dual-polarization
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57 Dual Polarization Weather Radar and Appli cation s
Reason for Dual polarization
Up to now, the particle is assumed as spheres.
The radar reflectivity is an equivalent factor by assumingspherical particles.
Not always true!
Dual Polarization Measurements
http://images.google.com/imgres?imgurl=http://blog.spokanetogo.com/blogs/kris/snowflake.jpg&imgrefurl=http://blog.spokanetogo.com/blogs/kris/2007/11/&h=342&w=485&sz=70&hl=en&start=77&um=1&tbnid=_0hvFT7nx2Jo2M:&tbnh=91&tbnw=129&prev=/images?q=melting+snow+flake&start=60&ndsp=20&um=1&hl=en&rls=com.microsoft:en-US&sa=Nhttp://images.google.com/imgres?imgurl=http://images.aad.gov.au/img.py/153a.jpg&imgrefurl=http://www.classroom.antarctica.gov.au/the-big-white/1-11-snowflakes&h=425&w=640&sz=43&hl=en&start=21&um=1&tbnid=UV3tAErDZUiS1M:&tbnh=91&tbnw=137&prev=/images?q=ice+crystal&start=20&ndsp=20&um=1&hl=en&rls=com.microsoft:en-US&sa=Nhttp://images.google.com/imgres?imgurl=http://www.norcalblogs.com/watts/images/snowflakes.jpg&imgrefurl=http://jalmzpix.blogspot.com/2007/09/snowflakes.html&h=320&w=300&sz=62&hl=en&start=2&um=1&tbnid=1j2Kq1qEGDUfJM:&tbnh=118&tbnw=111&prev=/images?q=ice+crystal&um=1&hl=en&rls=com.microsoft:en-UShttp://images.google.com/imgres?imgurl=http://www.learner.org/jnorth/images/graphics/t/ice_crystal_Greg_Rob.jpg&imgrefurl=http://www.learner.org/jnorth/tm/tulips/ColdExper_IceCrystal.html&h=405&w=467&sz=24&hl=en&start=4&um=1&tbnid=jQuZM2oc-xeEMM:&tbnh=111&tbnw=128&prev=/images?q=ice+crystal&um=1&hl=en&rls=com.microsoft:en-UShttp://images.google.com/imgres?imgurl=http://www.markcassino.com/b2evolution/media/IMGP0219.jpg&imgrefurl=http://www.markcassino.com/b2evolution/index.php?m=200701&h=405&w=400&sz=75&hl=en&start=3&um=1&tbnid=sAdhJTXDbBmJ-M:&tbnh=124&tbnw=122&prev=/images?q=snow+flake&um=1&hl=en&rls=com.microsoft:en-UShttp://images.google.com/imgres?imgurl=http://www.theweatherprediction.com/severe/gianthail/hail1.jpg&imgrefurl=http://www.theweatherprediction.com/severe/gianthail/&h=400&w=600&sz=33&hl=en&start=27&um=1&tbnid=WcRK0u_ZlKNpbM:&tbnh=90&tbnw=135&prev=/images?q=hail&start=20&ndsp=20&um=1&hl=en&rls=com.microsoft:en-US&sa=Nhttp://www.crh.noaa.gov/gid/Web_Stories/2001/weather/04-06/1hail.jpghttp://images.google.com/imgres?imgurl=http://www.theweatherprediction.com/severe/gianthail/hail3.jpg&imgrefurl=http://www.theweatherprediction.com/severe/gianthail/&h=404&w=600&sz=35&hl=en&start=10&um=1&tbnid=RIupdim8kUhUCM:&tbnh=91&tbnw=135&prev=/images?q=hail&um=1&hl=en&rls=com.microsoft:en-US&sa=Nhttp://images.google.com/imgres?imgurl=http://www.tornadochaser.net/images/hail.jpg&imgrefurl=http://www.tornadochaser.net/hail.html&h=426&w=700&sz=39&hl=en&start=2&um=1&tbnid=BBIBtGcRIX1CyM:&tbnh=85&tbnw=140&prev=/images?q=hail&um=1&hl=en&rls=com.microsoft:en-US&sa=N -
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58 Dual Polarization Weather Radar and Appli cation s
Dual Polarization Measurements
Solution: observe the target using two orthogoanl polarizations(normally orthogonal).
V
H
Dual Polarization Measurements
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Dual Polarization Measurements
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Dual Polarization Measurements
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Dual Polarization Measurements
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Dual Polarization Measurements
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Polarimetric Capability
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p y
Raindrops are nonspherical and the nonspherical shape isincreased more pronounced with size.
Therefore different reflectivities are seen at horizontal andvertical polarizations.
We can measure other parameters to characterize the raindrop
in more details shape, size, orientation.
v
h
Plane containingthe electric field
Direction ofpropagation
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Summary: Weather radar fundamentals
Scattering of electromagnetic wave by precipitationparticles
Range, direction, intensity, velocity
Radar as a system
Radar coordination: azimuth, elevation, range Radar equation for a single raindrop
Weather radar equation
Concept of reflectivity
Sensitivity
Resolution
Why dual polarization?