PPT.ppt

RADAR PRINCIPLES AND METEOROLOGY

1. Distinguish between continuous and pulsed wave RADARS. Explain the
working of wind profiler RADAR with a diagram.
2. Explain thunderstorm and environmental applications of wind profiler.
Write a note on RADAR signal processing.
3. Write a note on FM-CW radar. Give a few examples.
4. Explain the principle and working of pulsed Doppler radar. Obtain an
equation for received power from a pulsed Doppler radar.
5. Explain the aviation and tropical cyclone from pulsed radar observations.
6. Write down the mechanisms involved in Bragg and Rayleigh scaterring.
How synoptic and mesoscale atmosphere be studied by using WPR.
7. Describe the radar scattering and wind vector calculations.
QUESTION BANK:

INTRODUCTION TO RADAR
TYPES OF RADARS
Mono-static, pulsed radar, FM-CW Radar
BASICS PRINCIPLES OF PULSED (WIND
PROFILER) RADAR
Antenna Basics- radar signal processing
TYPES OF RADAR SCATTERING THEORY
WIND PROFILER APPLICATIONS
Aviation, Tropical Cyclone, Thunderstorm,
Meteorological (Synoptic and Mesoscale) and Environmental
SYLLABUS:
WIND VECTOR CALCULATIONS

RAdio Detection And Ranging
LIght Detection And Ranging
wawelength
RADIO RADAR
Buderi – 1996 : THE INVENTION THAT CHANGED THE WORLD
Young and Taylor – 1934 : PULSES OF ENERGY
INTRODUCTION

TYPES OF RADARS
Monostatic and Bistatic radar
CW and pulsed radar
Doppler radar
FM-CW radar
Wind profilers and aircraft radars
Airborn radar
Shipboard radar
Weather radar
Dual-wavelength radar
Polarization-diversity radar

RAdio Detection And Ranging (RADAR)
INTRODUCTION #1
 This term was first used by the U.S. Navy in 1940 and adopted universally in
1943. It was originally called Radio Direction Finding (R.D.F.) in England.
 The history of radar includes the various practical and theoretical discoveries of
the 18th, 19th and early 20th centuries that paved the way for the use of radio as
means of communication. Although the development of radar as a stand-alone
technology did not occur until World War II, the basic principle of radar detection
is almost as old as the subject of electromagnetism itself.
 Some of the major milestones of radar history are as follows:
 1842 It was described by Christian Andreas Doppler that the sound waves from a source coming
closer to a standing person have a higher frequency while the sound waves from a source going
away from a standing person have a lower frequency. That approach is valid for radio waves, too.
In other words, observed frequency of light and sound waves was affected by the relative motion
of the source and the detector. This phenomenon became known as the Doppler Effect
 1860 Electric and magnetic fields were discovered by Michael Faraday.
 1864 Mathematical equations of electromagnetism were determined by James Clark Maxwell.
Maxwell set forth the theory of light must be accepted as an electromagnetic wave.
Electromagnetic field and wave were put forth consideration by Maxwell.

 1886 Theories of Maxwell were experimentally tested and similarity between radio and light
waves was demonstrated by Heinrich Hertz.
 1888 Electromagnetic waves set forth by Maxwell were discovered by Heinrich Hertz. He showed
that radio waves could be reflected by metallic and dielectric bodies.
 1900 Radar concept was documented by Nicola Tesla as .Exactly as the sound, so an electrical
wave is reflected ... we may determine the relative position or course of a moving object such as
a vessel... or its speed.“
 1922 Detection of ships by radio waves and radio communication between continents was
demonstrated by Guglielmo Marconi.
 1922 A wooden ship was detected by using CW radar by Albert Hoyt Taylor and Leo C.Young.
 1925 The first application of the pulse technique was used to measure distance by G. Breit and
M. Truve.
 1940 Microwaves were started to be used for long-range detection.
 1947 The first weather radar was installed in Washington D.C. on February 14.
 1950 Radars were put into operation for the detection and tracking of weather phenomena such
as thunderstorms and cyclones.
 1990’s A dramatic upgrade to radars came in with the Doppler radar.
INTRODUCTION #2

The frequency of the em wave used depends on the
application. Some frequencies travel through clouds with
virtually no attenuation.
ALL em waves move at the speed of light

Pat Arnott, ATMS360
INTRODUCTION TO METEOROLOGICAL RADAR

ELECTRO-MAGNETIC WAVES
Radio & radar electro-magnetic radiation

c
f 
f : 1 Hz=1 cycle / second
c: m/s
: m

REFRACTIVE INDEX
u
c
n 
c : the speed of light in a vacuum
u: the speed of light in a medium
n: refractive index
cu (always) n1 (unitless parameter)
Actually, it has two components ; ik
n
m 
 1


i
k Absorption of coefficient of the medium
For air; m=1.003

REFRACTIVITY [N]
2
7
10
.
03
.
4
4810
6
.
77
f
N
T
e
P
T
N e









Atm. Press. (mb, hPa)
Temp. (°K)
Vap. Press. (mb, hPa)
Num. of free electron / m3
Freq. of the radar (Hz.)
Under normal atmospheric contions;
Ground N
Z N

There are two Types of Radar Detectors:
1. Pulse radar
2. Continuous-wave radar
TYPES OF RADARS

Brief explanation of major types of the radars are also
given below:
 Mono static Radars
Mono static radars use a common or adjacent antennas
for transmission and reception, where the radars
receiving antenna is in relationship to its transmitting
antenna. Most radar system are use a single antenna for
both transmitting and receiving; the received signal
must come back to the same place it left in order to be
received. This kind of radar is mono static radar.
Doppler weather radars are mono static radars.

Bistatic Radars
Bistatic radars have two antennas. Sometimes these are side by side
but sometimes the transmitter and its antenna at one location and the
receiver and its antenna at another. In this kind of radar the
transmitting radar system aims at a particular place in the sky where a
cloud or other target is located. The signal from this point is scattered
or reradiated in many directions, much of being in a generally
forward direction. Such receiving systems may also be called passive
radar systems

Air Surveillance Radars (ASR)
The ASR system consists of two subsystems:
primary surveillance radar and secondary surveillance
radar. The primary surveillance radar uses a continually
rotating antenna mounted on a tower to transmit
electromagnetic waves, which reflect from the surface of
aircraft up to 60 nautical miles from the radar. The radar
system measures the time required for the radar echo to
return and the direction of the signal. From this data, the
system can measure the distance of the aircraft from the
radar antenna and the azimuth or direction of the air craft
from the antenna.

The primary radar also provides data on six levels of rainfall
intensity. The primary radar operates in the range of 2700 to
2900 MHz. The secondary radar, also called as the beacon
radar, uses a second radar antenna attached to the top of the
primary radar antenna to transmit and receive aircraft data
such as barometric altitude, identification code and emergency
conditions. Military and commercial aircraft have
transponders that automatically respond to a signal from the
secondary radar with an identification code and altitude.

SAR is being used in air and space-borne systems for remote
sensing. The inherent high resolution of this radar type is achieved
by a very small beam width which in turn is generated by an
effective long antenna, namely by signal-processing means rather by
the actual use of a long physical antenna. This is done by moving a
single radiating line of elements mounted e.g. in an aircraft and
storing the received signals to form the target picture afterwards by
signal processing. The resulting radar images look like photos
because of the high resolution. Instead of moving radar relatively to
a stationary target, it is possible to generate an image by moving the
object relative to stationary radar. This method is called Inverse SAR
(ISAR) or range Doppler imaging.
Synthetic Aperture Radars(SAR)

Continuous Wave (CW) Radars.
The CW transmitter generates continuously un
modulated RF waves of constant frequency which pass
the antenna and travel through the space until they are
reflected by an object. The isolator shall prevent any
direct leakage of the transmitter energy into the receiver
and thus avoid the saturation or desensitization of the
receiver which must amplify the small signals received
by the antenna. The CW radar can only detect the
presence of a reflected object and its direction but it
cannot extract range for there are no convenient time
marks in which to measure the time interval. Therefore
this radar is used mainly to extract the speed of moving
objects. The principle used is the Doppler Effect.

The inability of simple CW radar to measure range is related to the relatively
narrow spectrum (bandwidth) of its transmitted waveform. Some sort of timing mark
must be applied to the CW carrier if range is to be measured. The timing mark
permits the time of transmission and the time of return to be recognised. The sharper
or more distinct the mark, the more accurate is the measurement of the transit time.
But the more distinct the timing mark, the broader will be the transmitted spectrum.
Therefore a certain spectrum width must be transmitted if transit time or range is to
be measured.
The spectrum of a CW transmission can be broadened by the application of
modulation, either by modulating the amplitude, the frequency or the phase. An
example of the amplitude modulation is the pulse radar.
FM-CW Radars

Pulse Radars
Pulse radar is primary radar which
transmits a high-frequency impulsive signal
of high power. After this a longer break in
which the echoes can be received follows
before a new transmitted signal is sent out.
Direction, distance and sometimes if
necessary the altitude of the target can be
determined from the measured antenna
position and propagation time of the pulse-
signal. Weather radars are pulse radars.

Doppler Radars
Conventional radars use MTI in order to
remove clutter as explained above. This processing
system is used almost entirely to eliminate
unwanted clutter from the background, selecting as
targets only those objects which move with some
minimum velocity relative to the radar or to the
fixed background. A more advanced type of system
is the pulse Doppler radar, defined as a pulsed
radar system which utilizes the Doppler Effect for
obtaining information about the target, such as the
target’s velocity and amplitude and not to use it for
clutter rejection purposes only.

TYPES OF RADARS
Monostatic Radars
Monostatic radars use a common or adjacent antennas for
transmission and reception, where the radars receiving antenna is in
relationship to its transmitting antenna. Most radar system are use a
single antenna for both transmitting and receiving; the received signal
must come back to the same place it left in order to be received. This
kind of radar is monostatic radar. Doppler weather radars are
monostatic radars.
Bistatic Radars
Bistatic radars have two antennas. Sometimes these are side by side
but sometimes the transmitter and its antenna at one location and the
receiver and its antenna at another. In this kind of radar the
transmitting radar system aims at a particular place in the sky where a
cloud or other target is located. The signal from this point is scattered
or reradiated in many directions, much of being in a generally forward
direction. Such receiving systems may also be called passive radar
systems.

TYPES OF RADARS
Air Surveillance Radars (ASR)
The ASR system consists of two subsystems: primary surveillance radar
and secondary surveillance radar. The primary surveillance radar uses a
continually rotating antenna mounted on a tower to transmit
electromagnetic waves, which reflect from the surface of aircraft up to 60
nautical miles from the radar. The radar system measures the time required
for the radar echo to return and the direction of the signal. From this data,
the system can measure the distance of the aircraft from the radar antenna
and the azimuth or direction of the aircraft from the antenna. The primary
radar also provides data on six levels of rainfall intensity. The primary radar
operates in the range of 2700 to 2900 MHz. The secondary radar, also
called as the beacon radar, uses a second radar antenna attached to the
top of the primary radar antenna to transmit and receive aircraft data such
as barometric altitude, identification code and emergency conditions.
Military and commercial aircraft have transponders that automatically
respond to a signal from the secondary radar with an identification code
and altitude.

27
55
2.14 dB
Dipole
360
0 dB
Isotropic
Beamwidth -
3 dB
Gain (over
isotropic)
Shape
Name Radiation Pattern
20
30
50
200
25
14.7 dB
10.1 dB
-0.86 dB
3.14 dB
7.14 dB
Parabolic
Dipole
Helical
Turnstile
Full Wave Loop
Yagi
Biconical Horn
15
15 dB
Horn
360x200
14 dB

What is an antenna?
 Region of transition between guided and free space propagation
 Concentrates incoming wave onto a sensor (receiving case)
 Launches waves from a guiding structure into space or air
(transmitting case)
 Often part of a signal transmitting system over some distance
 Not limited to electromagnetic waves (e.g. acoustic waves)

Wind Profiler Radar Doppler Weather Radar
OPERATION OF PULSED RADAR:

How does the radar wind profiler compare to Doppler
radar ?
What is a radar wind profiler? How does the radar wind
profiler work?
What do we use radar wind profilers for ?
 A first application of lower atmospheric wind profilers has been the study of flow in the
troposphere (i.e. below about 5 km height)), especially waves and turbulence
In meteorology, wind data may be used for continental-scale and local weather models, and air
pollution studies
In climatology, the long term variation of the wind field is of interest, as well as the occurrence of
specific events, statistics of waves, turbulence characteristics, etc.
In atmospheric research, a single profiler or a network may be used in connection with other
instruments and models to describe specific phenomena like fronts, topographic effects, exchange
of airmasses at great height, etc.
The real-time monitoring of wind profiles can provide information in connection with air pollution
and safety in high-risk areas such as chemical and nuclear plants.
INTRODUCTION TO WIND PROFILER RADARS

 Woodman and Guillen Jicamarca, Peru 1974
 First demonstration of unattended wind profiling, Platteville, 1978
 Colorado Wind Profiler Network, 1981
 MU Radar constructed in Japan, 1984
 Demonstration of wind profiler/ RASS, 1985
 Christmas Island 50 MHz profiler, 1986
 Development of UHF boundary layer radar, 1988
 First shipboard profiling, 1991
 ISS development for TOGA COARE, 1991-92
 National Profiler Network, central U.S., 1992
 National Profiler Network, Japan, 2000
MIESTONES IN THE DEVELOPMENT OF WIND
PROFILER RADAR

• Ray bending
– The index of refraction n is the ratio of the EM wave speed in vacuum (c) over that in air (u).
– N is the refractivity for air, N is:
–
– T is temperature (K), P is atmospheric pressure (mb) and e is the vapor pressure (mb)
– Snell’s law:
• Ray curvature (C), relative to height above sea level z:
–
– where R is the radius of the earth R=6374 km)
– In the atmosphere, dn/dz<0, so R’>R.
– R’~4R/3 (standard refraction)
 |dn/dz| is larger under stable conditions (inversion)
6
10
)
1
( 


n
N
u
c
n
i
r
n
n
r
i

)
sin(
)
sin(
nr>ni
i
r
'
1
1
R
z
n
R
C 


d
d











dz
de
T
dz
dT
R
g
T
dz
dn 4810
10
*
6
.
77 6


ELECTROMAGNETIC WAVE PROPAGATION:

 The echoing mechanisms that give rise to backscattering from
the clear-air atmosphere have been rather extensively
investigated.
 The main cause of radar returns from the clear air now has
been well established to be the inhomogeneities in refractive
index that result from turbulence.
 The radio refractive index n, at commonly used radar
wavelengths, is non-dispersive and depends on the atmospheric
temperature, humidity, and pressure.
BASIC THEORY OF WIND PROFILER RADAR :

• Electromagnetic pulse is sent into the Atmosphere
• Detection of the signal backscattered from refractive
index in-homogeneities in the atmosphere
• In clear air the scattering targets are the temperature and
humidity fluctuations produced by turbulent eddies
• Scale is about half of the wavelength for the
transmitted radiation (the Bragg Condition)
WIND PROFILER RADAR BASICS:

Radar beam refra
range vs height di
r
a
h
h is the height (m) above ground level (at the
radar site)
r is the radar range (m)
fo is the elevation angle:
fo
  '
sin
'
2
' 2
1
2
2
R
rR
R
r
h o 


 f
Earth
radius
R’
RADAR BEAM REFRACTIN RANGE vs HEIGHT DIAGRAM :

Anomalous propagation
• Subrefraction:
– High lapse rate (DALR), lower
– You may miss some thunderstorms within Rmax
– Thunderstorm tops may be underestimated
• Superrefraction:
– Stable conditions, high
– One can see further
– Echo heights are overestimated
– Under severe conditions, ground clutter may show up at range (ducting)

Backscatter energy for distributed targets
• Bragg scattering
– Refractivity:
– Scattering is due to turbulent motion (within the inertial subrange), causing changes in
refractivity
– Meteorologically, may be important, but only for longer wavelength radars (>5 cm)
• Particle scattering. (point or distributed targets) Amount scattered depends on:
– physical cross section (size & shape):
– scattering efficiency, which is a function of particle size/wavelength:
– state (liquid, frozen, mixed, dry, wet) - index of refraction
– concentration (number of particles per unit volume).
– Rayleigh scatter:
• targets whose diameter (D) is much smaller than the wavelength : (D < /16)
• Scattering efficiency Keff ~  -4
• Reflectivity ~ D6
– Mie: larger targets (D > /16)
Xsektion
physical
Xsektion
radar
Keff
_
_


SIMPIFIED BLOCK DIAGRAM OF A WIND PROFILR RADAR:

Basics of radar method
Pulses of EM radiation ~
1 µs long
Heterodyne detection
(Local Oscillator)
Doppler spectrum allows
velocity of target to be
measured
Polarisation of radar
beam can reveal target
shape
Height resolution for
distributed target z=½c
Pulse length 
TIME
H
e
i
g
h
t
z
 = 1 s  z = 150 m
 = 0.1 s  z = 15 m

Doppler method
Doppler shift from a moving target:
 = 2V/
When return signal is mixed with local
oscillator, the Doppler shift of the signal
is obtained.
To measure the spectrum, the signal is
sampled at intervals dt (several return
pulses combined). A Fourier transform
of N points then gives the spectrum.
dt determines the maximum
unambiguous velocity (Nyquist
frequency):
max = 1/(2dt)
Vmax = ½ max = /(4dt)
e.g.  = 6m, dt = ⅓s  Vmax = 4.5 ms-1
Number of points in FT, N, determines
separation of points in spectrum
Let T be the length of the FT; T=Ndt
dV = /(2T)
e.g.  = 6m, T = 10 s  dV = 0.3 ms-1
0
0.5
1.0
1.5
2.0
-5.0 -2.5 0 2.5 5.0
Mean Doppler shift
Spectral width
Frequency shift (velocity)
Power
spectral
density

Pat Arnott, ATMS360
Quick Overview
• Antenna emits series
of radio waves
• Listens for amount of
energy reflected back
• The better the target is
reflecting (i.e. more
raindrops) the stronger
the signal or echo will
be
Interesting Tidbit: The WSR-88D
takes about 0.0000016sec to emit a
pulse or radio wave. This means for
every hour, the Radar is “on” for 7
seconds and “listens” for the
remaining 59min and 53sec
From the National Weather Service
Greenville-Spartanburg, SC

•A wind profiler is a type of sensitive Doppler radar that uses
electromagnetic waves or sound waves to detect the wind speed
and direction at various elevations above the ground, up to the
troposphere (i.e., between 8 and 17 km above mean sea level)
•Detection of the signal backscattered from refractive
index in-homogeneities in the atmosphere
•In clear air the scattering targets are the temperature and
humidity fluctuations produced by turbulent eddies
WIND PROFILR RADAR:

• Let a pulsed electromagnetic wave be transmitted at time T1, the
pulse duration of this radar being τ . For simplification the pulse
shape is supposed to be rectangular, but in real applications it
may be a smoothed trapezoid or triangle or Gaussian shaped. In
a non-dispersive propagation medium the pulse travels with the
speed of the light c and reaches the range ra after time t1 = ra /C.
• A target at ra can scatter or reflect the radar signal in some
directions. A small fraction returns to the location of the
transmitter, where the radar echo will be received after time t2 =
2t1 = 2ra/c. This yields the basic relation r = c · t/2, which allows
the determination of the range r of any radar target by
measuring the round-trip time t.
WIND PROFILER RADAR PRINCIPLE:

Since the transmitted pulse has a finite duration τ , its
railing edge will reach the range ra at a time t1 + τ and
reach the receiver at t2 + τ . The pulse of duration τ , thus,
at one time illuminates a volume at ra extended along a
range ∆r = c · τ/2.
This is the range gate from which the radar echoes are
received. Therefore, the transmitter pulse length τ
determines the range resolution ∆r. In contrast, the
horizontal size of the scattering volume is obviously
defined by the antenna beam width.

In radar applications short pulses are normally transmitted
periodically, so that the nth pulse follows the (n – 1)th
pulse after a specific time. This time (Tn−Tn−1) is called
the interpulse period, TIPP. It’s inverse is called the pulse
repetition frequency, fPRF = 1/TIPP.
The off–on ratio of the transmitter TIPP,/τ − 1 determines
approximately the range from which radar echoes can be
unambiguously received (in unit of range resolution). It is
more customary, however, to use the ratio d = τ,/TIPP,
which is called duty cycle.

Because in normal radar operations the pulse repetition
frequency is kept constant (the transmitted pulse train is
periodic), range aliasing may occur. At time ta an echo is
received from the range ra ,and an echo is received from
range rb. Of course higher-order range aliasing can occur
from ranges rn = c · (t + (n − 1) TIPP)/2. Because these
echoes return from separate scatter volumes, the echo
signals are uncorrelated, but still their power accumulates
in the same receiver range gate. If special arrangements are
not being made (i.e., pulse-coding), the maximum
unambiguous range is rmax = c · TIPP,/2.

The minimum range rmin is obviously given by the
pulse duration τ , rmin = c·τ/2, plus some
instrumentally determined transition time between
transmission and reception. A point target within the
scattering volume defined by the antenna beam width
and pulse duration τ returns a signal whose
instantaneous voltage is
where A is the amplitude, ωc = 2πfc is the constant
carrier frequency, and ϕ is the phase relative to the
carrier phase. If the target is fixed, the phase is
constant and a function of the distance r from the
radar.
----------(3)

A moving target having a radial velocity VR returns a
signal whose phase varies with time and is given by
where λ is the incident radiation wavelength and r0 is
the initial distance. When the scattering volume
contains N point targets, the return signal is the
superimposition of individual returns.
----------(4)

The instantaneous return voltage is then
where An is the amplitude and ϕn is the phase of the return
signal from the nth scatterer.
The above expression assumes that secondary scattering
effects are negligible compared to the first-order scattering.
With the possible exception of heavy rain, snow, or hail, the
above expression is valid for atmospheric scattering.
----------(5)

The time rate of phase exchange, time derivative is
the angular frequency ωD = 4πVR/λ. It is therefore
equivalent to a Doppler frequency shift
Approaching targets have increasing phase with time,
which corresponds to a positive Doppler frequency
shift.
-----------(6)

In a pulsed Doppler radar system, the time functions
for point, or for distributed targets are available only at
discrete time intervals corresponding to the radar pulse
repetition period.
Therefore, if the radial velocity of the scatterers is
such that the phase changes by more than π (Doppler
frequency shift greater than one-half the pulse
repetition rate), an ambiguity in velocity exists.
This is equivalent to aliasing at the folding or Nyquist
frequency given by
----------(7)

where TIPP is the pulse repetition period or interpulse
period. If positive and negative frequencies can be
resolved, the unambiguous frequency range is doubled.
The unambiguous Doppler frequency range is then
From the previous equation, the maximum
unambiguous velocity is then
---------(8)
-----------(9)

A target having a cross-sectional area Ac located at
a distance r from the radar will intercept an amount
of power,
where Pt is the transmitted power and G is the
transmitting antenna gain factor .
---------(10)
WIND PROFILR RADAR EQUATION:

If the target reradiates Isotropically, the power intercepted
by the receiving antenna is
For a receiving antenna having an effective area Ae .The
relationship between effective area and its gain is
----------(11)
-------(12)

Since most targets do not scatter isotropically, it is
convenient to introduce the backscattering cross section
σ, defined as “the area intercepting the amount of power,
which, if scattered isotropically, would return an amount
of power equal to that actually received”; that is
Substituting the backscattered cross section for the
geometric cross section and replacing the effective area
with the return power becomes

where the constant Kr = PtG2λ2/64π3 depends only on the
particular radar system used and not on the scatterers.
For N targets, where σn is the cross section for the nth
scatterer, on average, the return power is
where r is the range to the center of the scattering
volume.
----------(13)
---------(14)

A slightly more useful meteorological form is obtained
by using the average radar cross section per unit
volume and multiplying by the volume, V, effectively
illuminated. This leads to
The quantity
is the so-called radar reflectivity.
---------(15)
-------(16)

Approximating the antenna pattern by a Gaussian beam
the gain is
where σϑ , σα are the standard deviations of the two-way
pattern (assumed to be at most a few degrees), ϑ and α
are, respectively, the off-axis horizontal and vertical
beam angles (assumed to be at most a few degrees), and
G0 is the on-axis gain factor.
---------(17)

Accounting for gain variations across the beam, the
exact form of the radar equation becomes
Introducing the radar reflectivity η = η (r, ϑ, α), the
summation can be expressed as a volume integral over
the pulse of the contribution region so that
----------(18)
---------(19)

Using the Gaussian beam approximation over a
volume having uniform reflectivity, integration leads
to
where τ is the pulse width and c is the propagation
speed of light.
--------(20)
The previous equation has been grouped according to the
constant (c/(1024π2 ln 2)). the measurable radar
parameters (Ptτλ2G2
0ϑ α) and target parameters (η/r2).

– Doppler Formula:
– Measurement of wind speed based on the
Doppler shift in the received signal:
– where Vr is the radial velocity of the scatterers
– Examples of Wind Profiler Doppler shift (radial
velocity 10m/s)
• 50MHz, wavelength 6m, Doppler shift 3.34Hz
• 449MHz, wavelength 0.66815m, Doppler shift 29.9Hz
• 1290MHz, wavelength 0.23m, Doppler shift 86Hz

r
D
V
f 2


DOPPLER SHIPT

2
*t
c
Range 
c = 3 x 108 m/sec
t is time to receive return
divide by 2 because pulse traveled to object and
back
DETERMING RANGE WITH WIND PROFILER RADAR

Comparison of reflectivity
• Reflectivity of scattering types:
– for perturbance in n, wavelength dependence -1/3
– backscatter from hydrometeors, dependency -4
1.00E-17
1.00E-16
1.00E-15
1.00E-14
1.00E-13
1.00E-12
0.01 0.1 1 10
Wavelength (m)
Cross-section
(1/m)
10-2 (mm6m-3) scattering from hydrometeors =dBz
10-3
10-4
Cn
2=10-14 clear air
Cn
2=10-15 clear air

Doppler Beam Swinging (DBS)
– DBS method for wind vector
calculations (u,v,w)
– radial scattered velocities
measured with one vertical
and 2 (4) off-zenith beams
– beam-pointing sequence is
repeated every 1-5 minutes
– Electronic beam pointing with
phase shifters using one
antenna
– local horizontal uniformity of
the wind field is assumed

Transmission pulse versus
resolution

Altitude of Radar Returns vs. Turbulence ( )
After Doppler Radar and Weather Observations
(1984), by Doviak and Zrnic

Basics.. Remote Sensing
– Cloud droplets are small enough to give a measurable signal
– During Precipitation backscattered signal from raindrops
may become comparable to the clear air contribution
– Backscattered signal is analyzed in the frequency domain to
extract the relative power, the Doppler shift and width of the
signal’s spectral peak
– The Doppler shift gives the radial component of the wind
velocity
– By using several antennas or electronic beam swinging the
radial velocity of the three components (u, v, w) of wind
vectors can be computed

• Scattering from atmospheric targets:
– irregularities in the refractive index of the air
– hydrometeors, particularly wet ones (rain, melting
snow, water coated ice)
• Scattering from Non-atmospheric targets:
– birds and insects (frequency dependant)
– smoke plumes
• Interfering signals:
– Ground and sea clutter
– Aircraft and migrating birds
– RFI (depends on frequency band)
Scattering Mechanism
RFI: Radio Frequency Interference is generated from spikes/surges
that usually come from - Lightning, man-made electrical equipment
noise and various transmitting equipment.

RANGE RESOLUTION
• Range Resolution:
• Long Pulse:
• Short Pulse:
2
c
4.7 ( 1410 )
s c m
  
1.57 ( 471 )
s c m
  

Typical frequencies used in wind profiling
45-65 MHz
404-482 MHz
915-924 MHz
1280-1357.5 MHz
The 915 MHz (33 cm, UHF) profiler measures the wind at low
levels, typically up to 1-3 km above ground level, depending on
atmospheric conditions, especially humidity. The top of the
atmospheric boundary layer marked by the entrainment zone is
very visible because the large humidity and temperature gradient
there cause a large change in index of refraction. The 915 MHz
profiler has fairly small antennas (at most 2x2 or 3x3 m), making
it transportable and less expensive.

A VHF wind profiler (50 MHz or 6 m) measures wind profiles
between 2 and 16, occasionally 20 km above the ground level
(AGL), but the antenna occupies 2 soccer fields (100x100m).
Frequency 50 MHz 405 MHz 915 MHz
Wavelength 600 cm 74 cm 33 cm
Antenna 100 m 13 m 2 m
The US NOAA operates a network of 400 MHz wind profilers.
These are smaller (antenna size about 10 x10 m). The higher
the frequency, the smaller the antenna, the smaller the turbulent
flow scale that is resolved.

Moments of the Average Doppler

The frequency spectra obtained for each
level are characterized by their moments:
• Doppler shift
• Spectral width
• Noise level
• Signal-to-noise ratio (SNR)
0

• Spectral Averaging
– Reduces data rate,improves detectability
• Estimation of Noise Level
• Identification of Doppler Signals
– Maximum Peak
• Construction of Doppler Profile
• Computation of Moments and SNR
Spectral Domain Processing……

Doppler Profile Analysis:
• The Doppler profiles from three beam directions from lower heights and
higher heights are available as inputs
• To analyse input data to generate the 6 minute and hourly wind profiles.
• In this process the input Doppler profiles are subjected extensive quality
assurance checks before generating the 6 minute and hourly wind profiles.
 Separation of Precipitation echoes
 Mode Merging
  Calculation of Radial velocity and height (6 min)
  Computation of Absolute Wind Velocity Vectors (UVW)
  Quality Assurance of sub-hourly velocity profiles
  Computation of Horizontal Wind Speed & direction (6 min)
  Computation of Hourly Averages

Basic Issues in Signal Processing….
Signal Detection
– Discrimination between signal and noise. (Hildebrand/Sekhon)
– Are one or more non-noise signals present in spectrum?
Signal Identification Signal Identification
– If more than one signal is detected, which one is due to the (clear (clear-air)
atmospheric return? air) atmospheric return?
– What kind of What kind of a-priori information priori information can be used
to select it?
– Can unwanted contamination be effectively filtered out without affecting
(biasing) the desired

Identification of Doppler Peaks…
• Basic Assumptions….
– There exist temporal and spatial continuities in a time
series of spectral profiles which can which can be be
employed.
– Echoes back-scattered from the atmosphere exhibit
continuity in time and height that can restrict the search
of restrict the search of signal peaks to a certain part of
the spectrum.

Identification of Doppler Peaks…
• Multiple Peak Identifications….
– Identify maximum 5 Spectral Peaks in each
range bin
– Mark spectral peaks which are below the noise
level threshold
– Compute three Moments for remaining spectral
peaks.
– Build the spectral chain across different range
bins using wind shear criteria

Doppler Peak Identification continued..
• Challenges …
– Identification of Atmospheric Targets but not
the Clear Air echoes
– Precipitation echoes
– Identification Interference Signal
– Identification of Clutter
– Identification of Non-Atmospheric Targets
– Birds, Planes, non-stationary objects from near by
buildings , roads (from Radar Side lobes)

Interferences….
• Interference from migrating birds:
– Birds act as large radar targets so that signals from birds overwhelm the weaker
atmospheric signals This can produce biases in the wind speed and direction
• Precipitation interference:
– During precipitation, the profiler measures the fall speed of rain
drops
• Ground clutter:
– Ground clutter occurs when a transmitted signal is reflected off of
objects such as trees, power lines, or buildings instead of the
atmosphere. Data contaminated by ground clutter can be
detected as a wind shift or a decrease in wind speed at affected
altitudes.
• RF Interference:
– The RF Interference signals looks similar to the CAT echoes and some times are
inseparable

Power Spectra : Vertical Beam with Precipitation echoes

Power Spectra : North Beam with Precipitation
echoes
During precipitation,
the profiler
measures the fall
speed of rain drops

Power Spectra : East Beam with Precipitation
echoes

Types of Radar scattering
When a pulse encounters a target...
It is scattered in all directions.
Of interest is the signal component
received back at the radar.
This signal is typically much weaker
than the original sent from the
transmitter and is called the "return
signal".
The larger the target, the
stronger the scattered signal.

• Scattering from atmospheric targets:
– irregularities in the index of refraction of the air
– hydrometeors, particularly wet ones (rain, melting snow,
water coated ice)
– birds and insects (frequency dependant)
– smoke plumes
• Multitude of targets may introduce serious errors
– the measured velocity is that of rain, not wind
• Interfering signals:
– ground and sea clutter
– aircraft and migrating birds
– RFI (depends on frequency band)
TYPES OF RADAR SCATTERING:

TYPES OF RADAR SCATTERING:
• Typical clear air Radar operating frequencies 50, 400, 1380
MHz.
• Clear air scattering mechanism – Bragg scatter
- λ^-1/3 dependence
• Precipitation - Rayleigh scatter - λ^-4 dependence
• If one equates the expressions for the volume reflectivity for
the two cases one obtains the relationship between the
equivalent reflectivity factor Ze & the Cn2 Thus
• dBZe = 10 log Ze = 10 {log 10 Cn2 + log λ11/3 +15.31}
where λ is in meters and Ze is in mm6/m3
• Equation is valid for scattering from water droplets
• Typical clear air Cn2 values are in the range of 10-15 to 10-18
m-2/3

– Identification of Atmospheric Targets but not the Clear
Air echoes
– Precipitation echoes
– Identification Interference Signal
– Identification of Clutter
– Identification of Non-Atmospheric Targets
– Birds, Planes, non-stationary objects from near
by buildings , roads (from Radar Side lobes)
Challenges …

Wind Measurements
Local right-hand Cartesian coordinate
Polar coordinate
x
y
U
V
W
O O
East
North
Up
M
speed
Wind
direction
Wind a

P ,
z ,

PPT.ppt

Recommended

Recommended

More Related Content

Similar to PPT.ppt

Similar to PPT.ppt (20)

Recently uploaded

Recently uploaded (20)

PPT.ppt