Radio%20Acoustic%20Sounding%20Techniques%20for%20Temperature%20Profiling

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Radio Acoustic Sounding Techniques for
Temperature Profiling

• Mrs Jyoti ChandeHead Atmospheric Remote Sensing
  DivisionSAMEER, IIT Campus, Powai, Mumbai 400076

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Why we need temperature profiles?

• For better understanding of Meteorological
  phenomenon
• Thermal perturbation excite gravity waves
• Temperature inversion layers prevent mixing of
  layers which causes trapping of hazardous
  chemicals

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Temperature profiling-Application areas-

• Meteorology
• Atmospheric research
• Study of thermal inversions,
• Measurement of heat flux
• Boundary layer research.
• Environmental monitoring applications

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Observation techniques of temperature profiles

• Direct (in-situ measurement)
• Radiosonde ( Height resolution 30 m , accuracy of
  0.5 deg K time interval 3hr)
• Remote sensing
• Radiometer
• RASS

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What is RASS?

• Radio Acoustic Sounding System
• - Combines Radio and acoustic probing
  techniques for obtaining continuous temperature
  profiles

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RASS concept
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RASS concept

• The basic concept of RASS is tracking of sound
  waves by means of electromagnetic radar.
• The compression and rarefaction of air due to
  transmitted sound waves alters the refractive
  index of air in periodic fashion causing the
  reflection of electromagnetic waves.
• For enhancing the reflected electromagnetic power
  it is essential that both acoustic and radio
  wavelength are BRAGG matched

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Bragg Matching condition

• Scattering of radio waves is intensified when
  the acoustic and radio wavelengths satisfy
  relation as follows
• ? e 2 ? a
• where ? e electrical wavelength
• ? a Acoustic wavelength

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RASS measurements

• Physical quantity inferred by the RASS is Ca
• Ca atmospheric sound velocity.
• The virtual temperature is related to speed of
  sound Ca is as follows
• Ca 20.047 ? Tv
• Tv virtual temperature
• Tv T(1 0.61x r)
• r the mixing ratio of water vapor in the air
  and T is the air temperature in deg K

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RASS realization

• RASS can be added to a
• wind profiler radar
• (Pulsed radar and FMCW acoustic)
• Acoustic sounder /Sodar system.
• (Pulsed Acoustic and CW radar)

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Windprofiler- RASS

• Three or four vertically pointing acoustic
  antennas are placed around the radar wind
  profiler's RF antenna
• Acoustic system is added which contains power
  amplifier Acoustic Signal generating unit.
• Acoustic antennas generate periodic scattering
  structure which is sampled by coherent pulsed
  electromagnetic radar.

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RASS added to an acoustic sounder

• The radar subsystems are added to transmit and
  receive radar signals and to process the
  reflected radar echo information.
• The sodar transducer are used to transmit the
  acoustic signals that produces the Bragg
  scattering of the radar signals.
• The speed of sound is measured by the CW
  electromagnetic radar

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Height Coverage

• The Maximum height coverage for Temperature
  profiles basically depends on
• System parameters (wave length, antenna Size,
  acoustic power and Radar Power)
• Atmospheric parameters ( turbulence, winds and
  humidity)
• Distance between the Acoustic and RF systems
• Acoustic attenuation

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Acoustic attenuation

• Sound is absorbed in air by several processes.
• Absorption is a complicated function of
• Frequency
• Temperature
• Humidity...

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Signal to Noise Ratio -for Wind profiler/ RASS
mode

• The back-scattered echo power is given as
• (c ?/2) P a Ga Pr
• Pr 3.7 x 10 -14 ------------------- x 10
  -?R/10 x I (? r R ) 2 B
• where ,
• Pr Averaged received power
• (c ?/2) radar range resolution (m)

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SNR

• c Speed of light (3 x 10 8 m/s) ? radar pulse
  width
• ? r radar wavelength in meters
• R range in meters
• Pa transmitted acoustic power in watts
• Ga gain of acoustic antenna
• Pr Transmitted radar power in watts
• B 2 ? b/Ca acoustic wave number bandwidth
  b acoustic frequency bandwidth
• ? acoustic attenuation
• The factor I in equation describes the
  attenuation of the received signal due to
  atmospheric effects

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Acoustic Excitation in pulsed radar

• CW acoustic excitation
• A short acoustic pulse completely enclosed within
  radar pulse.
• A Long acoustic pulse where only part of acoustic
  pulse lies within resolution volume
• FMCW acoustic excitation

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Q
Ca
A
?R
?a
I
T
Peak is always at Ca Transmitted acoustic freq
CW excitation and resulting phasor diagram
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Q
Ca
A
?R
?a
I
T
Peak is at Bragg freq
Short acoustic pulse and resulting phasor diagram
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Q
Ca
A
?R
?a
I
T
Two Peaks of approx equal magnitude at Ca
Bragg freq
Long Pulse and resulting phasor diagram
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FMCW
Q
Ca
A
?R
?a
I
T
Sharp peaks only at Bragg frequency
FMCW acoustic transmission
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RASS installed at India Meteorology Department
(IMD) Pune
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Atmospheric humidity

• The relationship between acoustic speed and
  atmospheric temperature for dry air is given by
• Ca A ?T
• Where Ca Acoustic Speed
• T Atmospheric Temperature in oK.
• Under the assumption that atmosphere is dry and
  obeys the ideal gas law We have equation
• A ? (? R / M) 20.053
• ? is ratio of specific heats
• R is the gas constant
• M is mean molecular weight of air.

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Effect of Atmospheric Parameters on Measurement
Accuracy of RASS

• Accuracy of the temperature profiles obtained by
  the RASS technique depends upon atmospheric
  variables
• Humidity
• Vertical winds..

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Humidity correction

• Assumption of dry and still atmosphere is not
  valid in the lower troposphere.
• It was observed that at a given temperature ,
  speed of sound varies with humidity .
• Ca 20.053 A ?T
• where
• A constant depending on Relative humidity ()
• For ex for 100 humidity A 1.0033

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Errors due to Vertical Wind Velocities

• The vertical winds introduce errors in the
  temperature measured by RASS.
• ?T 1.6 W
• where W is in m/sec.
• This error can be reduced by measuring the mean
  vertical velocity simultaneously and subtracting
  this from the acoustic speed at that height.

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Typical RASS spectrum
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Temperature profiles derived from RASS spectrum
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RASS implemented with WindprofilerSpecifications-

• Transmitted Acoustic Power is 100 W (electrical)
• Type of Antennas Parabolic reflector with
  acoustic transducer/ horn assembly
• Antenna gain 15 dB
• 3 dB beam width 16 degrees.
• No of Antennas Three ( switchable)
• Type of waveform FMCW

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Acoustic waveform design

• Range of acoustic frequencies to be transmitted
  depend on the variation of temperature in the
  desired range .
• The expected temperature variation is from -50 0C
  to about 50 oC. Sound velocities at these
  temperatures would be ranging from 298 m/s to
  356.65 m/s (? 30 m/s).
• The corresponding acoustic frequencies are 805
  Hz and 960 Hz .
• Thus a frequency modulated linear sweep of
  bandwidth 156 Hz ranging from 805 Hz to 961 Hz is
  required to be transmitted for getting Bragg
  matched conditions satisfied at all the range
  bins of our interest..

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Temperature resolution

• Temperature resolution depends on the ability of
  system to resolve Doppler frequencies
• For highest temperature ( 45 oC) the velocity
  resolution should be of the order of 0.16 m/s or
  the Doppler resolution should be of the order of
  0.45 Hz.
• This is achieved with Wind profiler system by
  keeping the data observation time for about 2
  sec.

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RASS II RASS implemented with acoustic sounder
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RASS II SPECIFICATIONS

• Radio Frequency 712.5 MHz
• Acoustic Frequency 1600-1700 Hz
• Range Resolution 50 meters
• Maximum Range 800-1000 meters
• Minimum range 50 meters
• Temperature measurement range -100 to 500 C
• Temperature resolution 0.30 K

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RASS SUBSYSTEMS

• 1. Tx and Rx RF Antennas
• 2. Transmitter (712 MHz)
• 3. Exciter
• 4. Receiver
• 5. Acoustic Source and Antenna
• 6. Digital Signal Processing

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Schematic block diagram of CW RASS
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ANTENNA

• Type Parabolic dish
• Frequency 712.5 MHz
• Diameter 1.5 m
• Gain 20 dB
• Bandwidth 20 MHz

Tx. Antenna
Rx. Antenna
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TRANSMITTER

• Frequency 712.5 MHz
• Power 25 W CW
• Harmonics lt 30 dBc
• Type Solid State
• Bandwidth 10 MHz

Radar Hardware
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EXCITER

• Reference Oscillator, OCXO (70 MHz)
• Generation of RF and LOs
• 642.5 MHz PLL

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RECEIVER

• Type Super Heterodyne
• Bandwidth lt 250 Hz
• Noise Figure lt 3 dB
• Receiver Sensitivity -131 dBm

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Acoustic Source Antenna

• Frequency 1600-1700 Hz
• Power 116 Watts (Peak)
• Pulse Width 120 ms (Variable)
• PRP 3 Sec. (Variable)
• Beam Width lt 100
• Transducer Eff. 25

Acoustic Antenna
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Temperature Data comparison with RS/RW
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CW RASS Outdoor Field equipment
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CW RASS equipment shelter
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Thank You