Radio Acoustic Sounding Techniques for Temperature Profiling Mrs Jyoti Chande Head Atmospheric Remote Sensing Division SAMEER, IIT Campus, Powai, Mumbai

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

Temperature profiling - Application areas- Meteorology Atmospheric research Study of thermal inversions, Measurement of heat flux Boundary layer research. Environmental monitoring applications

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

What is RASS? Radio Acoustic Sounding System - Combines Radio and acoustic probing techniques for obtaining continuous temperature profiles

RASS concept

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

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

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 =  Tv Tv: virtual temperature Tv = T( x r) r :the mixing ratio of water vapor in the air and T is the air temperature in deg K

RASS realization RASS can be added to a 1.wind profiler radar (Pulsed radar and FMCW acoustic) 2.Acoustic sounder /Sodar system. (Pulsed Acoustic and CW radar)

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.

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

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:

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

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 x 10 -  R/10 x I ( r R ) 2 B where, Pr : Averaged received power (c  /2) : radar range resolution (m)

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

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

RR I Q A T Ca a CW excitation and resulting phasor diagram Peak is always at Ca Transmitted acoustic freq

RR I Q A T Ca a Short acoustic pulse and resulting phasor diagram Peak is at Bragg freq

RR I Q A T Ca a Long Pulse and resulting phasor diagram Two Peaks of approx equal magnitude at Ca & Bragg freq

RR I Q A T Ca a Sharp peaks only at Bragg frequency FMCW FMCW acoustic transmission

RASS installed at India Meteorology Department (IMD) Pune

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 o K. Under the assumption that atmosphere is dry and obeys the ideal gas law We have equation A =  (  R’ / M) = –  is ratio of specific heats –R’ is the gas constant – M is mean molecular weight of air.

Accuracy of the temperature profiles obtained by the RASS technique depends upon atmospheric variables Humidity Vertical winds.. Effect of Atmospheric Parameters on Measurement Accuracy of RASS

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 = * A’  T where A’: constant depending on Relative humidity (%) For ex: for 100% humidity A’:

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.

Vertical Doppler (  2 Hz) Acoustic Frequency 970 Hz 805 HZ Frequency L O without Offset

Acoustic Frequency L O with 890 Hz Offset - 80 Hz+80 Hz

Acoustic Frequency Vertical Doppler L O with 445 Hz Offset

Typical RASS spectrum

Temperature profiles derived from RASS spectrum

RASS implemented with Windprofiler Specifications- 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

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 C to about +50 o C. Sound velocities at these temperatures would be ranging from 298 m/s to 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..

Temperature resolution Temperature resolution depends on the ability of system to resolve Doppler frequencies For highest temperature ( 45 o C) 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.

RASS II RASS implemented with acoustic sounder

RASS II SPECIFICATIONS  Radio Frequency : MHz  Acoustic Frequency: Hz  Range Resolution: 50 meters  Maximum Range: meters  Minimum range: 50 meters  Temperature measurement range: to 50 0 C  Temperature resolution: K

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

Transmitter Exciter Tx. Antenna Receiver Digital Signal Processing Rx. Antenna antenna Acoustic Source SODAR Antenna Fig.4 Block diagram of RASS system Transmitter Exciter Tx. Antenna Receiver Digital Signal Processing Rx. Antenna antenna Acoustic Source SODAR Antenna Fig.4 Block diagram of RASS system Schematic block diagram of CW RASS

ANTENNA  Type : Parabolic dish  Frequency: MHz  Diameter: 1.5 m  Gain: 20 dB  Bandwidth: 20 MHz Tx. Antenna Rx. Antenna

TRANSMITTER  Frequency :712.5 MHz  Power : 25 W CW  Harmonics : < 30 dBc  Type:Solid State  Bandwidth :10 MHz Radar Hardware

EXCITER  Reference Oscillator, OCXO (70 MHz)  Generation of RF and LO’s  MHz PLL

RECEIVER  Type:Super Heterodyne  Bandwidth: < 250 Hz  Noise Figure : < 3 dB  Receiver Sensitivity: -131 dBm

Acoustic Source & Antenna  Frequency: Hz  Power: 116 Watts (Peak)  Pulse Width: 120 ms (Variable)  PRP: 3 Sec. (Variable)  Beam Width: < 10 0  Transducer Eff.: 25% Acoustic Antenna

Temperature Data comparison with RS/RW

CW RASS Outdoor Field equipment

CW RASS equipment shelter

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