Radio Acoustic Sounding Techniques for Temperature Profiling

Mrs Jyoti ChandeHead Atmospheric Remote Sensing Division

SAMEER, IIT Campus, Powai, Mumbai 400076

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

Temperature Vs Height05.30 09/03/2002

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Temperature Tv (C)___RASS, -----RS/RW

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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

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 conditionScattering 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 = 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

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 10 -14 ------------------- 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

I

Q

A

T

Ca

a

CW excitation and resulting phasor diagram

Peak is always at Ca

Transmitted acoustic freq

R

I

Q

A

T

Ca

a

Short acoustic pulse and resulting phasor diagram

Peak is at Bragg freq

R

I

Q

A

T

Ca

a

Long Pulse and resulting phasor diagram

Two Peaks of approx equal magnitude at Ca & Bragg freq

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 humidityThe 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.

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 = 20.053 * A’ T• where

A’: constant depending on Relative humidity (%)

For ex: for 100% humidity A’: 1.0033

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 FrequencyVertical Doppler

525 365

445

L O with 445 Hz Offset

Typical RASS spectrum

Temperature 31/07/2002 (12GMT)

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1.5

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Temperature( deg c)

Hei

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t(km

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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 -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..

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.

RASS II RASS implemented with acoustic sounder

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

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 systemSchematic block diagram of CW RASS

ANTENNA

Type : Parabolic

dish

Frequency : 712.5 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

642.5 MHz PLL

RECEIVER

Type :Super Heterodyne

Bandwidth : < 250 Hz

Noise Figure : < 3 dB

Receiver Sensitivity : -131 dBm

Acoustic Source & Antenna

Frequency : 1600-1700 Hz

Power : 116 Watts (Peak)

Pulse Width : 120 ms

(Variable)

PRP : 3 Sec. (Variable)

Beam Width : < 100

Transducer Eff. : 25%

Acoustic Antenna

Temperature Vs Height17.30 06/03/2002

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Temperature Tv (C)___RASS, -----RS/RW

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Temperature Data comparison with RS/RW

CW RASS Outdoor Field equipment

CW RASS equipment shelter

Temperature Vs Height5.30 08/03/2002

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Temperature Tv (C)_____ RASS, ------ RS/RW

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Temperature Vs Height05.30 09/03/2002

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Temperature Tv (C)___RASS, -----RS/RW

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Thank You