Radio Acoustic Sounding Techniques for
Temperature Profiling
Mrs Jyoti Chande
Head 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-
Temperature Vs Height
05.30 09/03/2002
Meteorology
Atmospheric research
Study of thermal inversions,
Measurement of heat flux
Boundary layer research.
Environmental monitoring applications
1000
900
800
Height (m)
•
•
•
•
•
•
1100
700
600
500
400
300
200
100
0
0
10
20
30
40
Temperature Tv (C)
___RASS, -----RS/RW
50
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 = 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
( r R ) 2 B
where ,
• Pr : Averaged received power
• (c /2) : radar range resolution (m)
xI
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
Q
Ca
A
R
a
T
I
Peak is always
at Ca
Transmitted
acoustic freq
CW excitation and resulting phasor diagram
Q
Ca
A
R
a
T
I
Peak is at
Bragg freq
Short acoustic pulse and resulting phasor
diagram
Q
Ca
A
R
a
T
I
Two Peaks of
approx equal
magnitude at
Ca & Bragg
freq
Long Pulse and resulting phasor diagram
FMCW
Q
Ca
A
R
a
T
I
Sharp peaks
only at Bragg
frequency
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 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.
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..
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.
Acoustic
Frequency
Vertical Doppler
( 2 Hz)
Frequency
970
Hz
805
HZ
L O without Offset
Acoustic
Frequency
- 80 Hz
+80 Hz
L O with 890 Hz
Offset
Acoustic Frequency
Vertical
Doppler
445
525
365
L O with 445 Hz
Offset
Typical RASS spectrum
Temperature 31/07/2002 (12GMT)
3
Height(km)
2.5
2
1.5
1
0.5
0
5
10
15
20
25
30
Tem perature( deg c)
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.
2.
3.
4.
5.
6.
Tx and Rx RF Antennas
Transmitter (712 MHz)
Exciter
Receiver
Acoustic Source and Antenna
Digital Signal Processing
Rx. Antenna
Tx. Antenna
SODAR
Antenna
antenna
Rx. Antenna
Tx. Antenna
SODAR
Antenna
antenna
Transmitter
Receiver
Transmitter
Receiver
Exciter
Exciter
Acoustic
Source
Acoustic
Source
Digital Signal
Processing
Digital
Signal
Processing
Block diagram of RASS system
Fig.4
Block diagram
of RASS system
SchematicFig.4
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 Height
17.30 06/03/2002
1100
1000
900
800
Height (m)
700
600
500
400
300
200
100
0
0
10
20
30
40
50
Temperature Tv (C)
___RASS, -----RS/RW
Temperature Data comparison with RS/RW
CW RASS Outdoor Field equipment
CW RASS equipment shelter
Temperature Vs Height
5.30 08/03/2002
1100
Temperature Vs Height 1000
05.30 09/03/2002
900
800
Height (m)
1100
1000
900
Height (m)
800
700
600
500
400
700
300
600
200
500
400
100
300
0
0
200
10
20
30
40
Temperature Tv (C)
_____ RASS, ------ RS/RW
100
0
0
10
20
30
40
50
Temperature Tv (C)
___RASS, -----RS/RW
Thank You
50