This is the text of the poster presentation made at the Third International Symposium on Tropospheric Profiling: Needs and Technologies, Hamburg, Germany, Aug. 30- Sept. 2, 1994.
RASS temperature profiling at LaMP/OPGC.Cordesses R., G. Dubosclard, J. Fournet-Fayard, T. Ouazzani-Touhami and Y.B. Pointin : LaMP-OPGC, 24 avenue des Landais, 63177 Aubière, France
RASS = RADIO ACOUSTIC SOUNDING SYSTEM
Remote sensing technique for the measurement of virtual temperature profiles.
The speed of sound in air is given by : where is the virtual temperature
A tone - burst of sound, propagating upward in the troposphere is tracked by a Doppler radar (VHF profiler in our case). Therefore, is deduced from the radar measurement of .
Conditions for the detection of RASS echoes :
- the reflected signal must be focused on the antenna,
- the acoustic wavelength is half the radar wavelength (Bragg condition).
Limitations of the method :
- sound absorption by atmosphere (negligible in our case),
- horizontal translation of sound waves by the wind,
- distortion of sound waves by atmospheric turbulence.
In order to match the Bragg condition at any given height, a serie of short acoustic pulses is continuously transmitted (each pulse of this serie corresponding to a single frequency).
The acoustic signal is generated by a Direct Digital Synthesis (DDS) circuit controlled by a 80C552 microprocessor.
The RASS being operated in a suburban area, the transmitted acoustic power is limited to about 10 Watts.
In order to use the ST radar processing methods, the complex signal at the output of the receiver is multiplied by a 100 Hz reference signal.
Conditions of observations :
- one acoustic source near the center of radar antenna.
- no correction by vertical wind velocity.
Various parameter values
RASS OPERATING PARAMETERS
RADAR Frequency 45 MHz Pulse repetition frequency 6.4 kHz Pulse width 2.5 s Peak power 5 kW Antenna type CoCo array beamwidth 6o ACOUSTIC SOURCE Type of modulation FM CW Frequency sweeping Linear / Random Electrical power 100 W Acoustic power 10 W Acoustic antenna type woofer speaker + hornSIGNAL PROCESSING CHARACTERISTICS
Number of points for FFT 256 Number of coherent 128 256 512 1024 2048 integrations Maximum velocity (m.s-1) 83.2 41.6 20.8 10.4 5.2 Velocity resolution (m.s-1) 0.65 0.33 0.16 0.08 0.04
2. EXPERIMENTAL RESULTS
Measurements of the Doppler spectra have been made under various wind conditions and for different temperature vertical profiles. Simultaneously, PTU sounding balloons have been launched and the vertical profiles of the virtual temperature have been computed from the PTU measurements. Each virtual temperature profile (full line in the images) is then compared to the values deduced from the sound velocity measured by the RASS system (dots in the images). Just below, two examples can be obtained by a click on any underlined text in order to obtain the corresponding full scale GIF image.
(click on any underlined text in order to obtain the corresponding information)
Use of several acoustic sources around the radar antenna in order to partially compensate for the sound wave entrainment by the wind.
Signal processing with a 1024 points FFT in order to simultaneously measure the radial wind and the sound velocities.