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.

1. INTRODUCTION

Basic Principals

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.

Technical settings

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 +  horn                    

    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  

Schematic drawing (GIF image 10 Ko)

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.

• This is an example of the Doppler spectra (15 Ko) obtained under very light wind conditions and the corresponding virtual temperature comparison (5 Ko).

• This is an example of the Doppler spectra (15 Ko) obtained under light wind conditions and the corresponding virtual temperature comparison (5 Ko).

3. CONCLUSIONS

(click on any underlined text in order to obtain the corresponding information)

Future work

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.

Y.Pointin@opgc.univ-bpclermont.fr

Début de page

Mise à jour:15 mars 1996