| To get the temperature profile in
a height range comparable to the SODAR (several hundred meters) the RASS ('Radio Acoustic
Sounding System') measuring principle uses the reflections of
electromagnetic waves at refractivity fluctuations. These required refractivity
fluctuations occur in the natural atmosphere. They are generated by its turbulent
inhomogenities but have very different intensities. For this reason the RASS uses
artificial refractivity fluctuations generated by acoustic pulses from the vertical
antenna A3 of the SODAR system. This insures a reasonable intensity at a known
distribution of wave numbers. RADAR frequency and acoustic frequency must meet the Bragg
condition to guarantee a constructive interference. This means that the wavelength of the
acoustic waves is half of the wavelength of the electromagnetic waves.
Because the acoustic frequency may vary strongly with the
time and the height, which is caused by influence of temperature, the RASS does not use
only one single acoustic frequency but a signal covering a specific bandwidth. The mean
frequency of the transmitted acoustic signal is adjusted to the measured ground
temperature. For an electromagnetic frequency of 1274 MHz (ëe = 0.23 m) this corresponds
to a acoustic wavelength of ëa = 0.115 m resulting in a typical mean frequency of 2900 Hz
(depending on temperature). Similar to the measuring principle of the SODAR the reflected
RADAR waves will show a frequency shift in comparison with the transmitted waves, which is
a measure of the sound velocity. The air temperatures in the different heights can be
derived from the sound velocities which must be corrected for the vertical wind measured
by the SODAR. The analysing of the received signal is done by the same algorithms as
described above for the SODAR.
The electromagnetic transmitter is running in CW2-mode. The
used bandwidth is very small therefore and only limited by the frequency stability of the
oscillator. Due to the CW-mode the RASS uses separate vertical aligned electromagnetic
antennas for transmitting and receiving. |
