SODAR Measurement Principle                     

The measuring principle of the mobile Doppler SODAR PCS.2000 is similar to echo sounder or RADAR technique.

The SODAR transmits short and high powered acoustic pulses of a certain frequency into the atmosphere. A small fraction of the acoustic energy is scattered back from density fluctuations of the atmosphere. Because these micro turbulent density fluctuations are moved by the mean wind flow, the frequency of the backscattered signal is shifted according to the wind component parallel to the propagation of the acoustic waves (Doppler effect). This signals can be received and the frequency shift can be determined by a sensitive receiver. By means of the propagation time of the acoustic wave and the estimated acoustic velocity the distance (or the height range) of the measuring volume can be evaluated. Because the wind vector may be described by three wind components, three independent measurements of different orientations are required. Therefore, the PCS.2000 system consists of one physical acoustic antenna which can be seen as a set of 5 logical antennas. This logical antennas are used for transmission as well as for receiving mode. In order to reduce an interference with environmental noise and to suppress the influence of echoes from fixed targets, the acoustic antenna is lined with an absorbing material. Narrow acoustic beams are achieved by usage of a phased array of 64 resp. 24 high power loudspeakers. The speakers are switched to 4 different phases (0°, 90°, 180° and 270°) to pan the beam to one of the 5 directions, one of them is directed vertically upward.

The transmit frequency, signal power, pulse length and pulse repetition rate of the acoustic pulses of PCS.2000 are either manually adjustable or automatically controlled thus providing optimised parameter matching to the given conditions at the measuring site, to the actual backscattering conditions, to the desired height resolution resp. height range.

As a special characteristic of PCS.2000 a gaussian shaped time signal rather than a rectangular pulse is used for the acoustic pulses and the receiving windows. Therefore, the spectral characteristics of the signal and of the spectral analysis is easily to compute. The backscattered signal of the acoustic wave is processed by a low noise pre-amplifier and a band-pass filter in order to limit its spectral bandwidth.

Separately for each height step the digitised signals (16-bit-analogue/digital converter) are multiplied by a gaussian shaped filter function to avoid side lopes of the spectral signal. The signals are spectral analysed by a Fourier-Transformation. The resulting instantaneous power spectra (of each acoustic pulse) are averaged for each measuring height and for each logical antenna over the whole averaging interval (incoherent sampling). In order to estimate the spectral characteristics of the background noise, two independent measurements of the background noise are performed before the transmission of the acoustic pulses and analysed in the same way as the received signal. The derived noise spectra are averaged according to the averaging of the received signal. At the end of the averaging interval the mean noise spectra are subtracted from the mean signal spectra. Due to the averaging process the statistical significance of the averaged noise spectra is highly improved as against the instantaneous noise spectra. This results in a reliable determination and elimination of the environmental noise. Furthermore, the comparison of the averaged spectra of received signal and noise yields a reliable plausibility check of the data quality.

The incoherent sampling and averaging process improves the signal/noise ratio of the received signal. Therefore, the SODAR system PCS.2000 also can be used even under bad backscattering conditions, e.g. at locations characterised by high background noise.

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