Sodar (Sonic Detection And Ranging) is device for measuring remotely wind profiles from the ground by projecting sound waves. By sending an acoustic pulse away at sound velocity, it interacts with density fluctuations in the air and the frequency of backscattered signal is interpreted. This is possible due to the Doppler Effect that means frequencies between the sent signal and the backscattered one are different. Hence, wind speed can be interpreted. Hence, wind profiles (wind speed and wind directions in function of height) at that place can be obtained.
Reliable Sodar ranges are typically around several hundred meters and basically, it depends on ambient noises and atmospheric humidity. First Sodars were developed in late 1950’s, but the first commercial ones appeared in 70’s in California (AeroVironment, Inc). Prizes are around 50.000 USD per unit.
– Higher altitudes than met masts.
– Cheap and easy-to-install.
– Accurate at appropriated atmospheric conditions.
– Not accurate under rainy, noisy, and low humidity conditions (sound is attenuated more rapidly in dry air).
– Data are obtained by averaging the received data. Hence, wind speed and wind direction standards are not reliable.
– Not appropriate when high obstacles are nearby (buildings, trees, towers).
Monostatic Vs Bistatic
– Monostatic. The same antenna emits and receives the signal.
– Bistatic: Very convenient in complex terrains because vertical wind speed can be measured.
Most of the modern commercial Sodars use the multi-axes Doppler alternatives, that it means they can detect backscattered signals in three (or more) directions.
It must be weather-proof and most of Sodar differ in the antenna’s approach. Alternatives:
– Parabolic dish: The speaker is located at the focal point and commonly, it is shielded to reduce environmental interferences. When it is a multi-axes Sodar, one parabolic dish points vertically, and the other ones (generally two) are slightly inclined. All of them work at different frequencies and therefore, backscattered signals do not interfere with each other.
– Array: Small high-frequency speakers (tweeters) are interconnected in a tray. By adding more tweeters, the power of the antenna can be increased and therefore it makes a smarter configuration. Furthermore, its main advantage is to steer a unique sound beam in any direction by using the phased-array technology and hence, a single antenna array can be adjusted to obtain data along multiple axes.
The main consideration is to protect the array speakers from water under rainy situations. Basically, there are two approaches:
- Folded tweeters:
- Reflector board to avoid pointing upward.
– Single Frequency Systems: When Sodar has this configuration, a distinctive pinging noise is heard. This is more suitable at low highs (15-20 m) for its simplicity and accuracy.
– Frequency-coded pulse: The transmitted pulse is comprised of several frequencies and then it is emitted serially, that means a singing noise is heard. This configuration is very convenient to reach high altitudes. However, it is tempted to loose reliability and it makes more difficult to test the Sodar at the field.
Fast Fourier Transform (FFT) is the most common configuration to derivate the Doppler shift. However, there are some alternatives to improve the signal detection and reduce noises:
– By averaging the spectra of all pulses and then locating the region of maximum spectral energy.
– By locating the region of maximum spectral energy in each pulse and then averaging the results.
Data Storage and Presentation
Generally, every Sodar offers a different configuration. However, modern Sodars offer both text data and wind distribution graphs. Furthermore, it is very convenient that Sodars must be able to provide individual wind components for quality control purposes. Generally, only average data is recorded instead of the raw input signal in order to keep the large volume of data generated.
Typical error sources
On next figure, it is possible to realize the typical error sources such us; inadequate set-up, presence of reflecting structures, under complex terrains, rainy conditions, and lack of data under neutral conditions.
In order to mitigate these errors, some researches are proposing:
– To develop an algorithm that takes into consideration the changes of the backscattered signal’s frequency under rainy conditions. Preliminary results are promising when two frequencies are emitted and received simultaneously. High frequency Sodars (>3kHz) are promptly to work better, however they must be redesigned:
- Beam width must be different at different frequencies.
- Power spectra must be expressed as a function of radial velocity, not as a function of frequency, because the Doppler frequency shift is proportional to the emitted frequency.
- The antenna characteristics need to be known at the various transmitting frequencies.
– In order to minimize fixed echo among signals that they reduce the Sodar’s reliability, it should be choose the Sodar’s orientation and use a suitable frequency.
– Besides of minimizing fixed echo, baffle diffractions should be improved in order to achieve a suitable Sodar’s set-up. This is reached by choosing an effective tilt angle.
– In order to compare different Sodars under same conditions, a transponder system was designed.
– SALFORD SUMMARY ON WP 6.1, WP.6.2, WP 6.3 AND WP 6.4, May 2011