Posts Tagged ‘sodar’

Sodar: Operation and Limitations (1/2)

February 18, 2012

On last post I explained briefly some features related to this Remote Sensing (RS) apparatus, but I consider that it should be explained further. Therefore, I am going to detail other considerations related to Sodar’s technology through two posts based on the academic paper of S. Bradley, I. Antoniou et al. (2005). By now, physical principles and uncertainties will be described. Next post will focus on its calibration methods.

Sodar is a Remote Sensing apparatus that measures 3D wind speeds at high altitudes. By emitting vertical sound beams of sound, it is possible to interpreter the backscattering frequencies due to the Doppler Effect and thus, wind components can be decoded (See Fig. 1). Usually, three or five beams are necessary to obtain reliable raw data measurements. Each of them is usually tilted 15-20º (ϕ) to the vertical (See Fig. 2). Though the emitted signal produces a continuous backscattering echo after crossing the infinite turbulent layers in the atmosphere, the echo generated at the studied altitude (Z) can be recognized according to the following formula. This means that among the continuous echo signal received, the specific signal generated at the Scattering Volume at Z height is generated at t (time) moment. The same principle is used by pulsed lidars.

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Sodar’s Overview

January 3, 2012

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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.

Advantages:

– Higher altitudes than met masts.

– Cheap and easy-to-install.

– Accurate at appropriated atmospheric conditions.

Disadvantages:

– 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).

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