Radio acoustic sounding system

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radio acoustic sounding system

(Abbreviated RASS.) A ground-based remote sensing method that combines radar and acoustic techniques to determine the vertical profile of virtual temperature, often used in conjunction with wind profilers.

With the RASS technique, sound waves generated by an acoustic source located near the radar produce perturbations in the atmospheric refractive index. The perturbations propagate upward in the air at the local speed of sound and serve as a target for the radar. When the acoustic wavelength is matched to half the radar wavelength (the Bragg scattering condition), a resonance condition develops that leads to the production of a detectable radar echo. The Doppler shift of this echo is a measure of the sum of the speed of sound and any vertical air motion in the radar volume. Early work with RASS either ignored the vertical air motion or used time averaging to reduce its effect. Some modern RASS processors have the ability to correct for vertical air motion. The local speed of sound is related to the virtual temperature by
where Cs (m s-1) is the speed of sound, R is the gas constant of air, γ (= cp/cv) is the ratio of the specific heats of air, and Tv is the virtual temperature in degrees Kelvin. Using this relation, the vertical profile of sound speed can be converted to a profile of virtual temperature. The RASS technique depends on close matching of the acoustic wavelength to half the radar wavelength (called Bragg matching). To ensure that Bragg matching occurs throughout the range of temperature expected in the vertical profile, the acoustic source is swept in frequency over an interval broad enough to contain all the necessary spatial wavelengths. Bragg matching also requires that different acoustic frequency bands be used for different radar frequencies. Acoustic frequencies range from around 100 Hz for 50-MHz radars up to 2 kHz for radars operating around 1 GHz. Because acoustic attenuation in the atmosphere is strongly dependent on frequency, the height coverage or reach of RASS varies greatly with radar frequency. Typical small UHF boundary layer radars can measure RASS temperature profiles to about 1.5 km, whereas large VHF MST radars can measure the temperature up to 10 km or higher.
See also electromagnetic acoustic probe.

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