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Summer Research Fellowship Programme of India's Science Academies

Optimizing the height coverage of radio acoustic sounding system using acoustic ray tracing

Ahmer Mehmood

Undergraduate in Int.M.Tech. Geological Technology, Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India. E-mail: ahmer.mehmood191@gmail.com

Dr. T.V.C. Sarma

Scientist-SG , National Atmospheric Research Laboratory, Gadanki 517112, Tirupati, Andhra Pradesh, India. E-mail: tvcsarma@narl.gov.in

Abstract

Radio Acoustic Sounding System (RASS) is a ground based remote profiling instrument to obtain height profiles of atmospheric temperature. It consists of a wind profiling radar and collocated acoustic sources. The ambient temperature in air is a function of the local speed of sound. The speed of sound is measured using the radar. The wind profiler obtains backscatter from radio refractive index fluctuations in the atmosphere through Bragg scattering. Acoustic excitation at a wavelength that satisfies the Bragg scattering condition viz., half of radar wavelength super-imposes additional fluctuations due to contractions and rarefactions that propagate with the local speed of sound. Using the measurement of the speed of sound, temperature profile is obtained. In order to obtain useful signal from acoustic wavefronts propagating away from the radar, the backscatter should fall in the antenna array. However, due to variation of sound speed with height, the spherical acoustic wavefronts become ellipsoidal. Further, horizontal wind distorts the wavefronts. Due to these distortions, the backscatter from the acoustic wave fronts does not fall inside the antenna array most of the time. Consequently, the height coverage of RASS comes down. In order to improve the height coverage, it is important to compute the location of parts of acoustic wavefronts that would result in radar backscatter falling on the antenna array. Further, use of multiple acoustic sources has been shown to improve the height coverage of RASS under varying wind conditions. This work aims to compute the direction of useful portions of acoustic wave fronts at different heights so as to provide guidance for the use of optimum antenna beam directions. In addition, it is aimed to compute the optimum number of acoustic sources and their locations for maximizing the height coverage of RASS throughout the year. For this purpose, Lord Rayleigh’s acoustic propagation formulation along with temperature and wind speed data from radiosonde measurements are being used. The acoustic sources generally used in RASS have wide beam widths of about 100º or more. This results in high level of sound propagating in the horizontal direction. Consequently, neighbourhood experiences noise pollution. Recently, RASS using narrow beam acoustic sources based on parametric arrays has been reported. Parametric array speakers use non linear interaction of high power ultrasonic waves with air to generate audible frequencies. Ray tracing for such a speaker has also been atempted.

Keywords: wind profiler, radio acoustic sounding system, atmospheric temperature, radiosonde, acoustic ray tracing.

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