Layer model for assessing acoustic refraction effects in echo sounding

Abstract

Acoustic refraction effects in the atmosphere by wind‐speed and temperature gradients are investigated using a simple multilayer model for plotting the ray path of a signal transmitted by an acoustic echo sounder (AES) and scattered back at various heights to be received monostatically. It is found that while nonperfect backscattering has negligible effect on the scattering cross section for acoustic waves in a turbulent medium, the effect on the ’’angle of arrival’’ and the Doppler shift is important. A purely horizontal wind can produce a Doppler shift in a vertically pointing monostatic AES which for a wind of 20 m/sec at 1 km can simulate vertical wind speeds of 50 cm/sec. For nonvertical sounding, this effect is of little importance. The generally accepted first‐order approximation for the angle of arrival of the scattered signal appears, as a result of this more detailed analysis, to be incorrect. The problems of the height resolution of a horizontal scatter layer using a nonvertical AES are briefly considered in terms of the antenna characteristics as well as the length of the transmitted pulse.