EFFECT OF ATMOSPHERIC-TURBULENCE ON THE SPOT SIZE OF A RADIOACOUSTIC SOUNDING SYSTEM - A CALCULATION REVISITED

A theory developed almost 20 years ago suggests that atmospheric turbu lence does not significantly alter the average spot size of the radar return from a radio acoustic sounding system (RASS). This theory predi cts that the primary effect of turbulence is a reduction of the on-axi s intensity and the formation of extended tails in the spot profile, w ith little or no broadening of the central core of the spot. For a nar row-beam radar, the width of the central core was found to be proporti onal to the diameter of the radar transmitting antenna. We demonstrate that this conclusion is based on an erroneous numerical evaluation of a fourfold integral that rigorously describes the spot profile. We us e an alternate formulation to show that the spot profile can be expres sed as a convolution of the radar antenna illumination function and a function whose width depends on the strength of turbulence. This convo lution is governed by the ratio W/rho(0), where W is the horizontal di mension of the radar beam and rho(0) is the characteristic transverse scale of the turbulence-induced acoustic wavefront distortions, both e valuated at the range R of interest. In weak turbulence (i.e., when W much less than rho(0)) the spot profile approximates the antenna illum ination function. In strong turbulence (i.e., when rho(0) much less th an W) the spot profile is significantly broadened and has a diameter p roportional to lambda R/rho(0), where lambda is the radar wavelength. The spot broadening is consistent with recent theoretical and experime ntal work. The approximate spot profile as a function of turbulence st rength for a variety of circularly symmetric antenna illumination func tions is examined.