Determination of aircraft wake vortex radar cross section due to coherent Bragg scatter from mixed atmospheric water vapor

Abstract

Remote detection and tracking of wing-tip-generated wake vortices are important for hazard avoidance, especially near airports. Aircraft that fly through these hazardous vortices experience sudden induced roll. Experiments have demonstrated that there is sufficient radar cross section for remote detection at frequencies ranging from VHF to C band (100 MHz to 5 GHz). The mechanism that yields this radar cross section is Bragg scattering from the index of refraction variations due to the atmospheric water vapor being mixed by the wake vortex system. Refractive index variations of the size that correspond to half the operating radar wavelength produce the observed radar return. Previous analysis has postulated turbulence within the wake vortex to be the generator of the index of refraction variations. In this work, a new mechanism is identified that does not assume turbulence within the wake vortex system. This "laminar flow mechanism" mixes the stratified atmosphere as the wake vortex system swirls and descends, which causes refractive index structure that stretches into successively smaller spirals over time. The results are quantitatively consistent with experimental data.