Considerations for bistatic probing of clear-air winds in the atmospheric boundary layer

Bistatic radar measurement of clear-air winds in the atmospheric boundary layer is considered. The context is three-dimensional wind field measurement using dense networks of short-range radars configured to operate in bistatic geometries. Such networks exploit a combination of Rayleigh scattering from insects and Bragg scattering from refractive index turbulence, the latter exhibiting enhanced scattering intensity in forward scatter geometries compared to the monostatic case. Bistatic radar fundamentals are reviewed, and beam-limited scattering volumes are considered. Measurements with sufficient precision (<1 m s(-1)) are achievable with relatively low average powers (100 W) with reasonably short dwell times (1 s) for transmitters and receivers separated by as much as 15 km. For a fixed antenna aperture size, frequency dependence of sensitivity for the Bragg component of the composite scattered signal is weak (lambda(2/3)), provided that the Bragg-resonant wave number for the forward scattering geometry lies within the inertial subrange of refractive index turbulence. In contrast, the strong (lambda(4)) frequency-dependent Rayleigh insect echo dominates the scattered signal for short wavelengths (i.e., X band and higher frequencies) under many conditions except for small forward scatter angles. Owing to this dominance and to the tendency for refractive index turbulence and insects to occur together in the atmospheric boundary layer, reliance on the bistatic Bragg scattering mechanism is not warranted for short-range, short-wavelength radar networks.