Exponential Decay in Windblown Radar Ground Clutter Doppler Spectra: Multifrequency Measurements and Model

A new empirical model for windblown radar ground clutter Doppler spectra is developed based on many clutter spectral measurements obtained with Lincoln Laboratory's L-Band Clutter Experiment (LCE) and Phase One five-frequency (i.e., VHF, UHF, L-, S-, and X-band) instrumentation radars over spectral dynamic ranges reaching 60 to 80 dB below zero-Doppler peaks. The model includes both ac and dc spectral components. Ac spectral shape is specified as exponential, with the Doppler-velocity exponential shape factor strongly dependent on wind speed but independent of radar frequency, VHF to X-band. The exponential shape is intermediate in spectral extent between the Gaussian shape of historical usage (now acknowledged as being too narrow) and power-law shapes of more recent usage (which are too wide when extrapolated to levels 60 to 80 dB down). The ratio of dc to ac spectral power in the model is determined by an empirically derived analytic expression that captures the strong dependencies of dc/ac ratio on both wind speed and radar frequency in the measurement data. Many examples of windblown clutter spectra are provided and compared with model predictions, encompassing variations in wind speed and radar frequency, as well as in other parameters such as polarization, range, cell size, grazing angle, wind direction, measurement site, and season of the year. Although the exponential model is explicitly derived to be applicable to windblown trees, examples are also provided of measured clutter spectra from scrub desert, rangeland, and cropland vegetations which indicate that the model can also perform adequately for other windblown vegetation types by suitably adjusting its dc/ac term. In addition, the moving target indicator (MTI) improvement factor for a single delay-line canceller in exponentially distributed clutter is derived and compared with that for Gaussian clutter; and the theoretical and experimental literature on windblown radar clutter is reviewed, including the somewhat apocryphal evidence that spectral tails very much wider than exponential might exist at spectral power levels well below LCE and Phase One radar sensitivities.