Modeling the spectral structure of ducted sea clutter

A model has been developed to explain unusual sea clutter Doppler frequency spectral characteristics that were observed from search radars operating in ducting conditions. Sea returns from arctic coastal sites exhibit extreme mean Doppler frequency shifts, with the magnitudes of the means varying from one ocean area to another. The spectrum bandwidth for the individual returns was very narrow, even though the mean value was large. Clutter spectra collected from a Middle East site exhibited similar anomalies. Both data sets were collected during calm (almost glassy) seas, so breaking waves and surface turbulence were not responsible for the unusual characteristics. A physical mechanism is hypothesized to explain how the interaction between a moving ocean surface and propagating electromagnetic energy in a surface duct can induce large Doppler shifts in sea clutter spectra. Model simulations support the hypothesis and compare favorably with measured data