Estimation of surface wave spectra from nautical radar image sequences with a small azimuthal coverage

The radar backscatter from the ocean surface, called sea clutter, is modulated by the surface wave field. A method has been developed to estimate calibrated surface wave spectra from nautical radar image sequences. The algorithm is based on the three-dimensional fast Fourier transformation of the spatio-temporal sea clutter pattern in the wavenumber-frequency domain. The dispersion relation is used to define a filter to separate the spectral signal of the imaged waves from the background noise component caused by speckle noise. The signal-to-noise ratio (SNR) depends on the spectral energy of the surface wave field and on the image transfer function (ITF). With a proper parameterization of the ITF, after a calibration period, the nautical radar can be used as a stand alone instrument to obtain calibrated surface wave spectra with full directional information. The ITF depends on the azimuth angle between the wave propagation and the antenna look direction. The azimuthal dependence of the ITF has been analyzed with a data set taken by a ship based radar installation. The implications for wave measurements from land based installations, often allowing only a small azimuthal coverage, are outlined