Determination of the sea surface current field from the Doppler shift, of the coherent radar backscatter with grazing incidence

A conventional nautical radar detects only the amplitudes of the backscatter from the sea surface. With such an incoherent system the near-surface current must be deduced from the Doppler shift of the imaged waves. For the image analysis a large number of image pixels, each representing a radar resolution cell, is necessary. For standard operation a 128×128 pixel field covering an area of 1 km² is used and yields a mean current value for the analysed area. Therefore, the topography of the covered area must be homogeneous. However, especially in coastal regions, this condition often is not fulfilled. To obtain a better spatial resolution for the surface-current determination, a nautical radar was upgraded to also measure the phase information of the backscattered signal. With the upgraded system, the Doppler frequency shift of the backscattered signal can be directly detected for each image pixel. The coherent system therefore has a resolution of 10 m×10 m, equal to the size of a resolution cell. The current can also be determined in the absence of long waves, and the topography of the covered area must only be homogeneous within each resolution cell and not over the whole area covered. The radial velocity of the scatterers was calculated from the frequency shift of the peak power in the Doppler spectre. This magnitude is produced not only by the near-surface current, but also by the wind and the surface waves. Additionally, the antenna polarisation and incidence angle must be taken into account. To eliminate the influence of the sea state, a spectrogram of an image sequence taken at the Finnish Gulf was calculated