ISSN 2070-7401 (Print), ISSN 2411-0280 (Online)
Sovremennye problemy distantsionnogo zondirovaniya Zemli iz kosmosa


Sovremennye problemy distantsionnogo zondirovaniya Zemli iz kosmosa, 2022, Vol. 19, No. 2, pp. 212-222

On the estimation of surface current velocities from microwave sea surface measurements at moderate incidence angles

I.A. Sergievskaya 1, 2, 3 , S.A. Ermakov 1, 2, 3 , L.M. Plotnikov 1, 3 , I.A. Kapustin 1, 4, 3 , A.V. Ermoshkin 1, 3 
1 Institute of Applied Physics RAS, Nizhny Novgorod, Russia
2 Volga State University of Water Transport, Nizhny Novgorod, Russia
3 Lobachevsky State University of Nizhni Novgorod, Nizhny Novgorod, Russia
4 Volgograd State University, Volgograd, Russia
Accepted: 27.04.2022
DOI: 10.21046/2070-7401-2022-19-2-212-222
Estimation of surface current velocities and their structure from microwave radar data is one of the important problems of remote sensing of the sea surface. The paper presents some results of measurements of surface current velocities from the Doppler spectra obtained by polarization multifrequency radar (X-/C-/S-bands) at moderate incident angles (40–70 degrees) of radiation in the upwind direction. A comparison of measured velocities with independent measurements of surface currents using an Acoustic Doppler Current Profiler (ADCP) is also presented. The radar scatterer velocities were determined from the Doppler spectrum centroid (Doppler shift) of the backscattered signal at VV and HH polarizations, and then for the Bragg (polarized) and non-Bragg (nonpolarized) components. It is shown that the flow velocity retrieved from the Doppler shift of the Bragg component after subtraction of the Bragg wave intrinsic velocity agrees well with the independent measurements. For the nonpolarized component, the scatterers velocities are close to the sum of surface current velocity and decimeter wave velocity with lengths of 10–70 cm, a sharp change in the scatterer velocity is observed for wind speeds of the order of 5–7 m/s. The latter is due to the contribution to the backscattering of the nonpolarised component associated with small-scale breaking moving at the short decimeter wave velocities (20–30 cm) and individual strong wave breaking (foam crests) moving at the meter wave velocities. At wind speeds at which strong wave breaking appears at the surface, the weighted speed of the scatterers increases sharply due to the appearance of fast scatterers.
Keywords: three-frequency microwave polarization Doppler radar, Bragg (polarized) and non-Bragg (non-polarized) scatterers, surface current
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