Modulation of Ka-Band Doppler Radar Signals Backscattered From the Sea Surface

Type Article
Date 2018-05
Language English
Author(s) Yurovsky Yury Yu.1, Kudryavtsev Vladimir N.1, 2, Chapron BertrandORCID3, Grodsky Semyon A.4
Affiliation(s) 1 : RAS, Marine Hydrophys Inst, Fed State Budget Sci Inst, Sevastopol 299011, Russia.
2 : Russian State Hydrometeorol Univ, Satellite Oceanog Lab, St Petersburg 195196, Russia.
3 : Inst Francais Rech Exploitat Mer, F-29280 Plouzane, France.
4 : Univ Maryland, Dept Atmospher & Ocean Sci, College Pk, MD 20742 USA.
Source Ieee Transactions On Geoscience And Remote Sensing (0196-2892) (Ieee-inst Electrical Electronics Engineers Inc), 2018-05 , Vol. 56 , N. 5 , P. 2931-2948
DOI 10.1109/TGRS.2017.2787459
WOS© Times Cited 34
Keyword(s) Doppler shift, modulation transfer function (MTF), radar backscattering, sea surface, spikes, wave breaking
Abstract This paper presents dual copolarized (VV and HH) Ka-band radar measurements of joint modulation of normalized radar crass section (NRCS) and Doppler velocity (DV) performed from a sea research platform. NRCS and DV modulations are well correlated. NRCS modulations exhibit a spiky structure. HH modulations are stronger than VV ones leading to modulations of the polarization ratio. This suggests that an important portion of NRCS modulations is produced by nonpolarized radar returns from modulated wave breaking facets. DV modulations reveal that at incidence angles <50 degrees, NRCS spikes are attributable to rather slow moving facets, which may he interpreted as short wave breaking disturbances embedded in the water at crests of modulating waves. Using the DV as a proxy for wave gauge, a modulation transfer function (MTF) is estimated for both polarizations. The hydrodynamics component of the total MTF, hydro-MTF, combines NRCS modulations supported by Bragg waves and wave breaking. The contribution of each type of facets to the hydro-MTF is weighted by its partial contribution to the NRCS, and thus, hydro-MTF becomes dependent on radar polarization. Using hydro-MTF for HH and VV, Bragg wave and wave breaking modulations are separated. Wave breaking modulations are significant, with MTF amplitude varying from 5 to about 30 depending on wind speed. Ka-band Bragg waves are strongly modulated at low winds, but their modulation almost vanishes at moderate winds. Finally, we propose an empirical MTF parameterization based on polynomial fitting as a function of observation geometry and wind.
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