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Remote Sens. 2018, 10(6), 870; https://doi.org/10.3390/rs10060870

Low-Frequency Sea Surface Radar Doppler Echo

1
Marine Hydrophysical Institute Russian Academy Sci., 2 Kapitanskaya, Sevastopol 299011, Russia
2
Russian State Hydrometeorological University, Satellite Oceanography Laboratory, 98 Malookhotinskiy, St-Petersburg 195196, Russia
3
University of Maryland, Department of Atmospheric and Oceanic Science, College Park, MD 20742, USA
4
Institut Français de Recherche pour l’Exploitation de la Mer, 29280 Plouzané, France
*
Author to whom correspondence should be addressed.
Received: 20 April 2018 / Revised: 30 May 2018 / Accepted: 1 June 2018 / Published: 4 June 2018
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Abstract

The sea surface normalized radar backscatter cross-section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave peak. These NRCS and DV variations are coherent and thus may produce a bias in the DV averaged over large footprints, which is important for interpretation of Doppler scatterometer measurements. To understand the origin of LF variations, the platform-borne Ka-band radar measurements with well-pronounced LF variations at frequencies below wave peak (0.19 Hz) are analyzed. These data show that the LF NRCS is coherent with wind speed at 21 m height while the LF DV is not. The NRCS-wind correlation is significant only at frequencies below 0.01 Hz indicating either differences between near-surface wind (affecting radar signal) and 21-m height wind (actually measured) or contributions of other mechanisms of LF radar signal variations. It is shown that non-linearity in NRCS-wave slope Modulation Transfer Function (MTF) and inherent averaging within radar footprint account for NRCS and DV LF variance, with the exception of VV NRCS for which almost half of the LF variance is unexplainable by these mechanisms and perhaps attributable to wind fluctuations. Although the distribution of radar DV is quasi-Gaussian, suggesting virtually little impact of non-linearity, the LF DV variations arise due to footprint averaging of correlated local DV and non-linear NRCS. Numerical simulations demonstrate that MTF non-linearity weakly affects traditional linear MTF estimate (less than 10% for typical MTF magnitudes less than 20). Thus the linear MTF is a good approximation to evaluate the DV averaged over large footprints typical of satellite observations. View Full-Text
Keywords: radar; ocean; backscatter; Doppler shift; wave groups; non-linearity; modulation radar; ocean; backscatter; Doppler shift; wave groups; non-linearity; modulation
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Yurovsky, Y.Y.; Kudryavtsev, V.N.; Grodsky, S.A.; Chapron, B. Low-Frequency Sea Surface Radar Doppler Echo. Remote Sens. 2018, 10, 870.

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