Abstract
Values for Doppler center frequency are calculated from the echo signal at the satellite using the Doppler centroid method and so include the predicted Doppler frequency caused by the relative motion of the satellite and the Earth, which is the main component of Doppler center frequency and must be removed to obtain the Doppler frequency anomaly for ocean current measurement. In this paper, a new Doppler frequency anomaly algorithm was proposed when measuring surface currents with synthetic aperture radar (SAR). The key of the proposed algorithm involved mean filtering method in the range direction and linear fitting in the azimuth direction to remove the radial and the azimuthal component of predicted Doppler frequency from the Doppler center frequency, respectively. The basis is that the theoretical Doppler center frequency model of SAR exhibits an approximately linear characteristic in both the range direction and in the azimuth direction. With the help of the new algorithm for predicted Doppler frequency removal, the estimation error of Doppler frequency anomaly can be reduced by avoiding employing the theoretical antenna pattern and imperfect satellite attitude parameters in the conventional Doppler frequency method. SAR measurement results demonstrated that, compared to the conventional Doppler frequency with/without error correction method, the proposed algorithm allows for a pronounced improvement in the current measuring accuracy in comparison with the global ocean multi-observation (MOB) products. In addition, the effectiveness and robustness of the proposed Doppler algorithm has been demonstrated by its application in the high velocity current in the Kuroshio region.
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Data Availability Statement
The SAR images used in this study are distributed by ESA, and are available from https://earth.esa.int/eogateway/catalog/envisat-asar-ws-medium-resolution-11-asa_wsm_1p-. The global ocean multi-observation (MOB) products, MULTIOBS_GLO_PHY_REP_015_004, are distributed by CMEMS, and could be downloaded from http://marine.copernicus.eu.
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Acknowledgment
The authors would like to acknowledge the data made available by the European Space Agency and the Copernicus Marine Service for the global ocean multi observation products. We wish to thank the editor and anonymous reviewers for their helpful comments and suggestions.
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Supported by the National Natural Science Foundation of China (Nos. 42176174, 41706196), the Sichuan Science and Technology Program (No. 2018JY0484), the Natural Science Key Research Program of Education Department of Sichuan Province (No. 18ZA0103), the China Postdoctoral Science Foundation (No. 2020M683258), the Provincial Science and Technology Innovation Development Project of China Meteorological Administration (No. SSCX2020CQ), the Chongqing Technology Innovation and Application Development Special Project (No. cstc2020jscxmsxmX0193), and the Chongqing Meteorological Department Business Technology Research Project (No. YWJSGG-202017)
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Wang, L., Gao, Y., Lu, P. et al. A new Doppler frequency anomaly algorithm for surface current measurement with SAR. J. Ocean. Limnol. 40, 470–484 (2022). https://doi.org/10.1007/s00343-021-0492-4
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DOI: https://doi.org/10.1007/s00343-021-0492-4