RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 21, ES6004, doi:10.2205/2021ES000784, 2021

*V. Yu. Karaev, M. A. Panfilova, M. S. Ryabkova, Yu. A. Titchenko, E. M. Meshkov, and X. Li*

In October 2018, the Chinese-French satellite CFOSAT was launched into low-earth orbit, which carries the French SWIM spectrometer and the Chinese scatterometer RFSCAT. The SWIM (Ku-band) spectrometer measures the normalized radar cross section (NRCS) at small incidence angles (0°—11°) and for the first time measurements are performed at different azimuthal angles in a size-limited area where sea waves can be considered homogeneous. At small incidence angles, the backscattering mechanism is quasi-specular and the reflected field is calculated in the Kirchhoff approximation. Due to this, it becomes possible to retrieve the parameters of large-scale, in comparison with the length of the electromagnetic wave, sea waves (hereinafter referred to as large-scale waves). It has been shown that the common approach to measuring the mean square slopes (*mss*) of large-scale waves leads to large errors. For the measurement scheme at three azimuthal angles, an algorithm was developed for determining the direction of propagation of the dominant wave system, the *mss* of large-scale waves along the sounding direction and the unnormalized correlation coefficient between the slopes of large-scale waves along the *X* and *Y* axes (hereinafter referred to as the correlation coefficient), which requires solving a system of transcendental equations. If we neglect the correlation coefficient in comparison with the *mss* of large-scale waves, then the solution is in an analytical form. The results of the simplified algorithm are compared with the exact solution. The values of the *mss* of large-scale waves obtained with the simplified approach can be used both to estimate the intensity of sea waves and as the initial conditions for solving the transcendental equation. The developed algorithms will be used to process the data of the SWIM spectrometer, which will allow for the first time to retrieval a two-dimensional field of large-scale wave slopes.

Received 2 November 2021; accepted 2 December 2021; published 23 December 2021.

**Citation:** V. Yu. Karaev, M. A. Panfilova, M. S. Ryabkova, Yu. A. Titchenko, E. M. Meshkov, and X. Li (2021), Retrieval of the two-dimensional slope field by the SWIM spectrometer of the CFOSAT satellite: discussion of the algorithm, *Russ. J. Earth Sci., 21*, ES6004, doi:10.2205/2021ES000784.

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