Analysis of Sea Surface Backscatter Coefficient Errors and Its Effects for the CFOSAT Scatterometer

Noise is a key factor that affects the accuracy of spaceborne scatterometer backscatter coefficient (σ⁰), as well as the retrieved sea surface wind quality. In general, the scatterometer σ⁰ measurement error is attributed to both instrumental noise and geophysical noise, which are expressed in terms of normalized standard deviation (K_p). In this paper, the instrumental noise (K_pc) and the geophysical noise (K_pg) are analyzed as a function of sea surface wind speed, incidence angle, spatial resolution and offshore distance for the China-France Oceanography Satellite Scatterometer. The result shows that the variability of sea surface wind field is large under low wind conditions, so the geophysical noise dominates the measurement error of radar backscatters. Notably, the larger the grid size of Wind Vector Cell (WVC), the more inhomogeneous the sea surface wind, such that the K_pg value increases as the WVC size, but at the same time, the larger the wind cell grid and the larger the number of independent observation samples, the smaller the K_pc. While under high wind conditions, the variability of sea surface wind is small, and the contribution of instrument noise and geophysical noise is similar. Regarding the sensitivity of measurement error to the incidence Angle, K_pc and K_pg show a minimum value at the incidence angle of 40°, which is consistent with the antenna gain pattern. Finally, the relationship between the backscatter measurement error and the offshore distance is studied in order to clarify the feasibility of near shore wind field inversion for the CSCAT. The results show that the observation error generally increases as the offshore distance decreases, indicating that the near-shore geophysical noise is non-negligible for the scatterometer measurements approaching to the coastal line. In summary, the results presented in this paper reveal the influence factors of scatterometer σ⁰ measurements, which are relevant for better understanding the wind inversion and quality control of CSCAT, notably near the coastal areas.