Improvement of VV-polarization tilt MTF for Gaofen-3 SAR data of a tropical cyclone

In this paper, an essential modulation transfer function (MTF) on synthetic aperture radar (SAR), denoted as tilt modulation, is improved through five vertical-vertical (VV) polarized Gaofen-3 (GF-3) SAR images acquired in Global Observation (GLO) and Wide ScanSAR (WSC) mode that were captured during the 2017 tropical cyclone (TC) season in the western Pacific Ocean. These images are reprocessed to be Level-1B products and collocated with the wind fields from Global and Regional Assimilation and Prediction System - Typhoons Model. More than 9,000 matchups are exploited for studying the dependence of tilt on the wind speed and incidence angle. It is found that quantitative tilt MTF (herein denoted as the derivative scattering coefficient), which is proportional to first-order derivative normalized radar cross-section with respect to incidence angle, is inversely related with the incidence angle between 14 degrees and 46 degrees. In addition, the wind speed has an obvious influence on the derivative scattering coefficient due to each scattering facet essentially being modulated by strong wind-induced long-waves, whereas traditional tilt MTF does not conform to the actual situation. However, the sensitivity of wind speed on the derivative scattering coefficient weakens at larger incidence angles (>40 degrees). Following this rationale, the empirical tilt MTF in VV polarization has been developed. The reanalysis results indicate that the values from empirical tilt MTF could depict the role of wind. The improvement herein could overcome the limitation in tilt MTF, namely, the lack of a strong wind's influence, and it is expected to be implemented for SAR wave retrieval in TCs.