Backscattering characteristics and texture information analysis of typical crops based on synthetic aperture radar: A case study of Nong'an County, Jilin Province

Agriculture is the foundation of the national economy. Clarifying crop type, spatial and temporal distribution, and planting structure is an important scientific basis for the rational adjustment of agricultural framework. Optical remote sensing relies on solar radiation and is often subject to the influence of clouds. Therefore, optical remote sensing images cannot be obtained during cloudy weather. Synthetic aperture radar (SAR) images are not affected by cloud and fog and can provide all-day, all-weather data. Therefore, the backscatter characteristics and texture information of typical crops can be analyzed by using SAR images, which may improve accuracy of large area monitoring of crops in all weather conditions. In this study, Nong'an County in Jilin Province was taken as the study area, and 12 Sentinel-1B SAR images with dual-polarization were pre-processed. Then the SAR backscattering characteristics and texture information of typical crops with dual polarization were analyzed. The results showed that the co-polarization (VV) SAR backscattering coefficients of three crops (soybean, corn, rice 1 and rice 2) were higher than the cross-polarization (VH) coefficients during the growth period, and the ability of crop plants to change the polarization mode was about -25 to -15 dB. Over the whole growth seasons of crops, the backscattering coefficients showed large fluctuations. The backscattering characteristics of each growth stage were different. During the early growth stage of three crops, the soil played a major role in determining the backscattering characteristics, and the roughness of soil was small. Therefore, the three crops on the SAR images were dark in color. With the growth of the crops, the backscattering from crops was mainly due to canopy scattering and soil scattering. The backscatter coefficient value increased with crop growth, which meant that the color of the crops changed to a relatively bright shade on the SAR image. When crops reached the jointing stage or branching stage (after July 10), in addition to canopy scattering, their backscattering signal was strongly affected by the soil water content and its interaction with respective crops. The soil water content of rice was high. Therefore, the backscattering coefficient of rice considerably decreased after the jointing stage. The absorption of radar waves by rice was stronger than those of corn and soybean, which meant that the backscattering coefficient of rice was smaller than the coefficients for corn and soybean, especially in the VH polarization SAR images. The crop SAR texture information analysis showed that the mean, variance, and homogeneity could accurately identify crops on SAR images. The best texture information was the mean of VH polarization. The variance and homogeneity of VV polarization could also accurately identify the two kinds of rice. The best recognition phase for the crops on SAR images was from May 23 to July 10.