General BRDF Parameters for Normalizing GF-1 Reflectance Data to Nadir Reflectance to Improve Vegetation Parameters Estimation Accuracy

The Gaofen-1 (GF-1) wide-field view (WFV) data have wide view angles ranging from 0° to 48°, which generate notable angular effects for earth surface monitoring. However, the current angular correction method for GF-1 WFV data relies on bidirectional reflectance distribution function (BRDF) parameters derived from coarse resolution data, leading to limited correction accuracy. Therefore, this study aims to develop a general set of BRDF parameters for 16-m WFV data angular effect correction and improve the vegetation parameter estimation accuracy. First, more than 40 high-quality GF-1 WFV data in the North China Plain and Northeast China regions covering the typical vegetation types were collected to construct BRDF parameters. This study took into account three vegetation types (cropland, grassland, and forest) and the normalized differential vegetation index (NDVI) magnitude. Through the least square method, a set of BRDF parameters was estimated based on various NDVI levels. Then, the nadir reflectance was calculated to estimate leaf area index (LAI) and fractional vegetation cover (FVC). Finally, the evaluation of the corrected reflectance indicated that the developed BRDF parameters performed best for correcting the angular effect of cropland, followed by grassland. In addition, the validation indicated that the generated BRDF parameters effectively improved the LAI and FVC estimation accuracy.