Top-of-atmosphere radiometric reference model for stable land sites with relaxed temporal stability requirements and its application to GF6/WFI radiometric calibration and validation

Abstract

A top-of-atmosphere (TOA) reflectance model based on the temporal characteristics of stable land surface targets, such as pseudo-invariant calibration sites (PICSs), was developed to monitor the stability of satellite visible imagers without requiring ground measurements. This model accounts for variations in the viewing zenith angle, solar zenith angle, and atmospheric parameters. To better represent multiple scattering contributions between the surface and atmosphere, we introduced the scattering angle and applied a sinusoidal function with a 365-day period to model seasonal atmospheric variations. Notably, the model does not rely on long-term temporal stability of PICSs, as it primarily characterizes angular and atmospheric variations rather than site-inherent stability. A comprehensive uncertainty analysis, based on a metrology-driven approach, indicates that the model achieves an average uncertainty of better than 4 %. Furthermore, the modeled TOA reflectance maintains long-term radiometric consistency with observations from the Sentinel-2 A/B MSI and Landsat-8/OLI sensors. Across 177 matching observations, the relative difference between observed and modeled values is mostly within 3 %, demonstrating the model's effectiveness. The proposed model is further applied to calibrate the GF6/WFI sensor, with cross-validation conducted using Sentinel-2/MSI and Terra/MODIS observed TOA reflectance as references, while constraining imaging time and viewing geometry. The TOA reflectance calibrated using the proposed model shows improved consistency with the reference TOA reflectance compared to that obtained using official calibration coefficients.