Calculation of Viewing and Solar Geometry Angles for the Fengyun-4B Geostationary Satellite

The calculation of viewing and solar geometry angles is a critical first step in retrieving atmospheric and surface variables from geostationary satellite observations. Whereas the viewing angles for geostationary satellites are not time-varying, a primary source of inaccuracy in solar positioning is the use of a single timestamp. Since pixel scanning times can differ significantly across the field-of-view disk (e.g., by approximately 13 min for Fengyun-4B), this practice leads to errors of up to ±2° in solar zenith angle, which translates to ±50 W/m² in extraterrestrial irradiance; the errors in solar azimuth angle can exceed ±100°. Beyond scanning time, this work also quantifies the impact of other inputs—including altitude, surface pressure, air temperature, difference between terrestrial time and UT1, and atmospheric refraction—on the resulting angles. A comparison of our precise calculations with the official National Satellite Meteorological Center L1_GEO product shows an accuracy within 0.1°, confirming its utility for most retrieval tasks. To facilitate higher precision when required, this work releases the corresponding satellite and solar positioning codes in both R and Python.