How Does Topography Affect Land-Surface Observations from Geostationary Satellites?

In recent years, Earth observation using geostationary satellites has experienced remarkable development. In particular, third-generation geostationary satellites, beginning with Japan’s Himawari-8, have enabled sub-hourly measurements of the Earth. Following Himawari-8, the U.S. GOES-R series, China’s FY-4A, Korea’s GK-2A, and Europe’s MTG-1 have successively begun observations, making it possible to observe nearly the entire globe at sub-hourly temporal resolution. Along with these advances, research targeting terrestrial ecosystems such as forests, grasslands, and croplands has rapidly expanded beyond traditional meteorological applications. However, current studies on land-surface ecosystem observation with geostationary satellites still overlook an important but critical issue: topographic effects arising from the relatively large viewing angles of geostationary satellites compared with polar-orbiting satellites. As the distance from the satellite sub-satellite point increases, discrepancies grow between the latitude and longitude coordinates on the reference ellipsoid to which geostationary satellite data are projected and the actual geographic coordinates of the land surface. In addition, topographic features such as high mountains create invisible areas that are partially or entirely invisible to geostationary satellites. Despite these effects, few studies have considered topographic effects when comparing with in situ observations or polar-orbiting satellite data. In this study, we address this issue by using high-resolution digital surface model (DSM) data that are sufficiently detailed to represent sub-pixel topographic variability within individual geostationary satellite pixels. Specifically, we simulated topographic effects on geostationary satellite observations using a 30 m resolution DSM provided by the Japan Aerospace Exploration Agency (JAXA). Our results show that, due to topographic effects, the correspondence between ellipsoidal latitude–longitude coordinates and actual surface coordinates can be shifted by more than one pixel in some regions. We further confirmed that this spatial mismatch leads to differences in the seasonal variation patterns of vegetation indices. In addition, when attempting ray-matching with polar-orbiting satellite observations, we found significant differences between geostationary and polar-orbiting satellite data when topographic effects were not considered. These findings demonstrate that accounting for topographic effects is essential for accurate land-surface observation using geostationary satellites. Our results provide valuable guidance for future studies that aim to compare geostationary satellite data with in-situ observations or to perform data fusion with polar-orbiting satellites, and they will contribute to achieving more precise and reliable land-surface monitoring.