On-Orbit Spatial Performance Characterization for Thermal Infrared Imagers of Landsat 7, 8, and 9, ECOSTRESS and CTI

In this analysis of the spatial resolving power of thermal imagery products we focus on four satellite instruments that are used in research and applications, for example, to monitor land surface temperature and derive evapotranspiration. These are thermal imagers on Landsat 7, Landsat 8, and Landsat 9, as well as the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS). We compiled sets of close-in-time, day-time images of bridges surrounded by open water bodies, captured by each of the satellite imagers during cloud-free moments. Where possible, we also included some images captured by the Compact Thermal Imager (CTI), a technology demonstrator that was co-located with ECOSTRESS on the International Space Station in 2019. Bridges were found to provide a sufficient thermal contrast with the water surface to quantify the line-spread function of satellite-based thermal products. The full-width-at-half-max of a gaussian beam model fitted to this transect quantifies the on-orbit spatial resolution of different imagers. The results show some loss of spatial resolving power in the final product delivered to end-users as compared to the at-sensor characterization of spatial resolution. For Landsat 7, 8, and 9, the spatial resolution of the thermal bands is 1.5 times the ground sampling distance of 60 and 100 m respectively. For the ECOSTRESS the difference is up to twice the sampling distance of 78 by 69 m2. Since spatial resolution is a main driver for instrument design it is important to understand and communicate this discrepancy between pre-flight design parameters and the characteristics of the surface imagery delivered to the user community. The goal of this research is to facilitate an improved fusion of current and future satellite observations into harmonized products with superior temporal and spatial characteristics. This manuscript describes a method to verify the spatial resolution of thermal imagers after launch. Spatial resolution is a key driving requirement in thermal imager design and technology development. Many aspects that control spatial resolution are mathematically understood and can be calculated based on the design specifications of spaceborne imagers. However, the actual spatial resolution of the products delivered to users may not meet the design target precisely. In this paper we compiled a set of thermal images of the San Francisco Bay area. In these daytime images the bridges show up as clearly defined linear features contrasted with the relatively homogenous water surrounding them. From these images, we construct sharply defined profiles perpendicular to the bridge orientation. We then use these profiles as analogs of the linear cuts used to determine the line-spread function in laboratory tests of imagers. What we find is a much lower spatial resolving power in the final product delivered to end-users as compared to the at-sensor characterization of spatial resolution. The reduced spatial resolution we found compared to what may be expected from mission specifications points to the need of better communication of how imager specifications are conveyed into mission products. Day-time thermal contrast between a bridge and its surrounding waters is used to quantify the spatial performance of space-borne imagers Spatial resolution of thermal imagery products can be lower, to a factor of 2, than the nominal values calculated based on system design The divergence between pre-flight and on-orbit metrics should be factored into mission design trade studies