A Sight for Sore Eyes: The Return of True Color to Geostationary Satellites

Abstract

n 1967, at the dawn of the satellite era, the Applications Technology Satellite 3 (ATS-3) provided the first full-disk ?true color? images of Earth. With its depiction of blue oceans, golden deserts, and green forestlands beneath white clouds, the imagery captured the iconic Blue Marble in a way that resonates strongly with human perception. After ATS-3, the standard fare of geostationary satellites entailed a single visible band with additional infrared spectral channels. While single-band visible satisfied the basic user requirements of daytime imagery, the loss of true-color capability and its inherent capability to distinguish myriad atmospheric and surface features via coloration left a notable void. Nearly half a century later, with the launch of Japan?s Himawari-8 in October 2014, there is once again a geostationary sensor?the Advanced Himawari Imager (AHI)?containing the multispectral visible bands required notionally for true color. However, it soon became apparent that AHI?s ?green? band, centered at 0.51 ?m, was not aligned with the chlorophyll reflectance signature near 0.55 ?m. As a result, vegetation appears browner and deserts appear redder than legacy true-color imagery. Here, we describe a technique that attempts to mitigate these issues by blending information from a ref lective-infrared band at 0.86 ?m to form a ?hybrid? green band. When combining this method with Rayleigh corrections, AHI?s true-color performance is found to be consistent with that of the optimal 0.55-?m band, offering a stopgap solution adaptable to future satellites of similar design.