Geostationary Environment Monitoring Spectrometer (GEMS): Long-Term Radiometric Accuracy and Spectral Stability From 4.5 Years of On-Orbit Solar Irradiance Observations

Abstract

The Geostationary Environment Monitoring Spectrometer (GEMS), the first ultraviolet-visible (UV) hyperspectral imager in geostationary orbit, has been operating since its launch in February 2020. This article evaluates and discusses the on-orbit performance of GEMS using 4.5 years of irradiance measurements (300-500 nm), with a particular emphasis on the long-term changes in its radiometric accuracy and spectral stability. As part of this assessment, we examine various super-Gaussian forms used for deriving on-orbit slit functions and conclude that applying symmetry constraints to both shape and width parameters enhances fitting stability and accuracy. During the initial phase of operation, the full-width at half-maximum (FWHM) of the derived slit functions ranged from 0.56 to 0.58 nm across spatial and spectral dimensions. The wavelength shift remains spatially uniform and varies smoothly within +/- 0.05 nm, showing a consistent, repeating pattern in 300-400 and 400-500 nm. However, over time, instrument degradation has led to pronounced spectral and spatial variations, particularly at shorter wavelengths. Slit widths (FWHM) at the center pixels fluctuate between 0.54 and 0.60 nm, and a "frown-shaped" spatial pattern emerges in the wavelength shift. Radiometric offsets exhibit a steady increase-approximately 5% per year at shorter wavelengths below 330 nm and 0.2% above 460 nm. By 2024, irradiance values are up to 35% lower near 325 nm and about 10% lower above 460 nm compared to the solar reference. To isolate fine-scale uncertainties, we further evaluate the fitting residuals between the measured irradiance and the radiometrically adjusted solar reference. As the degradation progressed, residuals in 310-330 nm increased sharply from 0.4% in 2020 to 4% in 2024, while fitting uncertainties nearly doubled in 330-360 nm. This degradation feature indicates the emergence of instrumental artifacts in spectral structures that are unrelated to physical solar absorption features. Such artifacts are most pronounced at the central pixels and have gradually extended toward the southern pixels and adjacent spectral regions throughout the mission. Beyond 400 nm, radiometric offsets occur on a broadband scale, with no noticeable degradation in the fine spectral structures.