Comparative analysis of GOME and SCIAMACHY reflectance over Pseudo-Invariant Calibration Sites: implications for spectrometers cross-calibration

Abstract

Accurate radiometric cross-calibration is essential for ensuring the consistency and interoperability of multi-sensor satellite observations. Vicarious calibration, a widely adopted approach, relies on the temporal stability of desert-based Pseudo-Invariant Calibration Sites (PICS). However, these sites are limited in spatial extent and have not been systematically examined for cross-calibration with larger pixels, whose dimensions exceed those of PICS. This study established a statistical framework to advance cross-calibration of spectrometers over PICS and their surrounding areas. The framework included performance comparisons of different satellite instruments and the identification of reference and constrained sensors. The temporal stability of PICS across various spectral bands was reassessed using observations from the reference sensor. Stability scores (SS) were derived from a combination of statistical indicators designed to capture temporal variability, distribution symmetry, the occurrence of anomalies, and long-term shifts in the observations. Decades of top-of-atmosphere (TOA) reflectance data in the ultraviolet, visible, and near-infrared (UV/VIS/NIR) ranges, collected by the Global Ozone Monitoring Experiment (GOME) and Scanning Imaging Absorption Spectrometers for Atmospheric Chartography (SCIAMACHY) spectrometers over 20 PICS sites, were analyzed. The results revealed significant degradation in GOME, particularly in the UV band, during 2001 and toward the end of its mission, as evidenced by positively skewed, heavy-tailed distributions. In contrast, SCIAMACHY observations were more uniform and stationary, indicating their greater suitability for assessing PICS stability. The investigated sites were ranked based on the average of SS across the entire investigated spectrum as well as for each spectral band. The analysis revealed a wide range of SS among PICS, including intra-site variations across spectral bands. While some sites demonstrated consistently high stability, many were found to be 2–3 times less stable than the most stable sites. Among these robust sites, some have not been recommended by the calibration communities and should be given further consideration. In addition, the stability of the sites based on TOA reflectance showed a clear spectral band dependency, with enhanced stability in the NIR relative to the UV and VIS bands. These findings underscore the importance of regularly evaluating PICS and of considering spectral-band-specific performance when selecting calibration sites and performing the cross-calibration of spectrometers over PICS.