Extended Cross-Calibration Analysis Using Data from the Landsat 8 and 9 Underfly Event

Abstract

The Landsat 8 and 9 Underfly Event occurred in November 2021, during which Landsat 9 flew beneath Landsat 8 in the final stages before settling in its final orbiting path. An analysis was performed on the images taken during this event, which resulted in a cross-calibration with uncertainties estimated to be less than 0.5%. This level of precision was due, in part, to the near-identical sensors aboard each instrument, as well as the underfly event itself, which allowed the sensors to take nearly the exact same image at nearly the exact same time. This initial calibration was applied before the end of the on-orbit initial verification (OIV) period; this meant the analysis was performed in less than a month. While it was an effective and efficient first look at the data, a longer-term analysis was deemed prudent to obtain the most accurate cross-calibration with the smallest uncertainties. The three forms of uncertainty established in the initial analysis, dubbed “Phase 1”, were geometric, spectral, and angular. This paper covers Phase 2 of the underfly analysis; several modifications were made to the Phase 1 process to improve the cross-calibration results, including a spectral correction in the form of a spectral band adjustment factor (SBAF) and a more robust filtering system that used the statistics of the reflectance data to better include important data compared to the more aggressive filters used in Phase 1. A proper uncertainty analysis was performed to more accurately quantify the uncertainty associated with the underfly cross-calibration. The results of Phase 2 showed that the Phase 1 analysis was within its 0.5% uncertainty estimation, and the cross-calibration gain values in this paper were used by USGS EROS to update the Landsat 9 calibration at the end of 2022.