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.

Landsat 9 Transfer to Orbit of Pre-Launch Absolute Calibration of Operational Land Imager (OLI)

“…The trending analysis of these collects assisted in finding the earliest time after which the Landsat 9 OLI response stability approached a nominal steady-state. Additional special Landsat 9 OLI collects during commissioning that assisted in further re-affirming the results of the transfer to orbit (but not discussed in this paper) are the Landsat 8 and Landsat 9 cross-cal activities that involved both the underfly [6,7] as well as experimental near-simultaneous working solar diffusers and lunar collects.…”

Prelaunch Spectral Characterization of the Operational Land Imager-2

“…Small modifications were made to the DSS-derived prelaunch responsivities once on orbit based on the Landsat-8 underfly [26][27][28]. While the differences were all within the uncertainties of the process, it was decided to update the OLI-2 responsivities to be more consistent with OLI (Table 12 and Figure 30).…”

Landsat 8 decade of on-orbit operation summary of events and current state

“…Furthermore, during the commissioning of Landsat 9 over a period of 3 days the Landsat 8 and Landsat 9 had coincident ground collects that served to cross-calibrated that made Landsat 9 match Landsat 8 absolute calibration in both reflectance and radiance. [10] For TIRS the NEdT performance for both bands had been completely unaffected by the on-orbit events or aging. Evaluated for the noise at 300K both bands have <0.06K without any clear trend to the noise performance as the temporal trend is less than the evaluation scatter (Figure 18).…”