# The Ground-Based Absolute Radiometric Calibration of the Landsat 9 Operational Land Imager
## 6 Conclusions  
After an extensive ground-based field measurement campaign by various government and university groups during the first nineteen months of operation, it has been confirmed that the Landsat 9 OLI instrument is performing exceptionally well and has maintained its stability throughout this period. The ground-based data were compared to the L1 TP imagery available from the USGS through Earth Explorer. The results showed that, on average, the OLI instrument aligns with the design specifications for TOA spectral radiance (\(\pm\)5%) and TOA spectral reflectance (\(\pm\)3%) in all multispectral bands as well as the panchromatic band (band 8). The temporal analysis did not reveal any significant degradation pattern in the OLI instrument over time. Furthermore, the results obtained from the automated RadCaTS facility at Railroad Valley supported the findings of the ground personnel, with even slightly better agreement with the OLI than the in situ measurements conducted by on-site personnel. However, it is worth noting that the number of RadCaTS data sets was lower than expected throughout the year, primarily due to the unusually high frequency of poor weather conditions experienced during that period. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ratio of TOA Quantities to Ground Measurements—All Field Data** \\ \hline
**OLI Band** & **TOA Spectral Radiance** & **TOA Reflectance** \\
**(Center Wavelength)** & **(OLI/ground)** & **(OLI/ground)** \\ \hline
1. (443 nm) & 1.031 \(\pm\) 0.051 & 0.989 \(\pm\) 0.046 \\
2. (483 nm) & 1.014 \(\pm\) 0.046 & 0.994 \(\pm\) 0.041 \\
3. (561 nm) & 1.003 \(\pm\) 0.043 & 0.999 \(\pm\) 0.040 \\
4. (655 nm) & 1.018 \(\pm\) 0.046 & 1.020 \(\pm\) 0.045 \\
5. (865 nm) & 1.024 \(\pm\) 0.045 & 1.014 \(\pm\) 0.043 \\
6. (1609 nm) & 1.008 \(\pm\) 0.043 & 1.021 \(\pm\) 0.045 \\
7. (2201 nm) & 0.979 \(\pm\) 0.040 & 1.002 \(\pm\) 0.040 \\
8. (pan, 592 nm) & 1.011 \(\pm\) 0.043 & 0.995 \(\pm\) 0.041 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The radiometric calibration results for Landsat 9 OLI using all ground data (from November 2021 to June 2023). The radiometric calibration of the Landsat 9 OLI will continue to be measured and monitored throughout its operational life, employing reliable and established techniques that are currently in use by a wide array of calibration teams. These techniques include ground-based in situ measurements using both automated systems and on-site personnel, observations over invariant desert sites, lunar observations, and cross-comparison with other well-calibrated sensors. By utilizing these methods, the temporal accuracy and precision of Landsat 9 OLI data will remain well understood, thereby ensuring the continued production of high-quality and reliable Earth observation data for a wide range of applications. All the authors were involved in various aspects of the data collection and/or analysis during the period of this work. [PERSON]-[PERSON] and [PERSON] were involved in the in situ collection and analysis of field data, which included simultaneous field work with [PERSON] and [PERSON], who provided the NASA GSFC results. [PERSON] and [PERSON] were involved in on-site data collection at the SDSU calibration site, including supervision, and they also assumed responsibility for the subsequent processing and analysis of field data. All authors have read and agreed to the published version of the manuscript. The UArizona work was supported by the USGS cooperative agreement G20 AC00003, as well as NASA grants 80 NSSC21K1749 and 80 NSSC20K1872. The SDSU work was supported by USGS EROS grant G18 AS00001. Data from the RadCaTS are freely available to registered users on the Radiometric Calibration Network (RadCalNet) web portal: www.radcalnet.org (accessed on 1 January 2024). Landsat 8 and 9 OLI Collection 2 data are courtesy of the US Geological Survey DOI Object Identifier (DOI) number: /10.5066/P975 CC9B.