# Radiometric Cross-Calibration of GF-1 PMS Sensor with a New BRDF Model
## 6 Conclusions  
An innovative cross-radiation calibration method is proposed in this paper, which is used to cross-calibrate GF-1 PMS camera at Dunhuang and Golmud test site, respectively, taking MODIS and Landsat 8 OLI as reference sensors. Firstly, a BRDF model is constructed based on the TOA reflectance of MODIS in long time-series sunny day, which are extracted by the TOA BT and the variation coefficient of TOA reflectance. This \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Blue** & **Green** & **Red** & **NIR** \\ \hline Lab 1  & \(-\)0.2129 & 0.1688 & 0.1697 & 0.1748 \\ CRESDA 2  & 0.2247 & 0.1892 & 0.1889 & 0.1939 \\ AVE( 3  & 0.2249 & 0.1873 & 0.1845 & 0.1996 \\ AVE(M 4  & 0.2282 & 0.1881 & 0.1866 & 0.2060 \\ (1-L/Lab)\% 5  & \(-\)5.64\% & \(-\)10.96\% & \(-\)8.71\% & \(-\)14.17\% \\ (1-M/Lab)\% & \(-\)7.19\% & \(-\)11.48\% & \(-\)9.96\% & \(-\)17.85\% \\ (1-L/CRESDA)\% & \(-\)0.09\% & 1.00\% & 2.34\% & \(-\)2.93\% \\ (1-M/CRESDA)\% & \(-\)1.56\% & 0.54\% & 1.22\% & \(-\)6.24\% \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Laboratory calibration coefficients, site calibration coefficients and average cross-calibration coefficients and relative error between them. Figure 8: Time-dependent variation of cross-calibration coefficients in GF-1 PMS1 Blue band. ‘Lab’ represents the laboratory calibration coefficients of blue band; ‘CRESDA’ represents the site calibration coefficients of blue band. process is an important step of this paper. Then the TOA reflectance of MODIS images with large imaging angles is corrected by BRDF model. After that, the radiometric cross-calibration of PMS is carried out by using BRDF-corrected MODIS and OLI images, and the radiometric calibration coefficients are calculated by the least square method. Compared with previous studies, it is found that the accuracy of cross-radiation calibration has been significantly improved after directional correction of large observation angle MODIS images using the BRDF model constructed in this paper. Furthermore, the study also finds that although ground measured parameters of Dunhuang test site are used as substitution for the Golmud test site in cross-calibration, the calibration coefficients obtained are in good agreement with that of the Dunhuang site. By comparing with the site calibration coefficients and analyzing the uncertainty of this paper, the total calibration uncertainties of PMS using MODIS as reference sensor are about 2%-6%, which are similar with the accuracy of vicarious calibration. The result is promising, and in the future, the robustness and universality of the technique proposed in this paper should also be assessed by applying it to other radiometric calibration test sites and similar optical satellite sensors. In addition, as CRESDA has adjusted the gain of GF-1 PMS1 on 25 September 2016, this paper only studies the PMS1 data from the launch of GF-1 satellite on 26 April 2013 to 25 September 2016. The radiation performance and calibration accuracy of the gain-adjusted GF-1 PMS1 sensor will be studied in the follow-up study. [PERSON] designed the research and prepared the manuscript. [PERSON] carried out the modeling and data processing. All authors contributed to the scientific content, the interpretation of the results, and manuscript revisions. This research was funded by National Key R&D Program of China, No. 2018 YFB0504800 (No. 2018 YFB0504803), the National Key Research and Development Program of China, (No. 2018 YFB0504800), the National Key Research and Development Program of China (No. 2017 YFC0209704), the Common Application Support Platform for Land Observation Satellite of National Civil Space Infrastructure (No. 17 QFGW02 KJ), and the Integrated Space-ground Evaluation Technology of Satellite Application (No. Y7K00100 KJ).