# Radiometric Cross-Calibration of GF-1 PMS Sensor with a New BRDF Model
## 2 Satellites, Test Sites, and Datasets
### Datasets  
Two image triplets are used for cross-calibration. The first image triplet is collected at the Dunhuang test site. The second image triplet is imaged on Golmud test site. The image acquisition dates and times of PMS, MODIS, and OLI over Dunhuang and Golmud test sites are shown in Table 2.  
The ground data of Dunhuang test site are used for cross calibration in this paper. The ground reflectance and aerosol optical thickness of the Dunhuang test site were measured on 7 August and 12 August 2014. MODIS and PMS are imaged on 7 August and OLI is imaged on 12 August. The sounding data were also measured at the Dunhuang test site. The spectra vista corporation (SVC) HR1024 spectrometer was used to measure the ground reflectance and CE318 Sun Photometer was used to retrieve the aerosol optical thickness. Figure 3 shows the ground reflectance of the Dunhuang test site, which varied from 0.15 to 0.3. The reflectance is increased with the increase in wavelength. The variance coefficient (the ratio of standard deviation and mean reflectance) of the Dunhuang test site was about 5% as shown in Figure 3. The aerosol optical thickness was 0.367 when PMS1 overpassed Dunhuang test site, with the surface meteorological range of 10.2 km. The water vapor profile of the Dunhuang test site is used in MODTRAN calculation, with the water vapor column of 1.982 gm/cm\({}^{2}\). For OLI sensor, the surface meteorological range on 12 August was 30.6 km and the water vapor column was 1.243 gm/cm\({}^{2}\).  
As no ground data are available on the Golmud test site, the ground reflectance of the Dunhuang test site is used for SBAF (spectral band adjustment factors) correction. The surface meteorological range of Golmud is assumed to be 10 km and the water vapor column value of 0.2077 gm/cm\({}^{2}\) of mid-latitude winter atmosphere mode is used in MODTRAN calculation.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sensor**} & \multicolumn{3}{c}{**Dunhuang Test Site**} & \multicolumn{3}{c}{**Golmud Test Site**} \\ \cline{2-7}  & **PMS** & **MODIS** & **OLI** & **PMS** & **MODIS** & **OLI** \\ \hline Date & 7 August 2014 & 12 August 2014 & 24 February 2014 & 26 February 2014 \\ Time (UTC) & 04:57 & 05:10 & 04:26 & 04:50 & 04:00 & 04:21 \\ Solar Zenith (\({}^{\circ}\)) & 26.013 & 24.76 & 31.196 & 48.410 & 53.18 & 50.014 \\ Solar Azimuth (\({}^{\circ}\)) & 150.997 & 160.42 & 137.666 & 157.97 & 143.89 & 148.372 \\ Sensor Zenith (\({}^{\circ}\)) & 5.387 & 49.68 & 0 & 1.768 & 53.12 & 0 \\ Sensor Azimuth (\({}^{\circ}\)) & 96.131 & \(-\)74.08 & 98.5 & 87.776 & 95.64 & 98.5 \\ Atmosphere Model & \multicolumn{3}{c}{Mid-Latitude Summer} & \multicolumn{3}{c}{Mid-Latitude Winter} \\ Aerosol model & \multicolumn{3}{c}{Desert model} & \multicolumn{3}{c}{Desert model} \\ VIS (km) & 10.2 & 30.6 & 10 \\ Water column (gm/cm\({}^{2}\)) & 1.982 & 1.243 & & 0.2077 \\ Ground Altitude (km) & \multicolumn{3}{c}{1.252} & \multicolumn{3}{c}{2.894} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Input parameters of MODTRAN for spectral band adjustment factors (SBAF) calculation.  
Figure 3: Ground reflectance and variance coefficient of the Dunhuang test site.