# COMS Visible Channel Calibration Using Moon Observation Data
## 2 Data and Methods  
### Moon Observation Data of COMS MI  
The COMS Meteorological Imager (MI) is a multi-spectral imaging radiometer that measures radiated and reflected energy from sampled areas of the Earth and its atmosphere. The Imager sweeps an 8 km-wide path along an East-West axis using a two-axis gimbaled mirror scanning system. Table 1 summarizes the specifications of each channel, including the central wavelength, spatial resolution, dynamic range, and required noise performances. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Channel**} & **Spectral** & **Central** & **IFOV 1 ** & **Ipoint Range** & **SNR 2 [FOOTNOTE:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IFOOT:2]Footnote 2: IOT:2]Footnote 2: IFOOT:2]Footnote 2: IOT:2]Footnote 2: IFOOT:2]Footnote 2: IOT:2] 2: IFOOT:2]Footnote 2: IOT:2] 2: IFOOT:2]Footnote 2: IOT:2] 2: IFOOT:2]Footnote 2: IOT:2] 2: IFOOT:2]Footnote 2: IOT:2] 2: IFOOT:2]Footnote 2: IOT:2] 2: IOT:2] 2: IOT:2] 2: IOT:2] 2: IOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOT:2] 2: IOTOTOT:2] 2: IOTOT:2] 2: IOTOTOT:2] 2: IOTOTOT:2] 2: IOTOTOT:2] 2: IOTOTOT:2] 2: IOTOTOT:2] 2: IOTOTOTOT:2] 2: IOTOTOTOT:2] 2: IOTOTOTOT:2] 2: I COMS MI normally observes the Earth using three observation modes: FD (Full Disk), ENH (Extended Northern Hemisphere), and LA (Local Area) mode. The FD mode provides a full-disk scan of the Earth (8.7\({}^{\circ}\)), followed by star observations and a repeated black body calibration. This process is repeated every three hours. The ENH and LA modes are repeated every 15 min and 8 min, respectively. The LA mode can observe a target within any specific area inside the Earth disk and a target outside of the Earth disk with a fixed area size of 1000 \(\times\) 1000 km as COMS can scan to cold space up to 10.4\({}^{\circ}\) along the East-West axis; this provides sufficient margins in which to capture the Moon as part of the image. To observe the Moon image, COMS MI can use two observation modes: the first is the Moon image collected indirectly in FD mode when it scans the Earth disk as a result of its rectangular field. The second is LA mode, which observes the Moon directly as a specific target. For operational use to enable visible channel calibration, a special LA observation mode is used to obtain a lunar image twice a month. As observed via the Imager, the Moon's diameter ranges from 0.44\({}^{\circ}\) to 0.51\({}^{\circ}\); visible channel detectors have a sampling size of 16 \(\upmu\)rad (considering oversampling) along EW (East-West) and 28 \(\upmu\)rad along NS (North-South). Therefore, the Moon can be imaged within a maximum of 318 NS lines by 556 EW pixels, of which the numbers of pixels and sampling size correspond to approximately 0.51\({}^{\circ}\). An oversampling ratio (FOS) for the EW scan versus NS scan of the COMS MI has been incorporated into Equation (8). To investigate degradation of COMS MI visible channel performance, several Moon observations (typically one per month) must be obtained with the following geometric and radiometric conditions to integrate the full signal produced by the Moon: first, the Moon image must be global and include unilluminated regions; second, there should be no interference in the Moon image from the Earth, the atmosphere, the Sun (no straylight), or from any star. In addition, the Sun must be at an adequate distance: the absolute phase angle must be between 2\({}^{\circ}\) to 92\({}^{\circ}\).