# COMS Visible Channel Calibration Using Moon Observation Data
## 4 Discussion  
Long-term monitoring of COMS MI visible channel calibration was conducted using Moon observation data from April 2011 to December 2017 under GSICS frameworks. Annual drift using Moon observations and the GIRO model was 1.52 \(\pm\) 0.10% per year; this value is comparable with that of the DCC and integration method (i.e., 1.43 \(\pm\) 0.13% per year and 1.37 \(\pm\) 0.31% per year, respectively). Notably, the lunar calibration result was in good agreement with that provided by vicarious calibration, such as the DCC and integration method [13; 16]. However, the COMS MI annual drift value was different from that of other sensors (such as MSG/VIS06 of 0.48 \(\pm\) 0.02% per year [26] and MTSAT-2 of 2.71 \(\pm\) 0.15% per year) [30], as the sensor degradation rate depended on factors such as detector characteristics and the exposed space environment. This appears reasonable with respect to the relatively broad range of annual drift values, as the expected drifts were between one and several percent over five years, despite the very stable design of the imaging instrument [21].  
The irradiance ratio between GIRO and lunar observations showed strong seasonal variation due to seasonal variation in the phase angle of the Moon. Furthermore, the phase angle of the Moon showed a similar pattern every half year, which was related to Moon observation attitude and time. Phase angle dependence was corrected to remove seasonal variation of Moon irradiance. After angle correction was applied, the annual drift value remained the same but the standard deviation was reduced from 0.78 to 0.51.  
Sensitivity tests were conducted to investigate and quantify uncertainties that affected calibration accuracy using Moon observation and the GIRO model; these are described in Appendix A. In the sensitivity test described in Appendix A, the calibration uncertainty (around 7%), in accordance with the threshold used to determine moon pixels, was driven by measured irradiances by the MI (I\({}_{\text{Instrument}}\)). The uncertainty contained errors that were driven by the pixel numbers on the boundary area between the Moon and space. In addition, the uncertainty in calculating irradiance (Equation (7)) was derived from a cumulative error in determining the detector counts corresponding to photons from the Moon. This accounted for all detector artifact corrections and calibration uncertainties and was combined with the uncertainty of sky background subtraction. Summed irradiances were corrected for point-spread effects in the ground-based lunar observations; an analytic correction was developed from radial profiles of the sky background for a selected set of lunar images acquired over the ROLO operational range of phase angles.  
Uncertainty in lunar model absolute accuracy when scaled against SI (Systeme International d'unites) radiometric units remained at a level of 5-10%. Inter-band irregularities in model outputs required an adjustment to produce a smooth reflectance spectrum that was expected for the Moon. Therefore, the model's reliability for predicting irradiance variation with geometry far exceeded absolute accuracy; reducing such variation is a high priority to enable the use of the Moon as a transfer standard for inter-calibration of instruments which have dissimilar bands.