# COMS Visible Channel Calibration Using Moon Observation Data
Additionally, a NASA-sponsored project to accurately determine the irradiance and radiance of the Moon began routine operations in 1996 [18; 19; 20]. The surface of the Moon has reflectance properties that are virtually invariant over time. However, the Moon's brightness varies in a complex way with respect to the following: spatial variation of lunar albedo; physical and optical libration over periods of 1 month, 1 year, and 18 years, as well as a strong dependence on phase angle of the surface photometric function [21]. With the exception of dynamic range issues, all have been addressed by the ROLO (Robotic Lunar Observatory) program at the USGS (U.S. Geological Survey) in Flagstaff, Arizona, through development of the ROLO lunar model [20; 22]. The ROLO facility was designed for and is dedicated to radiometric observations of the Moon. Descriptions of the lunar calibration technique applied to spacecraft can be found in literature [17; 18; 23]. The USGS model can predict lunar irradiance with a precision of \(\sim\)1% over its full range of geometric coverage, irrespective of observation geometry (within the range of coverage, e.g., phase angles up to 90\({}^{\circ}\)). The absolute scale of the lunar model currently is undergoing refinement, and current uncertainties are within 5-10%. With improvements to the absolute scale, lunar calibration would enable a consistent radiometric scale for Earth-observing sensors that view the Moon as a common source. However, the utility of lunar calibration can only be realized if current and future instruments can observe the Moon, and visible channel imagers on geostationary meteorological satellites capture the Moon in normal operational images. In this respect, the value of regular lunar observations has been recognized; NOAA has instituted monthly scans of the Moon for both GOES-10 and GOES-12 [24]. With recent developments in numerical weather prediction models and related global climate change studies, there is an increasing demand from the user community for more accurate satellite measurements. With the possibility of obtaining superior lunar calibration accuracy, the objective of this study was to use the GSICS implementation of the ROLO model (GIRO) to monitor and improve the radiometric calibration accuracy of the visible channel. Recently, EUMETSAT has implemented a lunar calibration tool (GIRO) which has been made available to the GSICS community for activities that are focused on defining unified calibration references [24]. This paper reports the long-term characteristics of the COMS MI visible channel for the first time through that application of GSICS concepts, implementation of GIRO, and the use of approximately seven years of moon observation data. Section 2 briefly introduces characteristics of the COMS MI visible channel and associated data provided and describes the Moon observation operation. Section 3 describes the method used to compute radiances of the Moon (on the basis of the ROLO model) as reference values and observed radiances of the Moon from the imager. Degradation of the COMS MI visible channel is computed through the use of the ratio between two radiances (irradiance from a statistical model and one from the imager). Calibration results from a sensitivity study are shown in Appendix A; those of vicarious calibration methods using different stable targets are described in Section 4. The results of the sensitivity test used to determine on-Moon pixels, the threshold used in selection of Moon pixels, and ROLO Lunar Irradiance Model Coefficients of COMS MI are described in Appendices A and B.