Abstract
Chinese Fengyun-2 (FY-2) satellite is to ensure continuity of atmospheric observation from the geostationary orbit (GEO) at the longitude of 105E and 86.5E degrees as a part of the worldwide operational meteorological satellite system. FY-2C began to run operationally from March, 2005 until it was replaced by FY-2E on 25 November 2009. FY-2D and FY-2E are still operating on orbits and there will be more three members of FY-2 satellite family in the near future, including FY-2F, FY-2G and FY-2H. FY-2 spacecraft series has an on-board black body calibration mechanism, in which black bodies with known temperatures can be viewed. However, an absolute on-board calibration of the optical system is not possible, as part of the system consisting of the front optics is excluded from the calibration path. The current operational calibration of FY-2 imager is mainly based on the inter-calibration using Advanced Very High Resolution Radiometer (AVHRR) and High-Resolution Infrared Radiometer Sounder (HIRS) onboard NOAA satellite. But this vicarious technique can not be conducted frequently and provide the diurnal variation when the instrument temperature has significant change in eclipse seasons. But the observation digital counts of the black bodies can respond directly to the diurnal variation of the ambient spacecraft temperature and sensor sensitivity. Black body calibrations may be used to tune the vicarious calibration. The latest FY-2E operational calibration began to be replaced using the Global Space-based Inter-Calibration System (GSICS) results based on AIRS and IASI instead of AVHRR and HIRS on January 19, 2012. It shows the great improvement of this operational calibration based on GSICS correction. The new bias is only smaller than 0.2K and the deviation is smaller than 0.1 K except for eclipse seasons. We can also use this kind of GSICS correction for the FY-2 historical data for climate ultilization and provide an independent calibration coefficient. But GSICS cannot resolve all the calibration issues of FY-2 imagers for every time because of its limited collocated radiance in each day, especially GSICS meets difficulty in the completely diurnal cycle in eclipse phase. GSICS can provide the radiance reference in insolated time for FY-2 observation and it can be combined into the onboard blackbody model development. The black body observation of FY-2 is supplying a stable relative calibration data of all infrared channels frequently. The black body calibration sequence starts with a space view (hence mainly observing the self-emission of the front optical system) being taken, followed by a view of the black body itself. The viewing of the black bodies is performed by moving a mirror into the nominal optical path of the radiometer, between the optical block and the front optics. Therefore, the front optics of the radiometer is not included into the optical path of the black body calibration mechanism. In addition the viewing geometry is not similar for black body and Earth view. Hence, the pure black body calibration coefficients cannot be used directly for calibration of the infra red channels.The front optics model correcting for these issues is derived under the help of GSICS reference radiance in full optic. the front optics are not part of the optical path during a black body observation, and as the viewing geometry is different when performing a black body observation with respect to nominal Earth observation, a correction model has been designed allowing for these effects. The black body calibration gives a stable relative calibration of the infra red channels, while the tuning towards the GSICS calibration gives the absolute calibration level for both channels. Hence, the GSICS calibration of both infrared channels is used to tune the black body calibration for guaranteeing the absolute accuracy of calibration.
Onboard Blackbody Calibration Algorithm of Chinese FY-2 Geostationary Imager Based on GSICS Reference Radiance
Chinese Fengyun-2 (FY-2) satellite is to ensure continuity of atmospheric observation from the geostationary orbit (GEO) at the longitude of 105E and 86.5E degrees as a part of the worldwide operational meteorological satellite system. FY-2C began to run operationally from March, 2005 until it was replaced by FY-2E on 25 November 2009. FY-2D and FY-2E are still operating on orbits and there will be more three members of FY-2 satellite family in the near future, including FY-2F, FY-2G and FY-2H. FY-2 spacecraft series has an on-board black body calibration mechanism, in which black bodies with known temperatures can be viewed. However, an absolute on-board calibration of the optical system is not possible, as part of the system consisting of the front optics is excluded from the calibration path. The current operational calibration of FY-2 imager is mainly based on the inter-calibration using Advanced Very High Resolution Radiometer (AVHRR) and High-Resolution Infrared Radiometer Sounder (HIRS) onboard NOAA satellite. But this vicarious technique can not be conducted frequently and provide the diurnal variation when the instrument temperature has significant change in eclipse seasons. But the observation digital counts of the black bodies can respond directly to the diurnal variation of the ambient spacecraft temperature and sensor sensitivity. Black body calibrations may be used to tune the vicarious calibration. The latest FY-2E operational calibration began to be replaced using the Global Space-based Inter-Calibration System (GSICS) results based on AIRS and IASI instead of AVHRR and HIRS on January 19, 2012. It shows the great improvement of this operational calibration based on GSICS correction. The new bias is only smaller than 0.2K and the deviation is smaller than 0.1 K except for eclipse seasons. We can also use this kind of GSICS correction for the FY-2 historical data for climate ultilization and provide an independent calibration coefficient. But GSICS cannot resolve all the calibration issues of FY-2 imagers for every time because of its limited collocated radiance in each day, especially GSICS meets difficulty in the completely diurnal cycle in eclipse phase. GSICS can provide the radiance reference in insolated time for FY-2 observation and it can be combined into the onboard blackbody model development. The black body observation of FY-2 is supplying a stable relative calibration data of all infrared channels frequently. The black body calibration sequence starts with a space view (hence mainly observing the self-emission of the front optical system) being taken, followed by a view of the black body itself. The viewing of the black bodies is performed by moving a mirror into the nominal optical path of the radiometer, between the optical block and the front optics. Therefore, the front optics of the radiometer is not included into the optical path of the black body calibration mechanism. In addition the viewing geometry is not similar for black body and Earth view. Hence, the pure black body calibration coefficients cannot be used directly for calibration of the infra red channels.The front optics model correcting for these issues is derived under the help of GSICS reference radiance in full optic. the front optics are not part of the optical path during a black body observation, and as the viewing geometry is different when performing a black body observation with respect to nominal Earth observation, a correction model has been designed allowing for these effects. The black body calibration gives a stable relative calibration of the infra red channels, while the tuning towards the GSICS calibration gives the absolute calibration level for both channels. Hence, the GSICS calibration of both infrared channels is used to tune the black body calibration for guaranteeing the absolute accuracy of calibration.