NPOESS conical microwave imager/sounder: warm load calibration issues and progress

The Integrated Program Office (IPO) of the National Polar-orbiting Operational Environmental Satellite System (NPOESS) is currently engaged in developing the next-generation conically-scanned microwave radiometer known as the conical microwave imager/sounder (CMIS). CMIS heritage operational sensors are the special sensor microwave/imager (SSM/I) and its follow-on, the special sensor microwave imager sounder (SSMIS) that are currently flying on defense meteorological satellite system (DMSP) satellites. The conically-scanning CMIS sensor requires warm and cold calibration configurations stationed above the rotating canister. The exposure of the calibration configurations to non-uniform solar illuminations introduces errors in the calibration budgets. These errors were considered small for CMIS as well as for the heritage sensors. However, recent data from spaceborne radiometers indicate otherwise. Consequently, special considerations have been made in the CMIS design to ensure that overall calibration accuracy can be maintained during periods of Sun exposure and Sun shadow. This paper discusses mitigation of calibration errors associated with solar illumination.