<title>On-orbit calibration of the Special Sensor Ultraviolet Scanning Imager (SSUSI): a far-UV imaging spectrograph on DMSP F-16</title>

The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) is currently slated for launch on the Defense Meteorological Satellite Program (DMSP) F-16 in November 2001. This instrument consists of a scanning imaging spectrograph (SIS) whose field-of-view is scanned from horizon-to-horizon and a nadir-looking photometer system (NPS). It will provide operational information about the state of the atmosphere above 100 km. The unique problems incurred by the observational requirements (e.g. that we be able to make daytime and nighttime observations) and the design trade-offs needed to meet those requirements were strong drivers on calibration requirements. Those design trade-offs and the expectation that the instrument calibration will change appreciably in-flight have led to the requirement to perform a large instrument characterization in-flight using only natural sources. We focus, in this paper, on the flight characterization of the SSUSI instrument. This includes discussions of the stellar calibration approach for radiometric calibration, measurements of internally scattered light, sensitivity to the South Atlantic Anomaly, measurements of changing pulse height distributions, and measuring changing reflectivity of a nadir viewing scan mirror. In addition, the calibration of the NPS system using natural sources is addressed.