We describe the Special Sensor Ultraviolet Spectrographic Imager (SSUSI). 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). The SIS produces simultaneous multispectral images over the spectral range 1150 to 1800Å. The NPS consists of three photometers with filters designed to monitor the airglow at 4278Å and 6300Å and the terrestrial albedo near 6300Å. SSUSI will fly on the Defense Meteorological Satellite Program (DMSP) Block 5D3 satellites S-16 thru S-20 beginning in the later par of this decade. The instruments will be calibrated at the Applied Physics Laboratory's Optical Calibration Facility. 1. SCIENTIFIC OBJECTIVES The DMSP Block 5D3 Special Sensors will provide the first comprehensive operational space-based investigation of the physical and chemical processes in the Earth's upper atmosphere (above 80km). The upper atmosphere is the region that contains the mesophere, thermosphere, and ionosphere. This region is poorly understood due to the difficulty in carrying out in situ measurements, its inherent complexity, and the need to develop a comprehensive global picture of this environment. While the basic physics that controls this region is understood, it does represent a difficult region to model since the atmospheric temperature and temperature gradients reach their largest values, composition changes from molecular to predominantly atomic, complex chemical and electrodynamic processes become the major determinants of composition, and where the combination of these effects prevent an adequate global description of the upper atmospheric "weather". The study of the upper atmosphere has witnessed a major change in the last decade as ultraviolet technology has made the transition from spectroscopy to remote sensing. Traditionally the concern of optical aeronomers (scientists who study the structure, composition, and dynamics of the upper atmosphere using optical means) had been the identification of the excitation and emission mechanisms of spectral features. With the advent of an adequate physical description of the phenomena it has become possible to move beyond the simple identification of features to their interpretation in terms of geophysical parameters. APL is building four new state-of-the-art sensors for DMSP. These sensors, the SSUSI, are intended to provide information on the state of the upper atmosphere and the aurora on a global basis. In order for the data to be of use to the user community, rapid, efficient, and accurate operational algorithms must be developed to convert the radiance observations into environmental parameters. The development of the SSUSI data handling system and the operational uses of SSUSI data are described in another SPIE proceeding paper 1 and in greater detail in an APL Technical Report 2 .