The Global Ultraviolet Imager (GUVI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission will determine the variability in thermospheric composition, and its response to auroral inputs as well as measuring those inputs. GUVI is the result of twenty years of work in designing large field of regard far ultraviolet (1 10 -1 80 nm) imagers for spaceflight. These systems are based on the concept of a horizon-to-horizon "monochromatic" imager. The field of view of a spectrograph is swept from horizon to horizon using a scan mirror. The spectrograph uses a grating to spectrally disperse the light. A two-dimensional detector is used to record spatial and spectral information simultaneously. Images are obtained at discrete wavelengths without the use of filters; this reduces if not eliminates much of the concern about instrumental bandpasses, out-of-band rejection, and characterization of filter responses. Onboard processing is used to bin the spectral information into "colors" thereby reducing the overall data rate required. The spectral bandpass is chosen to lie in the far ultraviolet so that the sunlit and dark aurora can be imaged. We review the instrument's as delivered performance and the TIMED science requirments. TIMED will be launched May 18, 2000 and will inaugurate the Solar-Terrestrial Connections program at NASA.

The Global Ultraviolet Imager (GUV1) is an imaging spectrometer on the NASA TIMED spacecraft. GUVI produces simultaneous monochromatic images at five "colors" (121.6 nm, 130.4 nm, 135.6 nm, 140-150 urn, and 165-180 nm) as its field of view is scanned from horizon to horizon. The instrument consists of a scan mirror feeding a parabolic telescope and Rowland circle spectrometer, with a wedge-and-strip detector at the focal plane. We describe the design, and give an overview of the environmental parameters that will be measured. GUVI is a modified version of the Special Sensor Ultraviolet Spectrographic Imager (SSUSI), which will fly on the DMSP Block 5D3 satellites 5-16 through S-20. We present some results from the optical calibration of the five SSUSI units.

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