The Global Ultraviolet Imager (GUVI) is one of four instruments that constitute the TIMED spacecraft, the first mission of the NASA Solar Connections program. The TIMED spacecraft is being built by Johns Hopkins University Applied Physics Labratory and GUVI is a joint collaboration between JHU/APL and the Aerospace Corporation. TIMED will be used to study the energetics and dynamics of the Mesosphere and lower Thermosphere between an altitude of approximately 60 to 180 kilometers.

The other instruments that are a part of the TIMED mission are the Solar EUV Experiment (SEE), the TIMED Doppler Interferometer (TIDI), and Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) .

GUVI is a far-ultraviolet (115 to 180 nm), scanning imaging spectrograph that provides horizon-to-horizon images in five selectable wavelength intervals, or "colors." These colors (HI 121.6 nm, OI 130.4 nm, OI 135.6 nm, and N₂ Lyman-Birge-Hopfield bands 140 to 150 nm and 165 to 180 nm) are chosen in order to produce the GUVI key parameters.

GUVI is based on heritage from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI), an instrument previously built for Defense Meteorological Satellite Program Block 5d-3 satellites, which will fly on the DMSP Block 5D3 satellites F-16 through F-20. The instrument consists of a scan mirror feeding a parabolic telescope and Rowland circle spectrograph, with a wedge-and-strip detector at the focal plane.

GUVI will provide information on the ionosphere and thermosphere by monitoring three general regions: the daytime low to mid-latitude thermosphere, the nighttime low- to mid-latitude ionosphere and the high-latitude auroral zone. With the development of sophisticated remote sensing instruments capable of obtaining simultaneous monochromatic images at a number of wavelengths, and automated data processing techniques, it has become possible to routinely produce maps of the characteristics of the upper atmosphere.

GUVI products include maps of the auroral oval, the characteristic energy and flux of the electrons which excite it, F-region ionospheric electron density profiles, and dayside neutral composition information. These products will be useful to DoD and civilian users and can support many basic research activities as well. With fifteen years of continuous coverage of the composition and dynamics of the upper atmosphere, we will have the unprecedented opportunity to study the effects of global climate change on the upper atmosphere.