DMSP/S20, SESS Special Sensor Ultraviolet Limb Imager, SESS/SSUSI, The SUSSI provides Observations of the Extreme and Far UV radiation (vertical profiles) from the Earth's limb. Spectrograph for observation of Far UV and Extreme UV in the spectral range 80-170 nm inclusive of the H Lyman-alpha line at 121.6 nm, with 1.5 nm resolution, stemming from atomic oxygen and nitrogen, and molecular nitrogen, resulting in direct measurements of the electron density vertical profile as well as ion and neutral densities.