The Total Ozone Mapping Spectrometer (TOMS) originally flew aboard NASA's Nimbus-7 satellite and looked directly down at the atmosphere (nadir viewing) to measure the amount of backscattered ultraviolet (UV) radiation. TOMS had a fixed grating and an array of exit slits and step-scanned across the orbital track ±51º from the nadir in 3º steps with an instantaneous field of view (IFOV) of about 0.052 rad. At each scan position, Earth's radiance was monitored at six wavelengths between 310 and 380 nm to infer the total ozone amount. Following Nimbus-7, TOMS flew aboard several additional satellites.