by using a spectrophotometer to measure UV radiation. In this approach, scattered UV radiation is observed at multiple wavelengths—some strongly absorbed by ozone and the others weakly absorbed. The satellite total-column ozone retrievals were similar to the ground-based Dobson band-pair technique. The Nimbus-4 system did not include an onboard radiometric calibration source or stability monitor, and the BUV hardware was a direct current (dc) system. While consequent calibration challenges combined with problems concerned with the South Atlantic Anomaly and the nadir-only views gave a rough estimate of the global ozone climatology, the 2 years of measurements provided key lessons for later flights, which required and provided better coverage and accuracy.

The next-generation ozone measurements were obtained from the very successful Nimbus-7 Solar Backscatter UV (SBUV) and Total Ozone Mapping Spectrometer (TOMS), launched in October 1978. For more than 10 years the Sun-synchronous SBUV and TOMS on Nimbus-7 provided daily, near-global (the nighttime polar regions were not observed) maps and ozone profiles along the satellite track. These measurements included detailed mapping of the Antarctic “ozone hole” after it appeared. Improvements to the flight hardware included an alternating current (ac) chopper, a shared solar diffuser for reflective radiometric calibration, and swath coverage in the case of TOMS. The nature of BUV sensing (viewing the solar spectral irradiance and Earth’s spectral radiance, the Earth bidirectional reflectance distribution function (BRDF), through the same optical path) mitigated many calibration challenges. In addition, examination of the consistency of multiple band pairs (six UV wavelengths were sampled by the TOMS) helped to take out the total ozone drifts in time. The community is still struggling for consistency in the profiles in which absolute BRDF knowledge is used.

The Nimbus-7 observation time series was of fundamental importance. The more-than-10-year duration of the data set provided a unique climate record because of its long-term accuracy. The onboard calibration capability helped provide for stability of the measurement record.

The timing of the Nimbus-7 mission included a period of rapid deepening and discovery of the ozone hole. The significant lowering in total ozone over Antarctica caused a rethinking of the autonomous ground quality-assurance programs that otherwise would reject the “unrealistic” low values. The success of the mission exceeded expectations—exemplified at the 32,768th orbit, when the two-byte word for the orbit number no longer sufficed.

The Nimbus-7 SBUV legacy (November 1, 1978, through June 21, 1990) was extended into the operational realm starting with the NOAA (National Oceanic and Atmospheric Administration) -9, -10, -11, -14, -16, and -17 satellites, the present-day Polar-orbiting Operational Environmental Satellites (POESs). These satellites carry the SBUV/2, which includes an onboard solar diffuser stability monitor to help



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