Calibration of the Nimbus-7 scanning multichannel microwave radiometer (SMMR), 1979-1984

Graduation date: 1988Six years of Nimbus-7 Scanning Multichannel Microwave\ud Radiometer (SMMR) data from 1979 to 1984 are recalibrated\ud at the radiometric level. New statistical techniques are\ud developed to quantify the temporal and orbital data errors\ud and to generate numerical corrections for brightness temperatures\ud prior to estimation of geophysical parameters.\ud The foundations of passive microwave satellite\ud radiometry are reviewed. The Nimbus-7 spacecraft and the\ud SMMR experiment are described, along with the radiometric\ud data calibration methods. Sensor design weaknesses and\ud orbit-related problems are summarized. Physical temperatures\ud of various components of the SMMR sensor and housing\ud structure, analyzed for six selected days of data, show\ud systematic thermal changes throughout the orbit due to\ud direct and indirect effects of solar heating. Changes in\ud the operating temperatures, which impact the calibration,\ud also occur from orbit-to-orbit following instrument startup\ud on the SMMR's alternate sampling days. These errors and\ud others contaminate the radiometric data. Incidence angle\ud data, computed from measurements of spacecraft attitude,\ud are analyzed for 1983 and found to contain discontinuities\ud during every orbit due to the use of two different attitude\ud sensors, and temporal drifts and jumps.\ud The post-launch error corrections are developed from\ud the NASA CELL-ALL brightness temperature data. The data\ud are first converted to antenna temperatures, re-calibrated\ud with a bias and slope adjustment, and corrected for temporal\ud drifts. The cross-track bias and mean along-track\ud error for an average annual orbit are statistically derived\ud as well as a mean start-up bias for the first four orbits\ud of each sampling day. The corrections describe only systematic\ud errors found in global averages and are allowed to\ud vary from year to year. Comparisons with Seasat\ud radiometric data show that the errors are unique to the\ud Nimbus-7 SMMR.\ud The corrections are applied to the six years of data\ud and initial retrievals of integrated atmospheric water\ud vapor are examined in a preliminary analysis. Global\ud fields of 1°x 1° monthly averaged water vapor show that the\ud SMMR can map large scale features and document seasonal\ud events, such as the Asian monsoons, and interannual events,\ud such as the El Nino/ Southern Oscillation