The Nimbus 7 satellite has measured the solar total irradiance from November 1978 to July 1991 (153 months). These measurements are important both in solar physics and for climate change. To insure that the Nimbus 7 measurements are capturing the true behavior of the Sun, it is essential that the properties of the radiometer and its changes over time be understood. The calibration of the radiometer can be viewed as a process of removing instrumental influences from the raw measurements, leaving the experimenter with an estimate of solar variability. In this paper the changing radiometer pointing, the zero offsets, the stability of the gain, the temperature sensitivity, and the influences of other platform instruments are all examined and their effects on the measurements considered. Only the question of relative accuracy (not absolute) is examined. The resulting derived solar irradiances are compared to previous analyses of the Nimbus 7 radiometer and to the Solar Maximum Mission (SMM) measurements. Compared to previous analyses, the newly derived values are higher and somewhat less variable than the older values. Removal of the off‐axis pointing errors and a new treatment of the zero offsets are the major reasons for the changes. Compared to the SMM measurements, both agree quite well so long as any solar activity is present. When the Sun becomes quiet, so its irradiance variability is less than the Nimbus 7 radiometer resolution, the comparison to the SMM results breaks down. Between 1980 and 1988 the correlation of the daily values is 0.83, compared to 0.62 using previously published values from both satellites. The monthly means have a correlation of 0.90, indicating that about 80% of the longer‐term variance is in common. In 1980 when both satellites were operating without problems, Nimbus 7 was 0.3155% higher on average. For May 1984 to December 1988, Nimbus 7 was 0.3255% higher. Therefore if the Nimbus 7 satellite continues to operate properly until a period of about 1 year after the launch of UARS with its active cavity radiometer irradiance monitor (ACRIM), it appears that the differences between Nimbus 7, SMM ACRIM, and UARS ACRIM can be measured to within a few hundredths of a percent. A self‐consistent set of solar irradiance measurements from several satellites over nearly two solar cycles appears feasible.
AND THE NIMBUS 7 ERBThe development of ERB observational systems is traced from its beginnings in the late 1950's through to the current ERB on the NIMBUS 7 satellite. The instruments comprising the current 22-channel ERB experiment are described in some detail. Noteworthy are the inclusion in one solar channel, of a self-calibrating cavity to measure the solar constant and the use of biaxial scanning telescopes to determine the angular reflection and emission model required for processing the narrow-angle radiometric data. A fairly detailed description of the prelaunch and in-flight calibrations is given along with an analysis of the radiometric performance of the instruments. The data processing system is traced with the aid of a schematic flow diagram showing the steps required to produce the many tape and microfilm products archived. Future plans for improving the quality and accuracy of the data products are discussed. Finally, the upcoming Earth Radiation Budget Experiment (ERBE) is briefly mentioned. It will be capable of simultaneously measuring the radiation budget from three satellites, each having a different equator crossing time and angle. Kyle, andMaschhoff were elected members, and Coulson left the team because of other commitments. Ten papers in this JGR issue cover in more detail specific areas of the NIMBUS 7 ERB experiment and some preliminary analysis of the data. Analysis of the first year of earth radiation budget products is discussed by Jacobowitz et al. [this issue]. The in-flight adjustments to calibration are discussed from several aspects by Ardanuy and Jacobowitz [this issue], Ardanuy and Rea [this issue], Maschhoff et al. [this issue], and Kyle et al. [this issue]. In addition Arking and Vemury [this issue] analyze the difference in absolute calibration between the wide-field-of-view and narrow-field-of-view (scanner) earth radiation budget products. Taylor and Stowe [this issue] utilize the scanner data to obtain satellite altitude reflectance characteristics of various uniform earth and cloud surfaces. Vemury et al. [this issue] discuss some possible ways to improve albedo estimates from ERB narrow-field-of-view scanner data. Finally, Davis et al. [this issue] assess the correlation between the NIMBUS 7 shortwave scanner data and the narrow spectral bandwidth CZCS NIMBUS 7 data. 2. DEVELOPMENT OF ERB OBSERVATIONAL SYSTEMS Four factors can be identified as influences on the development of satellite instrumentation for measuring earth radiation budget: (1) spacecraft constraints--power, data storage, mode of stabilization, and satellite control; (2) viewing geometry--fixed wide/medium field-of-view radiometers and scanning medium/high-resolution radiometers; (3) spectral band-pass requirements--isolating the spectrum into its shortwave (0.2-4.0 •m) and longwave (5.0-50.0 •m) components; (4) on-board calibration--shortwave, using direct solar radiation and space, and longwave, using a warm blackbody source and cold space. These factors allow a logical breakdown of ERB observational sy...
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