Nimbus-6 earth radiation budget experiment

Abstract

This paper describes the Nimbus-6 earth radiation budget experiment including its prelaunch calibration and in-flight performance. A preliminary assessment of the data shows the ERB measurement of the solar constant to be 1392 W/m(2) which is 1.6% higher than the expected value of 1370 W/m(2). Both values are traceable to the cavity radiometer scale. There is a disagreement between the fixed wide-angle and scanning narrow-angle measurements of planetary outgoing longwave radiation flux. Since the scanning channels are calibrated in-flight and show good agreement with previous observations of the Nimbus-3 satellite, the discrepancy is believed to be due to erroneous wide-angle flux estimates. The erroneous estimates may be caused by the misinterpretation of the transfer function for the wide-angle-earth-flux sensing thermopile detectors when viewing the earth which, unlike the prelaunch calibration source, does not fill the field of view of the detector and is not an isotropic radiation source. A field of view factor for the wide-angle channels is determined using an in-flight calibration procedure using the night-time scanning channel longwave radiation flux measurements as the absolute standard. The planetary global albedoes, longwave radiation fluxes, and net radiation are about 30%, 240 W/m(2), and -4 W/m(2) for the months of July and August 1975, which is in good agreement with previous Nimbus-3 estimates.