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A primary goal of the radiometric calibration efforts for the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) is to provide consistent radiometric calibration of the NASA-provided aircraft and field instruments, particularly in the reflective (solar) portion of the spectrum. To this goal, a common source and traceability were chosen for use by all instruments for the primary calibrations: a 122 cm diameter hemisphere maintained by the Standards and Calibration office at GSFC. Among the primary multispectral reflective band instruments used in FIFE are the NS001 scanner on a C130 aircraft operated by Ames Research Center and several Barnes Modular Multispectral Radiometers (MMR's) operated on a helicopter and the ground. These instruments have 7 bands in the reflective portion of the solar spectrum: approximately the TM bandpasses and an additional band at about 1.2 um. The NS001 has a continuously variable gain setting; calibration is maintainable only by reference to its internal light source. Calibration of the NS001 data is affected by drift in the dark current level of up to 6 counts during a mirror scan at typical gain settings. Use of both samples of the dark current recorded by the instrument in performing the calibration, as opposed to the one currently used, generally decreases the uncertainty to less than 1 count, with the possible exception of channel 7. The apparent radiance of the internal source degraded an average of 4% over the 11 month period of January 1987 to December 1987 relative to the 76 cm sphere used to monitor it; this 76 cm source in turn apparently degraded 5-10% during the same period, suggesting an overall 10-15% degradation in the internal source. The MMR instruments are being used in their 1° degree field-of-view (FOY) configuration on the helicopter and 15° FOV on the ground. Pre- and post-season laboratory calibrations were supplemented by daily stability checks using a 30 cm integrating sphere source. The changes in calibration of the MMR instruments were related to the extent of their use. In the silicon channels of the MMR's , instruments used throughout the 4 field campaigns showed degradations of 3-4% between pre- and post-season calibrations; the one instrument used for 2 of the field campaigns degraded approximately half those values; and the instrument used just one day showed pre- and post-season calibrations within one percent of each other. Strong temperature sensitivity in the lead-sulfide channels (PbS) channels, about ±25% over a 15° C range, which was reduced to ±4% or less with temperature correction, led to greater uncertainty in these channels calibration, although pre- and post-season calibrations differences were no larger than for the silicon channels.
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