System-Level Pre-Launch Calibration of Onboard Solar Diffusers
R Barnes, Steven W. Brown, Keith R. Lykke, Gerald T. Fraser, James J. Butler
Onboard diffuse reflecting plaques are carried to orbit as radiometric reference standards for Earth-observing satellite instruments. For many instruments the reflectance properties of the plaque are characterized independent of the instrument, and the effects of scattering by the diffuser housing are determined through mathematical modeling. The pre-launch laboratory calibration of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) included a system-level calibration of the onboard diffuser using an external lamp and a reference plaque at the sensor's Earth-view port. The calibration of the onboard diffuser was made relative to the reference plaque using SeaWiFS as a transfer radiometer. Recent developments in laboratory light sources enable a significant improvement to the SeaWiFS calibration technique. These include sets of fiber-optically coupled tunable lasers that illuminate integrating spheres or the prime focus of a collimator, to produce uniform, high radiant flux Lambertian or collimated sources, respectively. In addition, newly developed, spectrally tunable supercontinuum-based light sources can provide sets of radiance spectra for the collimator to validate the laser-based diffuser calibration. An absolute calibration of the diffuser system in the laboratory also provides the first step in a two-part transfer-to-orbit experiment, in which the second step uses the illumination of the diffuser on orbit by the Sun. For hyperspectral instruments, the laboratory calibration must account for spectral artifacts in the diffuser material. And for on-orbit measurements, the calibration must account for the effects of Earth-shine as a contaminating source of irradiance illuminating the diffuser.