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Degradation nonuniformity in the solar diffuser bidirectional reflectance distribution function

机译：太阳漫射器双向反射分布函数中的退化不均匀性

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The assumption of angular dependence stability of the solar diffuser (SD) throughout degradation is critical to the on-orbit calibration of the reflective solar bands (RSBs) in many satellite sensors. Recent evidence has pointed to the contrary, and in this work, we present a thorough investigative effort into the angular dependence of the SD degradation for the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polarorbiting Partnership (SNPP) satellite and for the twin Moderate-resolution Imaging Spectroradiometer (MODIS) onboard Terra and Aqua spacecrafts. One common key step in the RSB calibration is the use of the SD degradation performance measured by an accompanying solar diffuser stability monitor (SDSM) as a valid substitute for the SD degradation factor in the direction of the RSB view. If SD degradations between these two respective directions do not maintain the same relative relationship over time, then the unmitigated use of the SDSM-measured SD degradation factor in the RSB calibration calculation will generate bias, and consequently, long-term drift in derived science products. We exploit the available history of the on-orbit calibration events to examine the response of the SDSM and the RSB detectors to the incident illumination reflecting off SD versus solar declination angle and show that the angular dependency, particularly at short wavelengths, evolves with respect to time. The generalized and the decisive conclusion is that the bidirectional reflectance distribution function (BRDF) of the SD degrades nonuniformly with respect to both incident and outgoing directions. Thus, the SDSM-based measurements provide SD degradation factors that are biased relative to the RSB view direction with respect to the SD. The analysis also reveals additional interesting phenomena, for example, the sharp behavioral change in the evolving angular dependence observed in Terra MODIS and SNPP VIIRS. For SNPP VIIRS the mitigation for this "SD degradation nonuniformity effect" with respect to angles relies on a "hybrid methodology" using lunar-based calibration to set the reliable long-term baseline. For MODIS, the use of earth targets in the major release Collection 6 to improve calibration coefficients and time-dependent response-versus-scan-angle characterization inherently averts the use of SD and its associated issues. The work further supports that having an open-close operational capability for the space view door can minimize SD degradation and its associated effects due to solar exposure, and thus provide long-term benefits for maintaining calibration and science data accuracy. (C) 2016 Optical Society of America
机译：太阳漫射器（SD）在整个退化过程中的角度依赖性稳定性的假设对于许多卫星传感器中的反射太阳带（RSB）的在轨校准至关重要。最近的证据指出了相反的情况，在这项工作中，我们对Suomi国家极地轨道合作伙伴（SNPP）卫星上的可见红外成像辐射计套件（VIIRS）以及卫星的SD衰减的角度依赖性进行了全面的调查工作。 Terra和Aqua航天器上的双分辨率中等分辨率成像光谱仪（MODIS）。 RSB校准中的一个常见关键步骤是使用由随附的太阳漫射器稳定性监视器（SDSM）测量的SD降级性能作为RSB视图方向上SD降级因子的有效替代。如果这两个方向之间的SD劣化不随时间保持相同的相对关系，则在RSB校准计算中对SDSM测量的SD劣化因子的正确使用将产生偏差，因此，派生科学产品中的长期漂移。我们利用在轨校准事件的可用历史来检查SDSM和RSB探测器对反射自SD的入射照度与太阳偏角的关系，并显示出角度依赖性，特别是在短波长下相对于时间。广义和决定性的结论是，SD的双向反射率分布函数（BRDF）相对于入射方向和出射方向均不均匀地退化。因此，基于SDSM的测量提供相对于RS相对于RSB视图方向有偏差的SD退化因子。该分析还揭示了其他有趣的现象，例如，在Terra MODIS和SNPP VIIRS中观察到的不断变化的角度依赖性中的剧烈行为变化。对于SNPP VIIRS，针对“角度”的“ SD降级不均匀效果”的缓解措施依赖于“混合方法”，该方法使用基于月球的校准来设置可靠的长期基线。对于MODIS，在主要版本集合6中使用地球目标来提高校准系数和与时间相关的响应-扫描角度特性固有地避免了SD及其相关问题的使用。这项工作进一步支持具有对空间查看门的开关操作能力，可以最大程度地减少SD退化及其因日光照射引起的相关影响，从而为保持校准和科学数据的准确性提供长期利益。 （C）2016美国眼镜学会

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