Calibration is a critical step to ensure data quality and to meet the requirement of quantitative remote sensing in a broad range of scientific applications. One of the least expensive and increasingly popular methods of on-orbit calibration is the use of pseudo invariant calibration sites (PICS). A spatial homogenous and temporally stable area of 34 km2 in size around the center of Kunlun Mountain (KLM) over Tibetan Plateau (TP) was identified by our previous study. The spatial and temporal coefficient of variation (CV) this region was better than 4% for the reflective solar bands. In this study, the BRDF impacts of KLM glacier on MODIS observed TOA reflectance in band 1 (659 run) are examined. The BRDF impact of KLM glacier with respect to the view zenith angle is studied through using the observations at a fixed solar zenith angle, and the effect with respect to the sun zenith angle is studied based on the observations collected at the same view angle. Then, the two widely used BRDF models are applied to our test data to simulate the variations of TOA reflectance due to the changes in viewing geometry. The first one is Ross-Li model, which has been used to produce the MODIS global BRDF albedo data product. The second one is snow surface BRDF model, which has been used to characterize the bidirectional reflectance of Antarctic snow. Finally, the accuracy and effectiveness of these two different BRDF models are tested through comparing the model of simulated TOA reflectance with the observed one. The results show that variations of the reflectances at a fixed solar zenith angle are close to the lambertian pattern, while those at a fixed sensor zenith angle are strongly anisotropic. A decrease in solar zenith angle from 50° to 20° causes an increase in reflectance by the level of approximated 50%. The snow surface BRDF model performs much better than the Ross-Li BRDF model to re-produce the Bi-Directional Reflectance of KLM glacier. The RMSE of snow surface BRDF model is 3.60%, which is only half of the RMSE when using Ross-Li model.
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机译：校准是确保数据质量的关键步骤，并满足广泛的科学应用中定量遥感要求。在轨道校准的最不昂贵且越来越流行的方法之一是使用伪不变校准站点（PICS）。通过我们之前的研究确定了在昆仑山（KLM）中心围绕昆仑山（KLM）中心的空间均匀和时间稳定的面积34 km2。对于反射太阳能带，该区域的空间和时间变异系数（CV）该区域优于4％。在这项研究中，研究了KLM冰川对MODIS的BRDF影响，观察了频段1（659次运行）的TOA反射率。通过使用固定的太阳能天性天顶角的观察结果研究了KLM冰川对视图角度的BRDF影响，并且基于在同一视角上收集的观察结果研究了相对于太阳的角度的效果。然后，将两个广泛使用的BRDF模型应用于我们的测试数据，以模拟由于观察几何形状的变化而产生的TOA反射率的变化。第一个是Ross-Li模型，它已被用于生产Modis Global BRDF Albedo数据产品。第二个是雪表面BRDF模型，已被用于表征南极雪的双向反射率。最后，通过将模拟Toa反射模型与观察到的模型进行比较来测试这两个不同BRDF模型的准确性和有效性。结果表明，固定的太阳能天顶角的反射变化靠近兰布尔图的图案，而固定传感器天顶角的角度是强大的各向异性的。从50°到20°的太阳能天性角度降低导致反射率的增加近似50％。雪表面BRDF模型比Ross-Li BRDF模型更好地重新生产KLM冰川的双向反射率。雪表面BRDF模型的RMSE为3.60％，仅使用Ross-Li模型时的RMSE的一半。
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