Measurement of land surface BRDF with spatial instability for vicarious calibration

The land surface for vicarious calibration, e.g. playa, desert sand and snowfield, seemed to be similar to lambertian surface, and the bidirectional reflectance factors were then assumed to be lambertian in many cases of vicarious calibration even if it actually showed the departure from lambertian behavior. Recently, the measurement of bidirectional reflectance distribution function (BRDF) of ground surface becomes important according to the improvement of the vicarious calibration method that leads to more precision coefficients. The scale of in-situ measured BRDF is very different from satellite based BRDF, which generally causes the problem of spatial instability in the in-situ measurement. To estimate the satellite based BRDF, it is better to measure the BRDF at the many points in the ground site. However, most of the previous instruments can measure only a few points at the overpass time of satellite, because the instruments are heavy and need a long time to measure the BRDF. In this study, new BRDF measurement system that has a potential to correct spatial instability using the spectroradiometer and CCD digital camera is proposed, and its capacity and limit are discussed using atmospheric and land surface radiative transfer models.