Measurements of Bidirectional Reflectance Distribution Functions of the Witness Samples of the Solar Diffuser in the Global Imager on the ADEOS-II
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概要
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Many optical sensors equipped on earth observation satellites have onboard calibration units. A solar diffuser is one of such units and a pattern of reflected radiance yielded by the solar diffuser directly affects the onboard calibration accuracy. In this paper, we present the results of our measurements of bidirectional reflectance distribution functions (BRDFs) for three witness samples of the solar diffuser in the Global Imager (GLI), a wide-field, multi-color radiometer equipped on the earth observation satellite ADEOS-II, to comprehend the scatter properties of the diffuser before degradation by UV radiation in space. Three witness samples are diffuser samples for GLI engineering model (EM) and pre-flight model (PFM1 and PFM2). The measurements of BRDFs were performed at wavelength of 490, 565, 666, 763, and 865nm by using the multidetector hemispherical optical scattering instrument. In the case of small angle of incidence (θ<SUB>i</SUB> ≤ 30°), our results show that little azimuth differences in BRDFs appear, in contrast with the clear appearance of zenith angle dependence of BRDFs. Furthermore, it is found that the strength of BRDF decreases as the zenith angle increases. On the other hand, at large angle of incidence (θ<SUB>i</SUB>=60°), a pattern of BRDF approaches that expected in a Lambertian surface, except in the certain direction related to the specular angle. At the direction close to the specular angle, the strength of BRDF increase remarkably as the zenith angle increases. Concerning a wavelength dependence of BRDF, we measured a relative reflectance from the wavelength of 400nm to 2.2μm by using a spectrophotometer. The results show no appearance of significant wavelength dependence of BRDFs in our sample. It is found, moreover, that the discrepancy of the intensity of BRDFs occurs between the EM witness sample and PFM samples. That is, the EM sample has systematically larger BRDF compared with BRDF of the PFM samples. The maximum difference between the BRDFs for the EM and PFM samples, i.e. about 20% in strength, occurs in the scattering angle larger than 30°. On the other hand, the BRDFs of two PFM samples were agreed within the 1% difference. Since our measured was performed for the non-degraded witness samples, measured BRDFs would be able to use as a reference to evaluate the degradation trend of the on-board diffuser.
- 社団法人 日本リモートセンシング学会の論文