In order to reduce the calibration uncertainty of the reflectance-based method brought by the assumption of the aerosol model, the irradiance-based method, known as improved reflectance-based method, was proposed. The irradiance-based method is described in this paper. The radiometric calibration field campaign was performed at Dunhuang test site on 27 August, 2014. A hyperspectral irradiance meter (HSIM) developed by Anhui Institute of Optics and Fine Mechanics (AIOFM) was used to measure the diffuse-to-global spectral irradiance ratio. The irradiance-based method and the reflectance-based method were performed to calibrate the first four bands of Moderate Resolution Imaging Spectroradiometer (MODIS). The results of two methods were compared with result of MODIS on-board calibrator. The comparison shows that the result of irradiance-based method has a good consistency with on-board calibration and reflectance-based method results. The difference of calibration coefficients between irradiance-based and on-board method was less than 1.4%. Due to the limitations of the irradiance-based method, a clear sky and stable atmospheric condition is required for the entire half of the calibration day to provide the data necessary for the extrapolation of diffuse-to-global ratio in viewing direction. A study on the effects of aerosol mode assumption on the final apparent reflectance was performed on both the irradiance-based method and the reflectance-based method by selecting different aerosol modes to predict the apparent reflectance. The results show that aerosol mode assumption has a great effect on the reflectance-based method, however slight effect on the irradiance-based method.
With the progress of quantitative remote sensing, the acquisition of surface BRDF becomes more and more important. In order to improve the accuracy of the surface BRDF measurements, a VNIR-SWIR Bidirectional Reflectance Automatic Measurement System, which was developed by Hefei Institutes of Physical Science (HIPS), is introduced that allows in situ measurements of hyperspectral bidirectional reflectance data. Hyperspectral bidirectional reflectance distribution function data sets taken with the BRDF automatic measurement system nominally cover the spectral range between 390 and 2390 nm in 971 bands. In July 2007, September 2008, June 2011, we acquired a series of the BRDF data covered Dunhuang radiometric calibration test site in terms of the BRDF measurement system. We have not obtained such comprehensive and accurate data as they are, since 1990s when the site was built up. These data are applied to calibration for FY-2 and other satellites sensors. Field BRDF data of a Dunhuang site surface reveal a strong spectral variability. An anisotropy factor (ANIF), defined as the ratio between the directional reflectance and nadir reflectance over the hemisphere, is introduced as a surrogate measurement for the extent of spectral BRDF effects. The ANIF data show a very high correlation with the solar zenith angle due to multiple scattering effects over a desert site. Since surface geometry, multiple scattering, and BRDF effects are related, these findings may help to derive BRDF model parameters from the in-situ BRDF measurement remotely sensed hyperspectral data sets.
In many research fields the coupling efficiency of single mode fiber is an important issue and is influenced by some
parameters. In this paper, the influence factors on coupling efficiency of single mode fiber were investigated, especially the influence of position of waist of laser beam is analyzed. For verifying these influence factors some numerical simulations were carried out. The results show that when the waist position being farther away from the laser emission system, the coupling efficiency is larger; when the waist position is located at infinite distance and the number of Airy speckles is 0, the relationship curve between the coupling efficiency and parameter a is unimodal; and also with parameter a tending towards zero the coupling efficiency is close to a bigger stable value, which differs from
computation result in previous reference. These conclusions have important directive significance to the engineering
design of coupling of single mode fiber.