The University of Arizona, Optical Sciences Center, Remote Sensing Group is involved with the vicarious calibration of satellite sensors in support of NASA’s Earth Observing System (EOS) program. Sensor calibration coefficients are calculated by comparing sensor DN values to top of the atmosphere (TOA) radiance values, calculated from radiative transfer code (RTC). The RTC output is based on measurements of site spectral reflectance and atmospheric parameters at a selected test site. The bidirectional reflectance distribution function (BRDF) which relates the angular scattering of a given beam of incident radiation on a surface, is an important factor in these radiative transfer calculations. The inclusion of BRDF data into RTC calculations improves the level of accuracy of the vicarious calibration method by up to 5% over some target sites. BRDF data is also valuable in the validation of Multi-Angle Imaging Spectroradiometer (MISR) data sets.
The Remote Sensing Group has developed an imaging radiometer system for ground-based measurements of BRDF. This system relies on a commercially-available 1024- by 1024-pixel silicon CCD array. Angular measurements are accomplished with a 8-mm focal length fisheye lens that has a full 180-degree field of view. Spectral selection is through four interference filters centered at 470, 575, 660 and 835 nm, mounted internally in the fisheye lens. This paper discusses the effect of calibration errors in this camera system on the retrieval of Hapke/Jacquemoud surface parameters from modeled BRDFs. The effect of these retrieved BRDFs on vicarious calibration results is discussed. Data processing schemes for the retrieval of these parameters from BRDF camera data sets are described. Based on these calculations, calibration requirements for digital camera BRDF-retrieval systems are presented.
Keywords: BRDF, CCD, Reflectance, Vicarious Calibration, Digital Camera