A field UAV system (FUAS) with dual spectrometers for BRDF measurement

This paper describes a Field UAV System (FUAS) with dual spectrometers for Bidirectional Reflectance Distribution Function (BRDF) measurement, which include the unmanned aerial vehicle (UAV) spectrometer measurement system, the ground-based spectrometer measurement system, the sun photometer and the all-sky imager. UAV was used to obtain the field multi-angular spectra in the hemispherical space. While the ground-based spectrometer was carried out to measure the solar diffuser, which continuously records the changes of the field diffuse light and illumination. Through a series of data processing such as data screening, geometric calibration, spectral correction, diffuse light correction, and model fitting, the bidirectional reflectance factor (BRF) in the field is calculated by combining the measured radiance data of the target and that of a solar diffuser with the same solar geometry. The UAV-measured multi-angular reflectance data of the Gould field were fitted based on the RossThick-LiSparse Reciprocal (RTLSR) model utilizing Bayesian inversion, and exhibits high simulating accuracy in characterizing BRDF features (RRMSE ~ < 5%, uncertainty < 1%). The BRDF calculated by the retrieved model parameter was compared with that derived from MODIS BRDF products, and the relative deviation between them can be maintained at about 5% at seven channels of MODIS, indicating that the constructed model based on BRDF data from FUAS can describe the directional reflectance characteristics of the calibration sites well. The FUAS exhibits significant potential for future applications in precise field calibration and environmental monitoring.