Experimental Study Of Field Measurement Method Of Thermal Infrared Directionality Over Forest Canopy Using Crane Platform

Forest canopy is an important intermediate process in water and energy cycle of terrestrial ecosystem which links the interactions and energy exchange between land surface and atmosphere. It is also one of the important driving forces in all kinds of land surface ecosystem. The temperature of forest canopy is a comprehensive reflection of the surface energy balance. However, values recorded from varied observation angle are directional different, which seriously limits the precision of temperature retrievals of remote sensing. To gain full understanding of the directional characteristics of thermal radiation and its changing mechanism, an field monitoring research was conducted in Huailai remote sensing test site of Chinese Academy of Sciences. Based on Airborne remote sensing scanning imaging principle, an oblique platform was developed. Wide-angle Infrared Dual-mode line/area Array Scanner (WiDAS) system was combined with a mobile tower crane, and also the infrared thermal imager within an 80°wide-angle lens were utilized as the most important parts of the platform. Pobulus davidiana, Platycladu orientalis (L.) Franco, and Pinus tabuliformis were monitored in the study. Under the current monitoring condition, the results were as follows:(1) Based on previous observations, this study proposed oblique observation project with four directions over forest canopy. A thermal radiant multi-angle observation tower platform was successfully set up on the basis of experimental site.(2) Based on geometric optical theory, the program of data processing and method of information extraction were put forward.(3) Clear directional difference of thermal radiant was found in the artificial pure forest canopy, with up to 7K temperature difference from different species and multi-angle observations, and 4K temperature difference between vertical and horizontal observation values.(4) Due to various factors, "hot spot" was found only in the canopy of Platycladu orientalis (L.) Franco, but not in pine and Aspen.(5) According to ground-based observations, the relationship between the component temperature difference of canopy-surface and trunk sap flow velocity of platycladus orientalis was established as F=I+S(Tc-Ts), in which the integrated variable parameters I was to be determined, fitted slope S ranges from -20 to 0, with mean value of -12.056. In addition, the component temperature difference correlation of tower crane observation and ground observations, and more precise parameter values are to be determined in the further observation.