Survey of Bi-directional Reflectance Factor of Black Spruce Forest in Alaska for Validation of GCOM-C Remote Sensing

The “Second-generation Global Imager (SGLI)” of the satellite “Global Change Observation Mission (GCOM)-C”, planned in 2014 or 2015, is a multi-viewing angle optical sensor. The sensor observes the reflected radiation from the land surface at 45 degree slant viewing angles (forward and backward along the orbit) in addition to the nadir. This function enables us to consider the Bidirectional Reflectance Function (BRF) of the forest, and to construct robust forest radiative transfer models for the simulation of the forest structure included Leaf Area Index (LAI) and above-ground biomass. To acquire in-situ BRF data of the forest for the validation of GCOM-C/SGLI data, we carried out the survey of BRF at a boreal forest in Alaska. A black spruce forest, a typical boreal forest in Alaska, located in the Poker Flat Research Range of University of Alaska Fairbanks (65°07’24”N, 147°29’15”W, 210 m MSL) was targeted. Since the forest homogeneously extends about 500 m wide and the terrain is relatively even, this forest site is highly suitable for the validation of the remote sensing measurement. Two kinds of the survey were carried out in early July, 2010. First survey was the forest census survey; we established a 30 m × 30 m quadrat in the forest and measured the position, diameter at breast height (DBH), and height of each tree stand in the quadrat. The tree stand density was about 4000 tree/ha, and the highest tree was 6.4 m. These data will be applied for configuring the forest radiative transfer model. The other survey was the observation of the BRF taken place around the noon of July 7 and 8, 2010 from the top of the tower (17 m) constructed in the forest. We measured the reflected irradiance from the forest by the spectroradiometer (MS-720; EKO Instruments) changing the viewing angle from 20 to 70 degrees and -20 to -70 degrees(off-nadir angle; positive and negative angles mean forward and back scatter angles, respectively) with 5 degrees interval in the principal plane and the orthogonal (cross) plane. The global radiation was simultaneously measured by the other spectroradiometer for the calculation of the reflectance. The BRF in the principle plane showed a kind of bowl-shape distribution with its minimum and maximum at approximately 30 and -70 degrees in visible and near-infrared bands, respectively, that is, the forward scatter was generally smaller than the back scatter. However the Normalized Difference Vegetation Index (NDVI) showed larger value in the forward scatter than in the back scatter. These results will be used for the validation of the forest radiative transfer model.