Photometric normalization of LROC WAC images

Monthly global Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC) observations of the Moon, acquired with varying emission and incidence angles, enable the precise derivation of spatially resolved Hapke photometric parameters [1]. The WAC global mosaics are stacked in a time series to enable phase curve fitting using a tile-by-tile method, with a wide range of phase angle in each tile. Tile-by-tile calculations provide photometric parameters for each tile (currently 1° by 1° from 80° to -80° latitude), resulting in resolved near-global photometric parameter maps (w, xi, Bco, and hc)[2]. Incidence, emission, and phase angles are computed using a new WAC stereometric digital terrain model (100 m/pixel)[3]. In the tile-by-tile method, we obtain low phase observations only near the equator resulting in an inability to accurately estimate Bco at higher latitudes (Hapke parameter controls the y-intercept of phase curve). We interpolated Bco poleward of +/- 5° latitudes, using a negative correlation between w and Bco observed in the equator +/- 5° latitudes. This interpolation method can decrease one free parameter, resulting in short calculation time and much less failed tiles. The normalized image using this parameter map shows almost no tile boundary, which shows that the tile-by-tile method works well. After the tile-by-tile method was applied with twenty months of data over a full range of beta angle, we observed an I/F offset with time. This offset results in reflectance differences at month-to-month boundaries in a global mosaic constructed from sequences acquired over different months (constructed to minimize incidence angle differences as a function of latitude). After significant tests of any possible parameters controlling the residual I/F value, incidence angle was revealed to be a dominant factor, indicating that the photometry model may not completely accounting for incidence angle. The main function dealing with incidence angle is the Lommel-Seeliger term, and LROC WAC photometric observation suggests that this term may require improvement.