The interpretation of the NISTAR data in terms of the global albedo needed for energy balance calculations is complicated by the large variation in albedo over the Earth's surface-atmosphere system, changes in albedo with solar zenith angle, and anisotropy of the reflected solar radiation (Smith, 1999). In addition to these general effects, there is the diurnal and daily variation in the amount of cloud cover and its reflectivity. An analysis of the problem shows that the effects can be summarized in an effective phase function for the various scene types typical of the Earth. Information concerning the distribution of albedo (clouds) and scene-dependent anisotropy of the reflected radiation can be provided by EPIC.
At least 3 EPIC channels will be required to compute the broadband albedo of Earth scenes. Data from a broadband instrument (e.g., CERES onboard Terra) will be used initially to calibrate and validate the broadband albedo data from EPIC. Since EPIC and NISTAR always have the same Earth viewpoint, EPIC data will then be used to account for the anisotropy of reflected solar radiation in the computation of global albedo from NISTAR measurements. Conversely, the NISTAR active cavity radiometer can provide ongoing calibration maintenance for albedo maps from the EPIC spectroradiometer.