To compute diffuse irradiance, the Page model performs clearness index variance comparisons on 30 ground measurement reference stations (Brunger and Thevenard, Proceedings from the 8th International Biannual Conference on Solar Energy in High Latitudes). Monthly global horizontal irradiance for a 12-month period is required to run the model. The monthly global horizontal irradiance values from the SSE data are used as input. SSE insolation and cloud parameters are calculated from the International Satellite Cloud Climatology Project (ISCCP) D-1 data set. The SSE data set is a continuous 10-year global climatology of insolation and meteorology data on a 2.5o equal-area grid system. Although the SSE data within a particular grid cell are not necessarily representative of a particular microclimate, or point, within the cell; the data are considered to be the average over the entire region of the cell. Comparisons have shown that SSE estimates of global horizontal radiation have bias differences less than 2% and RMS uncertainties less than 14% when monthly values of global horizontal radiation are compared with historic ground site measurements of the World Radiation Data Base.
Initial comparisons of DNR calculated from the Page method to DNR derived from 239 ground site measurements, show good agreement over the United States for January and July. These stations are National Weather Service stations that collected meteorological data for the period 1961 - 1990 and are derived from the National Solar Radiation Database. The 56 primary stations measured solar radiation from 1 to 27 years of the measurement period. However, over 90% of the radiation data are modeled. In January, the only site containing measured DNR is Seattle, Washington. And, in July, DNR is modeled for all of the stations. Investigations are ongoing to determine the accuracy of satellite-derived measurements of DNR and DFR over the globe using quality ground measurement data from the Baseline Surface Radiation Network and the Surface Radiation Budget Network.