The satellite data has the following limitations:
1. The sensors generally cannot distinguish between clouds and snow cover.
2. The measurements are less accurate near mountains, oceans or other large bodies of water.
3. All measurements are essentially made at the top of the atmosphere and require atmospheric models to estimate the solar radiation at the ground.
NASA estimates that their measurements of average daily solar radiation have an RMS error of 35 W/m2 (roughly 20% inaccuracy). The World Climate Research Program estimated that routine-operational ground solar radiation sites had end-toend inaccuracies of 6-12%, with the highest quality research sites in the range of 3- 6% inaccuracy.