Before the satellite era, solar irradiance was inferred from ground based radiometers using the Langley method. A major difficulty that limits the accuracy of the ground based exo-atmospheric solar irradiance estimates is the natural variability of the atmosphere. However, the limitation of the ground based measurements has never been quantitatively assessed. With the satellite measurements as true value, we will be able to estimate the uncertainty of ground based measurements. In this paper, we intercompare satellite and ground based observations of solar irradiance for two Cimel sunphotometer bands at 340 nm and 380 nm.