ABSTRACT
Solar radiation that arrives at the Earth’s surface having come directly from the Sun’s disk and striking a planar surface facing the Sun is defined as direct normal irradiance (DNI). Measuring DNI is the most expensive measurement in the field of broadband solar and infrared radiation. An alternative to pointing a pyrheliometer at the Sun is to calculate the DNI using measurements made with a rotating shadowband radiometer. The most accurate pyrheliometers are electrically self-calibrating absolute cavity radiometers. The uncertainties associated with satellite retrievals of DNI are about twice as large as those for the retrievals of global horizontal irradiance from satellites. The data produced have been used for validating and improving satellite retrievals of surface radiative fluxes, for comparisons to climate model calculations, and for monitoring subtle long-term changes in the surface radiative environment.
