Modeling Solar Radiation at the Earth’s Surface

Radiometer Uncertainty Sources

Sensitivity Functions

Combined uncertainties depend on the product of the sensitivity functions (par­tial derivatives of the response with respect to the measurement equation variables) and error source magnitudes, et. The largest contributions to shade-unshade uncer­tainty are from the evg and evd which must include estimates of the thermal offset (10-70 jiV) described below, and data acquisition measurement uncertainty (typi­cally <10 |iV). For example, for an all-black sensor pyranometer with a responsiv- ity of 7.0 mV per 1000Wm-2, a 70-pV offset corresponds to an irradiance error of -10Wm-2.

For the shade-unshade technique, with fixed direct beam error eB « 4.0Wm-2, zenith angle error ez « 0.06°, and diffuse irradiance error eD « 2.0Wm-2 (black - and-white sensor), the uncertainty in RS for a pyranometer is a function of: zenith angle, uncertainty in the pyranometer voltages, and magnitude of the beam; it ranges from about 1.0% at small zenith angles to >10% for zenith angles greater than 85°. The component-summation technique normally has lower total uncertainties, since the uncertainty in the direct beam is essentially the same as in the shade - unshade calibration, the uncertainty contribution from the diffuse measurement is rather low, and only one voltage measurement is involved for the instruments un­der test, as opposed to two voltage measurements in the shade-unshade technique (Myers et al. 2004).