Evaluating Radiometric Measurements using a fixed 45° Responsivity and Zenith Angle Dependent Responsivities

 
Poster PDF

Authors

Aron Habte — National Renewable Energy Laboratory
Mike Dooraghi — National Renewable Energy Laboratory

Category

Radiation

Description

Abstract: The Atmospheric Radiation Measurement (ARM) program provides high quality radiometric data traceable to International System of Units (SI), through the World Radiometer Reference (WRR) and World Infrared Standard Group (WISG). The National Renewable Energy Laboratory (NREL) and ARM through the Radiometer Calibration Facility (RCF) at the Southern Great Plains (SGP) provide calibration of broadband radiometers deployed in the SKYRAD, GNDRAD, and SIRS instrument platforms. Both NREL and ARM continue to improve radiometric calibration and measurement through the introduction of new methods and refinement of old methodologies. The poster seeks to demonstrate the importance and application of an existing but unused approach that ultimately reduces the uncertainty of radiometric measurements. Currently almost all broadband irradiance data, including those measured at the ARM facilities, collected from commercially available radiometers use a single responsivity value generated at the 45 degree solar zenith angle. However, based on our experience and research at the SRRL (Solar Radiation Research Laboratory) as well as from other experts in the radiometric science community, this method introduces significant uncertainty to the data. The reason for this uncertainty is the fact that the calibration is based on a single solar zenith angle which is only applicable to times of day that match that zenith angle. This is demonstrated in Reda, 1998 and Reda et al., 2008. Reda 1998 and Reda et al. 2008 also demonstrate that the uncertainty in the responsivity value can be reduced by as much as 50% through using responsivity as a function of solar zenith angle. In this poster we present the results of using the responsivity as a function of zenith angle versus using the responsivity at zenith angle at 45°.

Lead PI

Aron Habte — National Renewable Energy Laboratory