A method to Estimate Uncertainty in Radiometric Data using the Guide to the Expression of Uncertainty in Measurement (GUM) Method

 
Poster PDF

Authors

Ibrahim Reda — National Renewable Energy Laboratory
Manajit Sengupta — National Renewable Energy Laboratory
Aron Habte — National Renewable Energy Laboratory

Category

Radiation

Description

The Atmospheric Radiation Measurement (ARM) broadband radiometric data is traceable to International System of Units (SI), through the World Radiometer Reference (WRR). The National Renewable Energy Laboratory (NREL) and ARM through the Radiometer Calibration Facility (RCF) at the Southern Great Plains (SGP) continue to maintain and improve the accuracy and traceability of broadband radiometric data. Radiometric data with known and traceable uncertainty is essential for climate change studies to better understand cloud radiation interactions and the earth radiation budget. Further, adopting a known and traceable method of estimating uncertainty with respect to SI ensures that the uncertainty quoted for radiometric measurements can be compared based on documented methods of derivation. Radiometers such as pyranometers are used to measure downwelling or upwelling shortwave radiation whereas pyrheliometers are used to measure the direct beam. Currently, most radiometric data users rely on manufacturers’ specifications of calibration uncertainty to quantify the uncertainty of measurements. However, the accuracy of solar radiation measured by radiometers depends not only on the specifications of the instrument but also on (a) calibration procedure, (b) measurement setup and maintenance, and (c) location and environmental conditions. Therefore, statements about the overall measurement uncertainty can only be made on an individual basis, taking all relevant factors into account. This poster provides guidelines and recommended procedures for estimating the uncertainty in calibrations and measurements from radiometers. The approach follows the Guide to the Expression of Uncertainty in Measurement (GUM). The poster summarizes the process of estimating uncertainty using the following steps; define the measurement equation, determine the sources of uncertainty, calculate standard uncertainty for each source, derive the sensitivity coefficient using a partial derivative approach from the measurement equation, and combine the standard uncertainty and the sensitivity term using the root sum of the squares, and lastly calculate the expanded uncertainty by multiplying the combined uncertainty by a coverage factor.