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An Improved Calibration Method for a Chopped Pyrgeometer

An Improved Calibration Method for a Chopped Pyrgeometer An improved calibration method for a chopped pyrgeometer is presented. The chopped pyrgeometer is a system of two radiometers (target radiometer and reference radiometer) modulated by a common chopper. One radiometer measures the temperature of the chopper by modulating an internal blackbody source. A previously used calibration method needs two steps. The first step is the measurement of the ratio of the responsivities for the two radiometers; the second step is the calibration of the target radiometer against a blackbody source. The first step is based on the assumption of a thermal steady state during the measurement of (typically) a few minutes. Further investigations have shown that this assumption is too idealistic. The errors introduced to airborne measurements by neglecting thermal unstabilities during ratio measurement are calculated using a numerical model of the instrument. To overcome these problems a new calibration method is presented, which also performs the calibration in two steps. The first step is the calibration of the target radiometer using an isothermal instrument; that is, the temperature of the chopper is known and, most importantly, the whole measurement takes only a few seconds, which provides sufficient thermal stability. In a second step, the calibration of the reference radiometer is done with the help of the previously known responsivity of the target radiometer. The experimental data of a calibration due to the new scheme are discussed. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Atmospheric and Oceanic Technology American Meteorological Society

An Improved Calibration Method for a Chopped Pyrgeometer

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Publisher
American Meteorological Society
Copyright
Copyright © 1999 American Meteorological Society
ISSN
1520-0426
DOI
10.1175/1520-0426(2000)017<0096:AICMFA>2.0.CO;2
Publisher site
See Article on Publisher Site

Abstract

An improved calibration method for a chopped pyrgeometer is presented. The chopped pyrgeometer is a system of two radiometers (target radiometer and reference radiometer) modulated by a common chopper. One radiometer measures the temperature of the chopper by modulating an internal blackbody source. A previously used calibration method needs two steps. The first step is the measurement of the ratio of the responsivities for the two radiometers; the second step is the calibration of the target radiometer against a blackbody source. The first step is based on the assumption of a thermal steady state during the measurement of (typically) a few minutes. Further investigations have shown that this assumption is too idealistic. The errors introduced to airborne measurements by neglecting thermal unstabilities during ratio measurement are calculated using a numerical model of the instrument. To overcome these problems a new calibration method is presented, which also performs the calibration in two steps. The first step is the calibration of the target radiometer using an isothermal instrument; that is, the temperature of the chopper is known and, most importantly, the whole measurement takes only a few seconds, which provides sufficient thermal stability. In a second step, the calibration of the reference radiometer is done with the help of the previously known responsivity of the target radiometer. The experimental data of a calibration due to the new scheme are discussed.

Journal

Journal of Atmospheric and Oceanic TechnologyAmerican Meteorological Society

Published: Jan 29, 1999

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