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Friedrich Fergg and Peter Wendling

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.

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Dieter Lorenz, Peter Wendling, Peter Burkert, Friedrich Fergg, and Günther Wildgruber

Abstract

A new type of pyrgeometer, which uses modulation of the atmospheric radiation by a mechanical chopper, has been built for the use of ground-based and airborne measurement of broadband infrared atmospheric irradiances. The instrument basically consists of two radiometers—a target radiometer and a reference radiometer—using a specially developed chopper. The reference radiometer is used to measure the radiation of the chopper that has to be known additionally to calculate the atmospheric radiation from the signal of the target radiometer. The radiometer equations take into account the nonflat spectral response. A spectral correction factor is introduced to describe the relation between the weighted irradiance at the detector and the irradiance coming from the source. One important advantage of the chopped pyrgeometer is its nonsensitivity to the temperature distribution inside the instrument as well as its fast response.

The chopped pyrgeotmter has been used during a number of airborne missions. During the Pre-EUCREX Intercomparison Campaign flown in January 1992, two Eppley pyrgeometers, two pyrgeometers of the Foot-type, and a chopped pyrgeometer were flown, in addition to other instruments on three aircraft. During a rectangular flight pattern (box) the absolute irradiance values measured by the five instruments differ by about ±10% from the average value (≈85 W m−2) of all instruments the chopped pyrgeometer indicating the smallest values throughout.

Since a detailed error budget of all instruments was not available, special emphasis was placed on relative quantities (ratios) to describe the trends measured by the various instruments along the flight tracks. It has been found that the measurements of the chopped pyrgeometer show a much better correlation to the measurement of the Foot pyrgeometers than to the Eppley instruments. A comparison of four different model calculations with the measured data of all participating instruments at ground and at three flight levels is presented.

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