An Improved Calibration Method for a Chopped Pyrgeometer

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.