An improved calibration method for a chopped pyrgeometer is presented. The
chopped pyrgeometer is a system of two radiometers (target radiometer and r
eference 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; th
e second step is the calibration of the target radiometer against a blackbo
dy source. The first step is based on the assumption of a thermal steady sl
ate during the measurement of (typically) a few minutes. Further investigat
ions have shown that this assumption is too idealistic. The errors introduc
ed to airborne measurements by neglecting thermal unstabilities during rati
o measurement are calculated using a numerical model of the instrument.
To overcome these problems a new calibration method is presented, which als
o performs the calibration in two steps. The first step is the calibration
of the target radiometer using an isothermal instrument; that is, the tempe
rature 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 ex
perimental data of a calibration due to the new scheme are discussed.