To insure that long-term determinations of Doppler width and shift—derived from observations of atmospheric emissions—are internally consistent and reliable, we have developed a method to both continuously and nonintrusively determine and monitor the instrumental constants of the Fabry–Perot spectrometer making the observations. We have used this method at our isolated field experiment at South Pole, Antarctica, because the instrument is only accessible to us for a few days every year. Here we report both the method and the Fabry–Perot stability results for the past 22 years of operation. The method involves the description of real Fabry–Perot instrumental constants as a small departure from those of an ideal Fabry–Perot. In general, this model is applicable for most observations. However, experimentally, there are times when the small-departure model is not applicable, thus indicating how to best reduce the observations into physical quantities for the utmost consistency in the geophysical results.
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