Set of FTS interferograms from a typical data acquisition cycle. The interferogram power level is the radiance difference between the signal and the liquid nitrogen  reference load. The lower curve is with the signal from the ambient load, the curve above it is  from 1 Airmass.  The top 3 curves are interferograms from 2,3, and 4 airmasses. Note that the ambient load is much colder than the sky  at 1 Airmass.  This effect is due to the deep inversion layer which typically forms over the South Pole surface in wintertime.