Fourier transform spectroscopy is defined well on Wikipedia as a measurement technique whereby spectra are collected based on measurements of the coherence of a radiative source, using time-domain or space-domain measurements of the electromagnetic radiation or other type of radiation.
Operationally the ASSIST II collects ground level water vapor and carbon dioxide data simultaneous with the ASPECT aircraft flying at 10k feet collecting mid-level data and a satellite overhead collecting high level data. The two areas of operation are over the land and water of Lake Livingston and the land and water near Lamont, Oklahoma.
Data will be collected 4 times, summer, fall, winter and spring however the ASSIST II will collect continuously though out the 12 months.
To read more about EPA activities on Climate change please go to the EPA Climate Change site.