At Level 1A, the raw data from the spacecraft are decommutated and the instrument outputs (called interferograms) reconstructed. File headers also contain important state data such as time, date, spacecraft and target locations, and instrument pointing angle.

At Level 1B, the interferograms are phase-corrected, Fourier transformed to spectra, radiometrically-calibrated and re-sampled onto a common frequency grid. Certain data quality flags are added to the header at this juncture and the results passed to Level 2.

At Level 2, vertical concentration profiles of the selected species are extracted from the data through a process of retrieval.

At Level 3, globally covered Level 2 daily, weekly, or monthly data are used to generate global species maps at uniform spacial grids. These are useful browse products.

References:
ATBD's: http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/
Worden, J.; Sund-Kulawik, S.; Shephard, M. W.; Clough, S. A.; Worden, H.; Bowman, K.; Goldman, A. "Predicted errors of tropospheric emission spectrometer nadir retrievals from spectral window selection" J. Geophys. Res., Vol. 109, No. D9, D09308 10.1029/2004JD004522 15 May 2004