Sensor system noise needs to be characterized to determine the limits of detecting a feature from an observed source. For
passive infrared spectral sensors, the noise is characterized in terms of the noise equivalent spectral radiance, NESR. The
total NESR (NESRtotal) has two components, the internal NESR of the instrument (NESRinstr) and the external NESR of
the path being viewed by the sensor (NESRpath). In the case of an FTIR instrument, the NESRinstr is measured by viewing
a stable blackbody at close range thereby removing the effects of the path on the spectrally dependent noise. The
standard deviation of the sine transform of the interferogram is then computed to estimate NESRinstr. In our application,
however, the NESRpath is our signal, and it is measured by viewing an atmospheric scene and removing the effect due to
the instrument. A histogram of the spectrally dependent noise spectrum is then computed. The full-width of this
histogram is taken at the 1/e2 points and is driven by temperature and species concentration fluctuations along the path.
Both of these effects can dominate over the instrument noise. In the following, we compare preliminary values of path
spectral fluctuations determined from a ground-based FTIR for a selected slant path to measured values of the refractive
index structure constant (Cn2) along the same path.