29 May 2014 FTIR characterization of atmospheric fluctuations along slant paths
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Abstract
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.
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C. A. Martin, C. A. Martin, D. M. Brown, D. M. Brown, M. E. Thomas, M. E. Thomas, S. Strong, S. Strong, Michele B. Lohr, Michele B. Lohr, } "FTIR characterization of atmospheric fluctuations along slant paths", Proc. SPIE 9071, Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXV, 907117 (29 May 2014); doi: 10.1117/12.2050644; https://doi.org/10.1117/12.2050644
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