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QUAC Background

QUAC® Background

QUAC® determines atmospheric correction parameters directly from the observed pixel spectra in a scene, without ancillary information. QUAC performs a more approximate atmospheric correction than FLAASH® or other physics-based first-principles methods, generally producing reflectance spectra within the range of approximately 10 percent of the ground truth (Bernstein et al., 2012).

QUAC is based on the empirical finding that the average reflectance of diverse material spectra (excluding highly structured materials such as vegetation, shallow water, and mud) is not dependent on each scene. So processing is much faster compared to first-principles methods. QUAC also allows for any view angle or solar elevation angle. If a sensor does not have proper radiometric or wavelength calibration, or if the solar illumination intensity is unknown (with cloud decks, for example), QUAC can still retrieve reasonably accurate reflectance spectra as long as the following conditions are met:

  • There are at least 10 diverse materials in a scene.
  • There are sufficiently dark pixels in a scene to allow for a good estimation of the baseline spectrum.

ENVI uses the latest QUAC algorithm described in Bernstein et al. (2012). This implementation contains the following enhancements to improve the accuracy of atmospheric correction:

  • Applies mud filtering to exclude highly structured materials.
  • Selects endmembers based on a small subset of available bands for most sensors. When a sensor spans both the visible and NIR-SWIR spectral regions, the algorithm excludes bands in the visible region.
  • Constrains the gain curve to be constant for wavelengths below 650 nm.
  • Suppresses the effects of dense vegetation.
  • Removes cloud endmembers for hyperspectral sensors with 940 to 1020 nm water absorption bands.

 

References

Bernstein, L. S., X. Jin, B. Gregor, and S. Adler-Golden. "Quick Atmospheric Correction Code: Algorithm Description and Recent Upgrades." Optical Engineering 51, No. 11 (2012): 111719-1 to 111719-11.



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