QDOAS Projects Properties : Filtering page
Filtering attenuates components in the spectra with frequencies higher (low-pass filtering) or lower (high-pass filtering) than a given cut-off frequency. This simple method can improve the analysis of noisy spectra but may lead in the other hand to a loss of spectral information and should be used very carefully. Filtering corresponds to a convolution in the pixels domain. The most common filter functions are implemented :
Further details on these filters can be found in the literature. Low-pass filtering is applied to the spectra and to the cross-sections. Nevertheless, the filtering of cross-sections could be disabled individually in the Analysis windows properties dialog box. Cross-sections are filtered by subtracting (optical density fitting) or dividing (intensity fitting) a fitted polynomial or a smooth spectrum calculated by filtering the original cross-sections a high number of times. This can be an alternative to the Gram-Schmidt algorithm to calculate differential cross sections. For the moment, high-pass filtering is supported only in optical density fitting mode. It can be applied during the phase of calibration and/or during the analysis of spectra.
|