Fig. 2. The percentage difference between the forward model and the actual measurements are shown in blue. The statistical uncertainty is plotted in red. The four plots on top are the stratospheric forward model residuals, and the four plots at the bottom are the tropospheric forward model residuals. For digital counting systems, the Poisson distribution is appropriate and the variance of the measurements at each altitude is the number of photons at that altitude. However, the output signals of the analog channels do not follow a Poisson distribution and to find the variance a 3-point running filter is used. As a result the red line (which indicates the noise of measurements) for the analog channels is more structured then for the digital channels.
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