Getting the drift is important. Small instrument drifts, that is, when measuring wavelength differences on the order of one part in one hundred million, to determine upper atmospheric winds from Doppler shifts of airglow emission lines. Fabry-Pérot Interferometers (FPI) are widely used to perform this type of wind measurement from the ground at night. To keep track of instrument drifts, they typically interrupt the atmospheric observations to perform periodic measurements of a calibration source, which has a constant wavelength. The disadvantages of this approach include the shortening of atmospheric measurement time and the inability to capture any drift behaviors that cannot be fully characterized using these periodic calibrations. Grawe et al. propose to superimpose the calibration signal on the atmospheric signal to simultaneously measure the instrument drift and the atmospheric signal. While this approach has been used for this application by other types of spectrometers, it is new for FPIs. This paper presents practical approaches to superimpose the signals, and it quantifies the additional sources of uncertainties encountered while simultaneously retrieving atmospheric and calibration data. This work demonstrates that for several realistic measurement scenarios and drift behaviors, the proposed approach can lead to improved results.

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