The minimum detectable brightness temperature difference (left) and the noise equivalent radiometric sensitivity (right) for a shot-noise limited infrared heterodyne radiometer operating at with a 50% heterodyne efficiency. Figures appearing in the contour plots are in Kelvin.
Theoretical calculations of atmospheric ozone profile retrieval error (upper) and vertical resolution (lower) for an ideal infrared heterodyne receiver. Calculations were performed for two resolutions: and .
Plan view of the instrument optical bench showing the optical setup. The different modules described in the text are indicated.
The heterodyne signal recorded during laboratory measurements. The LO frequency was tuned by varying the QCL current. In sections I and II of the measurements the instrument was targeting a blackbody. A long gas cell filled with , of OCS was placed in the optical path in section I. The cell was removed between sections I and II, and during section III the instrument viewed a blackbody at ambient temperature. The QCL is switched off during section IV. The inset in section II shows the base line corrected signal used to calculate an estimate of the signal-to-noise ratio.
(a) In-phase heterodyne signal during temperature tuning of the LO frequency across two OCS absorption lines. The smoothed line is obtained by low pass filtering. The background was a blackbody. (b) Corresponding quadrature signal. The enlarged section (left) shows residual etalon fringes and the inset panel shows the corresponding relative variation in LO power.
OCS spectrum recorded under the same conditions as that in Fig. 5 but using a blackbody. In this case the OCS emission is greater than the background and the heterodyne radiometer resolves an OCS emission spectrum.
Calculated OCS emission spectrum corresponding to the experimental conditions of Fig. 6. The base line variation due to the LO power modulation was not included in the model.
Allan variance calculations for (a) the heterodyne signal and (b) the LO power signal. Both plots show a rollover at around .
Atmospheric transmission spectra obtained with the QCL-based LHR operating in solar occultation mode. Experimental and calculated spectra are shown, as well as the corresponding residuals.
Summary of experimental parameters during field observations on 21 September 2006, corresponding to spectra in Fig. 9
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