Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-7-9610
Timestamp: 2019-04-24 09:51:24+00:00

Document:
The performance of an interband cascade laser based laser heterodyne radiometer (LHR) is demonstrated in ground-based solar occultation mode. High-resolution (0.0033 cm−1) transmission spectra near 3.53 μm were obtained for simultaneous atmospheric observations of H2O and CH4. Combined with the preprocessed measurement data (acquired at Hefei, China, on June 21th 2016), an optimal estimation method based retrieval algorithm is developed for data retrieval and error analysis. By considering the corrected atmospheric parameters, vertical profiles of H2O and CH4 are retrieved. Finally, the measured total column abundance and XCH4 were calculated to be 1.87 ± 0.02 ppm and 1.88 ± 0.02 ppm, respectively. The interband cascade laser-based laser heterodyne radiometer that is demonstrated in this manuscript has high potential for use in the development of compact, robust, and unattended LHR for spacecraft, airborne or ground-based atmospheric sensing.
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Fig. 1 Schematic diagram of the IC-LHR setup and photo of the optical module. ICL: interband cascade laser; BS: beam splitter; LIA: lock-in amplifier; BP Filter: band-pass filter; USB: Universal Serial Bus.
Fig. 2 Relationships between laser wavenumber (red solid symbols), power (blue hollow symbols) and laser injection current.
Fig. 3 Band–pass filter used in the experiment and frequency spectrum analysis of the heterodyne, laser and background signals, the inset: the actually used instrument lineshape for data retrieval.
Fig. 4 (a) Calculated transmittance of atmospheric H2O vapor (black) and CH4 (red dots) based on the forward model; (b) Acquired IC-LHR (black), DC (DC output of detector #1, gray dots) and etalon signals (blue).
Fig. 5 Flow chart of the data retrieval. VMRs: volume mixing ratios; ILS: instrument lineshape; L-M: Levenberg–Marquardt.
Fig. 6 Temperature (a) and pressure (b) profiles used for the IC-LHR retrieval.
Fig. 7 LHR data retrieval results: (a) experimental (blue) and fitted (red) LHR spectra and the convergence of the iteration process (inset); (b) the residuals (pink); (c) and (d) the retrieved vertical concentration profiles of CH4 and water vapor, respectively.
Table 1 Definition of the state vector used in the data retrieval.
(1) y m =F( x,a,b,c )+ε.
(2) χ 2 = ( y−F ) T S ε −1 ( y−F )+ ( x i − x a ) T S a −1 ( x i − x a ).
(3) x i+1 = x i + [ ( 1+γ ) S a −1 + K i T S ε −1 K i ] −1 ×[ K i T S ε −1 ( y m − F i )− S a −1 ( x i − x a ) ].
(4) S m =G S ε G T , S s =( I n −A) S a ( I n −A) T .
(5) A= ∂ x ^ ∂x =GK=[ ( K T S ε −1 K+ S a −1 ) −1 K T S ε −1 ]K.
Definition of the state vector used in the data retrieval.
x1 (CH4) 1 30% Scale factor of a priori profile (ECMWF 45 levels).
x2 (H2O) 1 30% Scale factor of a priori profile (ECMWF 45 levels).
a a0 100% Polynomial coefficients of the baseline, a0, b0, and c0 are calculated by using a 2nd order polynomial fitting of the ratio of pre-processed LHR signal and model result (after eliminating the absorption part).

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