Fig. 1. Overall technical route of laser heterodyne system.

Fig. 2. Forward transmittance spectrum based on RFM simulation and selected detection window.

Fig. 3. Experimental results of the laser heterodyne: (a) The high-resolution heterodyne signal of CO₂; (b) the sunlight signal tracked in real time; (c) the DC signal of the laser.

Fig. 4. Principle of the wavenumber shift calibration: (a) Wavenumber shift between the reference signal and real signal; (b) the process of calculating the wavenumber shift. The inset shows the result of correlation coefficients.

Fig. 5. (a) Temperature and (b) pressure profiles.

Fig. 6. Data inversion flow chart. VMR, volume mixing ratio; ILS, instrument linear; LM, Levenberg–Marquardt.

Fig. 7. Inversion results of LHR data: (a) Experimental and fitted LHR spectrogram and convergence of iterative process (illustration); (b) residue; (c) and (d) obtained prior and inversion vertical concentration profiles of CO₂, respectively.

Fig. 8. (a) Retrieval results of the versus time from 11:00 to 13:00 on March 14, 2019; (b) time series of the LHR results and GOSAT results of .

Fig. 9. (a) The transmittance spectrum after adding white noise with the amplitude of 0.05; (b) change range of the lowest point of transmittance spectrum after adding 5000 times.

Fig. 10. Changes of , Max, Min, and SNR caused by the white noise amplitude increasing from 0.001 to 0.050.

Fig. 11. (a) The relationship between the minimum value and the change of profile; (b) the relationship between the change of column total and the minimum.

Fig. 12. The relationship between SNR and measurement error.