Fig. 1. Mixture absorption spectra of CO₂ and H₂O at sea level under six atmospheric models in MODTRAN based on HITRAN database. (a) Tropic; (b) mid-latitude summer; (c) mid-latitude winter; (d) sub-arctic summer; (e) sub-arctic winter; (f) 1976 U.S. standard

Fig. 2. Mixture gases absorption spectra of R16 line under standard condition of T=296 K and p=1013 hPa

Fig. 3. FWHM of spectral collision broadening and Doppler broadening at R16 line as functions of altitude. (a) CO₂; (b) HD¹⁶O

Fig. 4. Relative weight of CO₂ and HD¹⁶O as a function of pressure at different frequency positions

Fig. 5. One-way optical depth spectra of R16 as functions of altitude. (a) Mixture gases of CO₂ and HD¹⁶O; (b) CO₂

Fig. 6. Inversion errors of column concentration and range resolved concentration of CO₂ and HD¹⁶O as functions of altitude under the 1976 U.S. standard atmospheric model

Fig. 7. Concentration error as a function of frequency offset for +1 K temperature variation (Different colors represent errors at different relative frequencies and altitudes. The navy blue lines indicate the variation in errors with relative frequency at several typical altitudes, with the corresponding error values displayed on the right axis. The red dashed line represents the neutral point where the error value is zero). (a) CO₂ column concentration error; (b) HD¹⁶O column concentration error; (c) CO₂ range resolved concentration error; (d) HD¹⁶O range resolved concentration error

Fig. 8. Concentration error as a function of relative frequency for -0.5 hPa pressure variation (Different colors represent errors at different relative frequencies and altitudes. The navy blue lines indicate the variation in errors with relative frequency at several typical altitudes, with the corresponding error values displayed on the right axis. The red dashed line represents the neutral point where the error value is zero). (a) CO₂ column concentration error; (b) HD¹⁶O column concentration error; (c) CO₂ range resolved concentration error; (d) HD¹⁶O range resolved concentration error

Fig. 9. CO₂ column concentration error as a function of relative frequency for +0.5 hPa pressure variation (Different colors represent errors at different relative frequencies and altitudes. The red dashed line represents the neutral point with zero sensitivity. The solid and dashed lines in navy blue represent column concentration errors of CO₂ at altitudes of 0 km and 60 km, respectively, and the corresponding error values are displayed on the right axis)

Fig. 10. Concentration error as a function of relative frequency for +1 MHz frequency drift (Different colors represent errors at different relative frequencies and altitudes. The navy blue lines indicate the variation in errors with relative frequency at several typical altitudes, with the corresponding error values displayed on the right axis. The red dashed line represents the neutral point where the error value is zero). (a) CO₂ column concentration error; (b) HD¹⁶O column concentration error; (c) CO₂ range resolved concentration error; (d) HD¹⁶O range resolved concentration error

Fig. 11. Concentration error as a function of relative frequency for 5% H₂O mixing ratio variation under atmospheric models of tropic, mid-latitude summer, mid-latitude winter, sub-arctic summer, sub-arctic winter, and 1976 U.S. standard. (a)‒(f) CO₂ column concentration error; (g)‒(l) CO₂ range resolved concentration error

Table 1. Atmospheric parameters of MODTRAN model at sea level

Table 2. Spectral parameters of CO₂ and HD¹⁶O for R16 line^[29]