Abstract The convolution theorem fot obtaining the cumulative k -distribution of a gas mixture requires assumptions than the multiplicative property of band transmission; thus new experimental investigations of its effectiveness are undertaken. The convolution is found to be a useful speed optimization of k -distribution at high pressures. For low pressures a variety of mixing methods are compared, all relying in the fact that stratospheric lines are too narrow to overlap. The optimal altitude for changing over from the convolution to one othe methods is strongly correlated with the logarithm of the ratio of linwidth to line spacing. Some spectrally-integrated outputs of the resulting split-level method are compared with another method requiring much less computation.

Obtaining the cumulative k-distribution of a gas mixture from those of its components. [radiative transfer in stratosphere]

Space-based spectral radiation detection is one of the most effective approaches for detecting hypersonic vehicles. However, due to complex characteristics of the plasma sheath that is generated by aerodynamic thermal effect and envelopes hypersonic vehicle, space-based spectral radiation detection is still confronted with great challenges. Based on the mechanisms of spectral radiation and transmission, we establish a model to conduct optimal selection on spectral wavebands for detecting hypersonic vehicle. Through using the established model, we obtain the spectral radiation characteristics of plasma-sheath-enveloped vehicles (RAM-CII and HTV2) under different flight conditions (at 40–70-km altitude with 10–25-Mach velocity). The optimal spectral wavebands for space-based detection are analyzed and identified using the calculated spectral data, upon which the results reveal that significant differences exist in spectral radiation intensities of different flight status for those two vehicles. The overall radiation intensity of RAM-CII vehicle is higher than that of HTV2 vehicle, to which the optimized detection wavebands corresponding to velocity and altitude differ. Generally, at different altitudes, the optimal wavebands for both vehicles are 5.00–7.86, 2.59–3.44, and 1.81–2.10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> . At different flight speeds, the optimal bands are 0.78–1.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and 99–130 nm, respectively. Combined with the optimized speed and height bands, the optimal detection bands of RAM-CII vehicle and HTV2 vehicle are 5.00–7.86 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ \mu \text{m}$ </tex-math></inline-formula> , 2.59–3.44 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> , and 99–130 nm. The above findings can provide useful data for in-depth study of space-based spectral radiation detection. 

Optimizing Spectral Waveband Selection for Spectral Radiation Detection of Hypersonic Vehicle

Space-based spectral radiation detection is one of the most effective approaches for detecting hypersonic vehicles. However, due to complex characteristics of the plasma sheath that is generated by aerodynamic thermal effect and envelopes hypersonic vehicle, space-based spectral radiation detection is still confronted with great challenges. Based on the mechanisms of spectral radiation and transmission, we establish a model to conduct optimal selection on spectral wavebands for detecting hypersonic vehicle. Through using the established model, we obtain the spectral radiation characteristics of plasma-sheath-enveloped vehicles (RAM-CII and HTV2) under different flight conditions (at 40–70-km altitude with 10–25-Mach velocity). The optimal spectral wavebands for space-based detection are analyzed and identified using the calculated spectral data, upon which the results reveal that significant differences exist in spectral radiation intensities of different flight status for those two vehicles. The overall radiation intensity of RAM-CII vehicle is higher than that of HTV2 vehicle, to which the optimized detection wavebands corresponding to velocity and altitude differ. Generally, at different altitudes, the optimal wavebands for both vehicles are 5.00–7.86, 2.59–3.44, and 1.81–2.10 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>. At different flight speeds, the optimal bands are 0.78–1.13 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and 99–130 nm, respectively. Combined with the optimized speed and height bands, the optimal detection bands of RAM-CII vehicle and HTV2 vehicle are 5.00–7.86 <inline-formula> <tex-math notation="LaTeX">$ \mu \text{m}$ </tex-math></inline-formula>, 2.59–3.44 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, and 99–130 nm. The above findings can provide useful data for in-depth study of space-based spectral radiation detection.

The correlated k-distribution (CKD) is a fast radiative transfer model and is often used in atmospheric absorption simulation. In the paper, we apply two automatic CKD methods to satellite brightness temperature simulations from the Fengyun 4A Advanced Geostationary Radiation Imager (AGRI) in infrared channels, namely, the finding point method (FPM) and the re-optimized method (ROM). In the calculation, we used Radiative Transfer for the Television Observation Satellite Operational Vertical Sounder (RTTOV) as the comparison, and we use line-by-line (LBL) integration as the reference. Compared with LBL in the brightness temperature simulation of real profiles, the errors of FPM in 7.1 µm and 13.5 µm channels are 0.22 K, -0.13K for mean error and 0.3128 K, 0.2184 K for root mean square error (RMSE), respectively, which are larger than that of RTTOV, with 0.16 K, 0.02 K, 0.2144 K, and 0.1226 K, respectively. In the other channels, the results show that of ROM has the highest accuracy and RTTOV has the lowest accuracy. In general, FPM and ROM can achieve very good accuracy in satellite infrared remote sensing.

Finding point method and re-optimized method for the brightness temperature simulation from Fengyun 4A AGRI in infrared channels.

Recently, an automatic atmospheric radiation absorption parameterization scheme based on correlated k-distribution (CKD) has been proposed, namely, the finding point method (FPM). The FPM can avoid errors caused by manual operation and has higher accuracy when compared with other common CKD methods. In this paper, FPM is applied to the satellite brightness temperature simulation. In the simulation of the Fengyun 4A Advanced Geostationary Radiation Imager (AGRI), the root mean square errors (RMSE) of FPM in the brightness temperature simulation of the six infrared channels when compared with the standard result from the line-by-line integration are 0.1103, 0.2128, and 0.0459, 0.0266, 0.0328, and 0.1742 K, respectively, while the RMSE of Radiative Transfer for the Television Observation Satellite Operational Vertical Sounder (RTTOV) are 1.0879, 0.1120, 0.2204, 0.1567, 0.1164, and 0.3256 K, respectively. Therefore, FPM can achieve very good accuracy in satellite infrared remote sensing.

Evaluation of Finding Point Method for Fengyun-4A Satellite Infrared Longwave Remote Sensing

Several schemes have been proposed for handling gaseous overlapping bands in the context of the correlated k-distribution model (CKD), but they all require manual operation and the accuracy is limited. In this paper, we proposed two automatic methods for gas absorption calculation based on correlated k-distribution, namely finding point method (FPM) and re-optimized method (ROM), to improve the accuracy and the speed of gaseous absorption calculation. Compared with the line-by-line (LBL) results under standard profiles, the resulting accuracy of FPM is 0.09 Kday−1 in troposphere, -0.3 Kday−1 in stratosphere, -0.18 W/m2 for upward flux and -0.44 W/m2 for downward flux; the accuracy of ROM is 0.1 Kday−1 in troposphere, 0.3 Kday−1 in stratosphere, -0.35 W/m2 for upward flux and -0.18 W/m2 for downward flux. The accuracy of the two methods is higher than rapid radiative transfer model for general circulation models (RRTMG). Under realistic profiles, the accuracy of FPM and ROM is slightly lower than that their accuracy under standard profiles but still higher than RRTMG.

Automatic methods for gas absorption calculation based on correlated k-distribution

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Automatic methods for gas absorption calculation based on correlated k-distribution