Remote sensing using THz wave has irreplaceable advantage comparing to the microwave and the infrared waves, and study on the THz remote sensing become more and more popular in recent years. The major applications of the remote sensing in THz wavelengths are the retrieval of the atmospheric parameters and the microphysical information of the ice cloud. The remote sensing of the atmosphere is based on the radiation of THz wave along the earth-space path of which the most significant part is the upward radiation of the atmosphere. The upward radiation of the atmosphere in sunny day in the low latitude area is computed and analyzed in this paper. The absorption of THz wave by the atmosphere is calculated using the formulations illustrated in the Recommendation ITU-R P.676 to save machine hour, the frequency range is then restricted below 1THz. The frequencies used for the retrieval of atmospheric parameters such as temperature and water content are usually a few hundred GHz, at the lower end of THz wavelengths, so this frequency range is sufficient. The radiation contribution of every atmospheric layer for typical frequencies such as absorption window frequencies and peak frequencies are analyzed. Results show that at frequencies which absorption is severe, information about lower atmosphere cannot reach the receiver onboard a satellite or other high platforms due to the strong absorption along the path.
The rapid development of the communication technology and sensing applications calls for broader bandwidth and faster speed. The influence of atmosphere on the propagation character of THz wave can mainly fall into two categories: the scattering of the particles in the atmosphere and the absorption of atmosphere molecules which can be divided into two aspects: line-by-line and continuum absorption. The most controversial point is the calculation of continuum absorption. The specific attenuation of different humidity are calculated by AM model and the modified VVW line shape method and compared with that of ITU-R P.676-9. The continuum absorption of the AM model is calculated using the MT-CKD method, another method to calculate the continuum absorption is to introduce continuum-like terms of water vapor and dry air which is used in the modified VVW line shape method. The result shows a better agreement of the modified VVW line shape method than that of the AM model because the coefficient can be modified to fit the ITU-R P.676-9, and the difference between the AM model and the ITU-R P.676-9 is small, too. The best fit coefficients of modified VVW line shape method for different humidity are obtained. And the relation of the coefficients and the humidity can be expressed by two regressed equations, and the humidity modification is included in the new correction terms.