The THz atmospheric limb sounder (TALIS) is a microwave radiometer designed by the National Space Science Center, Chinese Academy of Sciences (NSSC, CAS), to sound the vertical distribution of the atmosphere. It mainly consists of the antenna and feed system, the radio frequency (RF) front-end receiver and intermediate-frequency (IF) modules, and the digital back-end spectrometer. The TALIS is used to measure the temperature and chemical composition of the Earth’s atmosphere at a range of 0–100 km with good vertical resolution. The offset paraboloidal antenna of the TALIS is composed of a single projected 1.6-m aperture reflector combined with four independent feeds at four frequencies. The receiver of the TALIS consists of four superheterodyne radiometers operating at room temperature with center frequencies of 118, 190, 240, and 640 GHz. The back-end spectrometer of the TALIS includes four fast Fourier transform spectrometers (FFTSs), each with an analysis bandwidth of 2 GHz and a spectral resolution of 2.3 MHz. In this study, preliminary performance analysis of the TALIS system was performed based on the power spectrum of cold and hot targets, a calibration method was proposed for the spectral lines, and sub-band sensitivity results were obtained. The results showed that for the four frequency band radiometers, the mean sub-band sensitivities were lower than 5.7 K and the standard deviations were lower than 0.8 K, which will have a positive impact on future retrieval applications.

The THz atmospheric limb sounder (TALIS) is a microwave radiometer developed by the National Space Science Center of the Chinese Academy of Sciences for the detection of atmospheric trace gases. The observation range of the instrument mainly focuses on the middle and upper atmosphere (10–100 km above the earth’s surface). The detection targets include the temperature, pressure, and more than 10 kinds of atmospheric components. Its scientific goal is to improve our comprehension of atmospheric chemical composition and dynamics, and to monitor environmental pollution and sources in the atmosphere. The TALIS instrument is composed of an antenna, superheterodyne radiometers, and digital fast Fourier transform (FFT) spectrometers. By measuring the atmospheric thermal radiance in the wide frequency band with 118, 190, 240, and 643 GHz as the center frequency, the required volume mixing ratio (VMR) of atmospheric chemical species can be obtained. This paper introduces the characteristics of the TALIS instrument, and establishes a simulation model for the TALIS spectrometer. Through a joint simulation with an atmosphere radiative transfer simulator (ARTS), the TALIS instrument performance is evaluated from the aspects of calibration, the imbalance of two sidebands, the spectrum resolution, and quantization. The simulation results show that the two-point calibration can well-restore the radiance spectrum of the scene target and remove the influence of the spectral response function (SRF); the double side band (DSB) receiver with a 2 MHz resolution can meet the sensitivity and spectrum resolution requirements. Finally, the sensitivity errors of different quantization bits are given by the simulation and the results show that at 8-bit, the sensitivity and its degradation ratio are 1.251 K and 1.036 at a 2 MHz spectrum resolution and 100 ms integration time, respectively.

THz Atmospheric Limb Sounder (TALIS) is a microwave radiometer designed by the National Space Science Center of the Chinese Academy of Sciences (NSSC, CAS) for measuring the vertical distribution of temperature and chemical constituents in the middle and upper atmosphere. The digital spectrometer is an important part of TALIS’ back end, which mainly realizes the function of spectral analysis. The radix 16 real-time complex fast Fourier transform (FFT) algorithm used in the digital spectrometer was obtained by improving and combining the parallel processing and complex processing of the FFT algorithm. In this study, the digital spectrometer parameter selection is systematically presented, and the effect of the digital spectrometer FFT algorithm on the TALIS system sensitivity is analyzed based on the TALIS system simulation model we established in the previous stage of this research. The results show that the actual full-band sensitivity obtained after using the FFT algorithm is consistent with the ideal full-band sensitivity of TALIS for different integration time, spectral resolutions, and quantization bits. However, the results of the comparison of the actual sub-band sensitivity after using the FFT algorithm with the ideal sub-band sensitivity show that the deterioration of the sub-band sensitivity will be caused by the FFT algorithm. The mean value of the deterioration ratio was found to be approximately 18%, and the maximum value of the deterioration ratio was approximately 33%.