## 1 Introduction  
Observation of the long-term evolution of the chemical species and the status of Earth's atmosphere is essential for scientists to understand the physical and chemical processes in the atmosphere and to assess the changes of climate. A satellite is the only platform which can provide the daily global coverage of this information. Generally, three ways of observation can be used to monitor atmosphere: nadir observation, occultation observation and limb observation. Limb sounding is more appropriate for atmospheric species detection, since it has good vertical resolution and wide altitude range [1]. Several frequency regions have been adopted to monitor the changes in the atmosphere, ranging from ultraviolet to terahertz. Among others, the terahertz frequency region has the advantage of being independent of the day-night cycle. There are already many infrared limb sounders, such as MIPAS and SCIAMACHY on the Envisat satellite, and occultation instruments such as SAGE, SciSat-FTS and GOMOS used for middle and upper atmosphere detection. Terahertz limb sounding techniques have been used for deriving the information of Earth's atmosphere for more than twenty years. The first Microwave Limb Sounder (MLS) onboard the UpperAtmosphere Research Satellite (UARS) launched in 1991 provided atmospheric data of temperature and species such as ClO, O\({}_{3}\), H\({}_{2}\)O, SO\({}_{2}\), CH\({}_{3}\)CN and HNO\({}_{3}\)[2; 3; 4]. This information helps scientists understand the physical and chemical processes which lead to the O\({}_{3}\) depletion. The Sub-Millimeter Radiometer (SMR) carried by the Odin satellite observed the atmospheric limb for half of the observation time, producing various species profiles in the middle atmosphere such as O\({}_{3}\), ClO, N\({}_{2}\)O, HNO\({}_{3}\), H\({}_{2}\)O, CO and NO, as well as isotopes of H\({}_{2}\)O, O\({}_{3}\) and ice clouds with a high vertical resolution of 1-2 km [5; 6; 7; 8]. MLS onboard the Aura satellite has heritage from the UARS-MLS experiment, and it is improved to measure more species with better global and temporal coverage, better spatial resolution and better precision [9; 10; 11; 12; 13]. Its major productions include OH, HO\({}_{2}\), H\({}_{2}\)O, O\({}_{3}\), HCl, ClO, HOCl, BrO, HNO\({}_{3}\), N\({}_{2}\)O, CO, HCN, CH\({}_{3}\)CN, SO\({}_{2}\) and the ice cloud [14]. The Superconducting Submillimeter-wave Limb-Emission Sounder (SMILES) onboard the Japanese Experiment Module (JEM) of the International Space Station (ISS) successfully demonstrated the performance of 4K-cooled Superconductor-Insulator-Superconductor (SIS) mixers, and gave observations of various chemical species [15; 16; 17; 18; 19; 20]. Since Aura-MLS and Odin-SMR have worked for more than ten years, and the observation may be stopped in a few years, the follow-on instruments are imperatively needed to fill the gap. Currently, Stratospheric Inferred Winds (SIW) and SMILES-2 are being developed in Sweden and Japan, respectively [21; 22; 23]. SIW can measure horizontal wind vectors within 30-90 km, and provide the profiles of temperature, O\({}_{3}\), H\({}_{2}\)O and other species. SMILES-2 is designed to measure the horizontal wind vectors, temperature and molecules at the range of 15-180 km. The THz Atmospheric Limb Sounder (TALIS) is the first Chinese microwave limb sounder being developed in the National Space Science Center, the Chinese Academy of Sciences (NSSC, CAS). It is designed to measure atmospheric temperature, pressure and various chemical species, from 10 to 100 km with good precision and resolution. TALIS has four microwave radiometers covering 118, 190, 240 and 643 GHz. Fast Fourier Transform (FFT) spectrometers of 2 GHz bandwidth with 2 MHz resolution are used to improve the measurement performance. The observation performance of TALIS has been estimated in our previous study [24]. In this study, the impacts of the instrument uncertainties, calibration errors and a priori errors on the retrieval are estimated.