Quality Assessment and Observation Error Estimation of Tianmu-1 GNSS Radio Occultation Bending Angle and Refractivity Retrievals

As China’s first commercial low Earth orbit meteorological constellation, Tianmu-1 (TM) radio occultation (RO) provides high-density observations, with approximately 30,000 profiles per day globally, offering new opportunities for research in observationally sparse regions. TM RO bending angle and refractivity retrievals from June to December 2023 are evaluated using ERA5 reanalysis, global radiosonde observations, and COSMIC-2 RO as reference datasets. TM observation errors are further estimated using the three-cornered hat (3CH) method. The results show that, in the upper troposphere and lower stratosphere, TM bending angle biases are within ±0.19%, ±0.40%, and ±0.37% relative to ERA5 (8–30 km), radiosonde (8–25 km), and COSMIC-2 (8–30 km), respectively. The corresponding refractivity biases are within ±0.09%, ±0.21%, and ±0.14%, respectively. Both bending angle and refractivity from TM RO exhibit high accuracy and precision. The observation errors estimated using the 3CH method show a clear latitudinal dependence. The observation errors in the tropics (±30° latitude) are larger than those in the middle and high latitudes. This study quantitatively characterizes the accuracy, precision, and error structure of TM RO bending angle and refractivity, providing a quantitative basis for future quality control and observation error specification in numerical weather prediction data assimilation.