Tianjin University-1 (TJU-1) is a small low-Earth-orbiting (LEO) meteorological satellite equipped with a multi-GNSS receiver that provides the dual-frequency pseudorange and carrier-phase measurements from GPS, GLONASS, and BDS, including BDS-2 and BDS-3. In this contribution, the quality of GNSS data during the period of 2022/31~2022/37 is assessed, and the precise orbit determination (POD) based on the onboard GNSS data is conducted. Assessing the pseudorange measurement accuracy, it reveals that the GPS and BDS-3 are comparable, followed by BDS-2, and GLONASS the worst. The POD experiments demonstrate that, the orbit precision of all single-GNSS solutions is almost at an equivalent level of 2.2~3.4 cm, in terms of the overlapped orbit consistency, and the GPS-based solution is slightly worse than the BDS-based one and the GLONASS-based one is worse than other two. According to the orbit differences between different solutions, the orbit accuracy of single-GNSS solutions is inferred to be at the level of about 4.7~6.2 cm. With the multi-GNSS fusion, the precision and inferred accuracy of orbits could be improved significantly to 1.7~2.1 cm and about 1.7~3.7 cm, respectively. Furthermore, the root mean square (RMS) of posteriori carrier-phase residuals for all the IGSO/MEO satellites is at the level of 7~15 mm in different single-GNSS and multi-GNSS fusion solutions. These performances have fully demonstrated that, the centimeter-level POD is easily achievable for small LEO satellites by using the multi-GNSS receiver.

Global Navigation Satellite System-Reflectometry (GNSS-R), as a favorable technology to provide large-scale soil moisture estimates, contributes to studies in climatology, hydrology, and agriculture. The Tianmu-1 Meteorological Mission (TM-1), currently runs 23 satellites in orbit (including one experimental satellite) with multi-GNSS compatibility, achieve shorter revisit periods and higher data acquisition frequencies compared with single-satellite missions. The hourly TM-1 surface soil moisture (SSM) products, offer affluent information for global soil moisture monitoring. This study provides the first comprehensive characterization and performance evaluation of TM-1 SSM products based on in-situ measurements and products of Soil Moisture Active Passive (SMAP), European Space Agency Climate Change Initiative (ESA CCI), and Global Land Data Assimilation System (GLDAS). The TM-1 SSM demonstrates expected spatiotemporal patterns at both regional and global scales. The in-situ validation results reveal its landcover-dependent accuracy, with superior performance over bare soils (unbiased Root Mean Square Error, ubRMSE of about 0.02 m³/m³) compared to vegetated regions (ubRMSE of around 0.07 m³/m³). Furthermore, Extended Triple Collocation (ETC) assessments using (1) TM-1, active, and ground observations and (2) TM-1, model, and ground observations triplets are conducted. The ETC-derived results present that TM-1 SSM achieve global correlation coefficient of 0.75 and random error standard deviation of 0.035 m³/m³. Overall, this study demonstrates the reliable accuracy of TM-1 SSM product, and provides valuable insights for its refinement and potential applications.