Centimeter-level orbit determination for China’s space station: a preliminary result

China’s space station Tiangong has been in operation since April 29, 2021. To support the onboard time-frequency relevant experiments, a multi-channel BDS/GPS (Beidou Navigation Satellite System/Global Positioning System) receiver onboard the Mengtian laboratory module was launched to Tiangong on October 31, 2022. Since then, the receiver enabled post-processing Precise Orbit Determination (POD) and time synchronization for Tiangong. Comparing to the typical low Earth orbit satellites with designated scientific research objectives, Tiangong is significantly heavier and larger, inducing more problematic challenges for POD. Therefore, efforts were first made to estimate the real BDS/GPS antenna reference point with respect to the varying center-of-mass of Tiangong. Based on a simplified macro-model excluding the movable solar arrays, the main non-gravitational forces were modeled as surface forces for which the un-explicit modeling was compensated by the co-estimated empirical accelerations. Results of POD for 40 days indicated that the internal consistency between the reduced-dynamic orbits and the kinematic orbits was 2–5 cm in the local orbital reference frame. The BDS/GPS phase residuals were about 4 mm and became smaller in east Asia, where BDS was better tracked. Inter-agency orbit comparisons among the four orbit solutions computed using different software packages showed an orbit agreement of within 10 cm. Finally, external validations by satellite laser ranging confirmed that 5-cm level orbit product was feasible. Such obtained orbit precision at the Technology and Engineering Center for Space Utilization of the Chinese Academy of Sciences (CSU, CAS) foresees strong pillar for supporting space utilizations such as time-frequency comparison and transmission between ground and space, space cold atomic clock verifications, or space weather investigations.