Kinematic phase-referencing VLBI for precise positioning of the Tianwen-2 probe

Abstract

China's Tianwen-2 probe, launched on 29 May 2025, will sample asteroid 2016 HO3 and exploring comet 311P. The precise positioning of the Tianwen-2 probe will benefit its mission goals. Phase-referencing VLBI imaging offers significant potential to improve the positional precision of deep space probes. However, traditional phase-referencing VLBI imaging assumes a nearly constant angular position, thereby limiting positioning precision for moving targets, particularly with sparse arrays or long-duration observations. To enhance the tracking precision of moving targets, an improved Kinematic Phase-Referencing (KPR) VLBI method is investigated. By directly integrating a kinematic orbital model—parameterized by polynomial coefficients—into the phase-referencing VLBI process, KPR-VLBI removes the need for the nearly constant-position assumption. Observations of the Tianwen-2 probe were carried out using the five stations of the Chinese VLBI Network to validate the KPR-VLBI method. Results indicate that the final angular position precision is at the sub-milliarcsecond level, which is primarily constrained by tropospheric and ionospheric errors as well as calibrator position errors, while the 1σ formal error of angular position is better than 10 micro-arcseconds. Furthermore, compared with conventional phase-referencing VLBI imaging, KPR-VLBI is able to effectively suppress sidelobes arising from the target motion in synthesized images, thereby leading to more reliable results.