A review of real-time multi-GNSS precise orbit determination based on the filter method

Stable and reliable high-precision satellite orbit products are the prerequisites for the positioning services with high performance. In general, the positioning accuracy depends strongly on the quality of satellite orbit and clock products, especially for absolute positioning modes, such as Precise Point Positioning (PPP). With the development of real-time services, real-time Precise Orbit Determination (POD) is indispensable and mainly includes two methods: the ultra-rapid orbit prediction and the real-time filtering orbit determination. The real-time filtering method has a great potential to obtain more stable and reliable products than the ultra-rapid orbit prediction method and thus has attracted increasing attention in commercial companies and research institutes. However, several key issues should be resolved, including the refinement of satellite dynamic stochastic models, adaptive filtering for irregular satellite motions, rapid convergence, and real-time Ambiguity Resolution (AR). This paper reviews and summarizes the current research progress in real-time filtering POD with a focus on the aforementioned issues. In addition, the real-time filtering orbit determination software developed by our group is introduced, and some of the latest results are evaluated. The Three-Dimensional (3D) real-time orbit accuracy of GPS and Galileo satellites is better than 5 cm with AR. In terms of the convergence time and accuracy of kinematic PPP AR, the better performance of the filter orbit products is validated compared to the ultra-rapid orbit products.