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Real-time precise orbits of low earth orbit (LEO) satellites are becoming indispensable with the rapid development of real-time Earth observation application and LEO enhanced precise point positioning. Currently, GNSS-based precise orbit determination is a widely used method for LEO onboard navigation. However, the real-time LEO satellite orbits are usually obtained by ambiguity-float solution even when the GNSS augmentation corrections are considered. In this study, we perform LEO ambiguity-fixed multi-GNSS real-time precise orbit determination (RTPOD) based on square root information filter. One month of onboard GPS + Galileo observations from Sentinel-6A and real-time products of the Centre National d’Etudes Spatiales (CNES) are used to investigate the contribution of integer ambiguity resolution (IAR). The benefit of dual-system combination on LEO RTPOD is firstly evaluated. The combination of GPS and Galileo dual-system contributes to more visible GNSS satellites and better observation geometry for LEO RTPOD, which results in an evident accuracy improvement of 19% over GPS and a convergence time reduction of 43% and 41% compared to the GPS and Galileo solutions respectively. Considering the short arc of onboard GNSS observations and imperfections in GNSS real-time products, we propose a strict IAR quality control method to avoid fixing the ambiguity to the wrong values. The results indicate that the IAR quality control method we used can effectively reduce the wrong fixing risk and increase the robustness of the IAR solution. Using onboard GPS and Galileo observations, the 3D orbit accuracy of the ambiguity-fixed solution is significantly improved from 5.17 to 3.61cm, by 30%, compared to the ambiguity-float solution. Furthermore, the application of IAR also achieves a faster convergence to the centimeter-level orbit. [ABSTRACT FROM AUTHOR]

Precise knowledge of the antenna phase center variations (PCVs) of global navigation satellite system (GNSS) is indispensable for GNSS high-precision applications. Currently, the PCV models of most GNSS satellites are exclusively estimated from ground observations, which reveals some drawbacks such as limited nadir coverage (usually < 14°) and high correlation with tropospheric parameters. Onboard observations from plenty of low Earth orbit (LEO) satellites launched in recent years offer us a great chance to investigate the LEO-based GNSS PCV calibration. We estimate GPS PCV model using ambiguity-fixed carrier phase observations from multiple LEO satellites to obtain more reliable GPS PCVs. Onboard data of eight LEO satellites from day of year (DOY) 122, 2019 to DOY 180, 2019 are processed. We first evaluate the contribution of integer ambiguity resolution (IAR) to the LEO PCV calibration. Compared with the traditional ambiguity-float PCVs, the ambiguity-fixed PCVs can contribute to the maximum reduction of 6.2% in carrier phase residuals and 8.0% in satellite laser ranging (SLR) residuals. After that the LEO-based GPS PCV calibration using different LEO satellites is performed and the impact of different estimation strategies and datum definitions is assessed in detail. The results show that the IAR can contribute to the stability improvement in GPS PCVs by (24.5%, 41.5%, 42.4%, and 45.2%) for four BLOCKs. As the number of LEO satellites increases, GPS PCVs gain an evident stability improvement. With the inclusion of eight LEO satellites, the PCV stability is mainly within 0.3 mm with the maximum improvement of 63.8% and the PCV difference w.r.t igs14.atx is less than 1 mm. Meanwhile, we also find that the LEO-based GPS PCV calibration is hardly influenced by the datum definition which is applied to decouple GPS PCVs and LEO PCVs. Validation results indicate that our GPS PCV estimates slightly outperform PCV model from igs14.atx with the LEO kinematic orbit accuracy improvement of 5.1% and the precise point positioning (PPP) error reduction of 1.1%. The reference frame scale of our GPS PCV estimates is also consistent with that from igs14.atx. [ABSTRACT FROM AUTHOR]

Precise satellite clock product is an indispensable prerequisite for the real-time precise positioning service. To meet the requirement of numerous time-critical applications, real-time satellite clock corrections need to be broadcast to users with an update rate of 5 s or higher. With the rapid development of global navigation satellite systems (GNSS) over the past decades, abundant GNSS tracking stations and modern constellations have emerged, and the computation for multi-GNSS real-time clock estimation has become rather time-consuming. In this contribution, an efficient strategy is proposed to achieve high processing efficiency for multi-GNSS real-time clock estimation, wherein undifferenced method based on sequential least square is adopted. In the proposed strategy, parallel data processing and high-performance matrix operations are introduced to accelerate the processing of multi-GNSS clock estimation. The former is based on OpenMP (Open Multi-Processing), while the latter is achieved by the implementation of the Schur complement and the open-source library OpenBLAS. Multi-GNSS observations from 85 globally distributed tracking stations are employed for the generation of real-time precise clock products. The average elapsed time per epoch with the proposed strategy is 0.35, 0.68, and 2.30 s for GPS-only, dual-system, and quad-system solutions, respectively. Compared to the traditional serial strategy, the computation efficiency is significantly improved by 76.0%, 77.3%, and 77.7%, respectively. The accuracy of the estimated clocks is evaluated with respect to IGS final GPS clock products and GFZ final multi-GNSS clock products (GBM0MGXRAP), and multi-GNSS real-time precise point positioning (PPP) experiments are further carried out. All the results indicate that the proposed strategy is efficient, accurate, and can promise high-rate multi-GNSS real-time clock estimation. [ABSTRACT FROM AUTHOR]