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@ARTICLE{2025RemS...17.1125H,
       author = {{Hou}, Cong and {Jin}, Xiaojun and {Yang}, Xiaopeng and {Xiao}, Tong},
        title = "{Toward Robust GNSS Real-Time Orbit Determination for Microsatellites Using Factor Graph Optimization}",
      journal = {Remote Sensing},
     keywords = {microsatellite, real-time orbit determination, GNSS, factor graph optimization, sliding window},
         year = 2025,
        month = mar,
       volume = {17},
       number = {7},
          eid = {1125},
        pages = {1125},
     abstract = "{Extended Kalman Filter (EKF) is extensively employed in Global
        Navigation Satellite System (GNSS)-based real-time orbit
        determination (RTOD) for microsatellites due to its low
        complexity. However, the performance of EKF-RTOD is markedly
        degraded when the microsatellite deviates from a stable Earth-
        pointing attitude and employs a low-cost receiver. Factor graph
        optimization (FGO), which addresses nonlinear problems through
        multiple iterations and re-linearization, has demonstrated
        superior accuracy and robustness compared to EKF in challenging
        environments such as urban canyons. In this study, we introduce
        a novel FGO-based RTOD (FGO-RTOD) approach, which integrates
        state transfer factors to establish temporal connections between
        state nodes across multiple epochs. Real-time processing is
        achieved through a sliding window mechanism combined with
        marginalization. This paper evaluates the performance of the
        proposed algorithm in a regular scenario using data from GRACE-
        FO-A, which maintains the Earth-pointing attitude and employs a
        high-performance receiver. The positioning results of GRACE-FO-A
        indicate that FGO-RTOD marginally outperforms EKF-RTOD in
        accuracy. Furthermore, the performance of FGO-RTOD is assessed
        in challenging scenarios using simulation data and on-orbit data
        from Tianping-2B microsatellite, which is not in an Earth-
        pointing attitude and employs a low-cost receiver. The
        simulation results reveal that FGO-RTOD reduces the Root Mean
        Square (RMS) of positioning error by 79.0\% relative to EKF-RTOD
        and exhibits significantly enhanced smoothing. In the
        Tianping-2B experiments, FGO-RTOD reduces the RMS of carrier-
        phase ionosphere-free combination residuals from 2 cm to 1 cm
        relative to EKF-RTOD, alongside a substantial improvement in the
        ratio of valid observations. These findings underscore the
        effectiveness of FGO-RTOD in managing outlier measurements in
        challenging scenarios.}",
          doi = {10.3390/rs17071125},
       adsurl = {https://ui.adsabs.harvard.edu/abs/2025RemS...17.1125H},
      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}