## 1 Introduction  
The demand for spaceborne GNSS (Global Navigation Satellite System) real-time orbit determination technology is growing steadily in fields such as spacecraft orbit control, precise positioning, and flight monitoring. As satellite science and technology advance, the accuracy requirements for spaceborne GNSS orbit determination are becoming increasingly stringent. A technical framework based on post-mission precise orbit determination (POD) and real-time precise orbit determination (RTOD) has been developed [1]. In recent years, post-mission precise orbit determination (POD) has advanced significantly, with orbit determination accuracy reaching centimeters. In 1982, LANDSAT-4 became the first resource satellite with a GNSS receiver, as well as the first LEO satellite to successfully use GNSS-based orbit determination. Although its accuracy was limited to several tens of meters, it was a significant advancement in the field [2]. In 1992, the TOPEX/Poseidon satellite, developed in collaboration between the United States and France, carried a GPS receiver and achieved the first centimeter-level orbit determination using post-processed ground techniques, demonstrating the potential for high-precision satellite positioning [3]. Germany's GFZ launched the CHAMP satellite in 2000, which was equipped with NASA's Jet Propulsion Laboratory's (JPL) Black Jack GPS receiver. [PERSON] et al. (2003) used a reduced-dynamic orbit determination technique using ionosphere-freephase combinations to show that CHAMP's 3D orbit accuracy approached sub-decimeter levels [4]. [PERSON] et al. (2005) achieved centimeter-level orbit precision with the CHAMP and GRACE satellites [5]. [PERSON] et al. (2013) improved the study of relative orbit determination for GPS satellite formations, achieving an accuracy of 5 mm [6]. [PERSON] et al. (2013) confirmed the HY-2A orbit determination results using Satellite Laser Ranging (SLR), obtaining an accuracy of 1-2 cm [7]. [PERSON] et al. (2015) used a reduced-dynamic method and obtained a mean radial accuracy of 1.3 cm and a 3D accuracy of 6.2 cm [8]. [PERSON] (2019) combined geometric and dynamic approaches to achieve centimeter-level precision [9]. [PERSON] (2020) determined a long-arc orbit for the high-altitude satellite TJS-2. The non-differenced overlapping orbit approach had a precision of 1.3 m, but the epoch-differenced method successfully reduced slowly varying geometric errors, increasing orbit determination accuracy to more than 0.7 m [10]. With the rapid advancement of space technology, real-time orbit determination research evolved in response to the growing demand for autonomous satellite navigation, notably the independent functioning of low Earth orbit (LEO) satellites. This field has made steady international advancement. For example, [PERSON] et al. (2001) at NASA's Goddard Space Flight Center used dual-frequency GPS pseudorange measurements from the TOPEX/Poseidon satellite to obtain meter-level real-time orbit accuracy [11]. [PERSON] et al. (2022) at JPL obtained an accuracy of about 1.5 m using onboard dual-frequency GPS data from the SAC-C satellite [12]. [PERSON] et al. (2008) developed a method for absorbing mistakes from broadcast ephemerides by modeling carrier phase ambiguities as random-walk parameters, resulting in a real-time accuracy of 0.5 m [13]. More recently, [PERSON] et al. (2022a) used onboard GPS and Galileo observations from the Sentinel-6A satellite to achieve a real-time orbit determination accuracy of 0.1 m, outperforming other satellites with onboard GNSS real-time orbit determination capabilities [14]. In recent years, the development of multi-frequency, multi-constellation GNSS systems (such as GPS and BDS) has accelerated research into real-time orbit determination utilizing spaceborne GNSS data. Several research groups have made significant progress, particularly in the integration of multi-source observations, model refining, and filtering strategy optimization.