# Present and Future Marine Microwave Satellite Missions in China
## 3 Progress History of China's Marine Microwave Satellite Remote Sensing Technology
### Precise Orbital Determination Technology  
For the precise orbital determinations of marine dynamic environment satellites, a simplified dynamic method is generally used to estimate the parameters through non-differential state estimations and non-differential ambiguity fixation [61; 62]. It has been found that the accuracy of the orbital determinations and positioning results can be improved using such a method [63]. In terms of DORIS and laser precise orbital determinations, a dynamic method is usually adopted to achieve the precise orbit determinations of medium orbit ephemeris (MOE) and precision orbit ephemeris (POE) satellites. The applications of the aforementioned orbital determination methods have resulted in the precise orbital determinations of China's marine dynamic environment satellites reaching the international advanced level [64].  
[PERSON] et al. [65] examined an inter-epoch differential processing method that differed from the traditional phase observation data processing methods. The phase observational data were converted into distance change rate observational data, and the relative errors were corrected. Then, a perturbation model of the macro-model and empirical force from the HY-2 satellite was established. In addition, based on the dynamic orbital determination principle, precise orbital determinations were obtained using DORIS phase observational data. Orbital determinations were successfully carried out using the DORIS measured data from the HY-2 satellite. The preliminary calculation results showed that the radial orbital errors were smaller than 2 cm and the three-dimensional position errors were approximately 10 cm [66]. [PERSON] et al. [16] summarized the DORIS calibration technology, which is one of the main orbital determination technologies of the HY-2A satellite, and verified the accuracy of the DORIS orbital determinations using satellite laser ranging (SLR) verifications and independent orbital comparisons. The results confirmed that the DORIS precise orbital determination technology could be applied to the HY-2 series satellites. [PERSON] et al. [64] established dual-frequency GPS precise orbital determination technology using the dual-frequency GPS carried by the HY-2 satellites. After verifications were completed, it was determined that the orbital determination accuracy using the dual-frequency GPS could meet the requirements. Subsequently, the HY-2B satellite system was not equipped with a DORIS orbital determination system, and only a dual-frequency GPS was used for operational orbit determinations. This had laid a foundation for the subsequent use of the Beidou System in China for precise orbital determinations.  
Based on the precise orbit determination technology of the HY-2 satellites, [PERSON] et al. [66] proposed a real-time precise orbital determination method constrained by ultra-fast precise ephemeris, which was aimed at the fact that the real-time filtering precise orbital determinations of the Beidou System satellite with broadcast ephemeris as the starting orbit often require a long convergence time. Next, based on the measured data of 51 stations in the MGEX (Multi-GNSS Experiment) tracking network for seven consecutive days, the real-time precise orbit of the Beidou System satellite was determined using a square root information filtering method. The real-time filtering orbital accuracy of the Beidou System satellite was evaluated with reference to a three-day post solution orbit. The current maximum accuracy is about 5 cm.