Satellite remote sensing systems require accurate, autonomous and real-time orbit determinations (RTOD) for geo-referencing. Onboard Global Positioning System (GPS) has widely been used to undertake such tasks. In this paper, a novel RTOD method achieving decimeter precision using GPS carrier phases, required by China’s HY2A and ZY3 missions, is presented. A key to the algorithm success is the introduction of a new parameter, termed pseudo-ambiguity. This parameter combines the phase ambiguity, the orbit, and clock offset errors of the GPS broadcast ephemeris together to absorb a large part of the combined error. Based on the analysis of the characteristics of the orbit and clock offset errors, the pseudo-ambiguity can be modeled as a random walk, and estimated in an extended Kalman filter. Experiments of processing real data from HY2A and ZY3, simulating onboard operational scenarios of these two missions, are performed using the developed software SATODS. Results have demonstrated that the position and velocity accuracy (3D RMS) of 0.2–0.4 m and 0.2–0.4 mm/s, respectively, are achieved using dual-frequency carrier phases for HY2A, and slightly worse results for ZY3. These results show it is feasible to obtain orbit accuracy at decimeter level of 3–5 dm for position and 0.3–0.5 mm/s for velocity with this RTOD method.
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机译：卫星遥感系统需要精确，自主和实时的轨道确定（RTOD），以进行地理参考。车载全球定位系统（GPS）已被广泛用于执行此类任务。本文提出了一种新型的RTOD方法，该方法利用中国的HY2A和ZY3任务需要使用GPS载波相位来实现分米精度。算法成功的关键是引入了一个称为伪歧义的新参数。此参数将GPS广播星历的相位模糊度，轨道和时钟偏移误差组合在一起，以吸收大部分组合误差。基于对轨道和时钟偏移误差特征的分析，可以将伪模糊度建模为随机游动，并在扩展的卡尔曼滤波器中进行估算。使用开发的软件SATODS进行了处理来自HY2A和ZY3的真实数据的实验，以模拟这两个任务的机载操作场景。结果表明，使用双频载波相位实现HY2A的位置和速度精度（3D RMS）分别为0.2-0.4 m和0.2-0.4 mm / s，而ZY3的结果则稍差一些。这些结果表明，使用这种RTOD方法在分米级3–5 dm和速度0.3–0.5 mm / s的分米水平上获得轨道精度是可行的。
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