Kinematic Precise Orbit Determination of Sentinel-6A Satellite with Spaceborne GPS/Galileo Observations
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摘要:
哨兵（Sentinel）-6A 海洋测高卫星搭载了GPS/Galileo双模接收机,为研究基于全球导航卫星系统多星座的低轨卫星精密定轨提供了契机。固定载波相位模糊度可提升低轨卫星的定轨精度,利用在轨实测数据研究GPS/Galileo双系统组合以及模糊度固定对低轨卫星运动学定轨精度的影响。分别采用欧洲定轨中心（Center for Orbit Determination in Europe, CODE）、法国国家空间研究中心（Centre National d’Etudes Spatiales, CNES）、德国地学研究中心（German Research Centre for Geosciences, GFZ）和中国武汉大学 (Wuhan University, WHU)发布的观测值偏差及对应的精密星历和钟差产品开展单接收机模糊度固定。结果表明：GPS/Galileo双系统组合可明显改善定轨几何构型。双系统组合浮点解轨道三维精度优于30 mm,相对于GPS单系统提升超过20%。模糊度固定显著提升了运动学定轨精度,组合固定解轨道精度优于20 mm,相对于GPS提升30%。基于CODE、CNES和GFZ产品的GPS和Galileo单系统模糊度固定率分别优于93%和95%,WHU产品的Galileo固定率则偏低。利用卫星激光测距（satellite laser ranging,SLR）观测数据对运动学定轨结果进行检核,单系统固定解轨道SLR残差均方根误差（root mean square, RMS）为13~15 mm,双系统组合固定解RMS则达到12~14 mm,提升超过10%。
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关键词:
- Sentinel-6A卫星 /
- GPS/Galileo /
- 运动学定轨 /
- 单接收机模糊度固定
Abstract:ObjectivesThe Sentinel-6A spacecraft is equipped with a GPS/Galileo dual-constellation global navigation satellite system (GNSS) receiver which provides an opportunity to investigate the precise orbit determination (POD) accuracy of low Earth orbit (LEO) satellites based on multi-GNSS. Ambiguity resolution plays an important role in GNSS-based precise positioning and orbit determination. The single receiver ambiguity resolution is explored and the GPS/Galileo measurements are combined to further improve the kinematic orbit determination accuracy.
MethodsObservation specific bias (OSB) product is employed to calibrate the satellite dependent phase delay, and single difference (SD) observation between GNSS satellites is applied to remove the phase delay of receiver. Combined with the related GNSS precise orbit and clock products, the wide lane and narrow lane ambiguities are fixed to integers. Then the SD ionosphere free (IF) ambiguities are recovered with the fixed ambiguities and are taken as pseudo observations to constrain the undifferenced IF ambiguities. The effect of GPS/Galileo combination and ambiguity resolution on kinematic orbit determination is analyzed with Sentinel-6A onboard data. GNSS products provided by the Center for Orbit Determination in Europe (CODE), Centre National d’Etudes Spatiales (CNES), German Research Centre for Geosciences (GFZ) and Wuhan University (WHU) are used for single receiver ambiguity resolution and POD. Different kinematic orbits including GPS‑only, Galileo‑only and GPS/Galileo combined solutions are generated. The reduced dynamic orbits with ambiguity resolution are also calculated to assess the accuracy of kinematic orbits.
ResultsResults show that the visible satellites and position dilution of precision are significantly improved in dual-GNSS solution. The three dimensional (3D) accuracy of the dual-constellation kinematic orbit with float ambiguity achieves 30 mm and shows an improvement of 20% when comparing with the GPS-only result. Fixing the ambiguity to integer significantly improves the POD accuracy. The 3D accuracy of the GPS/Galileo ambiguity fixed orbit is 20 mm, which is 30% better than that of the GPS-only result. With the products of CODE, CNES and GFZ, more than 93% of the GPS and 95% of the Galileo ambiguities are successfully fixed and is further improved to 97% in the case of dual-GNSS solution. The ambiguity fixing rate shows degraded performance when using the WHU product. Independent satellite laser ranging (SLR) observations are used to validate the kinematic orbits. The root mean square (RMS) of SLR residuals of GPS-only solution with fixed ambiguity is 13-15 mm while it is 12-14 mm for the orbits derived with dual-constellation observations and an average improvement of 10% is achieved by introducing the Galileo data.
ConclusionsFixing the ambiguity to integers improves the accuracy and stability of POD results. Compared with the GPS-only solution, GPS/Galileo combined solution improves the ambiguity fixing rate which then leads to an improvement of kinematic orbit determination accuracy.
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http://ch.whu.edu.cn/cn/article/doi/10.13203/j.whugis20220455
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图 1 GPS C1W/C2W和Galileo C1C/C5Q伪距多径
Figure 1. Code Multipath of GPS C1W/C2W and Galileo C1C/C5Q Observations
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图 3 运动学固定解定轨GPS和Galileo伪距和载波残差RMS分布
Figure 3. Code and Carrier Phase Residual RMS of GPS and Galileo Derived from Kinematic Orbit Determination with Ambiguity Resolution
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图 4 使用CODE产品不同定轨模式的宽巷和窄巷模糊度固定率
Figure 4. Wide Lane and Narrow Lane Ambiguity Fixing Rates of Different Orbit Determination Solutions Using CODE Product
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图 5 基于各分析中心产品的GPS运动学定轨宽巷和窄巷模糊度固定率
Figure 5. GPS Wide Lane and Narrow Lane Ambiguity Fixing Rates for Kinematic Orbit Determination Using Products of Different Analysis Centers
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图 6 基于各分析中心产品的Galileo运动学定轨宽巷和窄巷模糊度固定率
Figure 6. Galileo Wide Lane and Narrow Lane Ambiguity Fixing Rates for Kinematic Orbit Determination Using Products of Different Analysis Centers
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图 7 使用CODE和WHU产品时Galileo运动学定轨模糊度总数、固定的宽巷模糊度数和固定的窄巷与宽巷模糊度数的比值
Figure 7. Galileo Total Ambiguity, Fixed Wide-Lane Ambiguity and Ratio of Fixed Narrow-Lane to Wide-Lane Ambiguity Using CODE and WHU Products
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图 8 2021年年积日第175天运动学浮点解轨道与参考轨道的差异
Figure 8. Orbit Differences Between Kinematic Orbits with Float Ambiguity and Reference Orbits on Day of Year 175 in 2021
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图 9 使用CODE产品时运动学浮点解轨道与参考轨道的差异
Figure 9. Orbit Differences Between Kinematic Orbits with Float Ambiguity and Reference Orbits Using CODE Product
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图 10 使用CODE产品时运动学固定解轨道与参考轨道的差异
Figure 10. Orbit Differences Between Kinematic Orbits with Fixed Ambiguity and Reference Orbits Using CODE Product
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图 11 基于各分析中心产品的运动学固定解轨道与参考轨道的三维差异
Figure 11. Orbit Differences Between Kinematic Orbits with Fixed Ambiguity and Reference Orbits Using Products of Different Analysis Centers
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图 12 使用CODE产品时运动学浮点解轨道SLR残差
Figure 12. SLR Residuals of Kinematic Orbits with Float Ambiguity Using CODE Product
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图 13 基于各分析中心产品的固定解轨道SLR残差RMS
Figure 13. SLR Residuals RMS of Different Orbits with Fixed Ambiguity Using Products of Different Analysis Centers
表 1 Sentinel-6A接收机输出的GPS和Galileo观测值类型
Table 1 GPS and Galileo Observation Types Supported by Sentinel-6A Receiver
GNSS卫星 伪距 载波 GPS IIR C1C、C1W、C2W L1C、L2W GPS IIR-M、IIF、Ⅲ C1C、C2L L1C、L2L Galileo C1C、C5Q L1C、L5Q 
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表 2 星载GNSS天线PCO、卫星质心和SLR反射器在星固系下的坐标/mm
Table 2 Positions of Onboard GNSS Antenna, SLR Retroreflector and Center of Mass in Satellite Reference Frame/mm
设备名称 X Y Z GNSS天线参考点 +2 474.8 +0.1 －1 080.3 GNSS天线GPS PCO 0 0 +75.0 GNSS天线Galileo PCO 0 0 +93.0 SLR 反射棱镜 +1 624.8 －400.6 +664.8 卫星质心 +1 533.0 －7.0 +37.0
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表 3 Sentinel-6A卫星运动学定轨策略
Table 3 Sentinel-6A Precise Orbit Determination Strategy Based on Kinematic Method
模型 参数 说明 使用数据和产品 观测数据 非差无电离层组合 定轨弧长/h 24 采样间隔/s 10 截止高度角/（°） 3 GNSS轨道和钟差 CODE, CNES, GFZ, WHU MGEX产品 GNSS相位偏差 CODE, CNES, GFZ, WHU OSB产品 GNSS天线 igs14.atx 接收机天线 改正 相位缠绕 改正 相对论改正 IERS 2010 参数估计 卫星位置 随机游走,过程噪声5 m/s,每历元估计 接收机钟差 随机游走,过程噪声30 m/s,每历元估计 模糊度 常数,每跟踪弧段估计1个
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表 4 使用CODE产品时运动学定轨结果与参考轨道差异统计值/mm
Table 4 Statistics of Orbit Differences Between Kinematic Orbits and Reference Orbits Using CODE Product/mm
定轨模式 切向 法向 径向 三维 GPS浮点解 －3.1±22.5 －0.1±18.9 －1.1±24.5 38.4 Galileo浮点解 －2.7±30.5 －3.7±31.5 3.1±36.2 57.1 GPS/Galileo浮点解 －2.9±16.2 －2.2±15.1 1.5±17.7 28.6 GPS固定解 0.4±9.5 －1.0±10.4 －1.5±20.3 24.8 Galileo固定解 －0.8±9.2 －0.7±12.0 1.7±23.8 28.3 GPS/Galileo固定解 －0.3±6.0 －0.9±7.8 1.2±13.1 16.5
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