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武汉大学李星星教授团队：PPP-RTK综述：进展、挑战与机遇
The following article is from 卫星导航国际期刊 Author 李昕
标题：PPP-RTK综述：进展、挑战与机遇
作者：李星星，黄佳欣，李昕*，申志恒，韩俊杰，李林阳，王波
主题词：PPP-RTK；模糊度快速固定；多频多系统GNSS；智能设备；多源传感器融合
（图片来自作者）
引用文章:
Li, X. X., Huang, J. X., Li, X. et al. Review of PPP–RTK: achievements, challenges, and opportunities. Satell Navig 3, 28 (2022). https://doi.org/10.1186/s43020-022-00089-9
PDF文件下载链接：
https://satellite-navigation.springeropen.com/articles/10.1186/s43020-022-00089-9
Review of PPP-RTK
The PPP-RTK method, which combines the concepts of Precise of Point Positioning (PPP) and Real-Time Kinematic (RTK), is proposed to provide a centimeter-accuracy positioning service for an unlimited number of users. Recently, the PPP-RTK technique is becoming a promising tool for emerging applications such as autonomous vehicles and unmanned logistics as it has several advantages including high precision, full flexibility, and good privacy.
This paper gives a detailed review of PPP-RTK focusing on its implementation methods, recent achievements as well as challenges and opportunities. The recent efforts and progress such as improving the performance of PPP-RTK by combining multi-GNSS and multi-frequency observations, single-frequency PPP-RTK for low-cost devices, and PPP-RTK for vehicle navigation are addressed. The main issues that impact PPP-RTK performance are highlighted and the new opportunities brought by the rapid development of low-cost markets, multiple sensors, and new-generation LEO navigation constellation are discussed.
本文亮点
1.系统介绍了PPP-RTK的模型、方法与应用，总结了相位小数偏差估计、精密大气增强改正数提取与建模、非差模糊度快速固定等关键技术的研究进展，概述了北斗三号精密单点定位服务（PPP-B2b）、QZSS-CLAS、伽利略HAS以及各大商业公司的PPP-RTK系统的建设情况和服务性能。
2.重点总结了PPP-RTK技术从单/双系统至多系统、从双频至多频、从后处理至实时车载导航应用的发展历程与最新进展。基于武汉大学测绘学院自主研发的GREAT（GNSS+ REsearch, Application and Teaching）软件，评估了PPP-RTK在不同应用场景下的定位性能。
3.讨论了现阶段PPP-RTK技术面临的挑战，包括电离层活跃、复杂地形条件下的高精度大气建模，城市环境下多路径与NLOS误差探测与识别，服务端与用户端完好性监测等。指出了新兴的低轨星座以及多传感器融合为PPP-RTK带来的机遇。
内容简介
PPP-RTK技术集成了传统精密单点定位（PPP）与实时动态定位（RTK）的优点，可为海量用户提供基于单接收机的实时快速厘米级位置服务。近年来，得益于服务范围广、精度高、隐私性好等优势，PPP-RTK已经成为自动驾驶、无人机、移动机器人等新兴应用领域的首选技术。本文从研究背景、国内外研究现状、模型与方法、最新研究进展以及发展前景等方面对PPP-RTK技术进行了综述。介绍了相位小数偏差估计、精密大气增强改正数提取与建模、非差模糊度快速固定等关键技术，概述了北斗三号精密单点定位服务（PPP-B2b）、QZSS-CLAS、伽利略HAS以及各大商业公司的PPP-RTK系统的建设情况和服务性能。重点总结了PPP-RTK技术从单/双系统至多系统、从双频至多频、从后处理至实时车载导航应用的发展历程与最新进展。最后，讨论了现阶段PPP-RTK技术面临的挑战与机遇。
I PPP-RTK系统构成与服务流程
PPP-RTK系统主要由服务端和用户端两大部分组成。如图1所示，服务端接收全球参考站网的观测数据实现精密轨道、钟差、相位小数偏差等产品的估计、生成与播发；同时接收区域参考站网的观测数据,基于非差模糊度固定技术提取高精度大气增强改正信息并播发给用户。在用户端，接收到来自服务端的各类产品之后便可以实现基于单台接收机的快速精密定位。
图 1 PPP-RTK系统服务流程。
II PPP-RTK增强系统建设
随着实时定位技术的快速发展，基于PPP-RTK模式的增强系统纷纷建立。在政府层面，日本首先基于QZSS系统建立了厘米级增强服务系统（CLAS）。该系统利用日本的密集地面参考网络生成GPS、Galileo和QZSS卫星的各类定位增强改正数，并通过QZSS卫星的L6频段广播播发，为日本及周边区域的用户提供一分钟内收敛的厘米级定位服务。中国新一代北斗三号系统以及欧盟的Galileo系统也计划建立各自的PPP-RTK增强系统。根据2019年发布的北斗卫星导航系统应用服务体系白皮书，北斗三号的PPP服务将利用GEO卫星的PPP-B2b信号播发各类增强改正数以实现不同精度和收敛时间的精密单点定位。系统的第二阶段计划结合地基参考站网提供的大气增强改正数来实现区域增强PPP-RTK服务。Galileo则规划了高精度服务（HAS）系统。该系统将通过Galileo E6-B信号和网络播发两种模式为用户提供覆盖全球的实时PPP服务和覆盖欧洲地区的PPP-RTK服务。
图 2 北斗精密单点定位服务系统。
近年来，许多商业公司也开始提供基于PPP-RTK的高精度定位服务，如天宝的CenterPoint-RTX Fast、诺瓦泰的TerraStar-X、Ublox的PointPerfect等。中国的千寻、合众思壮以及中海达等企业也在建立了相关的系统，提供相应的高精度定位服务。这些系统的服务区域遍布世界各地，被广泛应用于自动驾驶、无人机、精密农业、行人导航等大众市场。
III 多系统PPP-RTK
考虑到多系统融合在提升卫星数目、改善几何构型等方面的优势，许多学者开始研究多系统融合的PPP-RTK方法。2015年，Ojidk等人首先实现了基于GPS和BDS双系统融合的PPP-RTK；Nadarajah等人则于2018年提出了融合GPS/Galileo/BDS三系统的PPP-RTK方法。此后，Ma和Li等人进一步将PPP-RTK方法拓展至GPS/GLONASS/Galileo/BDS四大卫星导航系统(Ma et al.2020; Li et al.2021a)。Li(2021a)等人的研究验证了多系统融合PPP-RTK在加快收敛速度、提升模糊度固定成功率和定位精度方面的优势，尤其是在观测条件受限的情况下。论文中通过设置不同的截止高度角模拟观测环境受限的场景，不同系统组合的单历元模糊度固定成功率如图3所示。随着截止高度角的不断升高，单系统PPP-RTK的模糊度固定成功率迅速下降，当截止高度角超过30°时，模糊度固定率已不足50%；与之相反的，四系统融合PPP-RTK方案的固定率始终保持在98%以上。结果表明相较于单系统，多系统融合PPP-RTK可以显著提升模糊度固定成功率从而保障定位服务的稳定性。
近年来新发射的GPS Block-IIF、北斗、Galileo等卫星都可以传输三种频率以上的导航信号，使得基于多频信号的精密单点定位成为可能。相关的研究已经表明多频观测值组合可以加速模糊度固定，缩短初始化时间(Geng et al.2013; Gu et al.2015; Li et al.2019)。在此基础上，Li 等人在2022年提出了一种基于GPS/Galileo/BDS三频GNSS观测值组合的PPP-RTK方法并验证了在车载动态场景下的定位性能(Li et al.2022d)。图4展示了在城市环境下，双频动态PPP、双频PPP-RTK和三频PPP-RTK定位序列的比较结果。在信号受遮挡较为频繁的场景下，PPP几乎无法完成收敛；双频PPP-RTK虽然能够快速固定，但也无法避免频繁的重新固定和错误固定情况的出现。而三频PPP-RTK定位序列更加稳定，尤其是当观测卫星数量急剧下降时，三频PPP-RTK的错误固定情况更少，从而保障了定位服务的可靠性。
图 4 城市环境下的双频动态PPP、双频PPP-RTK和三频PPP-RTK定位序列比较。
V PPP-RTK车载动态导航
随着自动驾驶的快速兴起，实时高精度位置服务愈发重要。在城市复杂环境下，单一的GNSS手段已经难以满足连续高精度定位的需求。关于PPP-RTK的研究也开始转向多源传感器融合(Gu et al.2021; Li et al.2021d),图5比较了城市环境下PPP-RTK和PPP-RTK/INS紧组合的定位序列。结果表明在GNSS信号遮挡严重的情况下，惯性导航系统的辅助使得PPP-RTK的定位序列更加稳定，保障了在信号短时中断情况下的定位服务的持续性。然而民用惯性导航系统的精度发散较快，也无法应对长时间信号中断的挑战，于是学者们开始进一步引入视觉导航系统(Gu et al.2022; Li et al.2022e)。图6展示了PPP-RTK、PPP-RTK与低成本MEMS组合(PPP-RTK/MEMS)、PPP-RTK与战术级惯导(PPP-RTK/TINS)以及PPP-RTK加MEMS加视觉(PPP-RTK/MEMS/Vision)组合的定位结果。定位序列表明视觉信息的加入可以进一步提升PPP-RTK定位的精度和可靠性，使得采用低成本MEMS的多源融合PPP-RTK方案可以达到与PPP-RTK加战术级惯导增强方案相媲美的效果。这将大大降低多源传感器融合PPP-RTK的成本从而拓宽其应用市场。
图 6 PPP-RTK、PPP-RTK/MEMS、PPP-RTK/TINS、PPP-RTK/MEMS/Vision组合定位序列比较。
近年来，低轨卫星以其星座与信号方面的独特优势，受到导航领域的关注与青睐，成为下一代卫星导航系统发展的关注重点。低轨卫星具有几何变化快，信号强度高等优点，为缩短精密单点定位的收敛时间带来新的机遇(Ge et al.2018,2022; Li et al.2019b,2019c)。而PPP-RTK与低轨卫星相结合则有望实现在稀疏参考站网情况下的瞬时模糊度固定，从而扩展PPP-RTK的覆盖范围。图7展示了在800千米尺度的参考站网情况下，不同的低轨卫星星座对PPP-RTK定位的仿真增强效果。实验分别仿真了包含72颗、144颗、和360颗卫星的低轨星座并用于增强PPP-RTK定位。实验结果表明了加入低轨卫星观测值可以在地面参考站较为稀疏的情况下显著提升PPP-RTK的定位性能。与无低轨增强的PPP-RTK相比，加入360颗卫星的低轨星座观测值可以将首次固定时间从161秒缩短至12秒，这极大的提升了PPP-RTK在稀疏参考网情况下的应用潜力。
图 7 800千米尺度参考网情况下的低轨增强PPP-RTK仿真定位结果。
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