Study On The Theoretical Methodology And Applications Of Undifferenced And Uncombined PPP-RTK Based On Regional Reference Network

Precise point positioning(PPP)enhanced by a regional reference network,namely,the PPP-RTK(PPP real-time kinematic),is a new precise positioning technique with the global navigation satellite system(GNSS)after the RTK(real-time kinematic),NRTK(network RTK),and PPP technique.In the PPP-RTK technique,the precise state-space representation(SSR)corrections are first calculated on the network side based on a sparse regional reference network,and then the network-derived corrections are broadcast to a larger number of users to achieve the fast integer ambiguity resolution in PPP.PPP-RTK has both the advantages of PPP and NRTK leading to widespread application prospects in the fields of precision measurement,precision agriculture,and autonomous driving.At present,there two main models for PPP-RTK:the undifferenced and combined mode and the undifferenced and uncombined(UDUC)mode.In the undifferenced and combined PPP-RTK,precise satellite products are first estimated based on the linear combination code and phase observations of global GNSS stations,and then regional atmosphere delays are obtained from the raw code and phase observations of regional GNSS stations.However,in the UDUC PPP-RTK,precise satellite products and regional atmosphere delays are simultaneously estimated using the raw code and phase observations based on the rigorous mathematical model which is reconstructed from the raw code and phase observation equations by the S-basis theory.Compared with the undifferenced and combined PPP-RTK,the UDUC PPP-RTK effectively avoids the theoretical technical difficulties to select optimal observation combinations from the multi-frequency observations and has technical advantages such as low observation noise,good self-consistent among parameters,and strong model scalability.Therefore,the UDUC PPP-RTK is more suitable for the main trend of muti-frequency and multi-GNSS data processing.However,for the rapid precise positioning demands of massive users in a large regional reference network with the between-station distance above 100 km,both the combined and uncombined PPP-RTK models are facing the theoretical technical bottleneck,including extracting ionospheric delays from a large regional reference network,modeling finely precise atmosphere delays,and simplifying the broadcast scheme of precise regional corrections,which directly restricts the development and applications of the PPP-RTK It is concretely shown in the following three aspects:(1)the receiver code and phase biases are highly coupled with the ionospheric delays in the observation equations and the receiver code biases are usually treated as the time-invariant parameters.But the short-term variations of the receiver code bias have been proved to be remarkable and may affect the precise extraction of the ionospheric delays.Therefore,how to realize the precise processing of the biases is the primary problem to estimate precisely ionospheric delays.(2)In the traditional estimable form of the ionospheric parameters,different ionospheric parameters contain different receiver code biases,which limits the precision modeling of the ionospheric slant delays.How to unify the receiver code biases of ionospheric parameters is the key problem to modeling accurately atmosphere delays.(3)In the real-time PPP-RTK,the network-derived precise regional corrections need to be sent to users through the Internet.How to simplify the broadcast scheme of precise regional corrections and ensure the accuracy of the regional ionospheric model is the key issue to achieve the single-receiver fast precise positioningTherefore,based on the rigorous UDUC PPP-RTK model and S-basis theory,this paper focuses on the above-mentioned theoretical and technical problems related to ionospheric delay estimation,modeling,and broadcasting and carries out studies on the models,algorithms,and software platform of the multi-frequency and multi-GNSS post/real-time PPP-RTK.The main work and innovations of this paper are as follows1.Analysis of the characteristics and causes of the short-term variations of multi-GNSS RDCB,as well as its impact on the accuracy of ionospheric extractionFor the problem that the constant assumption of the receiver differential code biases(RDCB)within one day affects the accuracy of ionospheric estimates,this paper carres out the following analysis.First,the multi-GNSS modified carrier-to-code leveling method(MCCL)is developed based on the S-basis theory,which takes into account the short-term variations of the RDCB.Then,the epoch-by-epoch RDCB variations relative to the first epoch and ionospheric slant delays are extracted using the MCCL,followed by analyses of the significance of the short-term variations of the RDCB for different receiver and antenna types,the correlation between the intraday variations of the RDCB and temperature,and the impact of the short-term variations of the RDCB on ionospheric extraction.Results show:The short-term variations of the RDCB have a great correlation with the receiver types but have no obvious correlation with the receiver antenna types;the intraday variations of the RDCB have a high correlation with the intraday variations of temperature,and they can be expressed by a linear function which provides a reference for temperature modeling to constrain GNSS parameter estimation;the intraday variations of the RDCB have a greater impact on the accuracy of extraction of the ionospheric slant delays but have little effect on the ionospheric vertical delays.The main reason is that the model error of the projection function is greater than the short-term variations of the RDCB.Therefore,in precise applications,the ionospheric slant delays should be directly modeled instead of the ionospheric vertical delays2.Models and algorithms of the ionosphere-weighted UDUC PPP-RTKFor the problem that the estimable ionospheric parameters are biased by different receiver code biases which limit the ionospheric fine modeling and the characteristic that the ionospheric delays experienced by different regional reference stations to the same satellite are similar,this paper proposes an ionosphere-weighted UDUC PPP-RTK model enhanced by between-station single-difference ionospheric pseudo-observations In this paper,the multi-frequency and multi-GNSS UDUC PPP-RTK model is first theoretically derived,and then the ionosphere-weighted multi-frequency and multi-GNSS UDUC PPP-RTK model is developed based on the S-basis theory by introducing between-station single-difference ionospheric pseudo-observations.The between-station single-difference ionospheric pseudo-observations not only enhance the PPP-RTK network model but also unifies the receiver code bias datum contained in the biased ionospheric parameters.In this way,the receiver code bias datum can be completely absorbed by the code/phase bias estimates of the PPP-RTK user model to avoid causing systematic errors in user observation equations.Furthermore,the kinematic positioning performance of the ionosphere-weighted UDUC PPP-RTK model is preliminarily evaluated based on a sparse regional reference network from Michigan,USA by comparing with the PPP-AR/RTK/IW-RTK.In addition,the ionospheric delays play an important role in the ionosphere-weighted UDUC PPP-RTK model,and its accuracy has a great impact on the positioning performance of the PPP-RTK.Therefore,the impact of the ionospheric disturbance is also considered to more fully show the kinematic positioning performance of the ionosphere-weighted UDUC PPP-RTK model.3.Modeling of atmospheric delays and designing of the regional precise product message formats for the real-time ionosphere-weighted UDUC PPP-RTK,as well as its theoretical verification based on the BDS3 PPP-B2b productsFor the problem that the size of data transfer of the regional precise products is large and the accuracy of current ionospheric models are not enough to satisfy the demand for precise positioning,this paper proposes a single-satellite ionospheric slant delay model.In this way,the large number of regional ionospheric corrections are replaced by several model coefficients which reduce the size of products to be transmitted.At the same time,taking into account the planning of BDS3 PPP-B2b to be upgraded to the PPP-RTK,this paper has determined the ranges and least significant bits of the regional precise products and has particularly designed its message format according to the propagation law of errors.Based on the theories above,this paper first carried out theoretical verification experiments of the PPP-RTK using the precise satellite products of the BDS3 PPP-B2b,which can provide a reference for the BDS PPP-RTK technique.4.Design and development of a distinctive post/real-time multi-frequency and multi-GNSS PPP-RTK software platform(NASDAK)A flexible,stable,and efficient software platform is the basis to study the theories and applications of the PPP-RTK.In this paper,a set of post/real-time multi-frequency multi-GNSS PPP-RTK software platform NASDAK(Network Augmented Satellite Data Analysis Kits)is designed and developed based on the rigorous UDUC PPP-RTK model and systematic data processing strategies.NASDAK software platform takes into account theoretical research and engineering application requirements.Its modularized algorithms have high scalability,its resilient program frame adapts to a variety of application scenarios,and its concise user program can be used to the embedded development platform.In addition,the NASDAK software platform is composed of many subprograms,which not only play an important foundation for studies of theories and applications of the PPP-RTK but also provide important technical support for other relevant GNSS data processing.