Multi-frequency And Multi-GNSS PPP-RTK Method And Quality Control Under Ionospheric Scintillation

With the development of emerging technologies such as 5G communications and autonomous driving,the mass market urgently need a fast,precise,reliable,continuous,and robust Global Navigation Satellite System(GNSS)location service.Recently,a PPP-RTK method that exploits the precise atmospheric corrections from a regional network to enable precise positioning and rapid Ambiguity Resolution(AR)was proposed.It is very suitable for the requirement of the mass market in terms of positioning accuracy,convergence time,service area,communication bandwidth,and product quality detecting,which make it favored by many commercial companies.However,most PPP-RTK services only use dual-frequency GNSS observations,and its positioning results in urban environments are questionable.With the advent of the multi-frequency and multi-GNSS era,there are currently more than 140 in-orbit operation GNSS satellites,and the new generation of which can transmit GNSS signals on three or more frequencies.The abundant signals can enhance the performance of PPP-RTK in urban environments.Even though with many special advantages,PPPRTK still hardly maintains stable and reliable results under ionospheric scintillation.Scintillation will reduce the positioning accuracy from a few decimeters to tens of meters.Worse still,ionospheric scintillation occurs more than 100 days a year in southern China.Therefore,it is very important to deeply study the enhancement of the multi-frequency and multi-GNSS observations on PPP-RTK,and reduce the effects of ionospheric scintillation,finally provide real-time,high-precision,low-cost,lowbandwidth,and wide-area GNSS positioning services for the mass market.This paper focuses on the multi-frequency and multi-GNSS PPP-RTK model,and analyzes the influence of ionospheric scintillation on PPP-RTK,finally research indepth on the quality control method under ionospheric scintillation of PPP-RTK.To realize a fast and precise positioning method,which takes both the urban environments and the space condition of ionospheric scintillation into account,with the multifrequency and multi-GNSS PPP-RTK.The main work of this paper is as follows:First,a new multi-frequency and multi-GNSS PPP-RTK method aims to achieve rapid AR and precise positioning.Then,vehicle experiments are conducted to evaluate the performance of the proposed method.The results show that the multi-frequency GEC PPP-RTK achieves centimeter-level positioning within 1s,with a fixing percentage of 93.7%.Compared with the dual-frequency single-GPS solution,the positioning accuracy is improved by 87.58%,and the fixing percentage is improved by25.1%.Even in an urban environment where GNSS signals are blocked frequently,the new method can still realize a positioning accuracy within 5 cm in the horizontal direction,and 15 cm in the vertical direction in 3s.The multi-frequency and multiGNSS PPP-RTK can greatly improve the accuracy,reliability,and continuity of GNSSassisted vehicle navigation in urban environments.Secondly,the effect of ionospheric scintillation on PPP-RTK is analyzed in detail,which mainly includes: 1)Decreasing the quality of GNSS observations,which means the scintillation will reduce the signal-to-noise ratio of GNSS observations and increase the pseudorange residual.2)Increasing the misjudgment of a cycle slip,which means the scintillation will lead to the sharply increasing of the number of satellites and the epoch with cycle slip.3)Decreasing the accuracy and the number of the GEC corrections by 64.7%,64.0%,and 247.5%,respectively,and the average number of which is reduced by 4.5.4)Destroying the positioning accuracy on the user side of PPPRTK by 36.5%,and the average number of satellites decrease by 3.2.Worse still,the positioning errors in the vertical direction of multi-frequency solution even up to 2dm,with accuracy is decreased by 281.7%.Thirdly,the cycle slip detection method under ionospheric scintillation is improved,in which the double-difference GF combination is introduced into the method.Based on the analysis of the GNSS observations from the Hong Kong regional network in 2021,the cycle slip detection thresholds of the single-and double-difference GF combinations were extracted,which finally proved to be 0.18 m and 0.09 m,respectively.The number and epoch of cycle slip caused by ionospheric scintillation are significantly reduced by utilizing this threshold for cycle-slip detection.Finally,this thesis optimizes PPP-RTK in cycle slip detection,stochastic model,and scintillation correction,finally,a PPP-RTK quality control(QC)method under ionospheric scintillation is proposed.The QC method is verified by using the Hong Kong GNSS observations during the scintillation period.The results show that the fixing percentage is improved,the position dilution of precise(PDOP)is increased,and the number of atmospheric corrections is 1-3 more than before.The positioning accuracy of PPP-RTK also is improved.Take HKPC Station as an example,the fixing percentage is increased from 88.2% to 94.5%,and the positioning accuracy is increased by 34.3%,17.1%,and 4.6% in the east,north,and vertical directions,respectively.The QC method reduces the influence of ionospheric scintillation on PPP-RTK effectively,finally leading to a fast,accurate,continuous,and robust positioning of PPP-RTK under ionospheric scintillation.In addition,several methods to reduce the influence of ionospheric scintillation are proposed from the perspective of actual operation,and it is found that increasing the observable constellation can greatly improve the PPP-RTK positioning performance under the ionospheric scintillation.