# Evaluation of Ionospheric Delay Effects on Multi-GNSS Positioning Performance  
[PERSON]  
1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China; [EMAIL_ADDRESS]  
2 University of Chinese Academy of Sciences, Beijing 100049, China  
3 School of Remote Sensing and Geomatics Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China  
4 Institute of Communications and Navigation, German Aerospace Center, Kalkhorstweg 53, 17235 Neustrelitz, Germany; [EMAIL_ADDRESS]  
5 [PERSON]L_ADDRESS]; Tel. : +13-167-075-8225516 January 2019  
parameter of the NTCM is the solar radio flux index F10.7. Global Ionosphere Maps (GIMs) are routinely estimated by the International GNSS service (IGS) Analysis Centers (ACs) and have an accuracy of 2-8 TECU (total electron content unit). With the rapid development of current navigation constellations and construction of more stations, GNSS represents a new era and has wide applications all over the world. In this new multi-frequency and multi-GNSS environment, observations from different signals and constellations are processed together, however, a consideration of the error sources is necessary and mandatory to ensure that the positioning model is consistent. Standard point positioning (SPP) and precise point positioning (PPP) are two types of positioning modes with a stand-alone GNSS receiver that have been used to fulfill different requirements. SPP has been widely used in many fields such as land surveying and vehicle navigation. In the case of the single-frequency SPP approach, the GNSS observations are corrected with existing ionospheric models to ensure a positioning accuracy of meters. The PPP technique has also proved to be an efficient technique due to its high accuracy, global coverage and cost-efficiency. The ionosphere-free (IF) combination is an effective and popular way to eliminate the first order of ionosphere delay in dual-frequency PPP [6]. Another approach is to use the undifferenced and uncombined (UC) observations in dual-frequency PPP processing to extract ionospheric delays and avoid noise amplification [7]. The ionospheric delay is estimated by employing a prior ionosphere model and has the potential to improve positioning performance for wider applications [8; 9]. For instance, the experimental tests of [10] showed that the convergence time of dual-frequency PPP was reduced by 30% with the IGS GIM product constraint. Along with dual-frequency PPP, single-frequency PPP has also attracted increasing attention due to its low cast and high accuracy. Similarly, IF combination on a single frequency, known as the Group and Phase Ionospheric Correction (GRAPHIC) can be utilized to eliminate the ionospheric delay in single-frequency PPP [11]. The rank-deficient mathematical problem exists in the GRAPHIC approach, which utilizes the average of the carrier phase and pseudo-range observations. Ionospheric delays can also be estimated simultaneously along with other parameters when proper constraints are given by the ionospheric models. Several studies have been done to improve the convergence time of single-frequency PPP with the GIM [8; 12; 13]. The quality of ionospheric delay derived from the models will affect the positioning performances. Thus, ionospheric delay is a key issue to resolve in multi-GNSS combined positioning. In this study, we first present the ionospheric processing strategies for current multi-GNSS single point positioning and summarize the positioning models. Multi-GNSS combined positioning models consist of single-frequency SPP/PPP and dual-frequency PPP. Thereafter, three types of ionospheric correction models, including the broadcast ionospheric model (BIM), NTCM and GIM are introduced for multi-GNSS positioning scenarios. Then, the data and processing strategy is given. A unified time, varying weight, ionospheric scheme for PPP is presented to adapt three ionospheric models.