Abstract
The determination of slant total electron content (STEC) between satellites and receivers is the first step for establishing an ionospheric model. However, the leveling errors, caused by the smoothed ambiguity solutions in the carrier-to-code leveling (CCL) method, degrade the performance of ionosphere modeling and differential code bias (DCB) estimation. To reduce the leveling errors, an uncombined and undifferenced precise point positioning (PPP) method with ambiguity resolution (AR) was used to directly extract the STEC. Firstly, the ionospheric observables were estimated with CCL, PPP float-ambiguity solutions, and PPP fixed-ambiguity solutions, respectively, to analyze the short-term temporal variation of receiver DCB in zero or short baselines. Then, the global ionospheric map (GIM) was modeled using three types of ionospheric observables based on the single-layer model (SLM) assumption. Compared with the CCL method, the slight variations of receiver DCBs can be obviously distinguished using high precise ionospheric observables, with a 58.4% and 71.2% improvement of the standard deviation (STD) for PPP float-ambiguity and fixed-ambiguity solutions, respectively. For ionosphere modeling, the 24.7% and 27.9% improvements for posteriori residuals were achieved for PPP float-ambiguity and fixed-ambiguity solutions, compared to the CCL method. The corresponding improvement for residuals of the vertical total electron contents (VTECs) compared with the Center for Orbit Determination in Europe (CODE) final GIM products in global accuracy was 9.2% and 13.7% for PPP float-ambiguity and fixed-ambiguity solutions, respectively. The results show that the PPP fixed-ambiguity solution is the best one for the GIM product modeling and satellite DCBs estimation.
Highlights
Ionospheric delay is one of the main error sources in the positioning and navigation applications of global navigation satellite systems (GNSSs) [1,2,3,4]
The undifferenced and uncombined precise point positioning (PPP) with fixed ambiguity resolution improves the performance of positioning and improves the accuracy of ionosphere parameter estimation
Compared with Center for Orbit Determination in Europe (CODE) final global ionospheric map (GIM) products, the results prove that the largest differences between self-generated and CODE-generated vertical total electron contents (VTECs) products are located in areas where there are no reference stations
Summary
Ionospheric delay is one of the main error sources in the positioning and navigation applications of global navigation satellite systems (GNSSs) [1,2,3,4]. The influence of the ionospheric layer height in the thin layer ionospheric model is investigated [6] and an enhanced mapping function is proposed [7]. An improved empirical model is used for ionosphere modeling, considering the stochastic process of each satellite [8]. The higher-order ionospheric delay is demonstrated to mitigate its effects on precise point positioning (PPP) during disturbed ionospheric conditions [9]. The two-layer ionospheric model is proposed to better model the structure of the ionosphere [10]. The real-time ionospheric delays corrections are used in faster PPP to shorten the convergence time and obtain the fixed-ambiguities solutions [15,16,17]
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