On Inconsistent ROTI Derived From Multiconstellation GNSS Measurements of Globally Distributed GNSS Receivers for Ionospheric Irregularities Characterization

This study for the first time presents an investigation into the exploitation of multi-Global Navigation Satellite System (GNSS) observations (Global Positioning System, Globalnaya Navigazionnaya Sputnikovaya Sistema, Galileo, and BeiDou) to characterize ionospheric plasma irregularities, based on the rate of change of total electron content index (ROTI) sampled at 1 s. It is demonstrated that the multi-GNSS ROTIs can represent temporal evolutions of ionospheric plasma irregularities during a large geomagnetic storm. However, an inconsistency in the magnitudes of multi-GNSS ROTIs is found among a variety of GNSS receivers (i.e., Javad, Leica, Trimble, and Septentrio). Through cross comparisons between GNSS receivers installed at zero/short baselines and validations by receivers distributed separately, it is observed that the magnitude of ROTI corresponding to each system differs between closely installed and even collocated receivers of different models, as well as between the GNSS signals on the same frequency. From 1-year (i.e., 2015) data analysis, it is found that the magnitudes of multi-GNSS ROTIs exhibit the dependence on the receiver type. Among the four GNSS receiver types, the largest discrepancy in the multi-GNSS ROTIs is observed from Septentrio receivers, while the smallest one is shown by Trimble receivers. A one-to-one comparison indicates that the ROTI difference is noticeable and can increase to 4–6 TECu/min under ionospheric irregularities conditions, that is, in the postsunset period of 18–02 local time. To investigate the inconsistency, the effect of adopting different equivalent noise bandwidths in the tracking loop design is discussed, via cross comparisons between a GNSS software-defined receiver and a collocated Septentrio receiver. The result shows that adopting 15- and 2-Hz noise bandwidths in the tracking loop can cause 0.2- to 0.5-TECu/min differences in the magnitude of ROTI, suggesting that the diverse tracking techniques deployed by various receivers are very likely a major contributor to the inconsistency of multi-GNSS ROTIs.