Abstract
Users of the global navigation satellite system (GNSS) operating with a single-frequency receiver must use an ionospheric correction algorithm (ICA) to account for the delay introduced on radio waves by the upper atmosphere. Galileo, the European GNSS, uses an ICA named NeQuick-G. In an effort to foster the adoption of NeQuick-G by final users, two implementations in C language have been recently made available to the public by the European Space Agency (ESA) and the Joint Research Centre (JRC) of the European Commission (EC), respectively. The aim of the present contribution is to compare the slant total electron content (STEC) predictions of the two aforementioned implementations of NeQuick-G. For this purpose, we have used actual multi-constellation and multi-frequency data for several hundreds of stations distributed worldwide belonging to the Multi GNSS Experiment (MGEX) network of the International GNSS Service (IGS). For each first day of the month during year 2019, the STECs of the two NeQuick-G versions were compared in terms of accuracy, consistency, availability, and execution time. Our study concludes that both implementations of NeQuick-G perform equivalently. Indeed, in over 99.998% of the 2125 million STECs computed, the output is exactly coincident. In contrast, 0.002% of the whole set of STECs for those rays are tangent to the Earth, the behavior of both implementations differs. We confirmed the discrepancy by processing radio-occultation actual measurements from a COSMIC-2 low Earth orbit satellite. We selected the JRC version of the Galileo ICA to be integrated into the GNSS LABoratory (gLAB) tool suite, because its open license and its processing speed (it is 13.88% faster than the ESA version). NeQuick-G outperforms the GPS ICA in STEC residuals up to 12.15 TECUs (percentile 96.23th) and in the 3D position errors, up to 5.76 m (percentile 99.18th) for code-pseudorange positioning.
Highlights
The ionosphere of the Earth is a partly ionized region located between altitudes of 60 and beyond 2000 km [1]
The examined 24 h contains a total of 142,778 slant total electron content (STEC), in which 24 rays at elevations comprised from −19◦ to −27◦ cannot be computed in the Joint Research Centre (JRC) implementation, zero to for those cases
We examined the Klobuchar model, which uses a total of 184 bytes of memory, including the memory allocated for the 8 parameters broadcast in the global positioning system (GPS) navigation message
Summary
The ionosphere of the Earth is a partly ionized region located between altitudes of 60 and beyond 2000 km [1]. JRC NeQuick-G and ESA NeQuick-G are two independent implementations of the Galileo NeQuick-G ICA that should provide, a priori, the same STEC results They are equivalent, when comparing both software distributions, some differences arise. In the JRC NeQuick-G code, clear examples of such modularity are given as follows: different modules deal with time handling, coordinate transformation, or numerical integration; there is a file for the calculation of the contribution of each ionospheric layer (E, F1, and F2 layers). The main outcome of this study for the public domain (and the remote sensing community in particular) will be the availability of an update of the gLAB tool that integrates the official Galileo ICA This will hopefully contribute to a wider adoption of the NeQuick G model at the user level in light of the outperformance of Galileo ICA against GPS ICA.
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