Galileo Ionospheric Correction Algorithm Integration into the Open-Source GNSS Laboratory Tool Suite (gLAB)

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.