Assessment of the Galileo NeQuick-G Model for ionospheric correction of single-frequency GNSS observations from low earth satellites

Kurzfassung

The ionosphere plays a crucial role in Global Navigation Satellite Systems (GNSS) accuracy. This electrically atmospheric layer can lead to errors in positioning up to several tens of meters for single frequency users. For Galileo, the ionosphere model proposed is NeQuick-G, a 3- dimensional model that describes latitude, longitude and altitude dependence of the electron density and computes the Slant Total Electron Content (STEC) by integrating along the ray path. The Galileo system transmits 3 coefficients (a0, a1, a2) dedicated to this model which are issued daily. They define an effective ionization level Az as a second-order polynomial of the receiver latitude. With Galileo expecting to reach its Full Operational Capability soon, could NeQuick-G be utilised for correcting single-frequency observations made by terrestrial users as well as made on platforms on orbit? The major objective of this study is to asses and quantify the performance of NeQuick-G in a spaceborne receiver. The research has been divided into two parts: study of the quality of NeQuick-G as an ionosphere model itself and analysis of its performance in the positioning domain. This has been done for two years: 2014 which presents high ionospheric activity; and 2017 which shows the opposite. Operational data from the Swarm mission has been used as reference. 13% and 2% of the samples for 2014 and 2017, respectively, were found to be bellow the correction capability index performance indicator of 70% defined as a Galileo standard for terrestrial users. Regarding positioning, an error correction of 50% RMS has been obtained for 2014, and 20% RMS for 2017. By using a simplified version of NeQuick-G this correction was found to be 38% for 2014. However, the error is worsened by a 17% RMS for 2017. The impact of using the NeQuick-G model to correct the ionospheric error of single-frequency GNSS observations from low earth satellites is significant although it is even more significant for terrestrial users.