Single-frequency GNSS retrieval of vertical total electron content (VTEC) with GPS L1 and Galileo E5 measurements

The European Galileo satellite navigation system offers a signal in space that will enable us to deduce range measurements of unprecedented precision: the E5 broadband signal. These new code range measurements would be up to three or four times more accurate compared to nowadays GPS L1. However, E5 will be the only Galileo signal of that outstanding performance. For this reason, a single-frequency single-site ionospheric delay estimator is experimented with to retrieve absolute VTEC data. A single-frequency VTEC retrieval algorithm was developed and tested. It is following the principles published in Xia (1992), Issler et al. (2004), and Leick (1995, 2004). During the validation phase, we devoted considerable time to process GPS L1 measurements in order to assess the level of precision obtainable with current real-world GNSS measurements. We found the global average RMS to be close to 4 TECU (1.5–2.5 TECU at mid- and higher latitude stations) what is considered to be a promising result. The very limited Galileo satellite constellation present during the time of this study did not allow us to run the absolute VTEC retrieval algorithm with real Galileo data. However, we can demonstrate the significant improvements related to the Galileo E5 signal with the help of satellite-specific ionosphere retrieval results from the Galileo experimental satellite GIOVE-B, although only a limited set of data was available for that purpose. In addition, we are presenting a test case based on simulated data that also underlines that the precision figures will clearly improve when using Galileo E5 data. This could make single-frequency ionosphere retrieval more attractive in the future.