Operational multi-GNSS global ionosphere maps at GFZ derived from uncombined code and phase observations

Global navigation satellite system (GNSS) networks with multi-frequency data can be used to monitor the activity of the Earth's ionosphere and to generate global maps of the vertical total electron content (VTEC). This article introduces and evaluates operational GFZ VTEC maps. The processing is based on a rigorous least squares approach using uncombined code and phase observations, and does not entail leveling techniques. A single-layer model with a spherical harmonic VTEC representation is used. The solutions are generated in a daily post-processing mode with GPS, GLONASS, and Galileo data, and are provided for the period since the beginning of 2000. A comparison of the GFZ VTEC maps with the final combined International GNSS Service (IGS) product shows a high consistency with the solutions of the IGS analysis centers. A validation with about four years of Jason-3 altimetry-derived VTEC data is provided, in which the GFZ solution has the smallest bias of 1.2 TEC units compared to the solutions of the IGS analysis centers, and with 3.0 TEC units one of the smallest standard deviations.