Satellites with dual-frequency Global Navigation Satellite Systems (GNSS) receivers can measure integrated electron density, known as slant Total Electron Content (sTEC), between the receiver and transmitter. Precise relative variations of sTEC are achievable using phase measurements on L1 and L2 frequencies, yielding around 0.1 TECU or better. However, CubeSats like Spire LEMUR, with simpler setups and code noise in the order of several meters, face limitations in absolute accuracy. Their relative accuracy, determined by phase observations, remains in the range of 0.1-0.3 TECU. With a substantial number of observations and comprehensive coverage of lines of sight between Low Earth Orbit (LEO) and GNSS satellites, global electron density can be reconstructed from sTEC measurements. Utilizing 27 satellites from various missions, including Swarm, GRACE-FO, Jason-3, Sentinel 1/2/3, COSMIC-2, and Spire CubeSats, a cubic B-spline expansion in magnetic latitude, magnetic local time, and altitude is employed to model the logarithmic electron density. Hourly snapshots of the three-dimensional electron density are generated, adjusting the model parameters through non-linear least-squares based on sTEC observations. Results demonstrate that Spire significantly enhances estimates, showcasing exceptional agreement with in situ observations from Swarm and Defense Meteorological Satellite Program (DMSP). The model outperforms contemporary climatological models, such as International Reference Ionosphere (IRI)-2020 and the neural network-based NET model. Validation efforts include comparisons with ground-based slant TEC measurements, space-based vertical TEC from Jason-3 altimetry, and global TEC maps from the Center for Orbit Determination in Europe (CODE) and the German Research Center for Geosciences (GFZ).

The prediction of natural disasters in general and earthquakes, in particular, is becoming increasingly critical in providing early warnings and mitigating catastrophes’ effects. This study investigates relations between earthquakes and ionospheric disturbances. Based on analyzing TEC disturbances, we search for the impact of the earthquake-related seismic waves on the ionosphere. The study is designed as cross-sectional investigations, in which the global earthquakes are randomly collected by the cluster sampling method. We use data of 54 permanent GNSS (Global Navigation Satellite Systems) stations and global earthquakes with magnitudes (Mw) from 4.0 to 9.0 in the period from 2006 to March 2020. The selected data ensure strict conditions such as accuracy, the distance from the GNSS stations to the epicenter, and the depth of hypocenter. Probability and statistics are applied to filter, classify and analyze data. The results indicate that TEC fluctuations at the regions occurring earthquakes with magnitudes greater than 6.0 Mw are significant. These TEC anomalies appear from 30 minutes to around two hours before the mainshocks, and the oscillations remain from five to eight minutes. These TEC variations occur from five to eleven days before the great earthquakes. The results reveal the relations between ionospheric anomalies and earthquake-related seismic waves. The findings are the base to filter TEC anomalies generated by earthquakes in building ionospheric models. The study also opens up a prospect for GNSS applications in studying TEC anomalies linked to earthquake precursors as well.

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 paper 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 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.