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).

We report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observations rather than dedicated short-baseline experiments. An analysis of phase delay observables from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hour VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. Position ties are also obtained for other short baselines between legacy antennas and nearby, newly built antennas. They are critical for maintaining a consistent continuation of the realization of the terrestrial reference frame, especially when including the new VGOS network. The phase delays of the baseline WETTZ13N–WETTZELL enable an investigation of sources of error at the sub-millimeter level. We found that a systematic variation of larger than 1 mm can be introduced to the up estimates of this baseline vector when atmospheric delays were estimated. Although the sub-millimeter repeatability has been achieved for the baseline vector WETTZ13N–WETTZELL, we conclude that long term monitoring should be conducted for more short baselines to assess the instrumental effects, in particular the systematic differences between phase delays and group delays, and to find common solutions for reducing them. This will be an important step towards the goal of global geodesy at the 1 mm level.

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.

This article focuses on the new generation of Very Long Baseline Interferometry (VLBI), the VLBI Global Observing System (VGOS), and measurements carried out during CONT17 campaign. It uses broadband technology that increases both the number and precision of observations. These characteristics make VGOS a suitable tool for studying the atmosphere. This study focuses on the effects of the ionosphere on VGOS signals using a model that incorporates and extends ideas originally published in Hobiger et al. (2006). Our investigation revealed that the differential Total Electron Content (dTEC) data product calculated with the VGOS post-processing software revealed a sign error in the inferred dTEC values. Fortunately, this error does not change the final values of the phase and group delay. Therefore, this study was a way to identify this problem within the VGOS data. After diagnosing and solving this problem, the Hobiger et al. (2006) model was modified such that instead of considering a single unknown for the latitude gradient of the ionosphere, a time series of latitude gradients were taken into account. For comparison purposes, time series of Vertical Total Electron Content (VTEC) at each station during CONT17 campaign were constructed from Global Ionosphere Map (GIM) data. In this way, it is shown that the final accuracy of estimating VTEC at each station using VGOS data was between 1.0 and 5.8 TEC Units (TECU).

The next-generation, broadband geodetic very-long-baseline interferometry system, named VGOS, is being developed globally with an aim to achieve 1~mm accuracy for station positions. Currently, the systematic errors in VGOS observations are still about one order-of-magnitude larger than this aim. In this study, we demonstrate that it is feasible to make images directly from VGOS observations without the need of complicated calibrations and determine the source structure effects in VGOS broadband delays through the process of model fitting to the structure phases from our imaging results. Source structure effects are investigated in detail, and it is shown that the systematic errors in VGOS observations are well explained by these effects. For instance, the root-mean-square (RMS) closure delays of the observations of sources 0016$+$731 and 1030$+$415 are 24.9~ps and 50.2~ps in session VO0034, respectively; by correcting source structure effects based on the images, the RMS values of the residual closure delays are 5.5~ps and 10.1~ps. The jumps in delay observables with magnitudes of several hundreds of picoseconds are found to be caused by 2$\pi$ phase shifts among the four bands due to strong source structure effects. The impact of the alignment of the images at the four frequency bands in VGOS is discussed. Our study provides a methodology of deriving images of radio sources at the four bands of VGOS observations and discusses the alignment of the four-band images, which is fundamental to mitigate systematic effects.

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.