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The customary procedure to compute the rate of total electron content index (ROTI) presents some limitations when using the geometry-free (GF) combination of Global Positioning System (GPS) L1 and L2 carriers tracked by geodetic receivers. First, the effect of the tracking strategy implemented by each receiver manufacturer to obtain the L2 carrier from code-less observations. Second, the impact of frequent cycle slips on the L2 carrier. These limitations hinder the monitoring and characterization of ionospheric scintillation. To overcome them, the present study proposes the calculation of ROTI from the individual (uncombined) L1 carrier-phase, ROTIL1, using the Geodetic Detrending (GD) post-processing methodology, in contrast to the conventional GF combination, ROTIGF. The analysis of the entire year 2020 shows that those two aforementioned limitations produce inconsistent ROTIGF values measured by pairs of close receivers from different manufacturers. In contrast, the distribution of ROTIL1 values shows a full consistency between different receivers, being significantly less affected by cycle slips and allowing a valid and well-grounded identification of scintillation. The study concludes that ROTIL1, calculated using a 60 s window from geodetic receivers operating at 1 Hz, provides a robust tool to monitor and characterize ionospheric scintillation world-wide and regardless of the type of receiver. In particular, a ROTIL1 threshold of 1.8 TECU/min is established as the minimum level of detectable scintillation in 2020, a year of low solar activity. The most intense scintillation periods in high-latitude regions are statistically characterized by the newly proposed ROTIL1., This work was supported in part by MCIN/AEI/10.13039/501100011033/Fondo Europeo de Desarrollo Regional (FEDER), Unión Europea (UE), under Project CNS2022-135383 and Project PID2022-138485OB-I00; and in part by European Space Agency (ESA) through the Open Space Innovation Platform program under Contract 4000137762/22/NL/GLC/ov, Peer Reviewed, Postprint (published version)

Recent studies have shown possible signatures or precursors of seismic activity in the ionosphere. Our group is focusing on ionospheric scintillation associated with seismic activity. By the time this study was conducted, the sudden volcanic eruption in the Spanish La Palma island, starting on September 19th, opened a possibility to study the impact of this exceptional, well time-defined seismic event on the iono-sphere and the radio-wave propagation through it. A complete study measuring scintillation on GNSS signals and its correlation to seismic activity is presented in this extended abstract. In particular, scintillation data from two ground stations in the Canary Islands, NASA CYGNSS GNSS-Reflectometry and Spire GNSS Radio-Occultation measurements have been used. A linear correlation analysis has been conducted between S4, and the earthquakes generated energy in 6 h intervals. Small, and slightly positive regression coefficients have been found with almost all methods. © 2022 IEEE., was supported by the Spanish Ministry of Science, Innovation and Universities and EFRD, ”Sensing with Pioneering Opportunistic Techniques” SPOT, grant RTI2018-099008- BC21/AEI/10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600., Peer Reviewed, Postprint (published version)

Scintillation is a particular type of space weather perturbation occurred when the signals from the Global Navigation Satellite System (GNSS) traverse irregularities in the ionosphere; the upper layer of the Earth atmosphere. This produces rapid variations on the refraction index and, when the size of the irregularities is close to the Fresnel length of the frequencies used in GNSS frequencies, can also produce diffractive effects affecting the signal amplitude and the tracking of the carrier-phase measurements. Under these conditions, techniques delivering a High Accuracy Service (HAS) are severely degraded in terms of positioning accuracy and availability. The research group of Astronomy and Geomatics has recently introduced the Geodetic Detrending to perform scintillation studies. The GD method removes slowly-varying effects (i.e., trends) in the GNSS signals, such as the satellite movement, the on-board atomic clock offset, and the tropospheric delay (the atmosphere layer from sea level up to 50 km in height). Once the trends are removed, the GD technique allows studying effects of shorter time-scale –rapid fluctuations–of the GNSS signals, with the focus on the identification and correction of discontinuities, the so-called “cycle-slips”, on the geodetically de-trended GNSS carrier-phase measurements. Ultimately, once the cycle-slips problem is mitigated, the HAS can be re-stored under scintillation conditions., The authors acknowledge the use of data and products provided by the International GNSS Service., Postprint (published version)

The ionosphere impacts radio-wave propagation, notably up to a few GHz. The main applications impacted by the ionosphere are GNSS positioning and timing, Earth Observations (especially low frequency SAR missions e.g. BIOMASS, and GNSS-R), and Space Weather. While most effects can be compensated by using dual-frequency receivers and circular polarization antennas, ionospheric scintillation (rapid intensity and phase fluctuations) cannot. Climatological models for the mean stable part of the electron density in the ionospheric layers (e.g. IRI or NeQuick) or for the magnetic field (e.g. WMM) have significantly improved in the past years. However, models of the inhomogeneous part, responsible for scintillation, can be improved, since they are based on relatively old data (e.g. WBMOD), or climatological inputs are limited to properly characterize all latitudes and solar conditions (e.g. GISM or WAM). This study first assesses the goodness of GISM, the model adopted by the ITU-R, by comparing GISM predictions and measured scintillation data. Then, the impact of measured intensity and phase scintillation on TDS-1 GNSS-R data is illustrated., This work was supported by ESA/ESTEC project 4000120868/17/NL/AF “Radio Climatology Models of the Ionosphere: Status and Way Forward,” by the Spanish Ministry of Science, Innovation and Universities, "Sensing with Pioneering Opportunistic Techniques" SPOT, grant RTI2018-099008-B-C21, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600. The authors want to thank the discussions held with Dr. J. Lemorton, V. Fabbro, A. Mainvis (ONERA), and Dr. R. Orús (ESA)., Peer Reviewed, Postprint (author's final draft)

This work presents recent and ongoing updates to the free and open-source advanced interactive multi-purpose package for processing and analysing Global Navigation Satellite System (GNSS) data, named GNSS-Lab Tool suite (gLAB). The updates have been performed in the framework of two projects funded by the European Space Agency (ESA), namely, the “gLAB upgrade for European Geostationary Navigation Overlay System (EGNOS) Data processing” and “Upgrade of gLAB Tool for Double Frequency Multi-Constellation (DFMC)”. We examine various sets of results obtained with actual data using the Satellite Based Augmentation System (SBAS) corrections and the Differential GNSS (DGNSS) mode. Specifically, we introduce the Global Monitoring System (GMS) that routinely assesses the performance of the SBAS and DGNSS solutions using multiple station networks. That is, the Stanford-ESA integrity diagram, the Worst Integrity Ratio (WIR) maps, continuity risk, among other types of performance monitoring. Lastly, we present the ongoing update to the gLAB tool that focusses on the implementation of multi-frequency and multi-constellation data processing capabilities., Peer Reviewed, Postprint (published version)

The characterization of the ionosphere and its impact on the propagation of radio-wave signals has received an increasing interest for satellite communications, Global Navigation Satellite Systems (GNSS) applications (i.e. navigation and timing), and Earth Observation (EO) missions, especially low frequency SAR missions and GNSS-Reflectometry (GNSS-R). Although the main climatological models for the mean stable part of the electron density in the ionospheric layers (e.g. IRI or NeQuick) or for the magnetic field (e.g. WMM) have been significantly improved in the past years, the inhomogeneous part, responsible for scintillation effects, can still be significantly improved, since models are based on relatively old data (e.g. WBMOD), or the climatological inputs are limited to properly characterize all latitudes and solar conditions (e.g. GISM or WAM). ESA's CLIM IONO study aims to use the experimental observations of the ionosphere collected in the past years to assess the performance of climatological ionosphere models, with the focus on scintillation models, in order to evaluate their ability to properly support future needs, to identify weak areas if any, to propose recommendations for improvements and to implement these improvements whenever possible in existing models. This work focuses on the capabilities of GNSS-R techniques to characterize ionospheric scintillation, notably at low and high latitudes., Peer Reviewed, Postprint (published version)

A generic software tool to evaluate the impact of ionospheric disturbances is presented, including low and high latitude physically-based models, and low and high frequency fluctuations. This tool has been developed specifically to assess the performance of navigation receivers, but it is ready to simulate other frequencies and even receivers in dynamic conditions, which allows it to be used in other applications such as communications, GNSS-R, radar altimetry or SAR., Peer Reviewed, Postprint (published version)

Applying a methodology developed and tested in previous studies, the contribution from the ionospheric and plasmaspheric regions to the total electron content (measured by ground receivers) is analyzed. The method is based in the electron density profiles retrieved from radio occultations observed with low Earth orbit satellites, combined with an accurate empirical modeling of the topside-ionosphere electron density. The results of a climatological study of the fractional electron content from the ionospheric region are presented for a year of low solar activity. It is shown that a simple parametric model can be used to reproduce the electron content variations in the ionosphere and the plasmasphere between sunrise and midday, the period of the day showing the largest electron content variability., Peer Reviewed, Postprint (author's final draft)

Fast precise point positioning (Fast-PPP) is a satellite-based navigation technique using an accurate real-time ionospheric modeling to achieve high accuracy quickly. In this paper, an end-to-end performance assessment of Fast-PPP is presented in near-maximum Solar Cycle conditions; from the accuracy of the Central Processing Facility corrections, to the user positioning. A planetary distribution of permanent receivers including challenging conditions at equatorial latitudes, is navigated in pure kinematic mode, located from 100 to 1300 km away from the nearest reference station used to derive the ionospheric model. It is shown that satellite orbits and clocks accurate to few centimeters and few tenths of nanoseconds, used in conjunction with an ionosphere with an accuracy better than 1 Total Electron Content Unit (16 cm in L1) reduce the convergence time of dual-frequency Precise Point Positioning, to decimeter-level (3-D) solutions. Horizontal convergence times are shortened 40% to 90%, whereas the vertical components are reduced by 20% to 60%. A metric to evaluate the quality of any ionospheric model for Global Navigation Satellite System is also proposed. The ionospheric modeling accuracy is directly translated to mass-market single-frequency users. The 95th percentile of horizontal and vertical accuracies is shown to be 40 and 60 cm for single-frequency users and 9 and 16 cm for dual-frequency users. The tradeoff between the formal and actual positioning errors has been carefully studied to set realistic confidence levels to the corrections., Postprint (author’s final draft)