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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)

Medium-scale traveling ionospheric disturbances (MSTIDs) are fluctuations in the plasma density that propagate through the upper layer of the atmosphere at velocities of approximately 100 m/s and periods reaching some tens of minutes. Due to their wavelengths, MSTIDs can degrade the performance of differential positioning techniques, such as real-time kinematics (RTK) or network-RTK (NRTK). This paper defines a novel methodology as a tool for relating the errors in NRTK positioning based on an MSTIDs indicator using the second difference in time of the slant total electron content (STEC). The pro-posed methodology performs integer ambiguity resolution (IAR) on the undifferenced measurements instead of using double-differenced carrier-phase measurements, as it is usual in RTK and NRTK. Statistical tests are applied to evaluate the degradation in the position errors caused by the impacts of MSTIDs on RTK and NRTK positioning over a data set spanning one year gathered from the CATNET network; a dual-frequency network of fixed permanent GNSS receivers located at the mid-latitudes of northeastern Spain. With the development of the proposed methodology for measuring the position degradation, another results of the present research are the establishment of thresholds for the proposed MSTIDs index, which can be used to monitor the positioning solution and to warn users when the measurements are affected by MSTIDs events, relating the position error to MSTIDs that affect not only the user receivers but also of the reference receivers within the network., Peer Reviewed, Postprint (published version)

Global Navigation Satellite Systems (GNSSs) have improved the efficiency and accuracy of transportation means. In terms of safety, the European Geostationary Navigation Overlay System (EGNOS) is providing vertical and lateral guidance to civil aviation since 2011. Furthermore, EGNOS can support applications in other sectors such as maritime, railways and road.The present contribution aims to assess the use of EGNOS for maritime navigation compliant with the International Maritime Organization (IMO) requirements established in its Resolution A.1046 (27) for harbour entrances, harbour approaches and coastal waters: 99.8% of Signal Availability, 99.8% of Service Availability, 99.97% of Service Continuity and 10 m of Horizontal Accuracy, The authors would like to thank the Armed Institute of the Spanish Civil Guard for their assistance with the installation of equipment on-board their vessels that allowed collection of GNSS data, as well as for the navigation days across the Canary waters. The authors acknowledge the use of products provided by the International GNSS Service. EGNOS messages have been downloaded from SERENAD Data Server that belong to National Centre for Space Studies (CNES). The research reported in this paper was financially supported by the European GNSS Agency under contract GSA/OP/12/16/Lot1/SC1, Postprint (published version)

The main goal of the Marie Sklodowska-Curie Action (MSCA) titled “High Accuracy Navigation under Scintillation Conditions (NAVSCIN)” is to develop an improved strategy to mitigate scintillation –a particular type of space weather perturbation–tailored for satellite-based navigation techniques, in close collaboration with users and manufacturers of these technologies. Indeed, once the scintillation effect is correctly detected and mitigated, the availability and accuracy of the Global Navigation Satellite Systems (GNSSs) receivers will dramatically improve.Scintillation is one of the most challenging problems in GNSS. This phenomenon appears when the signal pass through ionospheric irregularities, producing rapid changes on refraction index and, depending on the size of such irregularities, also diffractive effects affecting the signalamplitude and the tracking of the carrier-phase measurements. In this work, we present the results being achieved within NAVSCIN to deal with scintillation effects on GNSS signal, exploiting the evidence that low and high latitudes present different characteristics.At low-latitude, we observe an increase of the carrier phase noise and a fade on the signal intensity that can produce frequent cycle-slips in the GNSS signal and, in extreme conditions, it can lead to the loss of GNSS signals. The detection of these cycle-slips associated with scintillation condition is a challenging problem for precise navigation. In the current state of the art, these uncorrected discontinuities can produce meters of position error. In contrast, we show that high accuracy isstill possible for dual-frequency users, if the cycle-slips are detected in a reliable way. In high latitude, the size of the ionospheric irregularities is typically larger than the Fresnel scale. Therefore, the main effects are related with the fast change on the refractive index associated to the fast movement of the irregularities (which can reach up to several km/s). Consequently, t, Postprint (published version)

Ionospheric scintillation causes rapid fluctuations of measurements from Global Navigation Satellite Systems (GNSSs), thus threatening space-based communication and geolocation services. The phenomenon is most intense in equatorial regions, around the equinoxes and in maximum solar cycle conditions. Currently, ionospheric scintillation monitoring receivers (ISMRs) measure scintillation with high-pass filter algorithms involving high sampling rates, e.g. 50 Hz, and highly stable clocks, e.g. an ultra-low-noise Oven-Controlled Crystal Oscillator. The present paper evolves phase scintillation indices implemented in conventional geodetic receivers with sampling rates of 1 Hz and rapidly fluctuating clocks. The method is capable to mitigate ISMR artefacts that contaminate the readings of the state-of-the-art phase scintillation index. Our results agree in more than 99.9% within¿±¿0.05 rad (2 mm) of the ISMRs, with a data set of 8 days which include periods of moderate and strong scintillation. The discrepancies are clearly identified, being associated with data gaps and to cycle-slips in the carrier-phase tracking of ISMR that occur simultaneously with ionospheric scintillation. The technique opens the door to use huge databases available from the International GNSS Service and other centres for scintillation studies. This involves GNSS measurements from hundreds of worldwide-distributed geodetic receivers over more than one Solar Cycle. This overcomes the current limitations of scintillation studies using ISMRs, as only a few tens of ISMRs are available and their data are provided just for short periods of time., Peer Reviewed, Postprint (published version)

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 research of this dissertation is studying the ionosphere by GPS radio occultation (RO) techniques. It is mainly divided into two parts. The first part focuses on the methodology in electron density profile retrievals from RO measurements. It aims to get more precise profiles from measured data. Classic Abel inversion is a methodology widely used in RO retrievals, and the error introduced by the spherical symmetry assumption is also well recognised. Separability Method (SM) was developed to eliminate this error in previous studies. In this work, the improvement brought by SM corresponding to classic method is checked and validated. The SM does have better performance excluding the Lack of Collocation (LoC) error. The precision of the results is also shown to depend on the accuracy of the supported GIMs, i.e., the more accurate GIMs are used, the better results can be obtained. The error in SM, introduced by the mis-modelling of using the Vertical Total Electron Content (VTEC) instead of ECLEO, the electron content below LEO height, is also checked. The result shows that it has only minor impact on the retrievals. The second part is the climatological study of topside ionosphere/bottomside plasmasphere based on the RO retrievals using SM, and aims to give a general picture of characteristics and features of these two regions in different solar periods, 2007 -- solar minimum, and 2014 – solar maximum. The empirical two-components models of topside profiles, STIP model and CPDH model, used to separate the ionospheric and plasmaspheric contribution to the VTEC measured from ground to global positioning system (GPS) satellite altitudes, are studied and validated. The conditions of applicability of the STIP model are also discussed. The same as other existing empirical models, it shows the picture of topside ionosphere till some limited altitude, which is decided by the Low Earth Orbit (LEO) satellite height used to observe the RO. The model is used to derive transitio, La recerca feta en aquesta dissertació consisteix en l'estudi de la ionosfera mitjançant tècniques d'Ocultació Ràdio (RO) de GPS. La primera part s'enfoca en la metodologia d'extracció de perfils de les mesures RO. L'objectiu és l'obtenció de perfils més precisos mesurats a partir de les dades. La inversió clàssica d'Abel és una metodologia emprada àmpliament en l'obtenció de RO, tant mateix l'error introduït per l'assumpció de simetria també és reconegut. En ordre d'eliminar aquest error en estudis previs el Separability Method (SM) va ser desenvolupat. En el present treball es revisa i es valida la millora en el mètode clàssic aportada per SM. SM té un millor comportament inclús quan s'exclou l'error de Lack of Collocation (CoL). La precisió dels resultats depenen també de l'exactitud del GIM's suportats (p.ex. Com més exacte són els GIM's emprats millor són els resultats obtinguts). L'error de SM introduït pel modelatge incorrecte en utilitzar el Vertical Total Electron Content (VTEC) en lloc del ECLEO (el contingut d'electrons per sota de l'altura LEO) també és revisat. Els resultats mostren que només tenen un impacte mínim en les mesures extretes de RO. La segona part és un estudi de la climatologia de la part superior de la ionosfera I la part inferior de la plasmaesfera basat en l'extracció RO mitjançant SM amb l'objectiu de donar una imatge general de les característiques I tres d'aquestes dues regions en diferents períodes solars; el mínim solar de 2007 i el màxim solar de 2014. Dos models empírics bi-component, el model STIP i el model CPD, utilitzats per separar la contribució de la ionosfera i la plasmaesfera al VTEC, s?han estudiat i validat. Addicionalment també es discuteix sobre les condicions de l'aplicació de STIP. També es fa el mateix amb altres models empírics. Es mostra la imatge de la part superior de la ionosfera fins a un límit definit d'altitud el qual és determinat per l'altitud de satèl·lit Low Earth Orbit (LEO) que s?ha utilitzat per obs, Postprint (published version)

Ionospheric anomalies, like large ionospheric gradients, might produce a difference between the ionospheric error experienced bythe Ground Based Augmentation System (GBAS) reference station and the aircraft on approach. This ionospheric delay differencecould lead to hazardous position errors if undetected.For that reason, the GBAS Approach Service Types (GAST) C and D provide solutions against this threat, but the methodsemployed still face challenges by limiting the availability in certain cases, especially in regions with severe ionospheric conditions.This issue is caused by the use of very conservative ionospheric threat models derived based on the worst-ever-experiencedionospheric gradients in the relevant region. However, these worst-case gradients occur very rarely.Therefore, this paper proposes a methodology capable of detecting ionospheric gradients in real-time and estimating theirparameters in near real-time by using a wide area network of dual-frequency and multi-constellation GNSS monitoring stations.Hence, the GBAS stations could use this information to update the threat model currently applied in their algorithms, which wouldresult in an improvement of the GBAS availability in regions where it is degraded.The detection and estimation algorithm is initially theoretically explained. Then, the performance of this algorithm is evaluatedwith simulated gradients and with a real gradient, utilizing for both the real measurements recorded by a reference networkin Alaska. The synthetic gradients are simulated over the nominal real measurements from this network and all the gradientparameters are modified within their ranges in the already existing threat models. In this way, we assess the performance of ouralgorithm by comparing the differences between the known simulated gradient parameters and the parameters estimated by ouralgorithm. Additionally, we also evaluate our algorithm with one real ionospheric gradient measured by the same network inAlaska to study, Peer Reviewed, Postprint (author's final draft)

Peer Reviewed, Postprint (author's final draft)

The Abel inversion method is the classic approach to retrieve electron density profiles from radio occultation measurements, assuming that the electron density is only dependent upon height. In the early 2000s, the total electron content (TEC)-aided inversion method was introduced, where horizontal gradients in the electron density distribution are taken into account by using information from an external model of total electron content (TEC). We show how the quality of the chosen external ionospheric model influences the radio occultation retrievals. Using a two-layer TEC model, the 68% percentiles of the global error distributions of retrieved critical frequency and bottomside ionospheric electron content for 2014 improve more than 30%, with regard to the classic Abel inversion results. In a well-sounded region like Europe, this improvement raises to about 40–45%, while 10% improvement is achieved using two-layer maps with regard to the retrievals using single-layer TEC maps. We point out that using the TEC to describe the horizontal gradient incurs an implicit mismodeling that, until now, has not been taken into account. A new technique is presented that, thanks to TEC modeling by means of two layers, can assess the impacts of such mismodeling. From this technique, a method to select the most accurate radio occultation retrievals is applied to demonstrate that the resulting errors in the retrieved critical frequencies for the European region are smaller than 7% in nearly 82% of the cases, very similar to the relative difference between measurements of two nearby ionosondes., Peer Reviewed, Postprint (published version)

Scintillation is one of the most challenging problems in Global Navigation Satellite Systems (GNSS) navigation. This phenomenon appears when the radio signal passes through ionospheric irregularities. These irregularities represent rapid changes on the refraction index and, depending on their size, they can produce also diffractive effects affecting the signal amplitude and, eventually producing cycle slips. In this work, we show that the scintillation effects on the GNSS signal are quite different in low and high latitudes. For low latitude receivers, the main effects, from the point of view of precise navigation, are the increase of the carrier phase noise (measured by s¿) and the fade on the signal intensity (measured by S4) that can produce cycle slips in the GNSS signal. With several examples, we show that the detection of these cycle slips is the most challenging problem for precise navigation, in such a way that, if these cycle slips are detected, precise navigation can be achieved in these regions under scintillation conditions. For high-latitude receivers the situation differs. In this region the size of the irregularities is typically larger than the Fresnel length, so the main effects are related with the fast change on the refractive index associated to the fast movement of the irregularities (which can reach velocities up to several km/s). Consequently, the main effect on the GNSS signals is a fast fluctuation of the carrier phase (large s¿), but with a moderate fade in the amplitude (moderate S4). Therefore, as shown through several examples, fluctuations at high-latitude usually do not produce cycle slips, being the effect quite limited on the ionosphere-free combination and, in general, precise navigation can be achieved also during strong scintillation conditions., 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)

This manuscript presents a method to identify the occurrence of Equatorial Plasma Bubbles (EPBs) with data gathered from receivers of Global Navigation Satellite System (GNSS). This method adapts a previously existing technique to detect Medium Scale Travelling Ionospheric Disturbances (MSTIDs), which focus on the 2nd time derivatives of total electron content estimated from GNSS signals (2DTEC). Results from this tool made possible to develop a comprehensive analysis of the characteristics of EPBs. Analyses of the probability of occurrence, effective time duration, depth of the depletion and total disturbance of the EPBs show their dependence on local time and season of the year at global scale within the latitude belt from 35°N to 35°S for the descending phase of solar cycle 23 and ascending phase of solar cycle 24, 2002–2014. These results made possible to build an EPBs model, bounded with the Solar Flux index, that simulates the probability of the number of EPBs and their characteristics expected for a representative day at given season and local time (LT). The model results provided insight into different important aspects: the maximum occurrence of bubbles take place near the equatorial anomaly crests, asymmetry between hemispheres and preferred longitudes with enhanced EPBs activity. Model output comparisons with independent observations confirmed its soundness., Postprint (published version)

This is a pre-print of an article published in GPS Solutions. The final authenticated version is available online at: https://doi.org/10.1007/s10291-018-0753-7. The study is funded by National Key Research and Development Program of China (2016YFB0501902), National Natural Science Foundation of China (41574025, 41574013, 41731069), Spanish Ministry of Science and Innovation project (CGL2015-66410-P), The Hong Kong RGC Joint Research Scheme (E-PolyU501/16) and State Key Laboratory of Geo-Information Engineering (SKLGIE2015-M-2-2)., The calibration errors on experimental slant total electron content (TEC) determined with global positioning system (GPS) observations is revisited. Instead of the analysis of the calibration errors on the carrier phase leveled to code ionospheric observable, we focus on the accuracy analysis of the undifferenced ambiguity-fixed carrier phase ionospheric observable determined from a global distribution of permanent receivers. The results achieved are: (1) using data from an entire month within the last solar cycle maximum, the undifferenced ambiguity-fixed carrier phase ionospheric observable is found to be over one order of magnitude more accurate than the carrier phase leveled to code ionospheric observable and the raw code ionospheric observable. The observation error of the undifferenced ambiguity-fixed carrier phase ionospheric observable ranges from 0.05 to 0.11 total electron content unit (TECU) while that of the carrier phase leveled to code and the raw code ionospheric observable is from 0.65 to 1.65 and 3.14 to 7.48 TECU, respectively. (2) The time-varying receiver differential code bias (DCB), which presents clear day boundary discontinuity and intra-day variability pattern, contributes the most part of the observation error. This contribution is assessed by the short-term stability of the between-receiver DCB, which ranges from 0.06 to 0.17 TECU in a single day. (3) The remaining part of the observation errors presents a sidereal time cycle pattern, indicating the effects of the multipath. Further, the magnitude of the remaining part implies that the code multipath effects are much reduced. (4) The intra-day variation of the between-receiver DCB of the collocated stations suggests that estimating DCBs as a daily constant can have a mis-modeling error of at least several tenths of 1 TECU., Peer Reviewed, Postprint (author's final draft)

This work reviews an ionospheric activity indicator useful for identifying disturbed periods affecting the performance of Global Navigation Satellite System (GNSS). This index is based in the Along Arc TEC Rate (AATR) and can be easily computed from dual-frequency GNSS measurements. The AATR indicator has been assessed over more than one Solar Cycle (2002–2017) involving about 140 receivers distributed world-wide. Results show that it is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity (i.e. DST or Ap), it is sensitive to the regional behaviour of the ionosphere and identifies specific effects on GNSS users. Moreover, from a devoted analysis of different Satellite Based Augmentation System (SBAS) performances in different ionospheric conditions, it follows that the AATR indicator is a very suitable mean to reveal whether SBAS service availability anomalies are linked to the ionosphere. On this account, the AATR indicator has been selected as the metric to characterise the ionosphere operational conditions in the frame of the European Space Agency activities on the European Geostationary Navigation Overlay System (EGNOS). The AATR index has been adopted as a standard tool by the International Civil Aviation Organization (ICAO) for joint ionospheric studies in SBAS. In this work we explain how the AATR is computed, paying special attention to the cycle-slip detection, which is one of the key issues in the AATR computation, not fully addressed in other indicators such as the Rate Of change of the TEC Index (ROTI). After this explanation we present some of the main conclusions about the ionospheric activity that can extracted from the AATR values during the above mentioned long-term study. These conclusions are: (a) the different spatial correlation related with the MOdified DIP (MODIP) which allows to clearly separate high, mid and low latitude regions, (b) the large spatial correlation in mid latitude, Postprint (published version)

A high-accuracy Global Ionosphere Model (GIM) is significant for precise positioning and navigating with the Global Navigation Satellite System (GNSS), as well as space weather applications. To obtain a precise GIM, it is critical to take both the ionospheric observable and mathematical model into consideration. In this contribution, the undifferenced ambiguity-fixed carrier-phase ionospheric observable is first determined from a global distribution of permanent receivers. Accuracy assessment with a co-located station experiment shows that the observational errors affecting the ambiguity-fixed carrier-phase ionospheric observables range from 0.10 to 0.35 Total Electron Content Units (TECUs, where 1 TECU =1016e-/m2 and corresponds to 0.162 m on the Global Positioning System, GPS L1 frequency), indicating that the ambiguity-fixed carrier-phase ionospheric observable is over one order of magnitude more accurate than the carrier-phase leveled-code one (from 1.21 to 3.77 TECUs). Second, to better model the structure of the ionosphere, a two-layer GIM has been built based on the above carrier-phase observable. Preliminary global accuracy evaluation demonstrates that the accuracy of the two-layer GIM is below 1 TECU and about 2 TECUs during low and high solar activity periods. Third, the single-frequency point positioning experiment is adopted to test the ionosphere mitigation effects of the GIMs. Positioning results demonstrate that the single-frequency positioning accuracy can be improved by more than 30% using the undifferenced ambiguity-fixed ionospheric observable-derived two-layer GIM, compared with that using the carrier-phase leveled-code ionospheric observable-based single-layer GIM., Peer Reviewed, Postprint (published version)

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The final publication is available at Springer via http://dx.doi.org/10.1007/s10291-017-0634-5, We address two main problems related to the receiver and satellite differential code biases (DCBs) determination. The first issue concerns the drifts and jumps experienced by the DCB determinations of the International GNSS Service (IGS) due to satellite constellation changes. A new alignment algorithm is introduced to remove these nonphysical effects, which is applicable in real time. The full-time series of 18 years of Global Positioning System (GPS) satellite DCBs, computed by IGS, are realigned using the proposed algorithm. The second problem concerns the assessment of the DCBs accuracy. The short- and long-term receiver and satellite DCB performances for the different Ionospheric Associate Analysis Centers (IAACs) are discussed. The results are compared with the determinations computed with the two-layer Fast Precise Point Positioning (Fast-PPP) ionospheric model, to assess how the geometric description of the ionosphere affects the DCB determination and to illustrate how the errors in the ionospheric model are transferred to the DCB estimates. Two different determinations of DCBs are considered: the values provided by the different IAACs and the values estimated using their pre-computed Global Ionospheric Maps (GIMs). The second determination provides a better characterization of DCBs accuracy, as it is confirmed when analyzing the DCB variations associated with the GPS Block-IIA satellites under eclipse conditions, observed mainly in the Fast-PPP DCB determinations. This study concludes that the accuracy of the IGS IAACs receiver DCBs is approximately 0.3–0.5 and 0.2 ns for the Fast-PPP. In the case of the satellite DCBs, these values are about 0.12–0.20 ns for IAACs and 0.07 ns for Fast-PPP., Peer Reviewed, Postprint (author's final draft)

Ionospheric scintillation produces strong disruptive effects on global navigation satellite system (GNSS) signals, ranging from degrading performances to rendering these signals useless for accurate navigation. The current paper presents a novel approach to detect scintillation on the GNSS signals based on its effect on the ionospheric-free combination of carrier phases, i.e. the standard combination of measurements used in precise point positioning (PPP). The method is implemented using actual data, thereby having both its feasibility and its usefulness assessed at the same time. The results identify the main effects of scintillation, which consist of an increased level of noise in the ionospheric-free combination of measurements and the introduction of cycle-slips into the signals. Also discussed is how mis-detected cycle-slips contaminate the rate of change of the total electron content index (ROTI) values, which is especially important for low-latitude receivers. By considering the effect of single jumps in the individual frequencies, the proposed method is able to isolate, over the combined signal, the frequency experiencing the cycle-slip. Moreover, because of the use of the ionospheric-free combination, the method captures the diffractive nature of the scintillation phenomena that, in the end, is the relevant effect on PPP. Finally, a new scintillation index is introduced that is associated with the degradation of the performance in navigation., Postprint (author's final draft)

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)

Fast Precise Point Positioning (Fast-PPP) provides Global Navigation Satellite System corrections in real-time. Satellite orbits and clock corrections are shown to be accurate to a few centimeters and a few tenths of a nanosecond which, together with the determination of the fractional part of the ambiguities, enable global high-accuracy positioning with undifferenced Integer Ambiguity Resolution. The new global ionospheric model is shown to provide corrections accurate at the level of 1 Total Electron Content Unit over well-sounded areas and Differential Code Biases at the level of tenths of a nanosecond. These corrections are assessed with permanent receivers, treated as rovers, located at 100 to 800 kilometers from the reference stations of the ionospheric model. Fast-PPP achieves decimeter-level of accuracy after few minutes, several times faster than single- and dual-frequency ionospheric-free solutions, using a month of Global Positioning System data close to the last Solar Maximum and including equatorial rovers., Postprint (author's final draft)

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)

© 2015, Springer-Verlag Berlin Heidelberg. The characterization of the accuracy of ionospheric models currently used in global navigation satellite systems (GNSSs) is a long-standing issue. The characterization remains a challenging problem owing to the lack of sufficiently accurate slant ionospheric determinations to be used as a reference. The present study proposes a methodology based on the comparison of the predictions of any ionospheric model with actual unambiguous carrier-phase measurements from a global distribution of permanent receivers. The differences are separated as hardware delays (a receiver constant plus a satellite constant) per day. The present study was conducted for the entire year of 2014, i.e. during the last solar cycle maximum. The ionospheric models assessed are the operational models broadcast by the global positioning system (GPS) and Galileo constellations, the satellite-based augmentation system (SBAS) (i.e. European Geostationary Navigation Overlay System (EGNOS) and wide area augmentation system (WAAS)), a number of post-process global ionospheric maps (GIMs) from different International GNSS Service (IGS) analysis centres (ACs) and, finally, a more sophisticated GIM computed by the research group of Astronomy and GEomatics (gAGE). Ionospheric models based on GNSS data and represented on a grid (IGS GIMs or SBAS) correct about 85 % of the total slant ionospheric delay, whereas the models broadcasted in the navigation messages of GPS and Galileo only account for about 70 %. Our gAGE GIM is shown to correct 95 % of the delay. The proposed methodology appears to be a useful tool to improve current ionospheric models., 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)

Preprint

We introduce a methodology to extract the separate contributions of the ionosphere and the plasmasphere to the vertical total electron content, without relying on a fixed altitude to perform that separation. The method combines two previously developed and tested techniques, namely, the retrieval of electron density profiles from radio occultations using an improved Abel inversion technique and a two-component model for the topside ionosphere plus protonosphere. Taking measurements of the total electron content from global ionospheric maps and radio occultations from the Constellation Observing System for Meteorology, Ionosphere, and Climate/FORMOSAT-3 constellation, the ionospheric and plasmaspheric electron contents are calculated for a sample of observations covering 2007, a period of low solar and geomagnetic activity. The results obtained are shown to be consistent with previous studies for the last solar minimum period and with model calculations, confirming the reversal of the winter anomaly, the hemispheric asymmetry of the semiannual anomaly, and the existence in the plasmasphere of an annual anomaly in the South American sector of longitudes. The analysis of the respective fractional contributions from the ionosphere and the plasmasphere to the total electron content shows quantitatively that during the night the plasmasphere makes the largest contribution, peaking just before sunrise and during winter. On the other hand, the fractional contribution from the ionosphere reaches a maximum value around noon, which is nearly independent of season and geomagnetic latitude., Peer Reviewed, Postprint (published version)

Peer Reviewed, Postprint (author's final draft)

Ground Based Augmentation System (GBAS) is a version of Differential Global Positioning System (DGPS) designed to reliably support airplane's all phase of approach, landing, departure and surface operations. Local ionospheric gradients are major threats to the GBAS, which prevents the advanced use of GBAS. A new monitor for detecting anomalous ionosphere to provide integrity assurance is implemented in this thesis. The monitor is based on the "Fast Initial Response" Cumulative Sum (FIR-CUSUM) quality-assurance method, which is sensitive to the small persistent shifts.

Postprint (published version)

In this project, the study of a methodology for the computation of a reliable estimation sigma_pr_gnd is study. The method presented is intended to be used for the calibration of the GBAS station, using only one day of data. The GBAS station is used for aircraft navigation in the vicinity of an airport, being used for the precision approaches and landing of the aircraft.

This work introduces a novel ionospheric activity indicator useful for identifying disturbed periods affecting performance for GNSS users, at regional level. This indicator is based in the “Along Arc TEC Rate (AATR) and can be easily computed from GNSS data. The AATR indicator has been assessed over more than one Solar Cycle (2002-2013) involving 140 receivers distributed world-wide. Results show that it is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity (i.e. DST, Ap), it is sensitive to regional behaviour the ionosphere and identifies specific effects on GNSS users. Moreover from a devoted analysis of EGNOS performances in different ionospheric conditions, it follows that the AATR indicator is able to predict SBAS user availability anomalies linked to the ionosphere. The AATR indicator has been chosen as the metric to characterise the ionosphere operational conditions in the frame of EGNOS activities. This indicator has been also proposed for joint analysis in the International SBAS-Ionosphere Working Group., Peer Reviewed, Postprint (published version)

In a recent paper, Salvador-Solé et al. have derived the typical inner structure of dark matter haloes from that of peaks in the initial random Gaussian density field, determined by the power spectrum of density perturbations characterizing the hierarchical cosmology under consideration. In this paper, we extend this formalism to the typical kinematics and triaxial shape of haloes. Specifically, we establish the link between such halo properties and the power spectrum of density perturbations through the typical shape of peaks. The trends of the predicted typical halo shape, pseudo-phase-space density and anisotropy profiles are in good agreement with the results of numerical simulations. Our model sheds light on the origin of the power-law-like pseudo-phase-space density profile for virialized haloes., Peer Reviewed, Postprint (published version)

In the three preceding papers in the series, we presented a model dealing with the global and small-scale structure and kinematics of hierarchically assembled, virialised, collisionless systems, which correctly accounted for the typical properties of simulated cold darkmatter (CDM) haloes. This model relied, however, on the spherical symmetry assumption. Here we show that the foundations of the model hold equally well for triaxial systems and extend it in a fully accurate way to objects that satisfy the latter more general symmetry. The master equations in the new version take the same form as in the version for spherically symmetric objects, but the profiles of all the physical quantities are replaced by their respective spherical averages. All the consequences of the model drawn under the spherical symmetry assumption continue to hold. In addition, the new version allows one to infer the axial ratios of virialised ellipsoids from those of the corresponding protoobjects. The present results generalise and validate those obtained in Papers I, II and III for CDM haloes. In particular, they confirm that all halo properties are the natural consequence of haloes evolving through accretion and major mergers from triaxial peaks (secondary maxima) in the primordial density field., Preprint

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A comparative study of the evolution of the Sun–Jupiter–Asteroid system near the 4:1 and 7:2 resonances is performed by means of two techniques that proceed differently from the Hamiltonian corresponding to the planar restricted elliptic three-body problem. One technique is based on the classical Schubart averaging while the other is based on a mapping method in which the perturbing part of the Hamiltonian is expanded and the resulting terms are ordered according to a weight function that depends on the powers of eccentricities and the coefficients of the terms. For the mapping method the effect of Saturn on the asteroidal evolution is introduced and the degree of chaos is estimated by means of the Lyapunov time. Both methods are shown to lead to similar results and can be considered a suitable tool for describing the evolution of asteroids in the Kirkwood gap and the group corresponding to the 4:1 and 7:2 Jovian resonances, respectively., Peer Reviewed, Postprint (published version)