References of "Wautelet, Gilles"
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See detailTropospheric jet stream as a source of traveling ionospheric disturbances observed by GPS
Wautelet, Gilles ULg; Warnant, René ULg

Poster (2014, May 02)

The integrity and the reliability of real-time precise positioning applications with Global Positioning System (GPS) are affected by the ionospheric variability with time and space. As a consequence ... [more ▼]

The integrity and the reliability of real-time precise positioning applications with Global Positioning System (GPS) are affected by the ionospheric variability with time and space. As a consequence, scientific community aims at describing, explaining and forecasting the occurrence and the amplitude of ionospheric irregularities observed by GPS. The use of the geometric-free combination of GPS dual frequency signals allows to retrieve the Total Electron Content (TEC) along the satellite-to-receiver path, which is the basic trans-ionospheric observable. Based on L1/L2 GPS phase measurements collected at a given station, the TEC high-frequency variability is isolated. A climatological study performed over 10 years in Western Europe shows that TEC irregularities are mostly observed daytime during quiet geomagnetic background in autumn and winter and correspond to classical Medium-Scale Traveling Ionospheric Disturbances (MSTIDs). The latter are generally understood as the ionospheric signature of Atmospheric Gravity Waves (AGWs), either generated in situ (solar terminator) or in the lower atmosphere and propagating upward. Because of its associated strong wind shears, the tropospheric jetstream, occurring mainly during autumn and winter months, constitutes an ideal candidate for AGW generation. This paper analyzes the spatial correlation between the presence of both MSTIDs and strong jetstream over Western Europe. This correlation is positive when the ionospheric pierce point of the satellite is located above regions of interest where wind shears are very large. In practice, we have selected regions for which wind speed is larger than 50 m/s. In addition, the propagation of AGWs up to the ionospheric layer is taken into account by assuming horizontal and vertical velocities of 200 and 50 m/s respectively. It comes that the region of interest of the correlation study is computed using an isotropic slant propagation of the AGW, which is supposed to be generated at a tropospheric level.Based on 30s GPS data collected over several stations in Belgium and on European Centre for Medium-Range Weather Forecasts (ECMWF) wind velocity maps, the correlation study covers a period ranging from January 2002 to December 2011. Preliminary results based on a limited number of cases show that large amplitude MSTIDs are generally observed during periods of strong wind speeds at an altitude corresponding to a pressure level of 250hPa (about 10 km). [less ▲]

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See detailClimatological study of ionospheric irregularities over the European mid-latitude sector with GPS
Wautelet, Gilles ULg; Warnant, René ULg

in Journal of Geodesy (2014), 88(3), 223-240

High-frequency variability of the ionosphere, or irregularities, constitutes the main threat for real-time precise positioning techniques based on Global Navigation Satellite Systems (GNSS) measurements ... [more ▼]

High-frequency variability of the ionosphere, or irregularities, constitutes the main threat for real-time precise positioning techniques based on Global Navigation Satellite Systems (GNSS) measurements. Indeed, during periods of enhanced ionospheric variability, GNSS users in the field – who cannot verify the integrity of their measurements – will experience positioning errors that can reach several decimeters, while the nominal accuracy of the technique is cm-level. In the frame of this paper, a climatological analysis of irregularities over the European mid-latitude region is presented. Based on a ten year GPS dataset over Belgium, the work analyzes the occurrence rate (as a function of the solar cycle, season and local time) as well as the amplitude of ionospheric irregularities observed at a single GPS station. The study covers irregularities either due to space weather events (solar origin) or of terrestrial origin. If space weather irregularities are responsible for the largest effects in terms of ionospheric error, their occurrence rate highly depends on solar activity. Indeed, the occurrence rate of ionospheric irregularities is about 9% during solar maximum, whereas it drops to about 0% during medium or low solar activity periods. Medium-Scale Ionospheric Disturbances (MSTIDs) occurring during daytime in autumn/winter are the most recurrent pattern of the time series, with yearly proportions slightly varying with the solar cycle and an amplitude of about 10% of the TEC background. Another recurrent irregularity type, though less frequent than MSTIDs, is the noise-like variability in TEC observed during summer nighttime, under quiet geomagnetic conditions. These summer nighttime irregularities exhibit amplitudes ranging between 8 and 15% of the TEC background. [less ▲]

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See detailGPS et topographie : erreurs, précision et perspectives futures
Wautelet, Gilles ULg

Conference given outside the academic context (2013)

En topographie moderne, le système GPS (Global Positioning System) permet de mesurer sa position en temps réel avec une précision centimétrique. Ce niveau de précision est atteint grâce à l'utilisation de ... [more ▼]

En topographie moderne, le système GPS (Global Positioning System) permet de mesurer sa position en temps réel avec une précision centimétrique. Ce niveau de précision est atteint grâce à l'utilisation de la technique appelée Real-Time Kinematics (RTK), qui se base sur un réseau de stations permanentes. La précision de la position calculée par RTK varie avec les conditions géométriques de la constellation GPS et dépend également de différentes sources d'erreurs, comme l'effet de l'atmosphère terrestre. Après un bref rappel théorique sur le fonctionnement du GPS, ces différentes sources d'erreur sont présentées, ainsi que leur impact sur le positionnement RTK. Enfin, l'avenir du RTK est évoqué avec, notamment, l'apport de la future constellation européenne Galileo. [less ▲]

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See detailLa précision dans les mesures RTK
Wautelet, Gilles ULg

Conference given outside the academic context (2013)

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See detailÉlements affectant la précision du GPS RTK
Wautelet, Gilles ULg

Conference given outside the academic context (2013)

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See detailCharacterization of ionospheric irregularities and their influence on high-accuracy positioning with GPS over mid-latitudes
Wautelet, Gilles ULg

Doctoral thesis (2013)

Over the last decade came major breakthroughs in satellite navigation and positioning, due to the development of precise positioning techniques based on Global Positioning System (GPS) signals. Modern ... [more ▼]

Over the last decade came major breakthroughs in satellite navigation and positioning, due to the development of precise positioning techniques based on Global Positioning System (GPS) signals. Modern processing methods, such as the Real-Time Kinematics (RTK), allow the GPS user to determine its position in real-time with an accuracy of a few centimeters. The success of these algorithms relies on the cancellation and/or the mitigation of the various errors affecting signal propagation. Among these errors is the delay due to the ionospheric refraction. More particularly, the presence of irregularities in the ionospheric plasma is responsible for positioning errors reaching several (deci)meters. As a result, there is a growing demand from GPS user communities (such as land surveyors or civil engineers) to be informed, if possible in advance, of the occurrence of irregularities that might impact on their positioning solution. Based on a ten years GPS dataset collected over Belgium, this thesis aims at assessing, understanding and modeling the occurrence of ionospheric irregularities as well as estimating their effects in terms of positioning accuracy. Firstly, we carry out a climatological study of irregularities to identify and characterize the most recurrent features. We can distinguish two main irregularity types: those due to space weather events (such as Coronal Mass Ejections – CMEs – or solar flares) and the others, constituting the bulk of irregularities observed at a single station and referred to as “quiet-time” irregularities, as they occur during quiet geomagnetic conditions. These latter are then divided into two groups: the first is made up of Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) which occur during autumn/winter daytime. In the second group are the summer nighttime irregularities, which are rapid fluctuations of the Total Electron Content (TEC) probably associated with spread-F phenomenon. Next, we develop a model of quiet-time irregularity occurrence, based on a statistical analysis of the aforementioned dataset. Computations rely on several mathematical modeling tools, such as the Principal Component Analysis (PCA), the Generalized Least-Squares (GLS) algorithm and the AutoRegressive and Moving Average (ARMA) method. The resulting climatological model is made up of two components reproducing daily profile as well as secular variations of the ionospheric variability for a typical GPS station in Belgium. The last part of this work deals with the impact of irregularities on relative positioning. This technique allows the measurement of the vector (called baseline) joining the receiver (user station) to a reference station whose position is accurately known. More precisely, we assess the effect of baseline length and orientation during the occurrence of MSTIDs and geomagnetic storms through the processing of the Belgian Dense Network, made up of 66 dual-frequency GPS stations. Finally, the relationship between positioning error and the presence of ionospheric irregularities detected at a single station is investigated, filling the gap between the scientific and the GPS-user communities. [less ▲]

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See detailMonitoring, tracking and forecasting ionospheric perturbations using GNSS techniques
Jakowski, Norbert; Béniguel, Yannick; De Franceschi, Giorgiana et al

in Journal of Space Weather and Space Climate (2012), 2(A22),

The paper reviews the current state of GNSS-based detection, monitoring and forecasting of ionospheric perturbations in Europe in relation to the COST action ES0803 ‘‘Developing Space Weather Products and ... [more ▼]

The paper reviews the current state of GNSS-based detection, monitoring and forecasting of ionospheric perturbations in Europe in relation to the COST action ES0803 ‘‘Developing Space Weather Products and Services in Europe’’. Space weather research and related ionospheric studies require broad international collaboration in sharing databases, developing analysis software and models and providing services. Reviewed is the European GNSS data basis including ionospheric services providing derived data products such as the Total Electron Content (TEC) and radio scintillation indices. Fundamental ionospheric perturbation phenomena covering quite different scales in time and space are discussed in the light of recent achievements in GNSS-based ionospheric monitoring. Thus, large-scale perturbation processes characterized by moving ionization fronts, wave-like travelling ionospheric disturbances and finally small-scale irregularities causing radio scintillations are considered. Whereas ground and space-based GNSS monitoring techniques are well developed, forecasting of ionospheric perturbations needs much more work to become attractive for users who might be interested in condensed information on the perturbation degree of the ionosphere by robust indices. Finally, we have briefly presented a few samples illustrating the space weather impact on GNSS applications thus encouraging the scientific community to enhance space weather research in upcoming years. [less ▲]

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See detailLocal climatological modeling of ionospheric irregularities detected by GPS in mid-latitude region
Wautelet, Gilles ULg; Warnant, René ULg

in Journal of Atmospheric & Solar-Terrestrial Physics (2012), 89

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See detailMonitoring the ionospheric activity using GNSS. From dual frequency GPS to multi-constellation multi-frequency GNSS
Warnant, René ULg; Bidaine, Benoît; Lonchay, Matthieu ULg et al

Scientific conference (2012, June 20)

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See detailIonospheric effects on relative positioning within GPS dense network
Lejeune, Sandrine; Wautelet, Gilles ULg; Warnant, René ULg

in GPS Solutions (2012), 16(1), 105-116

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See detailOn the impact of ionospheric variability and disturbances on GNSS-based positioning applications
Stankov, Stanimir; Wautelet, Gilles ULg; Lejeune, Sandrine et al

in Geophysical Research Abstracts (2011, April), 13(ABS. No EGU2011-12067),

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See detailA GPS/GNSS dense network used to monitor ionospheric positioning error
Wautelet, Gilles ULg; Lejeune, Sandrine; Warnant, René ULg

Poster (2010, December)

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See detailThe RMI Space Weather and Navigation Systems (SWANS) project
Warnant, René ULg; Lejeune, Sandrine; Wautelet, Gilles ULg et al

Conference (2010, July)

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See detailUnderstanding the occurrence of mid-latitude ionospheric irregularities by using GPS, ionosonde and geomagnetic measurements
Wautelet, Gilles ULg; Lejeune, Sandrine; Stankov, Stan et al

Conference (2010, June 11)

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See detailOn the local operational geomagnetic index K calculation
Stankov, Stan; Stegen, Koen; Wautelet, Gilles ULg et al

Poster (2010, May 06)

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See detailMonitoring the ionospheric positioning error with a GNSS dense network
Wautelet, Gilles ULg; Lejeune, Sandrine; Warnant, René ULg

Conference (2010, May 06)

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See detailInfluence of geometry in the case of relative positioning with GNSS
Wautelet, Gilles ULg; Lejeune, Sandrine; Lonchay, Matthieu ULg et al

Poster (2009, November 18)

Relative positioning with GNSS is generally used to achieve precise positions in the frame of critical applications (surveying, photo-control...). On this basis, we have developed a software which allows ... [more ▼]

Relative positioning with GNSS is generally used to achieve precise positions in the frame of critical applications (surveying, photo-control...). On this basis, we have developed a software which allows to compute a positioning error due to the ionosphere only using reference stations belonging to the Belgian Dense Network (BDN). This network consists in 66 GPS (dual-frequency) receivers over the whole Belgium. The drawback of this method is that this computation needs the design matrix which contains coefficients depending on satellite constellation geometry. Therefore, like for absolute positioning, a poor geometry (evaluated by the Dilution of Precision, or DOP) can also lead to large positioning error that cannot be separated from the one due to ionospheric effects, and in particular the small-scale structures. The main goal of this paper is to build a similar index to DOP for relative positioning in our software to be able to separate the ionospheric effects from the geometric ones. The final step is to study the feasability of a service for users of relative positioning using the BDN. The objective is to give in post-processing the positioning accuracy degradation for all BDN baselines and to associate a colour scheme to the different degradation classes created. [less ▲]

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See detailEffects of ionospheric small-scale structures on GNSS
Wautelet, Gilles ULg; Lejeune, Sandrine; Warnant, René ULg

in The Institution of Engineering and Technology 11th International Conference on Ionospheric Radio Systems and Techniques (IRST 2009) (2009, April)

Nowadays, Global Navigation Satellite Systems or GNSS allow to measure positions in real-time with an accuracy ranging from a few meters to a few centimeters mainly depending on the type of observable ... [more ▼]

Nowadays, Global Navigation Satellite Systems or GNSS allow to measure positions in real-time with an accuracy ranging from a few meters to a few centimeters mainly depending on the type of observable (code or phase measurements) and on the positioning mode used (absolute or differential). The best precisions can be reached in differential mode using phase measurements. In differential mode, mobile users improve their positioning precision thanks to so-called “differential corrections” provided by a fixed reference station. For example, the Real-Time Kinematic technique (RTK) allows to measure positions in real-time with a precision usually better than a decimeter. In practice, the ionospheric effects on GNSS radio signals remain the main factor which limits the precision and the reliability of real-time differential positioning. As differential applications are based on the assumption that the measurements made by the reference station and by the mobile user are affected in the same way by ionospheric effects, these applications are influenced by gradients in TEC between the reference station and the user. For this reason, local variability in the ionospheric plasma can be the origin of strong degradations in positioning precision. In this paper, we characterize local variability in the ionosphere which can pose a threat to high precision real-time differential positioning. GNSS carrier phase measurements can be used to monitor local TEC variability: small-scale ionospheric structures can be detected by monitoring TEC high frequency changes at a single station; as ionospheric disturbances are moving, we can expect that such structures will induce TEC temporal variability which can be detected at a single station. We applied this method (called the “one-station” method) to the GPS data collected at the permanent (mid-latitude) station of Brussels from 1994 to 2007 and performed a climatological study of the ionospheric structures on this period which covers more than one solar cycle. Two main types of structures have been observed: Traveling Ionospheric Disturbances (TID’s) and “noise-like” structures. TID’s have strong seasonal and solar cycle dependence when noise-like structures are “ionospheric variability” which is usually observed during geomagnetic storms. The largest Rate of TEC (RoTEC) detected at Brussels during the period considered in our study were observed during severe geomagnetic storms. Moreover, we found that strong irregularities occur even during solar minimum. This means that, even during periods where the probability of occurrence of ionospheric irregularities is very low, large RoTEC can occur. The one-station method allows to measure variability in time but GNSS differential applications are affected by variability in space between the user and the reference station. Therefore, in a second step, we measured TEC differential variability (using double differences of phase measurements) during few typical ionospheric conditions: quiet ionospheric activity, medium and large amplitude TID’s and noise-like variability due to a severe geomagnetic storm. We also analyzed the effects of the baseline length and orientation on the residual ionospheric term. As a last step, we developed a software which reproduces positioning conditions experienced by RTK users on the field. We used this software to assess positioning errors due to the different ionospheric conditions considered in the previous step. Again, the largest effects were observed during the occurrence of geomagnetic storms. [less ▲]

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