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See detailThe use of GNSS in geoscience
Wautelet, Gilles ULiege

Scientific conference (2019, February 04)

This presentation focuses on the use of Global navigation Satellite Systems (GNSS) in the frame of geoscience. From precise positioning to reflectometry, it also covers the study of the terrestrial ... [more ▼]

This presentation focuses on the use of Global navigation Satellite Systems (GNSS) in the frame of geoscience. From precise positioning to reflectometry, it also covers the study of the terrestrial ionosphere and plasmasphere from ground and space-based GNSS measurements. [less ▲]

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See detailAnalytic methods for the Abel transform of exponential functions describing planetary and cometary atmospheres.
Hubert, Benoît ULiege; Munhoven, Guy ULiege; Opitom, Cyrielle et al

Poster (2018, December 11)

Remote sensing of planetary and cometary atmosphere is one of the most important source of data and knowledge of the gas layers surrounding the celestial objects of our solar system, including our own ... [more ▼]

Remote sensing of planetary and cometary atmosphere is one of the most important source of data and knowledge of the gas layers surrounding the celestial objects of our solar system, including our own planet. Most of the instruments used up to now and that will be used in a near future study the emission of radiations directly produced by the atmosphere. Under optically thin conditions, this observation method provides the local volume emission rate (VER) originating from the atmosphere, integrated along the full line of sight (l.o.s.) of the instrument. Under a spherical or cylindrical symmetry assumption, the l.o.s. integration of the VER takes the form of the Abel transform of the vertical VER profile. The simplest analytical functions representing VER profiles in real planetary and cometary atmosphere include an exponential function of the altitude (or radial distance), giving the isothermal profile for a planet and the Haser model for a coma. The Abel transform of these functions can be computed analytically using combinations of special functions. Retrieving the vertical (radial) profile of the VER does however require to invert the observed Abel transform to account for possible departures from these idealized analytical expressions, so that indefinite integrals defined from the Abel integral (which we will call indefinite Abel transforms) are needed (or numerical integrations need to be performed). In this study, we present a new method to produce a workable series development allowing accurate computation of the indefinite Abel transforms that appear in the study of optically thin emissions of planetary and cometary atmospheres. Indeed, taking the Taylor series development of the exponential function to reduce the problem to a series of indefinite Abel transforms of polynomial functions (which can be carried analytically) does not work. It leads to the computation of the difference of large, nearly equal numbers, which cannot be done accurately. Our method rather relies on an appropriate series development of the Jacobian of the Abel transform. We show that the computation can be done reliably up to near machine precision, and that accuracy control can be enforced for tailored applications. Possible applications are considered, that include the study of comas and of the upper atmosphere of Mars and the Earth [less ▲]

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See detailSimulating the detection of traveling ionospheric disturbances with the ICON mission
Wautelet, Gilles ULiege; Hubert, Benoît ULiege; Gérard, Jean-Claude ULiege

Conference (2018, April 11)

Traveling Ionospheric Disturbances (TIDs) are of the utmost importance in energy and momentum transfer from the lower atmosphere to the ionosphere. The upcoming NASA’s ICON mission will address these ... [more ▼]

Traveling Ionospheric Disturbances (TIDs) are of the utmost importance in energy and momentum transfer from the lower atmosphere to the ionosphere. The upcoming NASA’s ICON mission will address these topics by performing remote sensing of ion and electron density, velocity and temperature from the bottom of the ionosphere up to the altitude of the spacecraft. More precisely, the ICON Far UltraViolet (FUV) instrument will image the ionospheric limb in two wavelength channels: the first one is dedicated to atomic oxygen and detects its emission at 135.6 nm. The second one studies the N2 Lyman-Birge-Hopfield (LBH) band around 155 nm. With an inclination of 27° and a circular orbit at an altitude of 550 km, the ICON mission will focus on low-latitudes only. Using ICON/FUV data, TID detection can be performed following two different approaches. The first possibility makes use of raw measurements (level-1) of the limb, corresponding to the line-of-sight integrated values of the O+ ion density. The second option consists in analyzing vertical profiles of the O+ density (level-2 product) derived from the inverse Abel transform of level-1 data. In this study, we simulate integrated O+ emission based on a background ionosphere provided by IRI-2016 on which we superimpose a TID of known characteristics: wavelength, period and velocity. This work investigates the retrieval of TID characteristics with algorithms using either level-1 or level-2 data. Given that the assumed spherical symmetry used in inverse Abel transform is rarely met in low-latitude regions, TID detection using ICON/FUV could prove to be more reliable using line-of-sight integrated values directly provided by the imager rather than using the inverted O+ profiles. We investigate the advantages and drawbacks of both above-mentioned methods in detecting TIDs and untangle possible ambiguities that may arise from the spherical symmetry hypothesis. [less ▲]

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See detailComputation of GPS P1–P2 Differential Code Biases with JASON-2
Wautelet, Gilles ULiege; Loyer, Sylvain; Mercier, Flavien et al

in GPS Solutions (2017)

GPS Differential Code Biases (DCBs) computation is usually based on ground networks of permanent stations. The drawback of the classical methods is the need for the ionospheric delay so that any error in ... [more ▼]

GPS Differential Code Biases (DCBs) computation is usually based on ground networks of permanent stations. The drawback of the classical methods is the need for the ionospheric delay so that any error in this quantity will map into the solution. Nowadays, many low-orbiting satellites are equipped with GPS receivers which are initially used for precise orbitography. Considering spacecrafts at an altitude above the ionosphere, the ionized contribution comes from the plasmasphere, which is less variable in time and space. Based on GPS data collected onboard JASON-2 spacecraft, we present a methodology which computes in the same adjustment the satellite and receiver DCBs in addition to the plasmaspheric vertical total electron content (VTEC) above the satellite, the average satellite bias being set to zero. Results show that GPS satellite DCB solutions are very close to those of the IGS analysis centers using ground measurements. However, the receiver DCB and VTEC are closely correlated, and their value remains sensitive to the choice of the plasmaspheric parametrization. [less ▲]

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See detailMapping and investigating phase anomalies in GPS data onboard Low Earth Orbiters
Wautelet, Gilles ULiege; Bruinsma, Sean; Loyer, Sylvain et al

Poster (2016, June 28)

To face important societal challenges like sea level variations, climate change and natural hazards management (tsunami detection, earthquakes, crustal deformations…), modern science rely more and more on ... [more ▼]

To face important societal challenges like sea level variations, climate change and natural hazards management (tsunami detection, earthquakes, crustal deformations…), modern science rely more and more on precise geodesy. Precise Orbit Determination (POD) is of major concern in the frame of altimetry or gravity recovery missions like GOCE or GRACE. Using the GPS receiver onboard, orbits at cm-level accuracy are generally achieved in both kinematic and reduced-dynamic approaches using dual frequency code and phase measurements. GPS data processing generally uses the Ionospheric-Free (IF) combination to get rid of the ionospheric delay, which is varying with the season, latitude, local time and solar activity. However, large discrepancies in the orbit determination are still observed over polar and equatorial regions, which turn into artefacts and errors in the derived scientific products (gravity field, sea surface height…). More precisely, large RMS values are strongly correlated to phase anomalies occurring on GPS receivers: cycle slips, data unavailability or enhanced measurement noise, especially on L2 signal. Phase anomalies are generally observed when the satellite orbit crosses regions where ionospheric scintillations occur, which are defined as rapid fluctuations in phase and amplitude of the GNSS signals. The occurrence of scintillations exhibits large day-to-day variations and depends mainly on geomagnetic latitude, season and local time. At low latitudes, maximum occurrence of scintillations is observed 15-20° on either side of the geomagnetic equator. Scintillations also occur at auroral and polar latitudes, where their intensity increases with increasing geomagnetic activity. This paper aims at detecting, mapping and understanding the phase anomalies experienced by LEO satellites and analyzing their correlation with geomagnetic activity, latitude, season and local time. Several LEO satellites at different altitudes are analyzed (e.g. SWARM, GRACE or JASON), which allows a multi-layer analysis of the underlying ionospheric phenomenon, including scintillation. The latter are generally measured with several indices, like the amplitude index S4 or the phase index SigmaPhi (σφ), which are usually derived from 100Hz measurements performed by dedicated scintillation monitors. In this study, we compute a similar index (called pseudo-σφ) using GPS phase data at 1Hz coming from POD GNSS antenna. A detailed study of the occurrence rate and the severity of pseudo-σφ, together with cycle slips and other spurious phase data, will be performed for different LEO satellites. [less ▲]

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See detailMonitoring the ionosphere using new GNSS
Warnant, René ULiege; Deprez, Cécile ULiege; Wautelet, Gilles ULiege et al

Conference (2016, June 27)

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See detailInfluence of the ionospheric model on DCB computation and added value of LEO satellites
Wautelet, Gilles ULiege; Lestarquit, Laurent; Loyer, Sylvain et al

Poster (2016, April)

In order to compute inter-frequency Differential Code Biases (DCBs), the Geometry-Free combination of a GNSS signal pair needs to be corrected from the ionospheric refraction effect. Such information is ... [more ▼]

In order to compute inter-frequency Differential Code Biases (DCBs), the Geometry-Free combination of a GNSS signal pair needs to be corrected from the ionospheric refraction effect. Such information is obtained using either Global Ionospheric Maps (GIMs) or local models. In this work we investigate the influence of GIMs on the final value and precision of DCB solution. The study covers different ionospheric conditions, ranging from very quiet ionospheric background up to a severe ionospheric storm. In a first step, the Slant Total Electron Content (STEC) between GIMs is assessed as a function of receiver latitude, elevation mask and ionospheric conditions. Then, daily DCBs are estimated using these different GIMs, receiver and satellite contributions being separated using a zero-mean constraint. At last, an independent estimation of DCBs is performed using Low Earth Orbit (LEO) observations (such as JASON's GPS data). This solution is compared with our ground network solution and with DCBs coming from Analysis Centers (ACs) of the International GNSS Service providing ionospheric and DCB solutions. [less ▲]

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See detailReflectometry with an open-source Software GNSS Receiver. Use Cases with Carrier Phase Altimetry.
Lestarquit, Laurent; Peyrezabes, Mathieu; Darrozes, José et al

in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing (2016)

An open-source GNSS software receiver allows to have full access to the signal processing and to make add-ons to the source code in order to obtain the desired GNSS reflectometry processing. The direct ... [more ▼]

An open-source GNSS software receiver allows to have full access to the signal processing and to make add-ons to the source code in order to obtain the desired GNSS reflectometry processing. The direct signal is processed in the standard way, its tracking loops replica are tapped to have a robust processing of the reflected signal in a master-slave configuration, with the very same carrier replica used to correlate the reflected signal. Additionally, the data bit sign is wiped off, which allows to extend the coherent integration time (CIT) well beyond the usual 20 milliseconds limit on the reflected way. This allows having a straightforward and accurate measurement of the Amplitude Ratio and Differential Carrier Phase between the direct and reflected signal. The possible applications are precise carrier phase altimetry and any application requiring signal amplitude ratio, or reflected signal Delay Map, with single or dual polarization, this includes code altimetry, humidity, biomass, soil roughness, ocean surface wind and wave height, snow and ice characteristics retrieval. This software is intended to be used as a research tool. It has been tested for carrier phase altimetry on real data sets collected in rather calm water conditions: at the 60 meter Cordouan Lighthouse, and during a 600 meter high ATR42 flight over a lake. In both case, continuous carrier phase measurement with a centimeter level precision were obtained when extending the CIT up to 500 ms. Increasing the CIT beyond 20 ms is the key to improve carrier phase altimetry robustness. [less ▲]

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See detailOrigin of high-frequency TEC disturbances observed by GPS over the European mid-latitude region
Wautelet, Gilles ULiege; Warnant, René ULiege

in Journal of Atmospheric and Solar-Terrestrial Physics (2015), 133

High-frequency variability of the ionospheric Total Electron Content (TEC) can strongly affect precise positioning with GNSS. The occurrence rate as well as the amplitude of such disturbances has been ... [more ▼]

High-frequency variability of the ionospheric Total Electron Content (TEC) can strongly affect precise positioning with GNSS. The occurrence rate as well as the amplitude of such disturbances has been extensively studied over the last decade. Mainly, one can distinguish disturbances due to space-weather events and the others, qualified as “quiet-time” as they are observed during quiet geomagnetic conditions. The latter, which represent more than 75% of the total number of disturbances over mid-latitudes, are then divided into two categories: the Winter Daytime (WD) and the Summer Nighttime (SN). The first category, representing the bulk of quiet-time disturbances, corresponds to classical Medium-Scale Traveling Ionospheric Disturbances (MSTIDs), that are the result of the interaction of gravity waves and the ionospheric plasma. On the other hand, SN disturbances are generally understood as non-classical MSTIDs of electrical origin. The paper investigates the origin of these two types of disturbance based on GPS measurements, ionospheric soundings and wind speed data at a tropospheric level. If one cannot exclude the solar terminator as a potential source of gravity waves responsible for WD events, it is thought that the major contribution comes from the lower atmosphere. More precisely, tropospheric jetstream is considered as the favorite candidate for daytime MSTIDs. Turning to SN disturbances, our analysis reveals that they are related to spread-F phenomenon, linked to the appearance of sporadic E-layers. The related instabilities are responsible for field-aligned irregularities in the F-region, which are thought to be responsible for noise-like fluctuations of the GPS TEC observed during SN events. [less ▲]

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See detailSpatial Analysis of GNSS Measurements from an Equatorial Ionospheric Scintillation Monitoring Receiver (ISMR) Network
Lonchay, Matthieu ULiege; Wautelet, Gilles ULiege; Cornet, Yves ULiege et al

Conference (2014, November 19)

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and ... [more ▼]

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and diffraction effects. In particular, small-scale ionospheric irregularities generated by different physical processes may cause scattering effects on GNSS signals, producing rapid fluctuations of the signal phase and amplitude as a result. Such scintillations of GNSS signals are responsible for critical consequences regarding applications, such as precise positioning, due to many resulting effects: cycle slips, signal power fading, receiver loss of lock and poor resulting satellite geometry. Ionospheric Scintillation Monitoring Receivers collect high-rate GNSS data. Specific scintillation parameters, such as the well-known S4 and Phi60 indices, are built on high-rate measurements performed on GNSS signals and provide additional information to characterize the intensity of such an event occurring at a specific geographic location at a given time. Spatial Statistics belong to the field of Spatial Analysis, Geography and GIS (Geographic Information System). This discipline allows to perform analyses of data which are localised in space. Ionospheric Scintillation observations achieved by ISMR stations can be characterized by a set of attributes (S4, Phi60, Rate of TEC, etc.) including also the geographic location of their respective Ionospheric Pierce Point (IPP). By combining the simultaneous Multi-GNSS ISMR measurements from a network of ISMR stations, we can obtain a spatially denser data set, able to support spatial statistics tests. The idea of our research is to provide a spatio-temporal analysis of ionospheric scintillation events over Equatorial regions by applying spatial statistics on ISMR Multi-GNSS measurements. In particular, by using spatial statistics, we aim to resolve specific issues regarding ionospheric scintillation data from an ISMR network established in Brazil. The research consists in spatially describing the data set, detecting and measuring potential spatial autocorrelation, determining the scale of the spatial dependency and finally producing an interpolated scintillation sky map at a given time. In terms of applicability of the methodology, our research project consists in exploiting the spatio-temporal analysis performed on ionospheric scintillation data in order to improve the performances and the reliability of Absolute GNSS Positioning algorithms under moderate ionospheric scintillation conditions. By assessing correlations existing between specific ISMR data and classic GNSS observations, the method could be extended to a more general usage which would be independent of ISMR measurements. [less ▲]

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See detailGNSS observational bias in the frame of ionospheric studies
Wautelet, Gilles ULiege; Warnant, René ULiege

Poster (2014, November 17)

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

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 ULiege; Warnant, René ULiege

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 detailGNSS meteorology and impact on NRT position
Brenot, Hugues; Wautelet, Gilles ULiege; Warnant, René ULiege et al

in ENC-GNSS 2014 (2014)

The analysis of GNSS signal and the use a dense network of ground-based stations allow to measure tropospheric parameters that can be used for near real-time (NRT) meteorological applications (e.g ... [more ▼]

The analysis of GNSS signal and the use a dense network of ground-based stations allow to measure tropospheric parameters that can be used for near real-time (NRT) meteorological applications (e.g. monitoring of the delay of the neutral atmosphere and the detection of blobs of water vapour). On the other hand, the meteorological activity can impact GNSS positioning solutions. For this reason, NRT indicators of the tropospheric activity related to the disturbance of GNSS signal are required. Using a dense network of GNSS stations, this study presents a new NRT indicator based on the double differences of the ionosphere-free combination. To validate this indicator, the impact of severe weather conditions on RTK positioning solutions is shown. [less ▲]

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

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 ULiege

Conference given outside the academic context (2013)

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

Conference given outside the academic context (2013)

Detailed reference viewed: 79 (4 ULiège)