[Show abstract][Hide abstract] ABSTRACT: High precision positioning and time transfer are required by a large number of scientific applications: seismic ground deformations, sea level monitoring or land survey applications require sub-centimeter precision in kinematic position; monitoring of stable atomic frequency standards requires an increasing sub –nanosecond precision. Differential GNSS is presently the best tool to reach such precisions, as it removes the majority of the errors affecting the GNSS signals. However, the associated need
[Show abstract][Hide abstract] ABSTRACT: Ice rises situated in the ice-shelf belt around Antarctica have a spatially confined flow regime with local ice divides. Beneath the divides, ice stratigraphy often develops arches with amplitudes that record the divide's horizontal residence time andsurface elevation changes. To investigate the evolution of Derwael Ice Rise, Dronning Maud Land, Antarctica, we combine radar and GPS data from three consecutive surveys, with a two-dimensional, full Stokes, thermomechanically-coupled, transient ice-flow model. We find that the surface mass balance (SMB) is higher on the upwind and lower on the downwind slopes. Near the crest, the SMB is anomalously low and causes arches to form in the shallow stratigraphy, observable by radar. In deeper ice, arches are consequently imprinted by both SMB and ice rheology (Raymond effect). The data show how arch amplitudes decrease as along-ridge slope increases, emphasizing that the lateral positioning of radar cross-sections is important for the arch interpretation. Using the model with three rheologies (isotropic with n = 3,4.5 and anisotropic with n = 3), we show that Derwael Ice Rise is close to steady-state, but is best explained using ice anisotropy and moderate thinning. Our preferred, albeit notunique, scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0.03 m a−1 over the last 3400 years. Independent of the specific thinning scenario, our modeling suggests that Derwael Ice Rise has exhibited a local flow regime at least since the Mid-Holocene.
No preview · Article · Feb 2015 · Journal of Geophysical Research: Earth Surface
[Show abstract][Hide abstract] ABSTRACT: Various scientific applications and services increasingly demand real-time information on the effects of space weather on Earth's atmosphere. In this frame, the Royal Observatory of Belgium (ROB) takes advantage of the dense EUREF Permanent GNSS Network (EPN) to monitor the ionosphere over Europe from the measured delays in the GNSS signals, and provides publicly several derived products. The main ROB products consist of ionospheric vertical Total Electron Content (TEC) maps over Europe and their variability estimated in near real-time every 15 min on 0.5° × 0.5°grids using GPS observations. The maps are available online with a latency of ~3 min in IONEX format at ftp://gnss.oma.be and as interactive web pages at www.gnss.be. This paper presents the method used in the ROB-IONO software to generate the maps. The ROB-TEC maps show a good agreement with widely used post-processed products such as IGS and ESA with mean differences of 1.3 ± 0.9 and 0.4 ± 1.6 TECu respectively for the period 2012 to mid-2013. In addition, we tested the reliability of the ROB-IONO software to detect abnormal ionospheric activity during the Halloween 2003 ionospheric storm. For this period, the mean differences with IGS and ESA maps are 0.9 ± 2.2 and 0.6 ± 6.8 TECu respectively with maximum differences (>38 TECu) occurring during the major phase of the storm. These differences are due to the lower resolution in time and space of both IGS and ESA maps compared to the ROB-TEC maps. A description of two recent events, one on March 17, 2013 and one on February 27, 2014 also highlights the capability of the method adopted in the ROB-IONO software to detect in near real-time abnormal ionospheric behaviour over Europe. In that frame, ROB maintains a data base publicly available with identified ionospheric events since 2012.
No preview · Article · Oct 2014 · Journal of Space Weather and Space Climate
[Show abstract][Hide abstract] ABSTRACT: Higher-order ionospheric effects (I2+) are one of the main limiting factors in very precise Global Navigation Satellite Systems (GNSS) processing, for applications where millimeter accuracy is demanded. This paper summarizes a comprehensive study of the I2+ effects in range and in GNSS precise products such as receiver position and clock, tropospheric delay, geocenter offset, and GNSS satellite position and clock. All the relevant higher-order contributions are considered: second and third orders, geometric bending, and slant total electron content (dSTEC) bending (i.e., the difference between the STEC for straight and bent paths). Using a realistic simulation with representative solar maximum conditions on GPS signals, both the effects and mitigation errors are analyzed. The usage of the combination of multifrequency L band observations has to be rejected due to its increased noise level. The results of the study show that the main two effects in range are the second-order ionospheric and dSTEC terms, with peak values up to 2 cm. Their combined impacts on the precise GNSS satellite products affects the satellite Z coordinates (up to +1 cm) and satellite clocks (more than +/- 20 ps). Other precise products are affected at the millimeter level. After correction the impact on all the precise GNSS products is reduced below 5 mm. We finally show that the I2+ impact on a Precise Point Positioning (PPP) user is lower than the current uncertainties of the PPP solutions, after applying consistently the precise products (satellite orbits and clocks) obtained under I2+ correction.
No preview · Article · Apr 2014 · Journal of Geophysical Research: Solid Earth
[Show abstract][Hide abstract] ABSTRACT: To better understand how receiver antenna calibration models contribute to GPS positioning error budget, we compare station positions estimated with different calibration models: igs05.atx, igs08.atx and individual antenna calibrations. First, the impact of switching from the igs05.atx antenna calibration model to the igs08.atx antenna calibration model is investigated using the EUREF Permanent Network historical data set from 1996 until April 2011. It is confirmed that these position offsets can be effectively represented by the igs05.atx to igs08.atx latitude-dependent model. Then, we demonstrate that the position offsets resulting from the use of individual calibrations instead of type mean igs08.atx calibrations can reach up to 1 cm in the up component, while in the horizontal, the offsets generally stay below 4 mm. Finally, using six antennas individually calibrated by a robot as well as in an anechoic chamber, we observe a position agreement of 2 mm in the horizontal component and a bias of 5 mm in the up component. Larger position offsets, dependent on the antenna/radome type, are, however, found when these individual calibrations are compared to type mean calibrations of two tested antennas.
[Show abstract][Hide abstract] ABSTRACT: This paper aims at providing an overview of latest advances in space weather modeling in an operational environment in Europe, including both the introduction of new models and improvements to existing codes and algorithms that address the broad range of space weather’s prediction requirements from the Sun to the Earth. For each case, we consider the model’s input data, the output parameters, products or services, its operational status, and whether it is supported by validation results, in order to build a solid basis for future developments. This work is the output of the Sub Group 1.3 “Improvement of operational models” of the European Cooperation in Science and Technology (COST) Action ES0803 “Developing Space Weather Products and services in Europe” and therefore this review focuses on the progress achieved by European research teams involved in the action
Full-text · Article · Apr 2013 · Journal of Space Weather and Space Climate
[Show abstract][Hide abstract] ABSTRACT: Since April 2011, the igs08.atx antenna calibration model is used in the
routine IGS (International GNSS Service) data analysis. The model
includes mean robot calibrations to correct for the offset and phase
center variations of the GNSS receiver antennas. These so-called "type"
calibrations are means of the individual calibrations available for a
specific antenna/radome combination. The GNSS data analysis performed
within the EUREF Permanent Network (EPN) aims at being as consistent as
possible with the IGS analysis. This also applies to the receiver
antenna calibrations. However, when available, individual antenna
calibrations are used within the EPN analysis instead of the "type"
calibration. When these individual calibrations are unavailable, then
the EPN analysis falls back to (type) calibrations identical as the ones
used within the IGS (igs08.atx). The aim of this study is to evaluate
the significance of the offset caused by using different receiver
antenna calibration models on the station position. Using the PPP
(Precise Point Positioning) technique, we first investigate the
differences in positioning obtained when switching between individual
antenna calibrations and type calibrations. We analyze the observations
of the 43 EPN stations equipped with receiver antenna individually
calibrated over the period covering from 2003 to 2010 and we show that
these differences can reach up to 4 mm in horizontal and 10 mm in
vertical. Secondly, we study the accuracy of the individual
calibrations models and we evaluate the effect of different sets of
individual calibrations on the positioning. For that purpose, we use the
data from 6 GNSS stations equipped with an antenna which has been
individually calibrated at two calibration facilities recognized by the
IGS: GEO++ and Bonn institute.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a new empirical model for predicting the daily mean ionospheric Total Electron Content (TEC) at a given latitude from only one solar index as input. For the development of the model we take advantage of the availability of 15 years of global GNSS-based TEC information and solar indices (Sunspot Number, F10.7 and derived F10.7P) including the 23rd solar cycle. Among all the tests, our preferred ionospheric climatological model to predict daily mean TEC presents yearly median differences with observed values of 1.4 ± 0.9 TECu (11.5 ± 2.9% for the relative differences) with no significant degradation during the different phases of the solar cycle. To realize this empirical model we used a least-square adjustment with (1) a combination of linear, annual and semi-annual terms between the TEC and F10.7P; (2) a discretization with respect to the phases of the solar cycle. The main differences between the modelled and the observed TEC occur during identified geomagnetic storms: the maximum differences (-3.2 ± 1.5 TECu) and relative differences (-19.6 ± 15.0%) occur one day after the storm onset. The typical time to retrieve the pre-storm conditions is 3-4 days after the onset. These results show a global picture of the effect of extreme Space Weather events on the Earth's upper atmosphere.
Full-text · Article · Apr 2012 · Journal of Space Weather and Space Climate
[Show abstract][Hide abstract] ABSTRACT: With the beginning of the 24th Solar cycle, the increased Solar activity
requires having a close eye on the ionosphere for better understanding
Space Weather physics and its effects on radio communications. In that
frame, near-real time ionospheric models over Europe are now routinely
generated at the Royal Observatory of Belgium (ROB). These models are
made available to the public through new interactive web pages at the
web site of the GNSS team (www.gnss.be) and the Solar Influences Data
Analysis Center (www.sidc.be) of ROB. The models are ionospheric
Vertical Total Electron Content (VTEC) maps estimated every 15 minutes
on a 0.5°x0.5° grid. They use the high-rate GPS observations of
the real-time stations in the EUREF Permanent Network (EPN) provided by
the ROB NTRIP broadcaster. The maps are published on the ROB web site
with a latency of 7-15 minutes with respect to the last GPS measurement
included in the 15-minute observation files. In a first step, this paper
presents the processing strategy used to generate the VTEC maps: input
data, parameter estimation, data cleaning and interpolation method. In
addition, the tools developed to further exploit the product are
introduced, e.g. on-demand animated VTEC maps. In a second step, the
VTEC maps are compared with external ionospheric products and models
such as Global Ionospheric Maps and IRI 2011. These new near-real time
VTEC maps will allow any user within the geographical scope of the maps
to estimate in near-real time the ionospheric delay induced along the
signal of any observed satellite. In the future, the web site will
continuously be updated in response to evolving user needs. This paper
opens doors to discussions with the user community to target their
[Show abstract][Hide abstract] ABSTRACT: More than 10 years (1996-2008) of weekly GPS solutions of 299 globally distributed stations have been used to quantify the impact of the reference frame definition and especially the size of the network on the estimated station positions, velocities, and residual position time series. For that purpose, weekly regional solutions (covering the European region) and global solutions have been respectively stacked to obtain regional and global station positions, velocities, and residual position time series. In both cases, the estimated long-term solutions have been tied to the ITRF2005 under minimal constraints using a selected set of reference stations. This study shows that: (1) regional position and velocity solutions can present biases with respect to each other and to global solutions, while in comparison, global solutions are much more stable; (2) the obtained residual position time series are affected by the size of the network with significantly reduced periodic signals in the regional networks, e. g. a 27% reduction of the annual signals in the height component.
[Show abstract][Hide abstract] ABSTRACT: Using dual-frequency data from 36 GPS stations from the EUREF Permanent Network (EPN), the influence of the October 30, 2003
Halloween geomagnetic storm on kinematic GPS positioning is investigated. The Halloween storm induced ionospheric disturbances
above the northern part of Europe and Scandinavia. It is shown that kinematic position repeatabilities for this period are
mainly affected for stations in northern Europe with outliers reaching 12cm in the horizontal, and 26cm in the vertical.
These magnitudes are shown to be possibly due to the second-order ionospheric delays on GPS signals, not accounted for in
the kinematic GPS positioning analysis performed. In parallel, we generate hourly TEC (Total Electron Content) maps on a 1°×1°
grid using the dense EPN network. These TEC maps do not use any interpolation but provide a high resolution in the time and
space and therefore allow to better evidence small structures in the ionosphere than the classical 2-hourly 2.5°×5° grid
Global Ionospheric TEC Maps (GIM). Using the hourly 1°×1° TEC maps, we reconstruct and refine exactly the zones of intense
ionosphere activity during the storm, and we show the correlation between the ionospheric activity and assess the quality
of GPS-based kinematic positioning performed in the European region.
KeywordsTEC maps–GPS kinematic positioning–Geomagnetic storm
[Show abstract][Hide abstract] ABSTRACT: The beginning of the 23rd solar cycle (May 1996 to December 2008) coincided with the start of the catalogue of global ionospheric modeling using GPS data. Comparison between solar activity parameters and GPS-derived Total Electron Content (TEC) is now possible for the whole of solar cycle 23. In this study, we compared the daily sunspot number and F10.7 cm flux with the daily mean global TEC values during the entire last solar cycle. In order to better understand the ionization response, we show correlations between the daily F10.7cm delivered by NGDC-NOAA (National Geophysical Data Center - National Oceanic and Atmospheric Administration) and the daily sunspot number from SIDC (Solar Influences Data Analysis Center) with the daily mean latitudinal TEC values extracted from CODE (Center for Orbit Determination in Europe) GPS-based global ionospheric maps for the period 1995-2009. The correlations were investigated for different daily mean latitudinal ionospheric TEC: (1) expressed in geographic and geomagnetic coordinates; (2) with respect to the seasons and; (3) with respect to the different phases of the solar cycle. In general, results show in the north and south hemispheres a different ionospheric response (TEC) to solar activity (F10.7cm). Moreover, the switch from geographic to geomagnetic coordinates does not change the observed correlation between TEC and solar parameters. Finally, a larger correlation is observed at N20°-30° during the transition phase in the solar cycle.
[Show abstract][Hide abstract] ABSTRACT: A good knowledge in near real-time of the state of the ionosphere, in terms of spatial and temporal variation of electron content, is now necessary for many scientific applications. Indeed, most of the applications using radio waves (e.g. communications, navigations systems and radio-sciences) are biased by the ionosphere. They consequently need information on the ionospheric electron content over a specific area at a given time e.g. to derive delay corrections. In this study, we present a method developed at the Royal Observatory of Belgium in order to produce each 15 minutes GPS-based ionospheric total electron content models over Europe on a 0.5°/0.5° grid for any user located in Belgium. The foreseen product will be available in near real-time and therefore the 120 stations of the EUREF Permanent Network providing near real-time GPS data are used. In a first step, the method is described (input data, parameter estimations) and the data cleaning and the spatial interpolation approaches are presented. In a second step, the results of the selected spline interpolation method are shown and the resulting models are compared to Global Ionospheric Maps produced by the CODE (Center for Orbit Determination in Europe) IGS analysis center. The quality of the product is evaluated during different periods: (1) active ionospheric state (October Geomagnetic Storm, 2003); (2) normal ionospheric state (close to the Solar Minimum in 2008); (3) and a recent period in 2010.
[Show abstract][Hide abstract] ABSTRACT: More than ten years (1996-2008) of weekly GPS solutions of 299 globally distributed stations have been used to quantify the impact of the reference frame definition and especially the size of the network on the estimated station positions, velocities, and residual position time series. For that purpose, weekly regional solutions (covering the European region) and global solutions have been respectively stacked to obtain regional and global station positions, velocities, and residual position time series. In both cases, the estimated long-term solutions have been tied to the ITRF2005 under minimal constraints using a selected set of reference stations. This study shows that: (1) regional position and velocity solutions can present biases with respect to each other and to global solutions, while in comparison, global solutions are much more stable; (2) the obtained residual position time series are affected by the size of the network with significantly reduced periodic signals in the regional networks, e.g. a 27% reduction of the annual signals in the height component. In summary, we will evidence the limitation of regional networks to produce reliable station positions, velocities and especially residual position time series.
[Show abstract][Hide abstract] ABSTRACT: The growing number of GNSS (GPS, GLONASS, Galileo) satellites and ground
receivers will in the next several years significantly increase the
number of radio navigation signals probing the Earth's atmosphere. In
this study, the added-value of using dense national GNSS networks, in
addition to the EUREF Permanent Network (EPN), is investigated for
performing ionospheric as well as tropospheric tomography over Europe.
For this purpose, the focus will be on the geometry of the
satellite-receiver ray distribution traversing the atmosphere. This
investigation shows that dense national networks mostly provide an
added-value for tropospheric tomography reducing significantly the empty
zones, while their interest for ionospheric tomography is less evident.
Additionally, 52% of the EPN stations are presently capable of tracking
GLONASS and 90% of new antennas installed in the EPN are able to observe
the three GNSS. The results show how GLONASS and/or Galileo
observations, in addition to GPS, not only increase the number of rays,
but also extend the observation zone, possibly contributing to a better
sounding of the atmosphere.
[Show abstract][Hide abstract] ABSTRACT: Ten years (1997–2006) of weekly GNSS solutions of 205 globally distributed stations have been used to investigate the impact of the reference frame definition on the estimated station velocities. For that purpose, weekly regional solutions (covering the European region) and global solutions have been, respectively, stacked to obtain regional and global velocity fields. In both cases, the estimated long-term solutions (station positions and velocities) were tied to the ITRF2005 under minimal constraints using a selected set of reference stations. Several sets of global and regional reference stations were tested to evaluate first the impact of the reference frame definition on the global and regional velocity fields and later the impact on the derived geodynamic interpretations.Results confirm that the regional velocity fields show systematic effects with respect to the global velocity field with differences reaching up to 1.3mm/year in the horizontal and 2.9mm/year in the vertical depending on the geographical extent of the network and the chosen set of regional reference stations.In addition, the estimations of the Euler pole for Western Europe differ significantly when considering a global or a regional strategy. After removing the rigid block rotation, the residual velocity fields show differences which can reach up to 0.8mm/year in horizontal component.In Northern Europe, the vertical ground motion is dominated by the Glacial Isostatic Adjustment (GIA). A proper modeling of this effect requires sub-mm/year precision for the vertical velocities for latitudes below 56°. We demonstrate that a profile of vertical velocities shows significant discrepancies according to the reference frame definition strategy. In the case of regional solutions, the vertical modeling does not predict any subsidence around 52° as predicted by the global solution and previous studies.In summary, we evidence the limitation of regional networks to reconstruct absolute velocity fields and conclude that when geodynamics require the highest precisions for the GNSS-based velocities, a global reference frame definition is more reliable.
No preview · Article · Apr 2010 · Journal of Geodynamics
[Show abstract][Hide abstract] ABSTRACT: "Patches" are regions of enhanced electron concentration that propagate through the polar-cap regions of the ionosphere at F-layer altitudes. They correlate well with the appearance of scintillations on trans-ionospheric radio links such as GPS broadcasts and can act as reflecting layers for HF radio links, allowing communication to regions which would be impossible to access in their absence. The aviation industry is particularly affected by both of these phenomena as it is increasingly reliant on GPS based systems for navigation purposes and on HF radio to communicate with aircraft on trans-polar flight paths. With the number of trans-polar commercial flights increasing annually, polar-cap patches present both an increasing problem (loss of accuracy using GPS) and a benefit (unexpected radio links to aircraft). The approach of a solar maximum, when the rate of occurrence of patches increases, exacerbates both of these [Hunsucker, 2003]. It is therefore of great practical as well as scientific interest to understand the formation, propagation, morphology and decay of polar-cap patches as well as the processes that generate scintillation causing electron concentration irregularities within them. Presented here is a technique for imaging the polar-cap ionosphere in terms of electron concentration and subsequently identifying and tracking polar-cap patches within these images in an automated manner. Ionospheric imaging of the polar cap based on GPS ground receiver data has been successfully demonstrated and used in a number of studies. The MIDAS 2.0 ionospheric imaging software uses a Kalman filter approach to weight GPS data assimilated into a background model against a predictive model of the polar convection partern in order to achieve a final solution for each image. When the images, which are fully three dimensional, are plotted as vertical TEC maps, patches can be readily identified by eye. However, automated identification and tracking of these features would allow analysis of patch climatology, statistical analysis of the tracks of patches as they convect and other studies leading to a better understanding of the phenomenon as a whole [R. Burston et al., 2009; Mitchell et al., 2005; Spencer and Mitchell, 2007; Weimer, 2001 ; Yin et al., 2009]. To identify and track the patches a method originally developed for tracking weather systems in model and Numerical Weather Prediction data has been adapted [K.I. Hodges, 1994; 1995; K. I. Hodges, 1999]. This involves taking the vertical TEC data and first removing the large scale background using a discrete cosine transform filter [Denis et al., 2002], spatial scales in the range 200-1000 km are retained. Representative points (feature points) are determined from the filtered TEC fields; this is done by finding the off-grid maxima to represent the feature points. The feature points are linked to form tracks initially using a nearest neighbour search; these are refined using a cost function minimization for track smoothness to produce the smoothest set of tracks. Once tracks have been determined for an extended time period, statistics for the density of tracks and their genesis and lysis are determined. Both the tracking and statistical estimation are carried out directly on the sphere to exclude the types of biases often introduced by using projections. Other properties along the tracks and intensity distributions are also determined. Results to date are presented, including a feasibility case-study based on the geomagnetic storm that occurred during 24-28th July 2004, using vertical TEC data as generated by the MIDAS 2.0 algorithm.
[Show abstract][Hide abstract] ABSTRACT: On February 15th 2009, Belgium together with the International Polar Foundation inaugurated a new Antarctic base named Princess Elisabeth in honour of the granddaughter of King Albert II of Belgium. The base, located in the Queen Maud Land, East Antarctica (lat = 71°57'S, long = 23°20'E) was built to fully operate with renewable energies, conditions motivated by present climatic issues. Among the wide range of ambitious scientific projects already initiated, a solid earth GIANT-LISSA project will be conducted by the Royal Observatory of Belgium to better understand the ongoing geodynamics affecting East Antarctica such as ice mass change and to shed light onto the past and present tectonics by investigating lithospheric structure and local and regional intra-plate seismicity. Here we present these scientific goals focussing particularly on the seismology experiment. We describe the technical aspects of the instrumentation to be shipped and installed during the coming BELARE 2009-2010 expedition: one surface and one borehole broadband seismometers in addition to two GPS stations. Absolute and relative gravity measurements will be undertaken the following year in collaboration with Luxembourg University. In regard of the excellent site conditions provided by the elongated nunatak outcrop hosting the Princess Elisabeth base, the scientific expectations are high allowing to envision further initiatives and collaborations.
[Show abstract][Hide abstract] ABSTRACT: Subduction zone deformation is difficult to fully constrain, as the plate boundary generally lies in deep waters, preventing the use of common GNSS based methods. While the subduction zone plate boundary in Central Vanuatu is indeed submerged, two shallow seamounts located on either side of the plate limit allow us measure vertical motions on both sides, using diver-installed seafloor pressure gauges. In 1999, we installed one gauge on the subducting plate, at Sabine Bank, and the other one on the overriding plate, at Wusi Bank. The two gauges have been recording nearly continuously since 1999. The signal we are looking for is small and on the same order of magnitude than sea level changes; thus we use satellite altimetry as an external reference to monitor sea surface changes. The Sabine Bank site is close to an ERS/Envisat crossing point, while the Wusi Bank site is close to a crossover point for Topex-Poseidon, Jason and Jason2. Seafloor pressure, converted to depth, and satellite altimetric measurements of the sea surface are combined to derive the height above ellipsoid of both seafloor points. In addition, kinematic GPS data on a floating platform above the pressure gauges, contribute to the calibration of the measurement system. We show the vertical deformation and uncertainties obtained at both underwater points and use these results to complement the deformation profile perpendicular to the plate boundary, obtained with on-land GPS measurements. The whole profile, comprising at sea and on land measurements is then modeled in 3D to investigate the role played by topographic features lying on the subducting plate on the locking of the subduction and on the vertical uplift of the overriding plate, which is responsible for the existence of both Santo and Malekula, the two largest islands of the Vanuatu archipelago.