Influence of ionospheric perturbations in GPS time and frequency transfer

The stability of GPS time and frequency transfer is limited by the fact that GPS signals travel through the ionosphere. In high precision geodetic time transfer (i.e. based on precise modeling of code and carrier phase GPS data), the so-called ionosphere-free combination of the code and carrier phase measurements made on the two frequencies is used to remove the first-order ionospheric effect. In this paper, we investigate the impact of residual second- and third-order ionospheric effects on geodetic time transfer solutions i.e. remote atomic clock comparisons based on GPS measurements, using the ATOMIUM software developed at the Royal Observatory of Belgium (ROB). The impact of third-order ionospheric effects was shown to be negligible, while for second-order effects, the tests performed on different time links and at different epochs show a small impact of the order of some picoseconds, on a quiet day, and up to more than 10 picoseconds in case of high ionospheric activity. The geomagnetic storm of the 30th October 2003 is used to illustrate how space weather products are relevant to understand perturbations in geodetic time and frequency transfer.Comment: 25 pages, 10 eps figures, 1 table, accepted in Journal of Advances in Space Research, Special Issue "Recent advances in space weather monitoring, modelling and forecasting

Modelling and assessing ionospheric higher order terms for GNSS signals

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 for dense GNSS observation networks is not fulfilled for many locations (e.g. Pacific, Africa). An alternative is to use Precise Point Positioning (PPP), but this technique requires correcting signal delays at the highest level of precision, including high order ionospheric effects. It is thus essential to accurately characterize the higher order ionospheric terms (I2+), i.e. I2, I3, I4, geometric bending and differential STEC bending, which is the goal of this paper. For that, we used a network of well-distributed GPS stations, and the Bernese v5.0 software. We have focused our attention in the I2+ terms, studying two approaches: A) Combining independent and simultaneous measurements of the same transmitter-receiver pair at three (or more) different frequencies, in order to remove the I2 term: it is theoretically possible to cancel out both I1 and I2 similarly as it is done typically in precise dual-frequency GNSS measurements for I1. It is shown that, as expected, due to the proximity of the corresponding frequencies in L-band, the high noise of the combinations makes this approach unpractical to either isolate or remove I2. B) Modelling the I2+ terms, in function of estimates of electron content, geomagnetic field and electron density values. Their characterization has been done in a realistic and full-control environment, by using the last version of the International Reference Ionosphere model (IRI2012) and International Geomagnetic Reference Model in its 11th version (IGRF11). Two metrics have been considered to assess the importance of the different higher order ionospheric corrections and their approximations: a) At the signal level, or range level, directly provided by the corresponding slant delays. b) At the geodetic domain level, provided by the impact of such values in the different geodetic parameters estimated consistently (i.e. simultaneously) from a global GNSS network.Peer ReviewedPostprint (author's final draft

Impact of higher order ionospheric delay on precise GNSS computation

Peer ReviewedPostprint (published version

High-resolution distributed vertical strain and velocity from repeat borehole logging by optical televiewer:Derwael Ice Rise, Antarctica

Abstract Direct measurements of spatially distributed vertical strain within ice masses are scientifically valuable but challenging to acquire. We use manual marker tracking and automatic cross correlation between two repeat optical televiewer (OPTV) images of an ~100 m-long borehole at Derwael Ice Rise (DIR), Antarctica, to reconstruct discretised, vertical strain rate and velocity at millimetre resolution. The resulting profiles decay with depth, from −0.07 a −1 at the surface to ~−0.002 a −1 towards the base in strain and from −1.3 m a −1 at the surface to ~−0.5 m a −1 towards the base in velocity. Both profiles also show substantial local variability. Three coffee-can markers installed at different depths into adjacent boreholes record consistent strain rates and velocities, although averaged over longer depth ranges and subject to greater uncertainty. Measured strain-rate profiles generally compare closely with output from a 2-D ice-flow model, while the former additionally reveal substantial high-resolution variability. We conclude that repeat OPTV borehole logging represents an effective means of measuring distributed vertical strain at millimetre scale, revealing high-resolution variability along the uppermost ~100 m of DIR, Antarctica.info:eu-repo/semantics/publishe

Study of space weather impact on Antarctica ionosphere from GNNS data

The impact of solar activity on the ionosphere at polar latitudes is not well known compare to low and mid-latitudes due to lack of experimental observations, especially over Antarctica. Consequently, one of the present challenges of the Space Weather community is to better characterize (1) the climatological behavior of the polar ionosphere in response to variations of the solar activity and (2) the different response of the ionosphere at high latitudes during extreme solar events and geomagnetic storms. For that, the combination of GNSS measurements (e.g. GPS, GLONASS and Galileo) on two separate frequencies allows determining the ionospheric delay between a ground receiver and a satellite. This delay is function of the integrated number of electrons encountered in the ionosphere along the signal ray path, called the Total Electron Content (TEC). It is thus possible to study the behavior of ionospheric TEC at different time and spatial scales from the observations of a network of permanent GNSS stations. In the frame of GIANT-LISSA and IceCon projects we installed since 2009 five GNSS stations around the Princess Elisabeth station. We used these stations additionally to other stations from the IGS global network to estimate the ionospheric TEC at different locations over Antarctica. This study presents this regional data set during different solar activity levels and discusses the different climatological behaviors identified in the ionosphere at these high latitudes. Finally, we will show few examples of typical TEC disturbances observed during extreme solar events

Fun at Antarctic grounding lines: Ice-shelf channels and sediment transport

Meltwater beneath the polar ice sheets drains, in part, through subglacial conduits. Landforms created by such drainages are abundant in areas formerly covered by ice sheets during the last glacial maximum. However, observations of subglacial conduit dynamics under a contemporary ice sheet are lacking. We present results from ice-penetrating radar to infer the existence of subglacial conduits upstream of the grounding line of Roi Baudouin Ice Shelf, Antarctica. The conduits are aligned with ice-shelf channels, and underlain by esker ridges formed from sediment deposition due to reduced water outflow speed near the grounding line. In turn, the eskers modify localice flow to initiate the bottom topography of the ice-shelf channels, and create small surface ridges extending onto the shelf. Relict features on the shelf are interpreted to indicate a history of these interactions and variability of past subglacial drainages. Because ice-shelf channels are loci where intense melting occurs to thin an ice shelf, these findings expose a novel link between subglacial drainage, sedimentation, and ice-shelf stability. To investigate the role of sediment transport beneath ice sheets further, we model the sheet-shelf system ofthe Ekstömisen catchment, Antarctica. A 3D finite element model (Elmer/ICE) is used to solve the transients full Stokes equation for isotropic, isothermal ice with a dynamic grounding line. We initialize the model with surface topography from the TanDEM-X satellites and by inverting simultaneously for ice viscosity and basaldrag using present-day surface velocities. Results produce a flow field which is consitent with sattelite and on-site observations. Solving the age-depth relationship allows comparison with radar isochrones from airborne data, and gives information about the atmospheric/dynamic history of this sector. The flow field will eventually be used to identify potential sediment sources and sinks which we compare with more than 400 km of seismic profiles collected over the floating ice shelves and the grounded ice sheet

St. Patrick’s Day 2015 geomagnetic storm analysis based on Real Time Ionosphere Monitoring

A detailed analysis is presented for the days in March, 2015 surrounding St. Patrick’s Day 2015 geomagnetic storm, based on the existing real-time and near real-time ionospheric models (global or regional) within the group, which are mainly based on Global Navigation Satellite Systems (GNSS) and ionosonde data. For this purpose, a variety of ionospheric parameters is considered, including Total Electron Content (TEC), F2 layer critical frequency (foF2), F2 layer peak (hmF2), bottomside halfthickness (B0) and ionospheric disturbance W-index. Also, ionospheric high-frequency perturbations such as Travelling Ionospheric Disturbances (TIDs), scintillations and the impact of solar flares facing the Earth will be presented to derive a clear picture of the ionospheric dynamicsPostprint (published version

Review of Environmental Monitoring by Means of Radio Waves in the Polar Regions: From Atmosphere to Geospace

The Antarctic and Arctic regions are Earth's open windows to outer space. They provide unique opportunities for investigating the troposphere–thermosphere–ionosphere–plasmasphere system at high latitudes, which is not as well understood as the mid- and low-latitude regions mainly due to the paucity of experimental observations. In addition, different neutral and ionised atmospheric layers at high latitudes are much more variable compared to lower latitudes, and their variability is due to mechanisms not yet fully understood. Fortunately, in this new millennium the observing infrastructure in Antarctica and the Arctic has been growing, thus providing scientists with new opportunities to advance our knowledge on the polar atmosphere and geospace. This review shows that it is of paramount importance to perform integrated, multi-disciplinary research, making use of long-term multi-instrument observations combined with ad hoc measurement campaigns to improve our capability of investigating atmospheric dynamics in the polar regions from the troposphere up to the plasmasphere, as well as the coupling between atmospheric layers. Starting from the state of the art of understanding the polar atmosphere, our survey outlines the roadmap for enhancing scientific investigation of its physical mechanisms and dynamics through the full exploitation of the available infrastructures for radio-based environmental monitoring

Etude du cycle sismique du Vanuatu par GPS

PARIS-BIUSJ-Thèses (751052125) / SudocSudocFranceF