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See detailOn the Application of Multipath Stacking Maps for Improved Severe Weather Tracking and Low-cost Antenna Calibration
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Duman, Hüseyin et al

Presentation (2022, July 07)

Modern cities all over the world are now more susceptible to flash floods as a result of a rise in the frequency and severity of meteorological events with significant precipitation. For minimizing the ... [more ▼]

Modern cities all over the world are now more susceptible to flash floods as a result of a rise in the frequency and severity of meteorological events with significant precipitation. For minimizing the risks due to these hydro-meteorological hazards, reliable fore- and now-casting of severe precipitation has become essential. Water vapor can be measured along slant paths by network of ground-based GNSS stations, providing real-time, three-dimensional (3D) distributions of vapor concentrations at an affordable cost. Consequently, these data provide an invaluable additional source of knowledge for monitoring and predicting events with flash flood potential. But site-specific multipath (MP) effects at GNSS stations do affect incoming signals, particularly at low elevations, as is widely known. The bulk of GNSS products for meteorology up to now are based on estimates of ordinary zenith total delay and horizontal gradients with little sensitivity to azimuthal variations, thus these products may not be best suited for estimating 3D distributions of water vapor during storm events. Using slant delays directly, can overcome this lack of azimuthal information. However, at low elevations, this approach is more susceptible to multipath errors. A thunderstorm event which occurred over Turkey and adjacent countries on July 27, 2017, resulting in flash floods and severe infrastructure damage, is used as an example to explore the effects of multipath-corrected slant wet delay (SWD) estimations on monitoring extreme weather events. To reconstruct the one-way SWD, we first added phase residuals derived from the GNSS one-way post-fit observations, which represent the anisotropic component of the SWD. This can also be interpreted as a higher order inhomogeneity component, which is not resolved by ordinary zenith or gradient products. For the generation of site-specific MP correction maps, we stacked the post-fit residuals derived from our Precise Point Positioning (PPP) processing strategy because the MP errors in the GNSS phase observables can adversely impact the SWD along the direction of individual satellites. The spatial stacking was performed in congruent cells as a function of elevation and azimuth. This enables each cell to receive roughly the same number of residuals, providing a better stacking result. The stacking of residuals for a single cell over several days allows the detection and reduction of systematic errors; random errors are minimal because the averaging is done over a suitably sufficient chosen time span. Finally, the one-way SWD were enhanced by applying these MP correction maps for the analysis of the meteorological event. Our study revealed that the anisotropic component contributed up to 11% of one-way SWD estimates. Furthermore, the spatio-temporal changes in SWD as derived from GNSS closely matched the moisture field from the ERA5 re-analysis linked to this weather event. As it turns out, the MP correction maps may also provide a “kind of calibration” for uncalibrated or low-cost GNSS antennas, even for those in mobile phones, as these devices are highly susceptible to MP errors. In turn, this would allow the application of low-cost sensors to accurately estimate SWD for severe weather monitoring in urban regions. [less ▲]

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See detailAnalysis of GNSS sensed slant wet delay during severe weather events in central Europe
Hunegnaw, Addisu UL; Duman, Hüseyin; Elgered, Gunnar et al

Scientific Conference (2022, May 26)

Over the last few decades, anthropogenic greenhouse gas emissions have increased the frequency of climatological anomalies such as temperature, precipitation, and evapotranspiration. It is noticed that ... [more ▼]

Over the last few decades, anthropogenic greenhouse gas emissions have increased the frequency of climatological anomalies such as temperature, precipitation, and evapotranspiration. It is noticed that the frequency and severity of the intense precipitation signify a greater susceptibility to flash flooding. Flash flooding continues to be a major threat to European cities, with devastating mortality and considerable damage to urban infrastructure. As a result, accurate forecasting of future extreme precipitation events is critical for natural hazard mitigation. A network of ground-based GNSS receivers enables the measurement of integrated water vapour along slant pathways providing three-dimensional water vapour distributions. This study aims to demonstrate how GNSS sensing of the troposphere can be used to monitor the rapid and extreme weather events that occurred in central Europe in June 2013 and resulted in flash floods and property damage. We recovered one-way slant wet delay (SWD) by adding GNSS post-fit phase residuals, representing the troposphere's higher-order inhomogeneity. Nonetheless, noise in the GNSS phase observable caused by site-specific multipath can significantly affect the SWD from individual satellites. To overcome the problem, we employ a suitable averaging strategy for stacking post-fit phase residuals obtained from the PPP processing strategy to generate site-specific multipath corrections maps (MPS). The spatial stacking is carried out in congruent cells with an optimal resolution in elevation and azimuth at the local horizon but with decreasing azimuth resolution as the elevation angle increases. This permits an approximately equal number of observations allocated to each cell. The spatio-temporal fluctuations in the SWD as measured by GNSS closely mirrored the moisture field associated with severe weather events in central Europe, i.e., a brief rise prior to the main rain events, followed by a rapid decline once the storms passed. Furthermore, we validated the one-way SWD between ground-based water-vapour radiometry (WVR) and GNSS-derived SWD for different elevation angles. [less ▲]

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See detailEvaluation of the Multipath Environment Using Electromagnetic-Absorbing Materials at Continuous GNSS Stations
Hunegnaw, Addisu UL; Teferle, Felix Norman UL

in Sensors (2022), 22(9), 1-23

o date, no universal modelling technique is available to mitigate the effect of site-specific multipaths in high-precision global navigation satellite system (GNSS) data processing. Multipaths affect both ... [more ▼]

o date, no universal modelling technique is available to mitigate the effect of site-specific multipaths in high-precision global navigation satellite system (GNSS) data processing. Multipaths affect both carrier-phase and code/pseudorange measurements, and the errors can propagate and cause position biases. This paper presents the use of an Eccosorb AN-W-79 microwave-absorbing material mounted around a GNSS antenna that reflects less than −17 dB of normal incident energy above a frequency of 600 MHz. To verify the feasibility and effectiveness of the Eccosorb, we installed two close stations by continuously operating multi-GNSS (BeiDou, GLONASS, Galileo and GPS) in a challenging location. One station is equipped with the Eccosorb AN-W-79, covering a square area of 3.35 m2 around the antenna, and the second station operates without it. The standard deviation reductions from single point positioning estimates are significant for all the individual GNSS solutions for the station equipped with microwave-absorbing material. The reductions are as follows: for GPS, between 15% and 23%; for Galileo, between 22% and 45%; for GLONASS, 22%; and for BeiDou, 4%. Furthermore, we assess the influence of multipaths by analysing the linear combinations of code and carrier phase measurements for various GNSS frequencies. The Galileo code multipath shows a reduction of more than 60% for the station with microwave-absorbing material. For GLONASS, particularly for the GLOM3X and GLOM1P code multipath combinations, the reduction reaches 50%, depending on the observation code types. For BeiDou, the reduction is more than 30%, and for GPS, it reaches between 20% and 40%. The Eccosorb AN-W-79 microwave-absorbing material shows convincing results in reducing the code multipath noise level. Again, using microwave-absorbing material leads to an improvement between 15% and 60% in carrier phase cycle slips. The carrier-phase multipath contents on the post-fit residuals from the processed GNSS solutions show a relative RMS reduction of 13% for Galileo and 9% for GLONASS and GPS when using the microwave-absorbing material. This study also presents power spectral contents from residual signal-to-noise ratio time series using Morlet wavelet transformation. The power spectra from the antenna with the Eccosorb AN-W-79 have the smallest magnitude, demonstrating the capacity of microwave-absorbing materials to lessen the multipath influence while not eliminating it. [less ▲]

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See detailFeasibility of ERA5 integrated water vapor trends for climate change analysis in continental Europe: An evaluation with GPS (1994–2019) by considering statistical significance
Yuan, Peng; Hunegnaw, Addisu UL; Alshawaf, Fadwa et al

in Remote Sensing of Environment (2021), 260(112416),

Although the statistical significances for the trends of integrated water vapor (IWV) are essential for a correct interpretation of climate change signals, obtaining accurate IWV trend estimates with ... [more ▼]

Although the statistical significances for the trends of integrated water vapor (IWV) are essential for a correct interpretation of climate change signals, obtaining accurate IWV trend estimates with realistic uncertainties remains a challenge. This study evaluates the feasibility of the IWV trends derived from the newly released fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5) for climate change analysis in continental Europe. This is achieved by comparing the trends derived from in-situ ground-based Global Positioning System (GPS)’s daily IWV series from 1994 to 2019 at 109 stations. The realistic uncertainties and statistical significances of the IWV trends are evaluated with the time series analysis on their noise characteristics and proper noise models. Results show that autoregressive moving average ARMA(1,1) noise model is preferred rather than the commonly assumed white noise (WN) or first-order autoregressive AR(1) noise for about 68% of the ERA5 and GPS IWV series. An improper noise model would misevaluate the trend uncertainty of an IWV time series, compared with its specific preferred noise model. For example, ARMA(1,1) may misevaluate the standard deviations of their trend estimates (0.1–0.3 kg m−2 decade−1) by 10%. Nevertheless, ARMA(1,1) is recommended as the default noise model for the ERA5 and GPS IWV series. However, the preferred noise model for each ERA5 minus GPS (E-G) IWV series should be specifically determined, because the AR(1)-related models can result in an underestimation on its trend uncertainty by 90%. In contrast, power-law (PL) model can lead to an overestimation by up to nine times. The E-G IWV trends are within −0.2–0.4 kg m−2 decade−1, indicating that the ERA5 is a potential data source of IWV trends for climate change analysis in continental Europe. The ERA5 and GPS IWV trends are consistent in their magnitudes and geographical patterns, lower in Northwest Europe (0–0.4 kg m−2 decade−1) but higher around the Mediterranean Sea (0.6–1.4 kg m−2 decade−1). [less ▲]

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See detailMulti-GNSS Slant Wet Delay Retrieval Using Multipath Mitigation Maps
Hunegnaw, Addisu UL; Ejigu, Yohannes Getachew; Teferle, Felix Norman UL et al

Scientific Conference (2021, April 24)

The conventional Global Navigation Satellite System (GNSS) processing is typically contaminated with errors due to atmospheric variabilities, such as those associated with the mesoscale phenomena. These ... [more ▼]

The conventional Global Navigation Satellite System (GNSS) processing is typically contaminated with errors due to atmospheric variabilities, such as those associated with the mesoscale phenomena. These errors are manifested in the parameter estimates, including station coordinates and atmospheric products. To enhance the accuracy of these GNSS products further, a better understanding of the local-scale atmospheric variability is necessary. As part of multi-GNSS processing, station coordinates, carrier phase ambiguities, orbits, zenith total delay (ZTD) and horizontal gradients are the main parameters of interest. Here, ZTD is estimated as the average zenith delay along the line-of-sight to every observed GNSS satellite mapped to the vertical while the horizontal gradients are estimated in NS and EW directions and provide a means to partly account for the azimuthally inhomogeneous atmosphere. However, a better atmospheric description is possible by evaluating the slant path delay (SPD) or slant wet delay (SWD) along GNSS ray paths, which are not resolved by ordinary ZTD and gradient analysis. SWD is expected to provide better information about the inhomogeneous distribution of water vapour that is disregarded when retrieving ZTD and horizontal gradients. Usually, SWD cannot be estimated directly from GNSS processing as the number of unknown parameters exceeds the number of observations. Thus, SWD is generally calculated from ZTD for each satellite and may be dominated by un-modelled atmospheric delays, clock errors, unresolved carrier-phase ambiguities and near-surface multipath scattering. In this work, we have computed multipath maps by stacking individual post-fit carrier residuals incorporating the signals from four GNSS constellations, i.e. BeiDou, Galileo, Glonass and GPS. We have selected a subset of global International GNSS Service (IGS) stations capable of multi-GNSS observables located in different climatic zones. The multipath effects are reduced by subtracting the stacked multipath maps from the raw post-fit carrier phase residuals. We demonstrate that the multipath stacking technique results in significantly reduced variations in the one-way post-fit carrier phase residuals. This is particularly evident for lower elevation angles, thus, producing a retrieval method for SWD that is less affected by site-specific multipath effects. We show a positive impact on SWD estimation using our multipath maps during increased atmospheric inhomogeneity as induced by severe weather events. [less ▲]

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See detailMonitoring and prediction of hurricane tracks using GPS tropospheric products
Ejigu, Yohannes Getachew; Teferle, Felix Norman UL; Klos, Anna UL et al

in GPS Solutions (2021), (76),

We have reconstructed integrated water vapor (IWV) using the zenith wet delays to track the properties of hurricanes and explore their spatial and temporal distributions estimated from 922 GPS stations ... [more ▼]

We have reconstructed integrated water vapor (IWV) using the zenith wet delays to track the properties of hurricanes and explore their spatial and temporal distributions estimated from 922 GPS stations. Our results show that a surge in GPS-derived IWV occurred at least six hours prior to the landfall of two major hurricanes (Harvey and Irma) that struck the Gulf and East Coasts of the USA in 2017. We observed enhanced IWV, in particular, for the two hurricanes landfall locations. The observed variations exhibit a correlation with the precipitation value constructed from GPM/IMERG satellite mission coinciding with hurricane storm front passage. We used GPS-IWV data as inputs for spaghetti line plots for our path predictions, helping us predict the paths of Hurricanes Harvey and Irma. Hence, a directly estimable zenith wet delay sourced from GPS that has not been previously reported can serve as an additional resource for improving the monitoring of hurricane paths. [less ▲]

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See detailMultipath Mitigation Maps feasibility and applicability as an International GNSS Service product
Hunegnaw, Addisu UL; Teferle, Felix Norman UL; Elgered, Gunnar et al

Scientific Conference (2020, December 17)

There have been many advances in the modeling of Global Navigation Satellite System (GNSS) observables when estimating position and other parameters of interest. Some of these bias models are related to ... [more ▼]

There have been many advances in the modeling of Global Navigation Satellite System (GNSS) observables when estimating position and other parameters of interest. Some of these bias models are related to improvements of reference frames, phase center offsets and variations of transmitter and receiver antennas, satellite orbits and clocks, and troposphere. Nonetheless, multipath remains for the most part an unmodelled source of error which causes range errors in the GNSS observations. The associated effects show highly localized features and have a different impact for each receiver and antenna. Multipath errors can propagate, can cause in-situ position biases and are also contributing to the prevalent draconitic harmonic signals. In order to mitigate the problem we generate site-specific corrections by employing a suitable averaging scheme for the stacking of carrier phase residuals. Our processing is based on globally distributed static multi-GNSS observations using several scientific GNSS software packages (Bernese GNSS Software, NAPEOS, GAMIT-GLOBK, and CSRS-PPP). Our multipath stacking maps (MPS) use the stacking of carrier phase residuals generated by variable azimuth cell size (congruent cells) and by allocating carrier phase residuals in each cell to generate the correction maps, unlike the standard fixed azimuth cell resolution approaches. This reduces the binning of fewer residuals at higher elevation angles. Before stacking, we also apply rigorous statistical outlier screening tests for each one-way post-fit carrier phase residual assigned to each of the congruent cells. We thus correct the multipath effects by subtracting the stacked multipath map from the post-fit carrier phase residual. Using this technique we produce a model available in the form of the Antenna Exchange (ANTEX) file format, that can potentially be implemented in routine GNSS analysis with no or little additional overhead for individual analysis centers (ACs). In this study, we assess the feasibility and applicability of the MPS maps as an International GNSS Service (IGS) product for routine GNSS analysis. We demonstrate the multipath stacking technique to result in a significant reduction of the variation in the one-way post-fit carrier phase residuals from multi-GNSS observations. [less ▲]

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See detailVertical Land Movements and Sea Level Changes on South Georgia, South Atlantic Ocean: Results from 7 Years of Geodetic and Oceanographic Observations on a Remote Island
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Hibbert, Angela et al

Scientific Conference (2020, December 16)

South Georgia Island in the South Atlantic Ocean, is a small remote land mass that supports various ground-based instrumental observations (Global Navigation Satellite System (GNSS), tide gauge ... [more ▼]

South Georgia Island in the South Atlantic Ocean, is a small remote land mass that supports various ground-based instrumental observations (Global Navigation Satellite System (GNSS), tide gauge, meteorological and seismic) in an otherwise largely under sampled oceanic region. Moreover, the South Atlantic Ocean plays an important role in global ocean circulation, con-necting the deep thermohaline circulation of the North Atlantic and Indian Oceans, whilst also linking to the Antarctic Circumpolar Current in the South, where the lack of continental barriers allows a free exchange of water between the major ocean basins. Hence, South Georgia po-tentially lies within a region susceptible to climatic changes before these can be felt further afield. In 2013 and 2014 a total of five GNSS stations were installed covering the area of the main island (approximately 170 x 50 km) with two of those being located close to the King Edward Point (KEP) Research Station and the GLOSS tide gauge (ID 187). Furthermore, precise levelling campaigns in 2013, 2014, 2017 and 2020 supported the analysis of local ground instabilities near the tide gauge. Through these activities the tide gauge datum within the Permanent Ser-vice for Mean Sea Level (PSMSL) has been established, which in turn, makes the derived KEP mean sea level (MSL) record highly valuable for long-term studies and satellite altimetry cali-brations. In this study, we will present the vertical land movement estimates from seven years of GNSS observations, five precise levelling campaigns, and will discuss their impact on the sea level record from the KEP tide gauge and nearby satellite altimetry sea surface heights. Our results confirm uplift all over South Georgia Island while the area at KEP and particularly the jetty with tide gauge are subsiding relative to the rest of the island. Using this information we correct the MSL record for the vertical land movements and investigate its signals together with those from nearby satellite altimetry tracks. [less ▲]

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See detailTest
Erkihune, Eshetu Nega UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

Poster (2020, December 11)

test

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See detailCross-Evaluation of Surface Meteorological Data and GNSS-derived Water Vapor with Re-analysis Information for South Georgia Island, South Atlantic Ocean
Erkihune, Eshetu Nega UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

Poster (2020, December 11)

As one of the most important components of the global hydrologic cycle, atmospheric water vapor shows significant variability in both space and time over a large range of scales. This variability results ... [more ▼]

As one of the most important components of the global hydrologic cycle, atmospheric water vapor shows significant variability in both space and time over a large range of scales. This variability results from the interactions of many different factors, including topography and the presence of specific atmospheric processes. One of the key regions for affecting global climatic variations lies in the sub-Antarctic zone over the Southern Ocean with its Antarctic Circumpolar Current and the associated Antarctic Convergence. There, in this cold and maritime region, lies South Georgia Island with its weather and climate being largely affected by both the dominating ocean currents and the strong east ward blowing winds in this zone. While the island forms an important outpost for various surface observations in this largely under-sampled and extremely remote region, it also forms a barrier for these winds due to its high topography, which, in turn, leads to various local meteorological phenomena, such as foehn winds. Surface meteorological data have been available for several stations near King Edward Point (KEP) on South Georgia for much of the 20th century. Since 2013 and 2014, Global Navigation Satellite System (GNSS) data have been available at five locations around the periphery of the island and during a few months in 2016 also radiosonde data have been collected at KEP. This study aims at investigating the consistency between the different surface meteorological data sets such as temperature, pressure and wind direction/speed that have been collected at KEP and a nearby GNSS station on Brown Mountain (BMT) for which we also compare the precipitable water vapor estimates. A cross-evaluation of these data sets with model values from the ERA-Interim re-analyses is carried out to further investigate the performance of both instruments and models. Overall, our preliminary results show high consistency between the surface meteorological observations and the re-analysis model values. It was our main objective to investigate the homogeneity and accuracy of the BMT observation time series through cross-evaluation with the series of the official WMO station at KEP. Air temperature and pressure at both sites from observation and model data are strongly correlated at hourly intervals, reaching correlation coefficients in the range of 0.966 - 0.968 for the former data set. The difference temperature time series shows seasonal variations but no obvious steps. The difference pressure time series is flat, also indicating no discontinuities. A cross-evaluation of the wind observations shows the distinct directional feature at KEP for a station in a valley where the winds are funneled through the valley. For BMT the wind observations confirm the main directions of winds but also show the openness of the station from all directions. The observations of temperature, pressure, humidity and GNSS-derived PWV clearly show the signatures of the frequent foehn events. [less ▲]

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See detailLow-cost GNSS for Meteorology: A receiver comparison
Monteiro, Cristiano; Hunegnaw, Addisu UL; Teferle, Felix Norman UL

Poster (2020, February 25)

GNSS, particularly the GPS and GLONASS satellite constellations are commonly used for accurate estimation of atmospheric parameters. The high accuracy is accomplished by sophisticated analysis methods and ... [more ▼]

GNSS, particularly the GPS and GLONASS satellite constellations are commonly used for accurate estimation of atmospheric parameters. The high accuracy is accomplished by sophisticated analysis methods and expensive high-end receivers and antennas along with precise products and bias corrections. The recent market introduction of low-cost dual frequency receivers that can produce raw data from multiple constellations offers an insight into the potential use of these receivers for meteorological applications. Here we demonstrate that GPS and GLONASS measurements from a low-cost dual-frequency receiver can be used to estimate the Zenith Total Delay ( ZTD) commensurate to meteorological applications. [less ▲]

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See detailMarkov Chain Monte Carlo and the Application to Geodetic Time Series Analysis
Olivares Pulido, German UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL

in Montillet, Jean-Philippe; Bos, Machiel (Eds.) Geodetic Time Series Analysis in Earth Sciences (2020)

The time evolution of geophysical phenomena can be characterised by stochastic time series. The stochastic nature of the signal stems from the geophysical phenomena involved and any noise, which may be ... [more ▼]

The time evolution of geophysical phenomena can be characterised by stochastic time series. The stochastic nature of the signal stems from the geophysical phenomena involved and any noise, which may be due to, e.g., un-modelled effects or measurement errors. Until the 1990's, it was usually assumed that white noise could fully characterise this noise. However, this was demonstrated to be not the case and it was proven that this assumption leads to underestimated uncertainties of the geophysical parameters inferred from the geodetic time series. Therefore, in order to fully quantify all the uncertainties as robustly as possible, it is imperative to estimate not only the deterministic but also the stochastic parameters of the time series. In this regard, the Markov Chain Monte Carlo (MCMC) method can provide a sample of the distribution function of all parameters, including those regarding the noise, e.g., spectral index and amplitudes. After presenting the MCMC method and its implementation in our MCMC software we apply it to synthetic and real time series and perform a cross-evaluation using Maximum Likelihood Estimation (MLE) as implemented in the CATS software. Several examples as to how the MCMC method performs as a parameter estimation method for geodetic time series are given in this chapter. These include the applications to GPS position time series, superconducting gravity time series and monthly mean sea level (MSL) records, which all show very different stochastic properties. The impact of the estimated parameter uncertainties on sub-sequentially derived products is briefly demonstrated for the case of plate motion models. Finally, the MCMC results for weekly downsampled versions of the benchmark synthetic GNSS time series as provided in Chapter 2 are presented separately in an appendix. [less ▲]

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See detailTracking Hurricanes using GPS atmospheric precipitable water vapor field
Ejigu, Yohannes Getachew; Teferle, Felix Norman UL; klos, Anna et al

in Beyond 100: The Next Century in Geodesy (2020)

Tropical cyclones are one of the most powerful severe weather events that produce devastating socioeconomic and environmental impacts in the areas they strike. Therefore, monitoring and tracking of the ... [more ▼]

Tropical cyclones are one of the most powerful severe weather events that produce devastating socioeconomic and environmental impacts in the areas they strike. Therefore, monitoring and tracking of the arrival times and path of the tropical cyclones are extremely valuable in providing early warning to the public and governments. Hurricane Florence struck the East cost of USA in 2018 and offers an outstanding case study. We employed Global Positioning System (GPS) derived precipitable water vapor (PWV) data to track and investigate the characteristics of storm occurrences in their spatial and temporal distribution using a dense ground network of permanent GPS stations. Our findings indicate that a rise in GPS-derived PWV occurred several hours before Florence’s manifestation. Also, we compared the temporal distribution of the GPS-derived PWV content with the precipitation value for days when the storm appeared in the area under influence. The study will contribute to quantitative assessment of the complementary GPS tropospheric products in hurricane monitoring and tracking using GPS-derived water vapor evolution from a dense network of permanent GPS stations [less ▲]

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See detailConsolidating Observation of Land and Sea Level Changes around South Georgia Island
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Hibbert, Angela et al

Poster (2019, December 13)

With its mid-ocean location in the Southern Atlantic Ocean South Georgia Island is in a key position for the oceanic and geodetic global monitoring networks. Since 2013 the tide gauge at King Edward Point ... [more ▼]

With its mid-ocean location in the Southern Atlantic Ocean South Georgia Island is in a key position for the oceanic and geodetic global monitoring networks. Since 2013 the tide gauge at King Edward Point (KEP) with GLOSS ID 187 has been monitored using a GNSS station nearby on Brown Mountain. By accurately geo-referencing the tide gauge and monitoring any vertical land movements, a continuous record of its datum within the Permanent Service for Mean Sea Level (PSMSL) can be established, which in turn makes the recorded and averaged sea levels useful for long-term studies and satellite altimetry calibrations. In 2014 another GNSS station was installed at KEP after local subsidence was sus-pected and later on three additional GNSS stations came to service at the periphery of the main island, making it possible to monitor uplift/subsidence wider afield. Further-more, together with four precise levelling campaigns of the KEP benchmark network in 2013, 2014 and two in 2017, it has also been possible to investigate the very local character of the vertical motions near KEP, i.e. the stability of the jetty upon which the tide gauge is mounted. In this study, we will present the results from the GNSS and precise levelling meas-urements, and will discuss their impact on the sea level record from the KEP tide gauge and nearby satellite altimetry sea surface heights. This study comes at a timely manner as during the Austral Summer 2019/2020 the jetty will be stabilized and en-larged, and consequently the current tide gauge will be replaced by a new one. Our measurements show that uplift is observed all over South Georgia Island while the ar-ea at KEP and particularly the jetty with tide gauge are subsiding relative to the rest of the island. In contrast, results for the tide gauge record show a lower magnitude of ob-served sea level rise than expected from nearby satellite altimetry. We will revisit all geodetic and oceanic observations in an attempt to improve the agreement between these measurements to summarize the status before the work at the jetty begins. [less ▲]

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See detailUsing the Vertical Land Movement estimates from the IGS TIGA combined solution to derive Global Mean Sea Level changes
Bogusz, Janusz; Hunegnaw, Addisu UL; Teferle, Felix Norman UL et al

Scientific Conference (2019, December 13)

Global mean sea level (GMSL) is now widely recognized to have risen between 1 to 2 mm/yr depending on location since the 20th century. Prior to the satellite altimetry era, GMSL was primarily estimated ... [more ▼]

Global mean sea level (GMSL) is now widely recognized to have risen between 1 to 2 mm/yr depending on location since the 20th century. Prior to the satellite altimetry era, GMSL was primarily estimated from a set of secular tide gauge records relative to coastal benchmarks. Recent measurements of GPS (Global Positioning System) have been demonstrated as a useful tool of a direct estimate of Vertical Land Motion (VLM) induced by both long and short-term geophysical and human-induced processes in a geocentric reference frame. This presentation will provide the results of a combination performed using the CATREF software of three independent GPS daily solutions provided by British Isles continuous GNSS Facility – University of Luxembourg consortium (BLT), German Research Centre for Geosciences (GFZ) and University of La Rochelle (ULR) under the auspices of the Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG), that results in a spatially comprehensive map of VLM near or close to tide gauge benchmarks. The combination was performed in accordance with the second re-processing campaign (repro2) of the IGS (International GNSS Service). Long coastal tide gauge records from the archives maintained at the Permanent Service for Mean Sea Level (PSMSL) were extracted for relative sea level estimates. To cross-compare the sea level rates over the years, we employed observations between 1900-2016. Then, the time series were cut and analyzed separately, ceteris paribus, for the period 1960-2016. This analysis was aimed at a cross-comparison of relative sea level trends and their changes over the years. The stochastic part of the tide gauge records was analyzed with Maximum Likelihood Estimation (MLE) and assumed several different combinations of noise models with the Bayesian Information Criterion (BIC) providing a means to identify the preferred one. The relative sea level estimates were corrected by the inverted barometric effect to the tide-gauge records using data from the 20th century Reanalysis project version V2C, the effect of wind stress on the surface of the ocean in both, zonal and meridional components, as well as Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO) influencing Pacific tide gauge records. The GPS-based velocities were corrected by Glacial Isostatic Adjustment (GIA) effect using ICE-6G(VM5a) model with associated geoid rate and post seismic decays using ITRF2014 estimates. Also, environmental loading models were employed to account for present-day elastic loading in VLM. The Mean Sea Level (MSL) trends from tide gauges and VLM-corrected MSL trends using GIA model (TG+GIA) and the TIGA combination (TG+TIGA) were determined. Our final reconstruction of GMSL based on the MSL records from 1900 to 2016 where the VLM uncertainty is smaller than 0.7 mm/yr indicate a long-term trend of 1.75 +/- 0.2 mm/yr and is in good agreement with several similar determinations. [less ▲]

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See detailTracking hurricanes Harvey and Irma using GPS tropospheric products
Ejigu, Yohannes Getachew; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

Poster (2019, December 10)

The 2017 Hurricanes season was one of the most powerful severe weather events producing catastrophic socio-economic and environmental effects on the east coast of the United States. Therefore, tracking ... [more ▼]

The 2017 Hurricanes season was one of the most powerful severe weather events producing catastrophic socio-economic and environmental effects on the east coast of the United States. Therefore, tracking their path accurately is extremely useful. Today Global Navigation Satellite Systems (GNSS) tropospheric products, such as Zenith Wet Delays (ZWD), and Integrated Water Vapor (IWV) are used as complementary data sets in Numerical Weather Prediction (NWP) models. In this study, we employed GPS-derived IWV and horizontal tropospheric gradient information to monitor and investigate the complicated characteristics of hurricane events in their spatial and temporal distribution using a dense ground network of GPS stations. Our results show that a surge in GPS-derived IWV occurred several hours prior to the manifestation of the major hurricanes Harvey and Irma. We used the derived GPS-derived IWV information as input to spaghetti lines weather models, allowing us to predict the paths of Harvey and Irma hurricanes. As such, a parameter directly estimated from GPS can provide an additional resource for improving the monitoring of hurricane paths [less ▲]

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See detailPresent-Day Land and Sea Level Changes around South Georgia Island: Results from Precise Levelling, GNSS, Tide Gauge and Satellite Altimetry Measurements
Teferle, Felix Norman UL; Dalziel, I W D; Hunegnaw, Addisu UL et al

Scientific Conference (2019, July 25)

South Georgia Island, the main land outcrop on the South Georgia microcontinent (SGM), is located approximately 1,400 km east of the Falkland Islands and approximately 1,400 km northeast of the ... [more ▼]

South Georgia Island, the main land outcrop on the South Georgia microcontinent (SGM), is located approximately 1,400 km east of the Falkland Islands and approximately 1,400 km northeast of the northernmost tip of the Antarctic peninsular. The SGM is believed to lie south of the North Scotia Ridge (NSR), which forms the boundary to the South America Plate, while to the south it is bordered by the Scotia Plate (SP). In its sub-Antarctic location, the island is largely covered by mountain glaciers which have been reported to be retreating due to climatic change. Furthermore, during past glaciation periods the island and its shelf area, stretching much of the SGM, have been ice covered as was revealed by scarring of the sub-oceanic topography. Together with ongoing tectonics along the NSR and recent seismicity at the SP boundary, these processes have the ability to produce significant uplift on local to regional scales. With its mid-ocean location in the Southern Atlantic Ocean South Georgia Island is in a key position for the oceanic and geodetic global monitoring networks. As these net-works suffer from a Hemisphere imbalance with the number of stations in the Northern Hemisphere outnumbering those in the Southern Hemisphere, operating these stations to the highest standards is of key scientific value. It is of particular interest to monitor the tide gauge (GLOSS ID 187) at King Edward Point (KEP) for vertical land movements to establish a continuous record of its datum within the Permanent Service for Mean Sea Level (PSMSL), which in turn makes it useful for long-term sea level studies and satellite altimetry calibrations. With the establishment of five GNSS stations on the islands by teams from Luxembourg, the UK and the USA during 2013 to 2015, and the scientific analysis of these data within a global network of stations, it has now become possible to study present-day vertical land movements and their impacts. Furthermore, together with four precise levelling campaigns of the KEP benchmark network in 2013, 2014 and two in 2017, it has also been possible to investigate the very local character of the vertical motions near KEP, i.e. the stability of the jetty upon which the tide gauge is mounted. In this study, we will present the still preliminary results from the GNSS and levelling measurements and will discuss their impact on the sea level record from the KEP tide gauge. Our measurements show that while South Georgia Island and the area around KEP are rising, the jetty and tide gauge are subsiding, leading to a lower magnitude of the observed sea level change than expected from satellite altimetry. In order to improve the agreement between these measurements both local and regional vertical land movements need to be monitored. [less ▲]

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See detailRecent Activities on Tristan da Cunha Island: Geodetic Installations, Local Tie Measurements and their Analysis
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Backes, Dietmar UL et al

Scientific Conference (2019, July 11)

During 2017 a team from the University of Luxembourg and the National Oceanography Centre, Liverpool, established a permanent Global Navigation Satellite System (GNSS) station and two new tide gauges on ... [more ▼]

During 2017 a team from the University of Luxembourg and the National Oceanography Centre, Liverpool, established a permanent Global Navigation Satellite System (GNSS) station and two new tide gauges on Tristan da Cunha Island in the South Atlantic Ocean. These installations were funded through various projects at both collaborating institutions under the umbrella of the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group and the Global Geodetic Observing System (GGOS) focus area on Sea Level Change, Variability and Forecasting. While this was the first scientific installation of a GNSS station on the main island within the Tristan da Cunha archipelago, IGS station GOUG, located on Gough Island which lies 412 km to the south, has been in operation since 1998. Unfortunately GOUG was decommissioned in 2018. Sea level observations on Tristan da Cunha have a longer history than GNSS with various tide gauges having been in operation since 1984. Tristan da Cunha also hosts a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) station which was established in 2012 after a previous installation was upgraded and moved to the current site. The antenna TCTA is located on the concrete monument of the previous DORIS antenna. Furthermore, in order for future International Terrestrial Reference Frame (ITRF) computations to fully benefit from the proximity of the sensors, the geodetic ties between the respective antennas (and reference markers in case of the tide gauges) need to be determined at the millimeter level using various terrestrial surveying methods and a local benchmark network. This contribution provides details of the activities on Tristan da Cunha including the installations, the established benchmark network, the terrestrial surveys of the geodetic ties and the analysis of these measurements in order to geometrically link the GNSS and DORIS antennas to each other as well as to the tide gauges. [less ▲]

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See detailImproved Monitoringand Tracking Hurricanes using GPS Atmospheric WaterVapour
Ejigu, Yohannes Getachew; Teferle, Felix Norman UL; klose, Anna et al

Poster (2019, April 09)

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See detailImpact of GPS Antenna Phase Center Models on Zenith Wet Delay and Tropospheric Gradients
Ejigu, Yohannes Getachew; Hunegnaw, Addisu UL; Abraha, Kibrom Ebuy UL et al

in GPS Solutions (2019), 23(5),

We demonstrate the potential for the Global Positioning System (GPS) to provide highly accurate tropospheric products for use in meteorological applications. Tropospheric products, in particular the wet ... [more ▼]

We demonstrate the potential for the Global Positioning System (GPS) to provide highly accurate tropospheric products for use in meteorological applications. Tropospheric products, in particular the wet delays, are treated as an unknown parameter in GPS processing and are estimated with other parameters such as station coordinates. In this study, we investigate the effects of Phase Center Correction (PCC) models on tropospheric zenith wet delay (ZWD), integrated water vapor (IWV) and gradient products. Two solutions were generated using the GAMIT software over the EUREF Permanent GNSS Network (EPN). The first (reference) solution was derived by applying the International GNSS Service (IGS) type-mean PCC model, while for the second solution PCC models from individual calibrations were used. The solutions were generated identically, except for the PCC model differences. The two solutions were compared, with the assumption that common signals are differenced out. The comparison of the two solutions clearly shows a bias in all tropospheric products, which can be attributed to PCC model deficiencies. Overall, mean biases of ±1.8, ±0.3, ±0.14 and ±0.19 mm are evident in ZWD, IWV, North-South and East-West gradients, respectively. Moreover, the differences between the two solutions show seasonal variations. For all antenna types, the ZWD and IWV differences are dominated by white plus power-law noise, with latter characterizing the low-frequency spectrum. On the other hand, the horizontal gradients exhibit a white plus first order autoregressive noise characteristic with less than 1% white noise. The individual PCC model provides a better fit to an external independent model in terms of gradient estimate and also provides up to 3 % more carrier phase ambiguity resolution. [less ▲]

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