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See detailVertical Land Movements and Sea Level Changes around South Georgia Island
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Abraha, Kibrom Ebuy UL et al

Poster (2018, April 09)

South Georgia Island in the Southern Atlantic Ocean is a key location for the seismic, geomagnetic and oceanic global monitoring networks. In its sub-Antarctic location, the island is largely covered by ... [more ▼]

South Georgia Island in the Southern Atlantic Ocean is a key location for the seismic, geomagnetic and oceanic global monitoring networks. 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 have been ice covered as was revealed by scarring of the sub-oceanic topography. Together with ongoing tectonics along the North Scotia Ridge, these processes have the ability to produce significant uplift on local to regional scales, affecting the measurements of the tide gauge (GLOSS ID 187) at King Edward Point (KEP). Furthermore, with its mid-ocean location, the tide gauge is of particular interest to satellite altimetry calibrations over the Southern Atlantic and Southern Oceans. With the establishment of five GNSS stations on the islands during 2013 to 2015 and the scientific analysis of these data within the global network of stations of the International GNSS Service Tide Gauge Benchmark Monitoring (TIGA) working group, it has now become possible to study present-day vertical land movements of the region and their impacts on, for example, regional sea level. 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, ie. 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 disagreement in the observed sea level change from the tide gauge and satellite altimetry. In order to improve the agreement between these sea level measurements both local and regional vertical land movements need to be monitored. [less ▲]

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See detailFirst Vertical Land Movement Estimates on South Georgia Island: An Impact Study on Sea Level Change from Tide Gauge and Altimetry Measurements
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Abraha, Kibrom Ebuy UL et al

Poster (2017, December 11)

South Georgia Island in the Southern Atlantic Ocean has been a key location for the seismic, geomagnetic and oceanic global monitoring networks. However, no permanent geodetic monitoring station had been ... [more ▼]

South Georgia Island in the Southern Atlantic Ocean has been a key location for the seismic, geomagnetic and oceanic global monitoring networks. However, no permanent geodetic monitoring station had been established there despite the lack of observations from this region within, for example, the International GNSS Service (IGS) network of Global Navigation Satellite System (GNSS) stations. Then, in 2013 the King Edward Point (KEP) Geodetic Observatory was established with a focus on sea level studies and in support of general geoscience applications. Currently, this observatory located roughly half-way along the main island and along its northern coastline, consists of two GNSS stations (KEPA and KRSA) with local benchmark networks, allowing the height determinations from the GNSS antennas to be transferred to the KEP tide gauge (GLOSS ID 187) and forming a height reference within the International Terrestrial Reference Frame. In late 2014, three additional GNSS stations (SG01, SG02 and SG03) were established, all on small islands at the perimeter of the main island. Together the stations provide the best possible opportunity to study various geophysical processes in the region. With the GNSS-derived position time series partly reaching over 4.5 years in length, it has become possible to provide first estimates of vertical land movements for the island and KEP with its surrounding area. Together with four precise levelling campaigns of the benchmark network in 2013, 2014 and two in 2017, it has also been possible to investigate the very local character of the vertical motions, ie. the stability of the jetty upon which the tide gauge is mounted. Our measurements show that while South Georgia Island and the area around KEP are rising, the jetty and tide gauge are subsiding. In this study, we will present the preliminary results from the GNSS and levelling measurements and will discuss their impact on the sea level record from the KEP tide gauge which is ideally situated in a mid-ocean location for satellite altimetry calibration over the Southern Atlantic and Southern Oceans. [less ▲]

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See detailRecent Activities at the King Edward Point Geodetic Observatory, South Georgia, in Support of TIGA Objectives
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Williams, Simon et al

Poster (2014, July 26)

In 2013 the King Edward Point (KEP) Geodetic Observatory was established in South Georgia, South Atlantic Ocean, in support of geoscience applications. South Georgia Island has been a key location for the ... [more ▼]

In 2013 the King Edward Point (KEP) Geodetic Observatory was established in South Georgia, South Atlantic Ocean, in support of geoscience applications. South Georgia Island has been a key location for the seismic, geomagnetic and oceanic global monitoring networks. However, no geodetic permanent monitoring station had been established there despite the lack of observations from this region within the International GNSS Service (IGS) network of Global Navigation Satellite System (GNSS) stations. Currently, the observatory consists of two state-of-the-art GNSS stations (KEPA 42701M001 and KRSA 42702M001) with local benchmark networks, allowing the height determinations from the GNSS antennas to be transferred to the tide gauge (Global Sea Level Observing System 187) and forming a height reference within the International Terrestrial Reference Frame. In this study, we will present an evaluation of the GNSS observations from the KEP Geodetic Observatory for the period from February 2013 to December 2013. We calculate multipath and positioning statistics and compare these to those from IGS stations. We report on the benchmark network and tide board installation, as well as, on the results from the two levelling campaigns carried out to date. For the future it is envisaged that the stations will contribute to the TIGA objective of monitoring vertical land movements at tide gauges, and that KEPA will contribute to the IGS network. [less ▲]

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See detailGeological interpretation of current subsidence and uplift in the London area, UK, as shown by high precision satellite-based surveying
Aldiss, Don; Burke, Helen; Chacksfield, Barrie et al

in Proceedings of the Geologists' Association (2014), 125(1), 1-13

Abstract Long term planning for flood risk management in coastal areas requires timely and reliable information on changes in land and sea levels. A high resolution map of current changes in land levels ... [more ▼]

Abstract Long term planning for flood risk management in coastal areas requires timely and reliable information on changes in land and sea levels. A high resolution map of current changes in land levels in the London and Thames estuary area has been generated by satellite-based persistent scatterer interferometry (PSI), aligned to absolute gravity (AG) and global positioning system (GPS) measurements. This map has been qualitatively validated by geological interpretation, which demonstrates a variety of controlling influences on the rates of land level change, ranging from near-surface to deep-seated mechanisms and from less than a decade to more than 100,000 years’ duration. During the period 1997–2005, most of the region around the Thames estuary subsided between 0.9 and 1.5 mm a−1 on average, with subsidence of thick Holocene deposits being as fast as 2.1 mm a−1. By contrast, parts of west and north London on the Midlands Microcraton subsided by less than 0.7 mm a−1, and in places appear to have risen by about 0.3 mm a−1. These rates of subsidence are close to values determined previously by studies of Quaternary sequences, but the combined GPS, AG and PSI land level change data demonstrate a new level of local geological control that was not previously resolvable. [less ▲]

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See detailFuture and development of the European Combined Geodetic Network ECGN
Poutanen, Markku; Ihdes, Johannes; Bruyninx, Carine et al

in Rizos, C.; Willis, Pascal (Eds.) Earth on the Edge: Science for a Sustanaible Planet (2014)

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See detailPotential Contributions to Geoscience from GNSS Observations of the King Edward Point Geodetic Observatory, South Georgia, South Atlantic Ocean
Teferle, Felix Norman UL; Hunegnaw, Addisu UL; Ahmed, Furqan UL et al

Poster (2013, September 01)

During February 2013 the King Edward Point (KEP) Geodetic Observatory was established in South Georgia, South Atlantic Ocean, through a University of Luxembourg funded research project and in ... [more ▼]

During February 2013 the King Edward Point (KEP) Geodetic Observatory was established in South Georgia, South Atlantic Ocean, through a University of Luxembourg funded research project and in collaboration with the United Kingdom’s National Oceanography Centre, British Antarctic Survey and Unavco, Inc. Due to its remote location in the South Atlantic Ocean, as well as, being one of few subaerial exposures of the Scotia plate, South Georgia Island has been a key location for a number of global monitoring networks, e.g. seismic, geomagnetic and oceanic. However, no geodetic monitoring station has been established, e.g. by the International Global Navigation Satellite System (GNSS) Service (IGS) community, despite the lack of such observations from this region. In this study we will present an evaluation of the GNSS observations from the KEP Geodetic Observatory for the period from February to August 2013. We calculate multipath and positioning statistics and compare these to those from IGS stations. The on-site meteorological data is compared to those from the nearby KEP meteorological station and global numerical weather models, and the impact of these data sets on delay and integrated water vapour estimates will be evaluated. We will discuss the installation in terms of its potential contributions to sea level observations using tide gauges and satellite altimetry, studies of tectonics, glacio-isostatic adjustment and atmospheric processes. [less ▲]

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See detailDetecting offsets in GPS time series: First results from the detection of offsets in GPS experiment
Gazeaux, Julien; Williams, Simon; King, Matt et al

in Journal of Geophysical Research. Solid Earth (2013), 118

The accuracy of Global Positioning System (GPS) time series is degraded by the presence of offsets. If these are not detected and adjusted correctly they bias velocities, and hence geophysical estimates ... [more ▼]

The accuracy of Global Positioning System (GPS) time series is degraded by the presence of offsets. If these are not detected and adjusted correctly they bias velocities, and hence geophysical estimates, and degrade the terrestrial reference frame. They also alter apparent time series noise characteristics as undetected offsets resemble a random walk process. As such, offsets are a substantial problem. A number of offset detection methods have been developed across a range of fields, and some of these are now being tested in geodetic time series. The DOGEx (Detection of Offsets in GPS Experiment) project aims to test the effectiveness of automated and manual offset detection approaches and the subsequent effect on GPS-derived velocities. To do this, simulated time series were first generated that mimicked realistic GPS data consisting of a velocity component, offsets, white and flicker noises (1/f spectrum noises) composed in an additive model. We focus on offset detection and together with velocity biases induced by incorrect offset detection. We show that, at present, manual methods (where offsets are hand -picked by GPS time series experts) almost always give better results than automated or semi-automated methods (two automated methods give quite similar velocity bias as the best manual solutions). For instance, the 5th percentile ranges (5% to 95%) in velocity bias for automated approaches is equal to 4.2mm/year,whereas it is equal to 1.8mm/yr for the manual solutions. However the True Positive detection rate of automated solutions is significantly higher than those for the manual solutions, being around 37% for the best automated, and 42% for the best manual solution. The amplitude of offsets detectable by automated solutions is greater than for hand picked solutions, with the smallest detectable offset for the two best manual solutions equal to 5mm and 7mm and to 8mm and 10mm for the two best automated solutions. The best manual solutions yielded velocity biases from the truth commonly in the range ±0.2mm/yr, whereas the best automated solutions produced biases no better than double this range. Assuming the simulated time series noise levels continue to be representative of real GPS time series, robust geophysical interpretation of individual site velocities lower than these levels is therefore not robust. Further work is required before we can routinely interpret sub-mm/yr velocities for single GPS stations. [less ▲]

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See detailMeasuring Changes in Ground Level at Tide Gauges using Continuous GPS and Absolute Gravimetry to Improve Estimates of Changes in Sea Level Around Britain
Bingley, Richard M; Teferle, Felix Norman UL; Dodson, Alan H et al

in Proceedings of the 41st Defra Flodd and Coastal Management Conference 2006 (2006)

Researchers studying climate change have used historical tide gauge measurements from all over the world to investigate the changes in sea level that have occurred over the last century or so. However ... [more ▼]

Researchers studying climate change have used historical tide gauge measurements from all over the world to investigate the changes in sea level that have occurred over the last century or so. However, such estimates are a combination of any true sea level variations and any changes in ground level at the specific tide gauge. For a tide gauge record to be used to determine the climate related changes in sea level it is necessary to correct for any changes in ground level. The development in geodetic techniques for monitoring changes in ground level at British tide gauges has been on-going at the IESSG and POL since 1990, based on research funded by both Defra and the Environment Agency. Since 1996, this research has focused on the establishment of continuous GPS (CGPS) stations and the use of absolute gravity (AG), as independent geodetic techniques for measuring changes in ground level. This paper details the results of Defra/EA R&D project FD2319, which is part of the Risk Evaluation and Understanding of Uncertainty Theme. The paper shows how CGPS and AG have been used on a national scale to monitor changes in ground level at tide gauges and obtain estimates of changes in sea level, decoupled from changes in ground level. [less ▲]

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See detailThe Effect of Processing Technique and Reference Frame Definition on Noise in CGPS Position Time Series
Teferle, Felix Norman UL; Williams, Simon; Kierulf, Halfdan et al

Poster (2005)

In this presentation we investigate the effects of GPS processing techniques and strategies, and the related reference frame realization, on the stochastic properties of continuous GPS (CGPS) position ... [more ▼]

In this presentation we investigate the effects of GPS processing techniques and strategies, and the related reference frame realization, on the stochastic properties of continuous GPS (CGPS) position time series. It was of particular interest to establish whether and how different GPS processing techniques and strategies, e.g. double differencing (DD) and precise point positioning (PPP), and the use of different orbit and clock products, and/or the definition of the reference frame (partly dependent on the applied strategy) affect the colored noise content of time series. We used CGPS position time series from 15 different solutions obtained from seven different analysis centers as part of the European Sea Level Service - Research Infrastructure project (ESEAS-RI) using the GIPSY OASIS II, GAMIT and Bernese GPS softwares. All time series analyzed have at least three years of data for the period between 2000 and 2005. Furthermore, a selected set of position time series was also analyzed using Empirical Orthogonal Function (EOF) analysis. The noise content of the first 15 modes, representing the solution-specific common mode time series for each of the selected solutions were then also investigated for colored noise. Using Maximum Likelihood Estimation (MLE) a white, a white plus flicker, a white plus power- law and a white plus first-order Gauss-Markov (FOGM) noise model were fitted to the position and EOF time series data. For both the position and EOF time series the parameter model included a constant, a rate and harmonic terms with annual, semi-annual, 4- monthly, 3-monthly, 2.4-monthly and 13.66 day periods. Position jumps were modeled at logged epochs or at visible discontinuities in the time series. The MLE showed that in most cases the best fitting noise model is a combination of white plus power-law noise with average spectral indices in the range between -0.5 and -1.4. This model is closely followed by the combination of white plus flicker and white plus FOGM noise. The noise properties of the EOF time series follow predominantly a white plus power-law character, with the first few modes indicating a white plus flicker noise behavior. In general, DD solutions contain less noise than PPP solutions and that regional reference frame definitions further reduce the amount of noise in the time series. [less ▲]

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