Assessment of BLT Tide Gauge Benchmark Monitoring (TIGA) repro2 solutions

Poster (2015)

In 2013 the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG) started their reprocessing campaign, which proposes to re-analyze all relevant Global Positioning ... [more ▼]

In 2013 the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG) started their reprocessing campaign, which proposes to re-analyze all relevant Global Positioning System (GPS) observations from 1994 to 2013. This re-processed dataset will provide high quality estimates of land motions, enabling regional and global high-precision geophysical/geodetic studies. Several of the individual TIGA Analysis Centres (TACs) have completed processing the full history of GPS observations recorded by the IGS global network, as well as, many other GPS stations at or close to tide gauges, which are available from the TIGA data centre at the University of La Rochelle (www.sonel.org). Following the recent improvements in processing models and strategies, this is the first complete reprocessing attempt by the BLT TIGA Analysis centre to provide homogeneous position time series. We report the quality of the multi-year daily solutions from the consortium of the British Isles continuous GNSS Facility (BIGF) and the University of Luxembourg TIGA Analysis Centres (BLT) based on the Bernese GNSS Software Version 5.2 using a double difference (DD) network processing strategy. [less ▲]

A Multi-Year Combination of Tide Gauge Benchmark Monitoring (TIGA) Analysis Center Products: Preliminary results

Poster (2014, December 16)

Impact of Station GNSS Antenna Phase Centre Calibrations on Satellite Orbits and Station Coordinates: Preliminary Results

Poster (2014, October 14)

The electromagnetic phase centre of a Global Navigation Satellite System (GNSS) antenna does not coincide with the physical one and depends on the azimuth and elevation of the incoming signals. To improve ... [more ▼]

The electromagnetic phase centre of a Global Navigation Satellite System (GNSS) antenna does not coincide with the physical one and depends on the azimuth and elevation of the incoming signals. To improve processing results, the International GNSS Service has developed antenna phase centre corrections (PCC), which include models for constant and variable biases, i.e. phase centre offsets and phase centre variations. By assuming that the small differences in PCCs between antenna/radome combinations of the same type/model are negligible, so called type-mean PCCs have been generated and, for practical reasons, have been widely applied. The type-mean PCCs are averaged corrections, which are composed from several individual PCCs. Contrary to the general assumption, individual PCCs may sometimes be significantly different for antennas of the same type and it can be argued, that the use of type-mean instead of individual calibrations may degrade GNSS-derived products, such as satellite orbits and station coordinates. Furthermore, through simple geometric considerations it can be shown that inaccuracies in the PCCs may propagate into time series of GNSS solutions and may contribute some power at the orbital frequencies and their harmonics. In this study we assess the impact of the applied antenna PCCs on satellite orbits and station coordinates. As the availability of individual PCCs for GNSS stations is very limited, we analyzed a global network with stations mainly located in Europe. We used 10 years of GPS data in our processing. Despite the limitations imposed by the poor network geometry, we observed improvements in orbit overlaps at day boundaries when individual antenna PCCs were used compared to the respective type-mean solutions. Additionally, we analyzed the impact of the applied PCCs on Precise Point Positioning (PPP) station coordinates using our computed orbits. Our preliminary results showed that out of two PPP runs, the one with individual PCCs and respective orbit set reduced the noise in solutions compared to the other PPP run. We conclude that the use of individual PCCs is advantageous for derived GNSS products, as improvements are observed both in the estimated satellite orbits and station coordinate time series. [less ▲]

Near Field Dynamic, Co-seismic and Post-seismic Deformations Associated with the 2013, M7.8, and 2003, M7.6, South Scotia Ridge Earthquakes Observed with GPS

Poster (2014, September)

The South Scotia Ridge (SSR) left-lateral transform/strike-slip (S-S) fault defines the Scotia plate’s (SP) southern boundary separating it from the Powell Basin (PB), South Orkney Microcontinent (SOM ... [more ▼]

The South Scotia Ridge (SSR) left-lateral transform/strike-slip (S-S) fault defines the Scotia plate’s (SP) southern boundary separating it from the Powell Basin (PB), South Orkney Microcontinent (SOM), and the Weddell Sea sections of the Antarctic plate (AP). The SP developed as a space filling accommodation zone for S. America-Antarctica relative motions, mostly during the last 40 m.y. The SSR also hosts several restraining and releasing bends. The SP, PB and SOM have complex evolution histories including large-scale displacement and stretching of the SOM, as well as other continental fragments within the SP, all of which were incorporated into a background of changing sea floor spreading geometries. The SOM defines an ~300 km segment of the SSR opposite a section of the SP that is primarily oceanic crust with a few small, stretched continental fragments. Two large earthquakes, M7.6 and 7.8, with aftershock zones largely confined to the northern SOM boundary, occurred on the SSR in 2003 and 2013. Moment tensor solutions show they occurred on faults dipping ~30 and 45° to the south. The 2013 event was almost pure, left-lateral strike-slip, while the 2003 event was oblique but predominantly strike-slip. This is an unusual combination of fault dip and slip direction for a strike-slip plate boundary. The half duration of both events is also relatively long. A continuous GPS (CGPS) station on Laurie Island is located immediately west of the rupture zone of the 2003 event and at the approximate center, and close to the surface projection, of the finite fault models for the 2013 earthquake. We present co-seismic static offsets and post-seismic transients for both earthquakes from GPS daily position estimates. In addition, the CGPS station now records at 1 Hz and we present the GPS displacement seismogram for the 2013 event. This record contains a complex signal that includes the passage of the Love and Rayleigh surface waves, with max displacements of ~70 cm, over an ~80 second time interval during which a ~50 cm static offset developed. [less ▲]

SUGAC: Sofia University GNSS Analysis Center

Poster (2014, June 25)

The Sofia University GNSS Analysis Centre (SUGAC, suada.phys.uni-sofia.bg) is a new analysis centre established via collaboration between the Department of Meteorology and Geophysics of Sofia University ... [more ▼]

The Sofia University GNSS Analysis Centre (SUGAC, suada.phys.uni-sofia.bg) is a new analysis centre established via collaboration between the Department of Meteorology and Geophysics of Sofia University, the IPOS - BuliPOS GNSS network in Bulgaria and the University of Luxembourg. In April 2014, the first processing campaign took place. One year GNSS data from 7 stations of the BuliPOS network are processed in collaboration with the University of Luxembourg. Tropospheric products (Zenith Total Delay and gradients) with 5 min temporal resolution are obtained using the NAPEOS software, developed by ESA. The tropospheric products from this campaign will be used for validation of the Weather Research and Forecasting (WRF) model as well as for case studies during intense precipitation events and fog. In this work the WRF model validation for Bulgaria will be presented. Future work will be the establishment of autonomous near real-time processing of the regional ground-based GNSS network in Southeast Europe in support of the EUMETNET E-GVAP and COST ES1206 ”Advanced Global Navigation Satellite Systems for Severe Weather Events and Climate” projects. [less ▲]

Geological interpretation of current subsidence and uplift in the London area, UK, as shown by high precision satellite-based surveying

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 ▲]

Comparative Analysis of Real-Time Precise Point Positioning Zenith Total Delay Estimates

Poster (2013, December 13)

The use of observations from Global Navigation Satellite Systems (GNSS) in operational meteorology is increasing worldwide due to the continuous evolution of GNSS. The assimilation of near real-time (NRT ... [more ▼]

The use of observations from Global Navigation Satellite Systems (GNSS) in operational meteorology is increasing worldwide due to the continuous evolution of GNSS. The assimilation of near real-time (NRT) GNSS-derived zenith total delay (ZTD) estimates into local, regional and global scale numerical weather prediction (NWP) models is now in operation at a number of meteorological institutions. The development of NWP models with high update cycles for nowcasting and monitoring of extreme weather events in recent years, requires the estimation of ZTD with minimal latencies, i.e. from 5 to 10 minutes, while maintaining an adequate level of accuracy for these. The availability of real-time (RT) observations and products from the IGS RT service and associated analysis centers make it possible to compute precise point positioning (PPP) solutions in RT, which provide ZTD along with position estimates. This study presents a comparison of the RT ZTD estimates from three different PPP software packages (G-Nut/Tefnut, BNC2.7 and PPP-Wizard) to the state-of-the-art IGS Final Troposphere Product employing PPP in the Bernese GPS Software. Overall, the ZTD time series obtained by the software packages agree fairly well with the estimates following the variations of the other solutions, but showing various biases with the reference. After correction of these the RMS differences are at the order of 0.01 m. The application of PPP ambiguity resolution in one solution or the use of different RT product streams shows little impact on the ZTD estimates. [less ▲]

The King Edward Point Geodetic Observatory in Support of Sea Level Research

Poster (2013, October 28)

During February 2013 the King Edward Point (KEP) Geodetic Observatory was established in South Geor- gia, 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 Geor- gia, 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 give details of the establishment of the KEP Geodetic Observatory, i.e. the installation of the continuous GNSS station KEPA on Brown Mountain and the estab- lishment of a new height datum for the tide gauge through a network of benchmarks at the KEP research station. We will present an evaluation of the GNSS positioning results for the period from February to August 2013 and of the survey/levelling work carried out for the height reference. 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 ▲]

Antenna phase centre calibration effects on position time-series: preliminary results

Poster (2013, September 01)

Advances in GPS error modelling and the continued effort of re-processing have considerably decreased the scatter in position estimates over the last two decades. The associated reduction of noise in ... [more ▼]

Advances in GPS error modelling and the continued effort of re-processing have considerably decreased the scatter in position estimates over the last two decades. The associated reduction of noise in derived position time-series has revealed the presence of previously undetected periodic signals. It has been shown that these signals have frequencies related to the orbits of GPS satellites. A number of potential sources for these periodicities at the draconitic frequency and its harmonics have already been suggested in the literature and include, e.g. errors in the sub-daily tidal models, multipath and unresolved integer ambiguities. Due to the geometrical relationship between an observation point and an orbiting satellite, deficiencies in the modelling of electromagnetic phase centres of receiving antennas have the potential to also contribute to the discovered periodic signals. The change from relative to absolute type mean antenna/radome calibrations within the international GNSS service (IGS) lead to a significant improvement and the use of individual calibrations could add further refinements to computed solutions. However, at this stage providing individual calibrations for all IGS stations is not feasible. Furthermore, antenna near-field electromagnetic effects might out-weight the benefits of individual calibrations once an antenna is permanently installed. In this study we investigate the differences between position estimates obtained using individual and type mean antenna/radome calibrations as used by the IGS community. We employ position time-series derived from precise point positioning (PPP) as implemented in two scientific GNSS software packages. Our results suggest that the differences in the employed calibrations propagate directly into the position estimates, affecting both sub-daily and daily results and yielding periodic variations. The sub-daily variations have periods close to half a sidereal day and one sidereal day with amplitudes of up to 10 mm in all position components. The stacked power spectra of the daily difference time-series reveal peaks at the GPS draconitic frequency and its harmonics, having the associated amplitudes of up to 1 mm. Although these results are still preliminary, they confirm that small differences between individual and type mean antenna/radome calibrations propagate into position time series and may be partly responsible for the spurious signals with draconitic frequency and its harmonics. [less ▲]

Assessing the Status of GNSS Data Processing Systems to Estimate Integrated Water Vapour for Use in Numerical Weather Prediction models

Poster (2013, September)

Modern Numerical Weather Prediction (NWP) models make use of the GNSS derived Zenith Total Delay (ZTD) or Integrated Water Vapour (IWV) estimates to enhance the quality of their forecasts. Usually, the ... [more ▼]

Modern Numerical Weather Prediction (NWP) models make use of the GNSS derived Zenith Total Delay (ZTD) or Integrated Water Vapour (IWV) estimates to enhance the quality of their forecasts. Usually, the ZTD is assimilated into the NWP models on hourly intervals but with the advancement of NWP models towards higher update rates, it has become necessary to estimate the ZTD on sub-hourly intervals. In turn, this imposes requirements related to the timeliness and accuracy of the ZTD estimates and has lead to a development of various strategies to process GNSS observations to obtain ZTD with different latencies and accuracies. Using present GNSS products and tools, ZTD can be estimated in real-time (RT), near real-time (NRT) and post-processing (PP) modes. The aim of this study is to provide an overview and accuracy assessment of various RT, NRT, and PP IWV estimation systems and comparing their achieved accuracy with the user requirements for GNSS meteorology. The NRT and PP systems are based on the Bernese GNSS Software v5.2 using a double-difference network and Precise Point Positioning (PPP) strategy, and the RT systems are based on BKG Ntrip Client 2.7 and PPP-Wizard both using PPP. One of the RT systems allows integer ambiguity resolution with PPP and therefore the effect of fixing integer ambiguities on ZTD estimates will also be presented. [less ▲]