Global Navigation Satellite System (GNSS) at Tide Gauge Installations in the South Atlantic Ocean

Learning material (2017)

Laboratory experiment for damage assessment using the DAD-method

in Conference proceedings SMAR 2017 (2017, September)

In the following, a new analytical method, the Deformation Area Difference (DAD) method for damage assessment of bridge structures is applied using experimental test results of a statically loaded beam ... [more ▼]

In the following, a new analytical method, the Deformation Area Difference (DAD) method for damage assessment of bridge structures is applied using experimental test results of a statically loaded beam. An essential prerequisite for the application of this method is a high precise measurement of the deflection line. In this paper, the results from a laboratory experiment using modern measurement techniques such as photogrammetry and displacement sensors are discussed. A reinforced concrete beam is stepwise loaded until reaching the ultimate limit state. The DAD-method is applied to the resulting data from the measurements and the outcome is discussed for further optimisation of the method. In principle, the measured deflection line of the beam contains already essential information on discontinuities which occur due to cracking. These entries are processed and visualised using the DAD-method. This study shows that a high accuracy of the measurement techniques in combination with the DAD-method can become an effective tool for damage detection. [less ▲]

On the Scientific Applications of IGS Products: An Assessment of the Reprocessed TIGA Solutions and Combined Products

Scientific Conference (2017, July 03)

Global sea levels have risen since the early 19th century and this rise is likely to accelerate through the 21st century and beyond. Much of the past information on sea level rise stems from the ... [more ▼]

Global sea levels have risen since the early 19th century and this rise is likely to accelerate through the 21st century and beyond. Much of the past information on sea level rise stems from the instrumental records of tide gauges, which measure changes in sea level relative to a tide gauge benchmark (TGBM) situated on land. In order to assess regional or global sea level changes the vertical land movements (VLM) at the tide gauge and its TGBM need to be monitored. GNSS, in particular GPS, has been recognized as one space-geodetic technique to provide highly accurate estimates of VLM in a geocentric reference frame for tide gauges and their TGBMs. As it turned out, this scientific application of GNSS poses the most stringent requirements on the consistency and homogeneity on the data, processing strategies, satellite products, bias models and reference frames used in the analysis of GNSS measurements. Under the umbrella of the International GNSS Service (IGS), the Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG) has the objective to provide highly-accurate positions and VLM estimates for a global network of tide gauges contributing to the Global Sea Level Observing System (GLOSS) and the Permanent Service for Mean Sea Level (PSMSL). As such TIGA forms an important contribution of the IGS to the goals of the Global Geodetic Observing System (GGOS), the Global Climate Observing System (GCOS) and the World Climate Research Programme (WCRP). To achieve the TIGA-WG objectives, five TIGA Analysis Centers (TACs) contributed re-processed global GPS network solutions to TIGA, employing the latest bias models and processing strategies in accordance with the second IGS re-processing campaign (repro2). These individual TAC solutions were then used to compute the combined products by the TIGA Combination Centre (TCC) at the University of Luxembourg using an in-house modified version of the CATREF software package. In this study, we present and internally evaluate the individual TAC and TIGA combined products. We investigate station positions, scale and origin biases, including their frequency content. We also externally evaluate the combined products, particularly the VLM estimates, using solutions from the ITRF2008, ITRF2014 and the glacial isostatic adjustment model ICE-6G (VM5a). Finally, we draw some conclusions on the recent advances and remaining limitations of the various IGS products required for the challenging application to sea level studies. [less ▲]

A New Global Vertical Land Movement Data Set from the TIGA Combined

Poster (2017, April 23)

Globally averaged sea level has been estimated from the network of tide gauges installed around the world since the 19th century. These mean sea level (MSL) records provide sea level relative to a nearby ... [more ▼]

Globally averaged sea level has been estimated from the network of tide gauges installed around the world since the 19th century. These mean sea level (MSL) records provide sea level relative to a nearby tide gauge benchmark (TGBM), which allows for the continuation of the instrumental record in time. Any changes in the benchmark levels, induced by vertical land movements (VLM) affect the MSL records and hence sea level estimates. Over the last two decades sea level has also been observed using satellite altimeters. While the satellite observations are globally more homogeneous providing a picture of sea level not confined to coastlines, they require the VLM-corrected MSL records for the bias calibration of instrumental drifts. Without this calibration altimeter instruments from different missions cannot be combined. GPS has made it possible to obtain highly accurate estimates of VLM in a geocentric reference frame for stations at or close to tide gauges. Under the umbrella of the International GNSS Service (IGS), the Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG) has been established to apply the expertise of the GNSS community to solving issues related to the accuracy and reliability of the vertical component to provide estimates of VLM in a well-defined global reference frame. To achieve this objective, five TIGA Analysis Centers (TACs) contributed re-processed global GPS network solutions to TIGA, employing the latest bias models and processing strategies in accordance with the second re-processing campaign (repro2) of the IGS. These solutions include those of the British Isles continuous GNSS Facility – University of Luxembourg consortium (BLT), the German Research Centre for Geosciences (GFZ) Potsdam, the German Geodetic Research Institute (DGF) at the Technical University of Munich, Geoscience Australia (AUT) and the University of La Rochelle (ULR). In this study we present to the sea level community an evaluation of the VLM estimates from the first combined solution from the IGS TIGA WG. The TAC solutions include more than 700 stations and span the common period 1995-2014. The combined solution was computed by the TIGA Combination Centre (TCC) at the University of Luxembourg, which used the Combination and Analysis of Terrestrial Reference Frame (CATREF) software package for this purpose. This first solution forms Release 1.0 and further releases will be made available after further reprocessing campaigns. We evaluate the combined solution internally using the TAC solutions and externally using solutions from the IGS and the ITRF2008. The derived VLM estimates have undergone an initial evaluation and should be considered as the primary TIGA product for the sea level community to correct MSL records for land level changes [less ▲]

An Evaluaton of Real-Time Troposphere Products Based on mult-GNSS Precise Point Posi)oning

Scientific Conference (2017, February 21)

When employing observations from multiple Global Navigation Satellite System (GNSS) the performance of real-time (RT) GNSS meteorology can be improved. In this paper, we describe an operational RT system ... [more ▼]

When employing observations from multiple Global Navigation Satellite System (GNSS) the performance of real-time (RT) GNSS meteorology can be improved. In this paper, we describe an operational RT system for extracting zenith tropospheric delay (ZTD) using a modified version of the PPP-wizard. Multi-GNSS, including GPS, GLONASS and Galileo, observation streams are processed using a RT PPP strategy based on RT satellite orbit/clock products from CNES. A continuous experiment for 30 days is conducted, in which the RT observation streams of 20 globally distributed stations are processed. The initialization time and accuracy of the RT troposphere products using single/multi-system observations are evaluated. The effect of RT PPP ambiguity resolution is also evaluated. The results reveal that the RT troposphere products based on single system observations can fulfill the requirements of meteorological application, in which the GPS-only solution is better than the GLONASS-only solution in both initialization and accuracy. The performance can also be improved by applying RT PPP ambiguity resolution and utilizing multi-GNSS observations. Specifically, we notice that the ambiguity resolution is more effective in improving the accuracy, whereas the initialization process can be better accelerated by multi-GNSS observations. Combining all systems, RT troposphere products with an average accuracy of about 8 mm in ZTD can be achieved after an initialization process of approximately 9 minutes, which supports the application of multi-GNSS observations and ambiguity resolution for RT meteorological applications. [less ▲]

Optimum stochastic modeling for GNSS tropospheric delay estimation in real-time

in GPS Solutions (2016)

In GNSS data processing, the station height, receiver clock and tropospheric delay (ZTD) are highly correlated to each other. Although the zenith hydrostatic delay of the troposphere can be provided with ... [more ▼]

In GNSS data processing, the station height, receiver clock and tropospheric delay (ZTD) are highly correlated to each other. Although the zenith hydrostatic delay of the troposphere can be provided with sufficient accuracy, zenith wet delay (ZWD) has to be estimated, which is usually done in a random walk process. Since ZWD temporal variation depends on the water vapor content in the atmosphere, it seems to be reasonable that ZWD constraints in GNSS processing should be geographically and/or time dependent. We propose to take benefit from numerical weather prediction models to define optimum random walk process noise. In the first approach, we used archived VMF1-G data to calculate a grid of yearly and monthly means of the difference of ZWD between two consecutive epochs divided by the root square of the time lapsed, which can be considered as a random walk process noise. Alternatively, we used the Global Forecast System model from National Centres for Environmental Prediction to calculate random walk process noise dynamically in real-time. We performed two representative experimental campaigns with 20 globally distributed International GNSS Service (IGS) stations and compared real-time ZTD estimates with the official ZTD product from the IGS. With both our approaches, we obtained an improvement of up to 10% in accuracy of the ZTD estimates compared to any uniformly fixed random walk process noise applied for all stations. [less ▲]

Error analysis of Tide Gauge Benchmark Monitoring (TIGA) Analysis Center stacked solutions

Poster (2016, December 12)

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 1995 to the end of 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 Centers (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 center at the University of La Rochelle (www.sonel.org). The TAC solutions contain a total of over 700 stations. This study focuses on the evaluations of any systematic error present in the three TIGA analysis center (TAC) SINEX solutions: the British Isles continuous GNSS Facility – University of Luxembourg consortium (BLT), the GeoForschungsZentrum (GFZ) Potsdam, and of the University of La Rochelle (ULR). We have analyzed the residual position time series of the individual TAC a combination of automatic and manual discontinuity identification, applying a post-seismic deformation model adopted from ITRF2014 for those stations that are affected by earthquakes, followed by the stacking of the daily solution of the individual TAC into a long term linear frame. We have carried out the error analysis using the Combination and Analysis of Terrestrial Reference Frame (CATREF) software package. The TIGA Combination Centre (TCC) at the University of Luxembourg (UL) is responsible for providing a combined solution with a global set of vertical land movement estimates. [less ▲]

Numerical investigation of bridges with the aim of condition assessment in applying the Deformation Area Difference method (DAD-method) and selecting appropriate measurement techniques

in 5th International Symposium on Life-Cycle Civil Engineering (IALCCE 2016), Delft (2016, October)

Condition assessment of existing road bridges gains ever increasing importance today as bridges are getting older and the inflow of heavy traffic is constantly increasing. The further development of ... [more ▼]

Condition assessment of existing road bridges gains ever increasing importance today as bridges are getting older and the inflow of heavy traffic is constantly increasing. The further development of recognized techniques and the development of new methods for early and accurate detection of damage to the structure are made possible by means of innovative technological progress. In this contribution, the principles of Defor-mation Area Difference Method (DAD-Method) for condition assessment of bridges are presented. This method is based on the further processing of measured and computed deformation values. The application of the DAD-Method requires a precise recording of the deflection of a load-deflection test. On the basis of theoretical cal-culations, this method has allowed to identify as well as to localise damage to a structure. The DAD-Method is independent of a reference measurement and insensitive to global influences such as temperature fluctuations. For precise detection of deformations, the most modern measuring instruments and methods like photogram-metry, total stations, displacement sensors, strain gauges and levelling are compared to each other. In collabo-ration with the appropriate measurement technology, the localisation of damage in bridges becomes possible. [less ▲]

A New Vertical Land Movements Data Set from a Reprocessing of GNSS at Tide Gauge Stations

Scientific Conference (2016, July 30)

The main objective of the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group is to provide accurate coordinates and changes in them in the form of long-term trends for ... [more ▼]

The main objective of the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group is to provide accurate coordinates and changes in them in the form of long-term trends for globally distributed Global Navigation Satellite System (GNSS) stations at or close to tide gauges (TGs). Mean sea level (MSL) records derived from TG observations measure sea level relative to benchmarks on the land and structures supporting the TGs. Therefore, any changes in land levels affect the MSL records and the computed estimates of sea level change, ie. the MSL trends. In order to compute regionally or globally averaged MSL required for climate studies, these MSL trends have to be corrected for the vertical land movements (VLMs) derived from the GNSS observations. In this study, we have estimated a new set of VLMs at or close to TGs from the recent reprocessing campaign “repro2” undertaken by British Isles continuous GNSS Facility and University of Luxembourg TIGA Analysis Center (BLT). The position time series of more than 700 stations distributed around the world have been reprocessed for the period 1994 to 2015 using the latest bias models and processing strategies following the conventions of the International Earth Rotation and Reference Frame Service (IERS). It is well known that position time series are affected by discontinuities, which stem from different sources such as earthquakes, hardware changes and other artificial offsets that do not reflect real geophysical events. Since uncorrected discontinuities adversely affect the trend estimates, we have, after applying all known offset epochs, inspected the time series of all stations manually and added any further offset epochs required during the analysis. We have included a total of 2500+ discontinuities of which two-thirds are from hardware changes, 4% from earthquakes and 9% from unknown sources. We fit a deterministic model (sum of linear trend and seasonal terms) to the position time series using the Hector software package. As expected the annual terms show the highest power with amplitudes of a few millimeters. The stochastic model for estimating trend and associated uncertainties follows a power-law noise process as has previously been described as optimal for GNSS-derived position time series. The new set of VLM estimates from our repro2 solution is evaluated through comparison with a published GNSS solution, the recent ICE-6G model of glacial isostatic adjustment and by application to the latest release of MSL trends from the Permanent Service For Mean Sea Level. [less ▲]

Impact of Limited Satellite Visibility on Estimates of Vertical Land Movements

in International Association of Geodesy Symposia (2016)

The number of Global Navigation Satellite System (GNSS) satellites and their geometry directly affect the quality of positioning and derived satellite products. Accordingly, the International GNSS Service ... [more ▼]

The number of Global Navigation Satellite System (GNSS) satellites and their geometry directly affect the quality of positioning and derived satellite products. Accordingly, the International GNSS Service (IGS) recommends GNSS antennas to be installed away from natural and man-made surfaces and structures, which may affect the incoming signals through severe multipath or obstructions. Following these recommendations, continuous GNSS (cGNSS) stations are generally located in low multipath environments with minimal signal obstructions. However, some applications require GNSS antennas to be installed at specific locations in order to measure local processes. In support of sea level studies, cGNSS stations are established at or close to tide gauges in order to accurately monitor the local vertical land movements experienced by the sea level sensors. However, the environment at the tide gauge might not be optimal for GNSS observations due to the aforementioned station-specific effects, which may degrade the quality of coordinate solutions. This study investigates the impact of severe signal obstructions on long-term position time series for some selected stations. A masking profile from an actually obstructed site is extracted, simulated and applied to unobstructed IGS sites. To investigate these effects, we imple- mented a new feature called azimuth-dependent elevation masking in the Bernese GNSS Software version 5.2. We present our preliminary results on the use of this new feature to study the impact of different obstruction scenarios on long-term GNSS position time series and vertical land movement estimates. The results show that a certain obstruction, with the effect being highly dependent on its severity and azimuthal direction, affects all coordinate components with the effect being more significant for the Up component. Moreover, it causes changes in the rate estimates and increases the rate uncertainty with the effect being site-specific. [less ▲]