Forschung in Geodäsie und Geoinformation an der Uni.lu

This is a summary of selected research carried out by the team GGE of the DoE in 2017-2022

Strategies for long-term monitoring of tide gauges using GPS

Changes in mean sea (MSL) level recorded relative to tide gauge benchmarks (TGBM) are corrupted by vertical land movements. Accurate estimates of changes in absolute sea level, require these MSL records to be corrected for ground level changes at tide gauge sites. For more than a decade, the Global Positioning System (GPS) has been used to determine positions of TGBMs and to monitor their position changes, i.e. station velocities, over time in the International Terrestrial Reference System (ITRS). This was initially carried out by episodic GPS campaigns and later on by continuous GPS (CGPS) or a combination of both. Highly accurate realizations of the ITRS, satellite orbits and models for the mitigation of systematic effects currently enable the determination of station positions using GPS at the centimetre or even millimetre level. It is however argued that accurate long--term estimates of changes in the vertical component at the 1mm/yr level cannot be achieved, making intercomparisons between GPS estimates and other techniques necessary. Daily processing and analysis of continuous GPS networks requires automated procedures. The modifications and improvements to the existing procedures at the IESSG are described. The newly developed tools include the monitoring and quality control of daily archived GPS observations and of processing results. A special focus is on the coordinate time series analysis and methodologies used to obtain the best possible estimates of vertical station velocities and associated uncertainties. The coordinate time series of 21 CGPS stations in the UK and France are analysed. Eight of these stations are co-located with tide gauges. The effects of two processing strategies and two realizations of the ITRS on the coordinate time series are investigated. Filtered coordinate time series are obtained by application of a regional filtering technique. Station velocity estimates are obtained by fitting a model including a linear and annual term, and offsets to the unfiltered and filtered coordinate time series. Realistic uncertainties for these velocities are obtained from the application of two empirical methods which account for coloured noise in the coordinate time series. Results from these are compared to the Maximum Likelihood Estimation (MLE), which allows for more rigorous and accurate, simultaneous estimation of the model parameters and their uncertainties. Strategies for coordinate time series analysis on a daily or monthly, and annual or bi-annual basis are defined. At two CGPS stations the dual-CGPS station concept is tested and compared to the single baseline analysis and the application of an adaptive filter. An empirical method to obtain coordinate time series specific filter parameters is described. This investigation shows that reliable relative vertical station velocity estimates can be obtained after much shorter observation spans than absolute vertical station velocity estimates. The availability of dual-CGPS station pairs allows a simplified processing strategy and a multitude of coordinate time series analysis methods, all contributing to a better understanding of the variations in the positions of CGPS stations. Vertical station velocity estimates for the unfiltered and filtered coordinate time series and different analysis strategies are compared for 17 of the CGPS stations and show disagreements of up to 2mm/yr. At the eight CGPS stations co-located with or close to tide gauges alternative estimates of vertical land/crustal movements from absolute gravimetry, geological information and glacial isostatic adjustment models are compared to the GPS estimates, and it is suggested that the latter are systematically offset. An alignment procedure is demonstrated, correcting the vertical station velocity estimates of all 17 CGPS stations for this offset. The correlation of the geology-aligned vertical station velocity estimates and the MSL records from eight tide gauges suggests changes in absolute sea level of approximately +1mm/yr around the UK

Strategies for long-term monitoring of tide gauges using GPS

Changes in mean sea (MSL) level recorded relative to tide gauge benchmarks (TGBM) are corrupted by vertical land movements. Accurate estimates of changes in absolute sea level, require these MSL records to be corrected for ground level changes at tide gauge sites. For more than a decade, the Global Positioning System (GPS) has been used to determine positions of TGBMs and to monitor their position changes, i.e. station velocities, over time in the International Terrestrial Reference System (ITRS). This was initially carried out by episodic GPS campaigns and later on by continuous GPS (CGPS) or a combination of both. Highly accurate realizations of the ITRS, satellite orbits and models for the mitigation of systematic effects currently enable the determination of station positions using GPS at the centimetre or even millimetre level. It is however argued that accurate long--term estimates of changes in the vertical component at the 1mm/yr level cannot be achieved, making intercomparisons between GPS estimates and other techniques necessary. Daily processing and analysis of continuous GPS networks requires automated procedures. The modifications and improvements to the existing procedures at the IESSG are described. The newly developed tools include the monitoring and quality control of daily archived GPS observations and of processing results. A special focus is on the coordinate time series analysis and methodologies used to obtain the best possible estimates of vertical station velocities and associated uncertainties. The coordinate time series of 21 CGPS stations in the UK and France are analysed. Eight of these stations are co-located with tide gauges. The effects of two processing strategies and two realizations of the ITRS on the coordinate time series are investigated. Filtered coordinate time series are obtained by application of a regional filtering technique. Station velocity estimates are obtained by fitting a model including a linear and annual term, and offsets to the unfiltered and filtered coordinate time series. Realistic uncertainties for these velocities are obtained from the application of two empirical methods which account for coloured noise in the coordinate time series. Results from these are compared to the Maximum Likelihood Estimation (MLE), which allows for more rigorous and accurate, simultaneous estimation of the model parameters and their uncertainties. Strategies for coordinate time series analysis on a daily or monthly, and annual or bi-annual basis are defined. At two CGPS stations the dual-CGPS station concept is tested and compared to the single baseline analysis and the application of an adaptive filter. An empirical method to obtain coordinate time series specific filter parameters is described. This investigation shows that reliable relative vertical station velocity estimates can be obtained after much shorter observation spans than absolute vertical station velocity estimates. The availability of dual-CGPS station pairs allows a simplified processing strategy and a multitude of coordinate time series analysis methods, all contributing to a better understanding of the variations in the positions of CGPS stations. Vertical station velocity estimates for the unfiltered and filtered coordinate time series and different analysis strategies are compared for 17 of the CGPS stations and show disagreements of up to 2mm/yr. At the eight CGPS stations co-located with or close to tide gauges alternative estimates of vertical land/crustal movements from absolute gravimetry, geological information and glacial isostatic adjustment models are compared to the GPS estimates, and it is suggested that the latter are systematically offset. An alignment procedure is demonstrated, correcting the vertical station velocity estimates of all 17 CGPS stations for this offset. The correlation of the geology-aligned vertical station velocity estimates and the MSL records from eight tide gauges suggests changes in absolute sea level of approximately +1mm/yr around the UK

Evaluation of the Multipath Environment Using Electromagnetic-Absorbing Materials at Continuous GNSS Stations

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

A Bayesian Monte Carlo Markov Chain Method for the Statistical Analysis of Geodetic Time Series

Geodetic time series provide information which help to constrain theoretical models of geophysical processes. It is well established that such time series, for example from GPS or gravity measurements, contain time-correlated noise which is usually assumed to be a combination of a long-term stochastic process (characterized by a power-law spectrum) and random noise. Therefore, when fitting a model to geodetic time series it is essential to also estimate the stochastic parameters beside the deterministic ones. In many cases the stochastic parameters have included the power amplitudes of both time-correlated and random noise as well as the spectral index of the power-law process. To date the most widely used method for obtaining these model parameter estimates is based on maximum likelihood estimation (MLE). We present a new Bayesian Monte Carlo Markov Chain (MCMC) method to estimate the deterministic and stochastic model parameters of geodetic time series. This method provides a sample of the likelihood function and thereby, using Monte Carlo integration, all parameters and their uncertainties are estimated simultaneously. One advantage of this method over MLE is that the computation time required increases linearly with the number of parameters, hence being very suitable for dealing with a large number of parameters. Another advantage is that the properties of the estimator used by the MCMC method do not depend on the stationarity of the time series. We assess the MCMC method through comparison with MLE, using a data set of 300 synthetic GPS-like time series and the JPL daily position time series for 90 GPS stations (the IGS core network)

BIM-Based End-of-Lifecycle Decision Making and Digital Deconstruction: Literature Review

This article is the second part of a two-part study, which explored the extent to which Building Information Modelling (BIM) is used for End-of-Lifecycle (EoL) scenario selection to minimise the Construction and Demolition Waste (CDW). The conventional literature review presented here is based on the conceptual landscape that was obtained from the bibliometric and scientometric analysis in the first part of the study. Seven main academic research directions concerning the BIM-based EoL domain were found, including social and cultural factors, BIM-based Design for Deconstruction (DfD), BIM-based deconstruction, BIM-based EoL within LCA, BIM-aided waste management, Material and Component Banks (M/C Banks), off-site construction, interoperability and Industry Foundation Classes (IFC). The analysis highlights research gaps in the path of raw materials to reusable materials, i.e., from the deconstruction to M/C banks to DfD-based designs and then again to deconstruction. BIM-based EoL is suffering from a lack of a global framework. The existing solutions are based on local waste management policies and case-specific sustainability criteria selection. Another drawback of these ad hoc but well-developed BIM-based EoL prototypes is their use of specific proprietary BIM tools to support their framework. This disconnection between BIM tools and EoL tools is reportedly hindering the BIM-based EoL, while no IFC classes support the EoL phase information exchange

Static load deflection experiment on a beam for damage detection using the Deformation Area Difference Method

A reliable and safety infrastructure for both transport and traffic is becoming increasingly important today. The condition assessment of bridges remains difficult and new methods must be found to provide reliable information. A meaningful in-situ assessment of bridges requires very detailed investigations which cannot be guaranteed by commonly used methods. It is known that the structural response to external loading is influenced by local damages. However, the detection of local damage depends on many factors such as environmental effects (e.g. temperature), construction layer (e.g. asphalt) and accuracy of the structural response measurement. Within the paper, a new so-called Deformation Area Difference (DAD) Method is presented. The DAD method is based on a load deflection experiment and does not require a reference measurement of initial condition. Therefore, the DAD method can be applied on existing bridges. Moreover, the DAD method uses the most modern technologies such as high precision measurement techniques and attempts to combine digital photogrammetry with drone applications. The DAD method uses information given in the curvature course from a theoretical model of the structure and compares it to real measurements. The paper shows results from a laboratory load-deflection experiment with a steel beam which has been gradually damaged at distinct positions. The load size is chosen so that the maximum deflection does not exceed the serviceability limit state. With the data obtained by the laboratory experiment, the damage degree, which can still be detected by the DAD method, is described. Furthermore, the influence of measurement accuracy on damage detection is discussed