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 ▲]

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 ▲]

Status of TIGA activities at the British Isles continuous GNSS Facility and the University of Luxembourg

in International Association of Geodesy Symposia (2016), 143

In 2013 the InternationalGNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group started their reprocessing campaign which proposes to reanalyse all relevant GPS observations from 1995 to ... [more ▼]

In 2013 the InternationalGNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group started their reprocessing campaign which proposes to reanalyse all relevant GPS observations from 1995 to the end of 2012 in order to provide high quality estimates of vertical land motion for monitoring of sea level changes. The TIGA Working Group will also produce a combined solution from the individual TIGA Analysis Centres (TAC) contributions. The consortium of British Isles continuous GNSS Facility (BIGF) and the University of Luxembourg TAC (BLT) will contribute weekly minimally constrained SINEX solutions from its reprocessing using the Bernese GNSS Software (BSW) version 5.2 and the University of Luxembourg will also act as a TIGA Combination Centre (TCC). The BLT will generate two solutions, one based on BSW5.2 using a network double difference (DD) strategy and a second one based on BSW5.2 using a Precise Point Positioning (PPP) strategy. In the DD strategy we have included all IGb08 core stations in order to achieve a consistent reference frame implementation. As an initial test for the TIGA combination, all TACs agreed to provide weekly SINEX solutions for a four-week period in December 2011. Taking these individual TAC solutions the TCC has computed a first combination using two independent combination software packages: CATREF and GLOBK. In this study we will present preliminary results from the BLT reprocessing and from the combination tests [less ▲]

Analysis of Global Climate Variability from Homogenously Reprocessed Ground-based GNSS Measurements

Poster (2015, June 26)

An analysis of global climate variability from homogeneously reprocessed GNSS measurements

Poster (2014, December 17)

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

in GPS Solutions (2014)

The continuous evolution of global navigation satellite systems (GNSS) meteorology has led to an increased use of associated observations for operational modern low-latency numerical weather prediction ... [more ▼]

The continuous evolution of global navigation satellite systems (GNSS) meteorology has led to an increased use of associated observations for operational modern low-latency numerical weather prediction (NWP) models, which assimilate GNSS-derived zenith total delay (ZTD) estimates. The development of NWP models with faster assimilation cycles, e.g., 1-h assimilation cycle in the rapid update cycle NWP model, has increased the interest of the meteorological community toward sub-hour ZTD estimates. The suitability of real-time ZTD estimates obtained from three different precise point positioning software packages has been assessed by comparing them with the state-of-the-art IGS final troposphere product as well as collocated radiosonde (RS) observations. The ZTD estimates obtained by BNC2.7 show a mean bias of 0.21 cm, and those obtained by the G-Nut/Tefnut software library show a mean bias of 1.09 cm to the IGS final troposphere product. In comparison with the RS-based ZTD, the BNC2.7 solutions show mean biases between 1 and 2 cm, whereas the G-Nut/Tefnut solutions show mean biases between 2 and 3 cm with the RS-based ZTD, and the ambiguity float and ambiguity fixed solutions obtained by PPPWizard have mean biases between 6 and 7 cm with the references. The large biases in the time series from PPP-Wizard are due to the fact that this software has been developed for kinematic applications and hence does not apply receiver antenna eccentricity and phase center offset (PCO) corrections on the observations. Application of the eccentricity and PCO corrections to the a priori coordinates has resulted in a 66 % reduction of bias in the PPP-Wizard solutions. The biases are found to be stable over the whole period of the comparison, which are criteria (rather than the magnitude of the bias) for the suitability of ZTD estimates for use in NWP nowcasting. A millimeter-level impact on the ZTD estimates has also been observed in relation to ambiguity resolution. As a result of a comparison with the established user requirements for NWP nowcasting, it was found that both the GNut/Tefnut solutions and one of the BNC2.7 solutions meet the threshold requirements, whereas one of the BNC2.7 solution and both the PPPWizard solutions currently exceed this threshold. [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 ▲]

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 ▲]

Status of TIGA activities at the British Isles continuous GNSS Facility and the University of Luxembourg

Scientific Conference (2013)

In 2013 the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group started their reprocessing campaign which proposes to re-analyse all relevant GPS observations from 1995 ... [more ▼]

In 2013 the International GNSS Service (IGS) Tide Gauge Benchmark Monitoring (TIGA) Working Group started their reprocessing campaign which proposes to re-analyse all relevant GPS observations from 1995 to the end of 2012 in order to provide high quality estimates of vertical land motion for monitoring of sea level changes. The TIGA Working Group will also produce a combined solution from the individual TIGA Analysis Centres (TAC) contributions. The consortium of British Isles continuous GNSS Facility (BIGF) and the University of Luxembourg TAC (BLT) will contribute weekly minimally constrained SINEX solutions from its reprocessing using the Bernese GNSS Software (BSW) version 5.2 and the University of Luxembourg will also act as a TIGA Combination Centre (TCC). The BLT will generate two solutions, one based on BSW5.2 using a network double difference (DD) strategy and a second one based on BSW5.2 using a Precise Point Positioning (PPP) strategy. In the DD strategy we have included all IGb08 core stations in order to achieve a consistent reference frame implementation. As an initial test for the TIGA combination, all TACs agreed to provide weekly SINEX solutions for a four-week period in December 2011. Taking these individual TAC solutions the TCC has computed a first combination using two independent combination software packages: CATREF and GLOBK. In this study we will present preliminary results from the BLT reprocessing and from the combination tests. [less ▲]

An Evaluation of Real-Time, Near Real-Time and Post-Processed Zenith Total Delay Estimates

Poster (2012, July)