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 ▲]

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 ▲]

The Combined Effect of Periodic Signals and Noise on the Dilution of Precision of GNSS Station Velocity Uncertainties

Poster (2016, April 05)

Station velocity uncertainties determined from a series of Global Navigation Satellite System (GNSS) position estimates depend on both the deterministic and stochastic models applied to the time series ... [more ▼]

Station velocity uncertainties determined from a series of Global Navigation Satellite System (GNSS) position estimates depend on both the deterministic and stochastic models applied to the time series. While the deterministic model generally includes parameters for a linear and several periodic terms, the stochastic model is a representation of the noise character of the time series in form of a power-law process. For both of these models the optimal model may vary from one time series to another while the models also depend, to some degree, on each other. In the past various power-law processes have been shown to fit the time series and the sources for the apparent temporally-correlated noise were attributed to, for example, mismodelling of satellites orbits, antenna phase centre variations, troposphere, Earth Orientation Parameters, mass loading effects and monument instabilities. [less ▲]

Total Impact of Periodic Terms and Coloured Noise on Velocity Estimates

Poster (2016, February 05)

The uncertainties of velocity estimates for position time series of Global Navigation Satellite System (GNSS) stations are mainly affected by a misfit of the deterministic model applied to this data ... [more ▼]

The uncertainties of velocity estimates for position time series of Global Navigation Satellite System (GNSS) stations are mainly affected by a misfit of the deterministic model applied to this data. Insufficiently modelled seasonal signals will propagate into the stochastic model and falsify the results of the noise analysis besides the velocity estimates and their uncertainties. In this presentation we derived the General Dilution of Precision (GDP) of velocity uncertainties. We define this dilution as the ratio between the uncertainties of velocities determined when different deterministic and stochastic models are applied. In this way we discuss, referring to previously published results, how insufficiently modelled seasonal signals influence station velocity uncertainties with white and coloured noise. Using simulated and real data from selected (115) IGS (International GNSS Service) stations we show that the noise character affects GNSS data more than seasonals for time series longer than 9 years. [less ▲]

Impact of Antenna Phase Centre Calibrations on Position Time Series: Preliminary Results

in Willis, Pascal; Rizos, Chris (Eds.) IAG 150 Years Proceedings of the 2013 IAG Scientific Assembly, Postdam,Germany, 1–6 September, 2013 (2016)

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

Advances in GPS error modelling and the continued effort of re-processing have considerably decreased the scatter in position estimates over the last decade. 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 the 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 the observing site and the 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) led to a significant improvement, but the use of individual calibrations could possibly 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 outweigh 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 calibration differences 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 peak-to-peak 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 with peak-to-peak 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 ▲]

The Status of GNSS Data Processing Systems to Estimate Integrated Water Vapour for Use in Numerical Weather Prediction Models

in Willis, Pascal; Rizos, Chris (Eds.) IAG 150 Years Proceedings of the 2013 IAG Scientific Assembly, Postdam,Germany, 1–6 September, 2013 (2016)

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 3-hourly to 6-hourly intervals but with the advancement of NWP models towards higher update rates e.g. 1-hourly cycling in the Rapid update Cycle (RUC) NWP, it has become of high interest to estimate 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 realtime (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 systems are based on Bernese GPS Software 5.0 and use a double-differencing strategy whereas the PP system is based on the Bernese GNSS Software 5.2 using the precise point positioning (PPP) strategy. The RT systems are based on the BKG Ntrip Client 2.7 and the PPP-Wizard both using PPP. The PPP-Wizard allows integer ambiguity resolution at a single station and therefore the effect of fixing integer ambiguities on ZTD estimates will also be presented. [less ▲]

Signal Obstructions at GNSS Stations: Benefits From Multi-GNSS Observations

Poster (2015, October 27)

The current accuracy of IGS products, few centimeter level, requires amongst other things that the location for GNSS antennas are nearly optimal for GNSS observations. This includes a low multipath ... [more ▼]

The current accuracy of IGS products, few centimeter level, requires amongst other things that the location for GNSS antennas are nearly optimal for GNSS observations. This includes a low multipath environment and little to no signal obstructions. However, this is not guaranteed for every station especially in urban areas and mountainous regions. As some applications such as GNSS for sea level studies or to monitor landslides require GNSS antennas to be installed at a specific site, it is clear that the environment might not be favourable for GNSS observations. In this study, we investigate the effect of signal obstructions on station positions, specifically the height component, based on simulated obstruction scenarios using a modified Bernese GNSS Software version 5.2 (BSW52). The behaviours of different obstruction scenarios and the impact of multi-GNSS (GPS+GLONASS for now) observations for both clear and obstructed stations are discussed. [less ▲]

Impact of Limited Multi-GNSS Visibility on Vertical Land Movement Estimates

Poster (2015, June 27)

The number of 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 ... [more ▼]

The number of 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. Hence, in support of sea level studies, cGNSS stations must be installed close to or at 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 degrade the quality of coordinate solutions.This first study investigates the impact of severe signal obstructions on long-term monitoring results by use of simulated and real observations for selected cGNSS stations, and evaluates if the use of multi-GNSS (GPS+GLONASS) constellations will benefit derived results. To investigate these effects, we implemented azimuth and elevation dependent masking in the Bernese GNSS Software version 5.2. We present our preliminary results on the impact of different obstruction scenarios and combined GPS and GLONASS solutions on coordinate and vertical land movement estimates. [less ▲]

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

Poster (2015, June 26)

Sofia University GNSS Analysis Center (SUGAC)

Scientific Conference (2015, May 20)

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, the University of Luxembourg and the Space Research and Technology Institute at the Bulgarian Academy of Sciences. In April 2014, the first processing campaign took place. One year of GNSS data from 7 stations of the BuliPOS network were processed in collaboration with the University of Luxembourg. Tropospheric products (Zenith Total Delay and gradients) with 5 min temporal resolution were obtained using the NAPEOS software, developed by ESA. The tropospheric products from this campaign were then 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 ▲]