On Improving Slant Wet delays for Tracking Severe Weather Events: An evaluation During Two Storms in Europe

Scientific Conference (2022, December 14)

Climate change has led to an increase in the frequency and severity of weather events with intense precipitation and, subsequently, a greater susceptibility of communities around the world to flash ... [more ▼]

Climate change has led to an increase in the frequency and severity of weather events with intense precipitation and, subsequently, a greater susceptibility of communities around the world to flash flooding. Networks of ground-based Global Navigation Satellite System (GNSS) stations enable the measurement of integrated water vapor along slant pathways, providing three-dimensional (3D) water vapor distributions at low-cost and in real-time. This makes these data a valuable complementary source of information for tracking storm events and predicting their paths. However, it is well established that residual modelling errors and multipath (MP) effects at GNSS stations do impact incoming signals, especially at low elevations and during storms when the atmospheric conditions change rapidly. Until now, the bulk of GNSS products for meteorology are estimates of the more conventional zenith total delays and horizontal gradients, but these products may not be most appropriate for determining 3D distributions of water vapor during convective storm events. In this study we investigate the impact of residual-phase-corrected and multipath-corrected slant wet delay (SWD) estimates on tracking extreme weather events using two events in Europe that led to flooding, damage to property and loss of life. We employed Precise Point Positioning (PPP) with integer ambiguity resolution to generate station-specific MP correction maps. The spatial stacking was carried out in congruent cells with an optimal resolution in elevation and azimuth at the local horizon but with decreasing azimuth resolution as the elevation angle increases. This permits an approximately equal number of observations allocated to each cell. In our analysis we recovered the one-way SWD by adding GNSS post-fit phase residuals, representing the non-isotropic component of the SWD, i.e., the higher-order inhomogeneity. Using the derived MP maps in a final step, the one-way SWD were improved to employ them for the analysis of the weather event. Moreover, we validated the SWD between ground-based water-vapor radiometry and GNSS-derived SWD for different elevation angles. Furthermore, the spatio-temporal fluctuations in the SWD as measured by GNSS closely mirrored the moisture field from the ERA5 re-analysis associated with this severe weather event [less ▲]

On the Application of Multipath Stacking Maps for Improved Severe Weather Tracking and Low-cost Antenna Calibration

Presentation (2022, July 07)

Modern cities all over the world are now more susceptible to flash floods as a result of a rise in the frequency and severity of meteorological events with significant precipitation. For minimizing the ... [more ▼]

Modern cities all over the world are now more susceptible to flash floods as a result of a rise in the frequency and severity of meteorological events with significant precipitation. For minimizing the risks due to these hydro-meteorological hazards, reliable fore- and now-casting of severe precipitation has become essential. Water vapor can be measured along slant paths by network of ground-based GNSS stations, providing real-time, three-dimensional (3D) distributions of vapor concentrations at an affordable cost. Consequently, these data provide an invaluable additional source of knowledge for monitoring and predicting events with flash flood potential. But site-specific multipath (MP) effects at GNSS stations do affect incoming signals, particularly at low elevations, as is widely known. The bulk of GNSS products for meteorology up to now are based on estimates of ordinary zenith total delay and horizontal gradients with little sensitivity to azimuthal variations, thus these products may not be best suited for estimating 3D distributions of water vapor during storm events. Using slant delays directly, can overcome this lack of azimuthal information. However, at low elevations, this approach is more susceptible to multipath errors. A thunderstorm event which occurred over Turkey and adjacent countries on July 27, 2017, resulting in flash floods and severe infrastructure damage, is used as an example to explore the effects of multipath-corrected slant wet delay (SWD) estimations on monitoring extreme weather events. To reconstruct the one-way SWD, we first added phase residuals derived from the GNSS one-way post-fit observations, which represent the anisotropic component of the SWD. This can also be interpreted as a higher order inhomogeneity component, which is not resolved by ordinary zenith or gradient products. For the generation of site-specific MP correction maps, we stacked the post-fit residuals derived from our Precise Point Positioning (PPP) processing strategy because the MP errors in the GNSS phase observables can adversely impact the SWD along the direction of individual satellites. The spatial stacking was performed in congruent cells as a function of elevation and azimuth. This enables each cell to receive roughly the same number of residuals, providing a better stacking result. The stacking of residuals for a single cell over several days allows the detection and reduction of systematic errors; random errors are minimal because the averaging is done over a suitably sufficient chosen time span. Finally, the one-way SWD were enhanced by applying these MP correction maps for the analysis of the meteorological event. Our study revealed that the anisotropic component contributed up to 11% of one-way SWD estimates. Furthermore, the spatio-temporal changes in SWD as derived from GNSS closely matched the moisture field from the ERA5 re-analysis linked to this weather event. As it turns out, the MP correction maps may also provide a “kind of calibration” for uncalibrated or low-cost GNSS antennas, even for those in mobile phones, as these devices are highly susceptible to MP errors. In turn, this would allow the application of low-cost sensors to accurately estimate SWD for severe weather monitoring in urban regions. [less ▲]

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

in Sensors (2022), 22(9), 1-23

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 ... [more ▼]

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. [less ▲]

Multi-GNSS Slant Wet Delay Retrieval Using Multipath Mitigation Maps

Scientific Conference (2021, April 24)

The conventional Global Navigation Satellite System (GNSS) processing is typically contaminated with errors due to atmospheric variabilities, such as those associated with the mesoscale phenomena. These ... [more ▼]

The conventional Global Navigation Satellite System (GNSS) processing is typically contaminated with errors due to atmospheric variabilities, such as those associated with the mesoscale phenomena. These errors are manifested in the parameter estimates, including station coordinates and atmospheric products. To enhance the accuracy of these GNSS products further, a better understanding of the local-scale atmospheric variability is necessary. As part of multi-GNSS processing, station coordinates, carrier phase ambiguities, orbits, zenith total delay (ZTD) and horizontal gradients are the main parameters of interest. Here, ZTD is estimated as the average zenith delay along the line-of-sight to every observed GNSS satellite mapped to the vertical while the horizontal gradients are estimated in NS and EW directions and provide a means to partly account for the azimuthally inhomogeneous atmosphere. However, a better atmospheric description is possible by evaluating the slant path delay (SPD) or slant wet delay (SWD) along GNSS ray paths, which are not resolved by ordinary ZTD and gradient analysis. SWD is expected to provide better information about the inhomogeneous distribution of water vapour that is disregarded when retrieving ZTD and horizontal gradients. Usually, SWD cannot be estimated directly from GNSS processing as the number of unknown parameters exceeds the number of observations. Thus, SWD is generally calculated from ZTD for each satellite and may be dominated by un-modelled atmospheric delays, clock errors, unresolved carrier-phase ambiguities and near-surface multipath scattering. In this work, we have computed multipath maps by stacking individual post-fit carrier residuals incorporating the signals from four GNSS constellations, i.e. BeiDou, Galileo, Glonass and GPS. We have selected a subset of global International GNSS Service (IGS) stations capable of multi-GNSS observables located in different climatic zones. The multipath effects are reduced by subtracting the stacked multipath maps from the raw post-fit carrier phase residuals. We demonstrate that the multipath stacking technique results in significantly reduced variations in the one-way post-fit carrier phase residuals. This is particularly evident for lower elevation angles, thus, producing a retrieval method for SWD that is less affected by site-specific multipath effects. We show a positive impact on SWD estimation using our multipath maps during increased atmospheric inhomogeneity as induced by severe weather events. [less ▲]

Multipath Mitigation Maps feasibility and applicability as an International GNSS Service product

Scientific Conference (2020, December 17)

There have been many advances in the modeling of Global Navigation Satellite System (GNSS) observables when estimating position and other parameters of interest. Some of these bias models are related to ... [more ▼]

There have been many advances in the modeling of Global Navigation Satellite System (GNSS) observables when estimating position and other parameters of interest. Some of these bias models are related to improvements of reference frames, phase center offsets and variations of transmitter and receiver antennas, satellite orbits and clocks, and troposphere. Nonetheless, multipath remains for the most part an unmodelled source of error which causes range errors in the GNSS observations. The associated effects show highly localized features and have a different impact for each receiver and antenna. Multipath errors can propagate, can cause in-situ position biases and are also contributing to the prevalent draconitic harmonic signals. In order to mitigate the problem we generate site-specific corrections by employing a suitable averaging scheme for the stacking of carrier phase residuals. Our processing is based on globally distributed static multi-GNSS observations using several scientific GNSS software packages (Bernese GNSS Software, NAPEOS, GAMIT-GLOBK, and CSRS-PPP). Our multipath stacking maps (MPS) use the stacking of carrier phase residuals generated by variable azimuth cell size (congruent cells) and by allocating carrier phase residuals in each cell to generate the correction maps, unlike the standard fixed azimuth cell resolution approaches. This reduces the binning of fewer residuals at higher elevation angles. Before stacking, we also apply rigorous statistical outlier screening tests for each one-way post-fit carrier phase residual assigned to each of the congruent cells. We thus correct the multipath effects by subtracting the stacked multipath map from the post-fit carrier phase residual. Using this technique we produce a model available in the form of the Antenna Exchange (ANTEX) file format, that can potentially be implemented in routine GNSS analysis with no or little additional overhead for individual analysis centers (ACs). In this study, we assess the feasibility and applicability of the MPS maps as an International GNSS Service (IGS) product for routine GNSS analysis. We demonstrate the multipath stacking technique to result in a significant reduction of the variation in the one-way post-fit carrier phase residuals from multi-GNSS observations. [less ▲]

Test

Poster (2020, December 11)

test

Low-cost GNSS for Meteorology: A receiver comparison

Poster (2020, February 25)

GNSS, particularly the GPS and GLONASS satellite constellations are commonly used for accurate estimation of atmospheric parameters. The high accuracy is accomplished by sophisticated analysis methods and ... [more ▼]

GNSS, particularly the GPS and GLONASS satellite constellations are commonly used for accurate estimation of atmospheric parameters. The high accuracy is accomplished by sophisticated analysis methods and expensive high-end receivers and antennas along with precise products and bias corrections. The recent market introduction of low-cost dual frequency receivers that can produce raw data from multiple constellations offers an insight into the potential use of these receivers for meteorological applications. Here we demonstrate that GPS and GLONASS measurements from a low-cost dual-frequency receiver can be used to estimate the Zenith Total Delay ( ZTD) commensurate to meteorological applications. [less ▲]

Tracking Hurricanes using GPS atmospheric precipitable water vapor field

in Beyond 100: The Next Century in Geodesy (2020)

Tropical cyclones are one of the most powerful severe weather events that produce devastating socioeconomic and environmental impacts in the areas they strike. Therefore, monitoring and tracking of the ... [more ▼]

Tropical cyclones are one of the most powerful severe weather events that produce devastating socioeconomic and environmental impacts in the areas they strike. Therefore, monitoring and tracking of the arrival times and path of the tropical cyclones are extremely valuable in providing early warning to the public and governments. Hurricane Florence struck the East cost of USA in 2018 and offers an outstanding case study. We employed Global Positioning System (GPS) derived precipitable water vapor (PWV) data to track and investigate the characteristics of storm occurrences in their spatial and temporal distribution using a dense ground network of permanent GPS stations. Our findings indicate that a rise in GPS-derived PWV occurred several hours before Florence’s manifestation. Also, we compared the temporal distribution of the GPS-derived PWV content with the precipitation value for days when the storm appeared in the area under influence. The study will contribute to quantitative assessment of the complementary GPS tropospheric products in hurricane monitoring and tracking using GPS-derived water vapor evolution from a dense network of permanent GPS stations [less ▲]

Using the Vertical Land Movement estimates from the IGS TIGA combined solution to derive Global Mean Sea Level changes

Scientific Conference (2019, December 13)

Global mean sea level (GMSL) is now widely recognized to have risen between 1 to 2 mm/yr depending on location since the 20th century. Prior to the satellite altimetry era, GMSL was primarily estimated ... [more ▼]

Global mean sea level (GMSL) is now widely recognized to have risen between 1 to 2 mm/yr depending on location since the 20th century. Prior to the satellite altimetry era, GMSL was primarily estimated from a set of secular tide gauge records relative to coastal benchmarks. Recent measurements of GPS (Global Positioning System) have been demonstrated as a useful tool of a direct estimate of Vertical Land Motion (VLM) induced by both long and short-term geophysical and human-induced processes in a geocentric reference frame. This presentation will provide the results of a combination performed using the CATREF software of three independent GPS daily solutions provided by British Isles continuous GNSS Facility – University of Luxembourg consortium (BLT), German Research Centre for Geosciences (GFZ) and University of La Rochelle (ULR) under the auspices of the Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG), that results in a spatially comprehensive map of VLM near or close to tide gauge benchmarks. The combination was performed in accordance with the second re-processing campaign (repro2) of the IGS (International GNSS Service). Long coastal tide gauge records from the archives maintained at the Permanent Service for Mean Sea Level (PSMSL) were extracted for relative sea level estimates. To cross-compare the sea level rates over the years, we employed observations between 1900-2016. Then, the time series were cut and analyzed separately, ceteris paribus, for the period 1960-2016. This analysis was aimed at a cross-comparison of relative sea level trends and their changes over the years. The stochastic part of the tide gauge records was analyzed with Maximum Likelihood Estimation (MLE) and assumed several different combinations of noise models with the Bayesian Information Criterion (BIC) providing a means to identify the preferred one. The relative sea level estimates were corrected by the inverted barometric effect to the tide-gauge records using data from the 20th century Reanalysis project version V2C, the effect of wind stress on the surface of the ocean in both, zonal and meridional components, as well as Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO) influencing Pacific tide gauge records. The GPS-based velocities were corrected by Glacial Isostatic Adjustment (GIA) effect using ICE-6G(VM5a) model with associated geoid rate and post seismic decays using ITRF2014 estimates. Also, environmental loading models were employed to account for present-day elastic loading in VLM. The Mean Sea Level (MSL) trends from tide gauges and VLM-corrected MSL trends using GIA model (TG+GIA) and the TIGA combination (TG+TIGA) were determined. Our final reconstruction of GMSL based on the MSL records from 1900 to 2016 where the VLM uncertainty is smaller than 0.7 mm/yr indicate a long-term trend of 1.75 +/- 0.2 mm/yr and is in good agreement with several similar determinations. [less ▲]

Present-Day Land and Sea Level Changes around South Georgia Island: Results from Precise Levelling, GNSS, Tide Gauge and Satellite Altimetry Measurements

Scientific Conference (2019, July 25)

South Georgia Island, the main land outcrop on the South Georgia microcontinent (SGM), is located approximately 1,400 km east of the Falkland Islands and approximately 1,400 km northeast of the ... [more ▼]

South Georgia Island, the main land outcrop on the South Georgia microcontinent (SGM), is located approximately 1,400 km east of the Falkland Islands and approximately 1,400 km northeast of the northernmost tip of the Antarctic peninsular. The SGM is believed to lie south of the North Scotia Ridge (NSR), which forms the boundary to the South America Plate, while to the south it is bordered by the Scotia Plate (SP). In its sub-Antarctic location, the island is largely covered by mountain glaciers which have been reported to be retreating due to climatic change. Furthermore, during past glaciation periods the island and its shelf area, stretching much of the SGM, have been ice covered as was revealed by scarring of the sub-oceanic topography. Together with ongoing tectonics along the NSR and recent seismicity at the SP boundary, these processes have the ability to produce significant uplift on local to regional scales. With its mid-ocean location in the Southern Atlantic Ocean South Georgia Island is in a key position for the oceanic and geodetic global monitoring networks. As these net-works suffer from a Hemisphere imbalance with the number of stations in the Northern Hemisphere outnumbering those in the Southern Hemisphere, operating these stations to the highest standards is of key scientific value. It is of particular interest to monitor the tide gauge (GLOSS ID 187) at King Edward Point (KEP) for vertical land movements to establish a continuous record of its datum within the Permanent Service for Mean Sea Level (PSMSL), which in turn makes it useful for long-term sea level studies and satellite altimetry calibrations. With the establishment of five GNSS stations on the islands by teams from Luxembourg, the UK and the USA during 2013 to 2015, and the scientific analysis of these data within a global network of stations, it has now become possible to study present-day vertical land movements and their impacts. Furthermore, together with four precise levelling campaigns of the KEP benchmark network in 2013, 2014 and two in 2017, it has also been possible to investigate the very local character of the vertical motions near KEP, i.e. the stability of the jetty upon which the tide gauge is mounted. In this study, we will present the still preliminary results from the GNSS and levelling measurements and will discuss their impact on the sea level record from the KEP tide gauge. Our measurements show that while South Georgia Island and the area around KEP are rising, the jetty and tide gauge are subsiding, leading to a lower magnitude of the observed sea level change than expected from satellite altimetry. In order to improve the agreement between these measurements both local and regional vertical land movements need to be monitored. [less ▲]