GNSS Measurements Research Articles

INTERFEROMETRIC SAR DEFORMATION MONITORING USING PASSIVE REFLECTORS AND ASCENDING AND DESCENDING PASSES

Abstract. The article concerns the estimation of land deformation in an area of copper mining using the Persistent Scatterer Interferometry (PSI) technique and C-band data coming from the Sentinel-1 satellite. The aim of the work is a comparison of two different PSI approaches exploiting passive corner reflectors deployed pairwise, oriented towards the satellites in ascending and descending orbits. The comparison was done in the line of sight geometry. In addition, a validation was carried out using GNSS measurements. The results achieved using the two PSI approaches are consistent and mutually coherent. They are also congruent with observations from GNSS stations. These results proves that the method proposed to identify pixels on multi-looked images is correct. In addition, the intermediary results on the corner reflectors shows that their responses, in terms of dispersion of amplitude and the coherence, is slightly different. This needs to be further studied.

ASSESSING THE EFFECT OF ALOS-2 DATA UTILIZATION ON THE ACCURACY OF ESTIMATION VERTICAL AND HORIZONTAL DEFORMATION COMPONENTS IN THE AREA OF HARD COAL MINING EXPLOITATION IN POLAND BY USING DIFFERENTIAL SYNTHETIC APERTURE RADAR INTERFEROMETRY

Abstract. Mining-induced deformations are characterized by high deformation rates. These deformations consist of both vertical and horizontal components, which should not be neglected during the monitoring of such areas. Unfortunately, Differential Satellite Synthetic Aperture Radar Interferometry (DInSAR) can only measure deformations in the line of sight (LOS) direction towards the satellite. Combining data from ascending and descending geometry allows reconstruction of the vertical and east-west deformation components. Theoretically, at least information from one ascending and one descending geometry is enough to reconstruct the vertical and east-west deformation components; however, for the area of interest three various Sentinel-1 datasets (one ascending and two descending) are available and have been used in the study. Furthermore, considering the availability of other SAR missions with slightly different geometries, the objective of this study was to evaluate the effect of ALOS-2 use on the accuracy of estimating the east-west and vertical cumulative deformation component estimated for a time span of approximately one year. Unfortunately, we have access to only five ALOS-2 scenes within the area of interest. However, the comparison of the decomposed results with GNSS measurements shows that the application of additional ALOS-2 data, even five scenes, allows the root mean square error (RMSE) to be decreased for the vertical and horizontal deformation component from 0.038m to 0.032m and from 0.031m to 0.018m, respectively. This means that the application of ALOS-2 data has a positive value on the accuracy of decomposed vertical and horizontal deformation components.

Алгоритм усреднения приращений координат и их ковариационных матриц при повторных ГНСС-измерениях

An algorithm for averaging part of GNSS measurements information obtained at different times and belonging to the same baseline is given. The averaging extends to the increments of such lines’ coordinates and the accompanying covariance matrix of the increments. It is implemented using the algorithm of the classic parametric version of data LS-optimization. The design matrix of the measured increasing is built with positive single blocks size 3×3; their total number is equal to that of the baseline observation sessions. Solving the system of relevant normal equations through the method of conversion, size 3×3, gives the vector of average increments of the base line coordinates. The reverse matrix of its normal equation ratios is an averaged covariance one of average increments (system roots). Processing observational materials is carried out in the Excel electronic computational table using a synthesized version of the parametric one of the data LS-optimization, taking into account the errors of the reference points’ coordinates. The two-way processing-determined coordinates and their RMSE (root mean square error) were identical both before and after averaging.

Optimizing the Use of RTKLIB for Smartphone-Based GNSS Measurements.

The Google Smartphone Decimeter Challenge (GSDC) was a competition held in 2021, where data from a variety of instruments useful for determining a phone’s position (signals from GPS satellites, accelerometer readings, gyroscope readings, etc.) using Android smartphones were provided to be processed/assessed in regard to the most accurate determination of the longitude and latitude of user positions. One of the tools that can be utilized to process the GNSS measurements is RTKLIB. RTKLIB is an open-source GNSS processing software tool that can be used with the GNSS measurements, including code, carrier, and doppler measurements, to provide real-time kinematic (RTK), precise point positioning (PPP), and post-processed kinematic (PPK) solutions. In the GSDC, we focused on the PPK capabilities of RTKLIB, as the challenge only required post-processing of past data. Although PPK positioning is expected to provide sub-meter level accuracies, the lower quality of the Android measurements compared to geodetic receivers makes this performance difficult to achieve consistently. Another latent issue is that the original RTKLIB created by Tomoji Takasu is aimed at commercial GNSS receivers rather than smartphones. Therefore, the performance of the original RTKLIB for the GSDC is limited. Consequently, adjustments to both the code-base and the default settings are suggested. When implemented, these changes allowed RTKLIB processing to score 5th place, based on the performance submissions of the prior GSDC competition. Detailed information on what was changed, and the steps to replicate the final results, are presented in the paper. Moreover, the updated code-base, with all the implemented changes, is provided in the public repository. This paper outlines a procedure to optimize the use of RTKLIB for Android smartphone measurements, highlighting the changes needed given the low-quality measurements from the mobile phone platform (relative to the survey grade GNSS receiver), which can be used as a basis point for further optimization for future GSDC competitions.

Correction of FAGN points velocities by results of GNSS measurements

The method of determining the velocities of the Fundamental Astronomical and Geodetic Network (FAGN) points from the coordinates time series obtained from processing GNSS measurements is considered. The criteria for estimation the quality of the solution, the issues of adequate estimation of accuracy and data control are presented. Proposals for the compilation of a priori weight matrix of measurements and velocities are considered. The importance of preliminary detection and exclusion of rough measurements is noted. The results of determining the OXTK point velocities are presented, the analysis of the results is demonstrated. It is proposed to use the decision analysis tools discussed in the article when automating the process of determining the FAGS points velocities, and the listed criteria for evaluating the results should be displayed in reports. It is recommended to publish on the RGS-center website not only the coordinates and velocities of the FAGS points, but also their average square errors.

Optimization of baseline configuration in a GNSS network (Nile Delta network, Egypt) – A case study

Abstract When starting any GNSS measurements, there is a need to establish a survey plan with the required optimal baselines. The optimal GNSS baselines can be chosen by solving the geodetic second-order design (SOD). The particle swarm optimization PSO is used widely to solve geodetic design issues. This work employed the particle swarm optimization (PSO) algorithm, a stochastic global optimization method, to select the optimal GNSS baselines. The optimal baselines satisfy the set criterion matrix at a reasonable cost. The fundamentals of the algorithm are presented. The effectiveness and usefulness of the technique are then demonstrated using a Nile Delta GNSS network as an example. In some cases, we have to observe many GNSS benchmarks with limited instrumentations. PSO represents a powerful tool for optimizing baseline to get the required accuracy with limited capabilities (like limited receivers). The PSO algorithm, a stochastic global optimization approach, was used in this paper to find the best observation weights to measure in the field that will match the predetermined criterion matrix with a fair degree of precision. The method’s fundamentals are presented with an actual geodetic network over the Nile delta in Egypt. In the current work, two survey strategies were applied. One represents a case with 9 GNSS receivers (high capability), and another one represents the tested survey plan with limited GNSS receivers (3 receivers, low capability) after applying PSO. By comparing two survey strategies, applying the PSO algorithm to a real Nile delta geodetic network shows its effectiveness on the obtained coordinate accuracy. This obtained accuracy ranged from 2 mm to 3 mm in X, Y, Z, and 3 mm in height. Also, the linear closure error between known and estimated coordinates improved to be 1.4 cm after applying PSO.

A Tightly Coupled Visual-Inertial GNSS State Estimator Based on Point-Line Feature.

Visual-inertial odometry (VIO) is known to suffer from drifting and can only provide local coordinates. In this paper, we propose a tightly coupled GNSS-VIO system based on point-line features for robust and drift-free state estimation. Feature-based methods are not robust in complex areas such as weak or repeated textures. To deal with this problem, line features with more environmental structure information can be extracted. In addition, to eliminate the accumulated drift of VIO, we tightly fused the GNSS measurement with visual and inertial information. The GNSS pseudorange measurements are real-time and unambiguous but experience large errors. The GNSS carrier phase measurements can achieve centimeter-level positioning accuracy, but the solution to the whole-cycle ambiguity is complex and time-consuming, which degrades the real-time performance of a state estimator. To combine the advantages of the two measurements, we use the carrier phase smoothed pseudorange instead of pseudorange to perform state estimation. Furthermore, the existence of the GNSS receiver and IMU also makes the extrinsic parameter calibration crucial. Our proposed system can calibrate the extrinsic translation parameter between the GNSS receiver and IMU in real-time. Finally, we show that the states represented in the ECEF frame are fully observable, and the tightly coupled GNSS-VIO state estimator is consistent. We conducted experiments on public datasets. The experimental results demonstrate that the positioning precision of our system is improved and the system is robust and real-time.

Verification of Satellite Railway Track Position Measurements Making Use of Standard Coordinate Determination Techniques

The article presents the results of satellite railway track position measurements performed by a multidisciplinary research team, the members of which represented Gdansk University of Technology and Gdynia Maritime University. Measuring methods are described which were used for reconstructing the railway track axis position and diagnosing railway track geometry deformations. As well as that, the description of the novel method developed by the authors to perform mobile GNSS measurements is included. The reported research aimed at assessing the uncertainty of railway track axis reconstruction making use of the dynamic GNSS method. To assess the applicability of this method, the obtained results were compared with those from the stationary measurement method used in railway business. The data used for comparison was recorded on the same railway track section during several measurement campaigns. In these campaigns, different types of GNSSs with different position recording frequencies (1–100 Hz) were used at different measurement speeds (5–70 km/h). The performed analysis has shown that the accuracy of railway track axis reconstruction making use of mobile GNSS measurements is sufficient for using this methodology in railway business.

Multi-technique geodetic detection of onshore and offshore subsidence along the Upper Adriatic Sea coasts

• InSAR-GNSS-Levelling data cross-validation procedure. • Unprecedented 20-years window estimation of onshore deformation along the Upper Adriatic Sea coastal areas. • Cross-validated deformation detected at an offshore platform for hydrocarbon extraction activity. We assess about 20 years of onshore and offshore subsidence along a sector of the Upper Adriatic Sea (Italy) coastal areas affected by natural soil compaction and intense anthropogenic activities such as aquifers exploitation and hydrocarbons extraction. Our approach is based on the synergistic use of independent remote sensing and in-situ geodetic data to detect and spatially characterise the deformation pattern by cross-validating the different available measurements. We collect extensive datasets from i) SAR images provided by Envisat, Cosmo-SkyMed and Sentinel-1 missions, ii) GNSS measurements from continuous stations managed by public institutions, local authorities and private companies and iii) Leveling surveys. The cross-validation analysis shows good agreement among all the independent datasets, thus providing a reliable assessment of the ongoing deformation. We detect an onshore and offshore subsidence peak of about −1/-1.5 cm/yr in the proximity of the coastline, close to Lido di Dante and Fiumi Uniti villages, and at the present offshore platform. The outcomes highlight how the integration of different remote sensing and in situ geodetic techniques is successful to retrieve deformation history in time and space in complex areas, where different natural and anthropogenic sources concur to the overall deformation pattern. Moreover, such approach provides a robust support to modelling studies for hazard assessment in both inland and shoreline areas.

An Effective Integrity Monitoring Scheme for GNSS/INS/Vision Integration Based on Error State EKF Model

It is well-known that the Global Navigation Satellite System/Inertial Navigation System (GNSS/INS) /Vision integration has been increasingly used for safety-critical applications like self-driving cars. However, the performance of the GNSS/INS/Vision integration degrades due to unknown faults. Therefore, it is essential to develop integrity monitoring algorithms for the integrated navigation system. A new protection level (PL) formula to ensure the integrity of the state estimation of the GNSS/INS/Vision integration is proposed, which can be calculated by the relation between the filter estimation error and faults. An integrity risk allocation tree is developed for each sensor fault hypothesis including the nominal hypothesis, and the specific PL is given in the case of GNSS, INS and visual measurement faults. In addition, the performance of the proposed PL is comprehensively evaluated. A field vehicular test was conducted to evaluate the performance of the integrity monitoring algorithm for the GNSS/INS/Vision integration. The results demonstrate that the proposed PL for the GNSS/INS/Vision integration can fit the position error against six different fault modes.

Seasonal and Multiyear Changes in CO2Degassing at Mammoth Mountain Explained by Solid‐Earth‐Driven Fault Valving

AbstractChanges in CO2emissions from volcanoes may evidence volcanic unrest. We use a multiyear time series of CO2flux collected at the Horseshoe Lake Tree Kill area on Mammoth Mountain, CA, to understand processes that cause variations in flux from this system. Seasonal variations are systematically lowest during the winter months and reach maximum values during the summer season. A persistent ∼20% reduction in CO2flux occurred during the Spring of 2017, coincident with the emergence of the area from drought and earthquake swarms in Long Valley Caldera. We used continuous GNSS measurements to calculate seasonal strains and stresses across the Mammoth Mountain area, and resolved resultant stresses onto the Mammoth Mountain Fault, which appears to facilitate gas transport to the surface. The normal stress changes are consistent with seasonal and multiyear changes in CO2flux, suggesting that fault valving by solid earth processes can alter surface gas fluxes.

All-Weather, sub-50-cm, Radar-Inertial Positioning

Deploying automated ground vehicles beyond the confines of sunny and dry climes will require sub-lane-level positioning techniques that use radio waves, rather than near-visible light radiation. Like human sight, LiDAR and optical cameras perform poorly in low-visibility conditions. We present and demonstrate a novel technique for robust, sub-50-cm, urban ground vehicle positioning based on all-weather sensors. The technique incorporates a computationally-efficient, globally-optimal radar scan registration algorithm within a larger estimation pipeline that fuses data from commercially-available, low-cost, automotive radars, low-cost inertial sensors, vehicle motion constraints, and, when available, precise GNSS measurements. We evaluate the performance of the presented technique on an extensive and realistic urban dataset derived from all-weather sensors. Comparison against ground truth shows that during 60 min of GNSS-denied driving in the urban center of Austin, TX, the technique maintains 95th-percentile errors below 50 cm in horizontal position and 0.5 in heading.

Comparison of the Estimates of the Monitoring Stations Coordinates of the Zagorsk PSPP-2, Obtained as a Result of Processing GNSS Measurements by the Double Differences Method and the PPP Method

Comparison of the Estimates of the Monitoring Stations Coordinates of the Zagorsk PSPP-2, Obtained as a Result of Processing GNSS Measurements by the Double Differences Method and the PPP Method

Deformations monitoring in complicated-surface areas by adaptive distributed Scatterer InSAR combined with land cover: Taking the Jiaju landslide in Danba, China as an example

Multi-temporal InSAR (MT-InSAR) technique provides a powerful tool for measuring large-scale surface displacements at high precision. However, traditional MT-InSAR approaches always employ a single global threshold to select measurement points (MPs), making it difficult to identify enough MPs in complex land cover areas (e.g., mountainous areas). In this study, to increase the number and spatial distribution density of distributed scatterer (DS) detection while maintaining its quality, we proposed a new MT-InSAR method, named Adaptive Distributed Scatterer InSAR Combined with Land Cover (ADSI-CLC). The key ideas of ADSI-CLC include the introduction of land cover maps as the constraint condition for DS adaptive selection and the employment of coherence weighted spatial adaptive filtering method to estimate optimal phase. Of these, DS adaptive selection method as the most pivotal idea realizes the unification of mathematical, physical, and geographical mechanism in the process of statistically homogeneous pixels (SHP) identification and adaptively adjusts the DS selection thresholds in different land cover areas in the process of DS selection. ADSI-CLC significantly increases the number and spatial distribution density of DS by using the adaptive selection method, and purifies the SHPs populations, and improves the efficiency of SHPs identification through introducing land cover maps. To demonstrate the effectiveness of the ADSI-CLC method, a case study of the Jiaju landslide using 19 scenes ALOS-1 PALSAR-1 L-band data (2006–2011) and 31 scenes Sentinel-1 C-band data (2014–2016) was carried out. The comparison between deformation measurements retrieved by ADSI-CLC and classical StaMPS-SBAS method showed that their distribution pattern is similar, but the ADSI-CLC method can provide more deformation details due to much higher (nearly 10 times) DS density. Quantitative comparisons between the ADSI-CLC results from ALOS-1 data and GNSS measurements also showed that the accuracy of this method is about 11.7 mm/yr in terms of root mean square error (RMSE). These results proved the reliability and effectiveness of ADSI-CLC and indicated its great potential for displacements monitoring in complicated-surface areas with large topographic fluctuations (e.g., mountainous areas).

Treatment of Tide Gauge Time Series and Marine GNSS Measurements for Vertical Land Motion with Relevance to the Implementation of the Baltic Sea Chart Datum 2000

Tide gauge (TG) time series and GNSS measurements have become standard datasets for various scientific and practical applications. However, the TG and geodetic networks in the Baltic Sea region are deforming due to vertical land motion (VLM), the primary cause of which is the glacial isostatic adjustment. Consequently, a correction for VLM, either obtained from a suitable VLM model or by utilizing space-geodetic techniques, must be applied to ensure compatibility of various data sources. It is common to consider the VLM rate relative to an arbitrary reference epoch, but this also yields that the resulting datasets may not be directly comparable. The common height reference, Baltic Sea Chart Datum 2000 (BSCD2000), has been initiated to facilitate the effective use of GNSS methods for accurate navigation and offshore surveying. The BSCD2000 agrees with the current national height realizations of the Baltic Sea countries. As TGs managed by national authorities are rigorously connected to the national height systems, the TG data can also be used in a common system. Hence, this contribution aims to review the treatment of TG time series for VLM and outline potential error sources for utilizing TG data relative to a common reference. Similar consideration is given for marine GNSS measurements that likewise require VLM correction for some marine applications (such as validating marine geoid models). The described principles are illustrated by analyzing and discussing numerical examples. These include investigations of TG time series and validation of shipborne GNSS determined sea surface heights. The latter employs a high-resolution geoid model and hydrodynamic model-based dynamic topography, which is linked to the height reference using VLM corrected TG data. Validation of the presented VLM corrected marine GNSS measurements yields a 1.7 cm standard deviation and −2.7 cm mean residual. The estimates are 1.9 cm and −10.2 cm, respectively, by neglecting VLM correction. The inclusion of VLM correction thus demonstrates significant improvement toward data consistency. Although the focus is on the Baltic Sea region, the principles described here are also applicable elsewhere.

Constraints on the hydrogeological properties and land subsidence through GNSS and InSAR measurements and well data in Salmas plain, northwest of Urmia Lake, Iran

Constraints on the hydrogeological properties and land subsidence through GNSS and InSAR measurements and well data in Salmas plain, northwest of Urmia Lake, Iran

A robust method for 3-D surface displacement fields combining GNSS and single-orbit InSAR measurements with directional constraint from elasticity model

A robust method for 3-D surface displacement fields combining GNSS and single-orbit InSAR measurements with directional constraint from elasticity model

GNSS observable-specific phase biases for all-frequency PPP ambiguity resolution

An unwritten rule to resolve GNSS ambiguities in precise point positioning (PPP-AR) is that users should follow faithfully the frequency choices and observable combinations mandated by satellite clock and phase bias providers. Switching to other frequencies of measurements requires that the satellite clocks be converted, albeit in a roundabout way, to agree with the new frequencies of code biases. Satellite phase biases, on the other hand, are prescribed conventionally as wide-lane and narrow-lane combinations, which prevents users from resolving other phase combinations in the case of multi-frequency observables. We therefore develop an approach to compute observable-specific phase biases (phase OSBs) in concert with the legacy, but ambiguity-fixed, satellite clocks to enable PPP-AR over any frequency choices and observable combinations at the user end, i.e., all-frequency PPP-AR. In particular, the phase OSBs on the baseline frequencies (e.g., L1/L2 for GPS and E1/E5a for Galileo) are estimated by decoupling the code OSBs pre-aligned with the satellite clocks; then satellite clocks are re-estimated by holding pre-resolved undifferenced ambiguities and phase OSBs on the baseline frequencies; finally, all third-frequency phase OSBs are determined by introducing the ambiguity-fixed satellite clocks above. We used a global network of multi-frequency GPS/Galileo data over a month to verify this approach. In dual-frequency PPP-AR using GPS L1/L2, L1/L5, Galileo E1/E5a, E1/E5b, E1/E5 and E1/E6 signals, over 95% of wide-lane and narrow-lane ambiguity residuals were within ±0.25 and ±0.15 cycles, respectively, after the code and phase OSB corrections on raw GNSS measurements. As a result, the ambiguity fixing rates reached around 95% in all PPP-AR tests, though it was only the satellite clocks aligned with the GPS L1/L2 and Galileo E1/E5a pseudorange that were applied throughout. We stress that the key to computing such phase OSBs for all-frequency PPP-AR is that the code OSBs have the same bias datum as that of the satellite clocks.

Smartphone GNSS Performance in an Urban Scenario with RAIM Application.

In an urban scenario, GNSS performance is strongly influenced by gross errors in the measurements, usually related to multipath and non-line-of-sight phenomena. The use of RAIM algorithms is a common approach to solve this issue. A significant amount of the existing GNSS receivers is currently mounted on smart devices, above all, smartphones. A typical drawback of these devices is the unavailability of raw measurements, which does not allow fully exploiting the GNSS potential; in particular, this feature limits the use of RAIM algorithms. Since 2016, for few smart devices, it has been finally possible to access GNSS raw measurements, allowing the implementation of specific algorithms and enabling new services. The Xiaomi Mi 8 is equipped with the Broadcom BCM47755 receiver, able to provide dual-frequency raw measurements from quad-constellation GPS, Glonass, Galileo, BeiDou. In this work, the performance in an urban area of the Xiaomi Mi8 GNSS was analyzed. An important issue of smartphone GNSS is related to the antenna, which is not able to protect from the multipath phenomenon; this issue has a large probability to emerge in hostile environments like urban areas. As a term of comparison, the high-sensitivity receiver NVS NV08C-CSM, connected to a patch antenna, was used. In particular, the considered receivers were placed in the same location, and their positions were estimated in single point positioning, applying a classical RAIM algorithm. An error analysis was carried out, and the obtained results demonstrated the effectiveness of RAIM when applied to Xiaomi Mi8 GNSS measurements.

ГНСС-НАБЛЮДЕНИЯ НА ГЕОДИНАМИЧЕСКОМ ПОЛИГОНЕ НЕФТЕГАЗОВОГО МЕСТОРОЖДЕНИЯ: МЕТОДИКА, ОБРАБОТКА ДАННЫХ И ИХ АНАЛИЗ

To ensure geodynamic safety within the scope of their license areas, surveying services of the oil and gas complex create geodynamic polygons. When using GNSS technologies as a method of geody-namic monitoring, the question arises of choosing a coordination technology for a new point inserted into the reference geodetic network. According to a number of regulatory documents, in the develop-ment of satellite geodetic networks, only the "network" method should be used, while the "radiant" method is considered unacceptable. The use of the "network" method, as the main one in the organiza-tion of GNSS measurements at geodynamic polygons, leads to an increase in the observation time, which is obviously less in the "radiant" method. The authors, using its own Excel-program as a compu-ting tool, they show that the accuracy of the "radiant" and "network" methods is identical. Theoretical-ly, the points of the reference geodetic network of the geodetic construction under study are consid-ered as mathematical constants whose values are not subject to LS-optimization. As an experiment, GNSS measurements were used to bind the new Langepas point, which is part of the network of the regional geodynamic polygon of LLC LUKOIL-Western Siberia, to six reference points of the state geodetic network. Processing of observational materials was implemented using the algorithm of the synthesized version of the parametric method (SVPM) of LS-optimization of geospatial data, taking into account the errors of the coordinates of reference points. The coordinates of the defined point and their average quadratic errors turned out to be, of course, identical in both solutions: "radiant" and "network". This testifies in favor of the theoretically justified use of the "radiant” method, as less labo-rious in terms of the volume of field work. Additionally, within the framework of the same SVPS al-gorithm, the GNSS measurements on the specified object were treated as a "free" network. In the latter case, the average quadratic errors ofthecoordinates of the new inserted item sharply decreased. Pro-cessing of the same data on the synthesized variant of the correlate method with additional parameters (SVCMAP) again confirmed the results of the "radiation" method.