Journal of Geodesy

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Soft clustering of GPS velocities from a homogeneous permanent network in Turkey

Thu, 08/01/2019 - 00:00
Abstract

Global positioning system (GPS) velocities have long and widely been used on various scales in revealing the deformations of the continental lithosphere. We present a homogeneous geodetic velocity field with high precision derived from ~ 10-year-long permanent GPS observations throughout Turkey. Without any apriori information or assumption, the cluster analysis might be applied upon the velocity fields for inspection, before going further in the analyses used prevalently in tectonic studies. We first “hard clustered” the velocities using k-means, hierarchical agglomerative clustering and Gaussian mixture models and examined how the cluster assignments change by tuning the algorithm-specific parameters. The Eurasian and the Arabian blocks which are separated from the Anatolian block with the strike-slip North and East Anatolian faults have been detected immediately. The Anatolian block itself has been divided into three blocks where the cluster assignments of the velocities at the transition zones might differ according to the chosen hard clustering algorithm. We then applied soft clustering using an appropriate Gaussian mixture model fit and created a probability map exhibiting the credibility of the cluster assignments. The detection capability of the cluster analysis has been demonstrated by comparison to various previously published block models of western Turkey. Cluster analysis detected the most pronounced blocks in western Turkey successfully, especially when the initially chosen number of clusters is not too large. The probability map of soft clustering can be used to modify the block boundaries together with the external validation.

Consistency and analysis of ionospheric observables obtained from three precise point positioning models

Thu, 08/01/2019 - 00:00
Abstract

Ionospheric observables based on Global Navigation Satellite System can be obtained by a variety of approaches. The most widely used one is the geometry-free combination of carrier-phase smoothed code measurements. This method, however, introduces leveling errors that substantially degrade the performance of ionospheric modeling and bias estimation. To reduce leveling errors, precise point positioning (PPP) model is preferred for obtaining the ionospheric observables. We aim to investigate whether the ionospheric observables obtained from three different PPP models are consistent and how the PPP-based ionospheric observables relates to the smoothed code method. The paper begins by formulating the ionospheric observables. We then explain the statistical evaluation methods used for analyzing the bias terms derived from these methods and assessing the leveling errors from the carrier-phase smoothed code method. Numerical analysis is then conducted to compare the bias terms in the ionospheric observables and evaluate the leveling errors. The ionospheric observables based on the three PPP models show strong consistency. Compared to leveling errors in the carrier-phase smoothed code method, the leveling errors using the uncombined PPP model are significantly reduced up to five times.

Rescue of the 1873–1922 high and low waters of the Porto Corsini/Marina di Ravenna (northern Adriatic, Italy) tide gauge

Thu, 08/01/2019 - 00:00
Abstract

The historical data corresponding to the period 1873–1922 of the Porto Corsini (northern Adriatic, Italy) tide gauge have been retrieved from the archive of the Italian Istituto Geografico Militare, in Florence (http://www.igmi.org). The data consist of daily high and low waters, which were recorded in six handwritten volumes. The 1873–1896 observations were, until now, totally unknown since they were never published, while, for the remaining years, only monthly mean values were available. We digitized and quality controlled the rescued data. Searching in different archives, we also found detailed information on the three sites occupied by the tide gauge within the Porto Corsini/Marina di Ravenna harbor. In addition, the rescue of the leveling data and of the datum definition made it possible to realize a homogeneous time series of monthly mean sea-level values, which now spans 144 years. Sea-level time series exceeding a century are rare, and they are most important because they provide key information for studying climate change-related processes. The Porto Corsini/Marina di Ravenna area is affected by natural subsidence, much enhanced by anthropogenic activities during the period 1940–1980, in particular. Using leveling, GPS and InSAR data we have modeled the subsidence behavior over the 1873–2016 period. After removing the subsidence from the series of monthly mean values, we estimated a long-period linear sea-level rise of + 1.25 ± 0.16 mm/year.

Triple-frequency PPP ambiguity resolution with multi-constellation GNSS: BDS and Galileo

Thu, 08/01/2019 - 00:00
Abstract

Multi-constellation GNSS (multi-GNSS) and multi-frequency signals open new prospects for fast ambiguity resolution (AR) of precise point positioning (PPP). Currently, all the BDS and Galileo satellites are capable of transmitting signals on three or more frequencies. In this contribution, we investigate the triple-frequency PPP ambiguity resolution with B1, B2 and B3 observations from BDS satellites and E1, E5a and E5b observations from Galileo satellites and evaluate the contribution of BDS + Galileo combination to triple-frequency PPP AR. The uncalibrated phase delay (UPD) products are estimated based on triple-frequency observations, and the temporal characteristic as well as the residual distributions are analyzed. Our results show that the extra-wide-lane (EWL) and wide-lane (WL) UPDs for BDS and Galileo satellites are both stable during the 30 days and the daily narrow-lane (NL) UPD series are also steady with no obvious fluctuation. The Galileo UPDs exhibit better performance than BDS UPDs due to the high-quality observations. It is also interesting to find that the EWL UPD corrections for all Galileo satellites are very close to the zero. With the precise UPD products, the triple-frequency PPP AR with BDS and Galileo observations was implemented in both static and kinematic modes. Compared to the ambiguity-float solution, the performance can be significantly improved by triple-frequency PPP AR with the positioning accuracy improved by 30–70% in both static and kinematic modes. Moreover, the triple-frequency PPP fixed solutions also present better performance than the dual-frequency PPP fixed solutions in terms of time to the first fix and positioning accuracy, especially for the Galileo-only and BDS + Galileo solutions. And the fusion of multi-GNSS (BDS and Galileo) can further improve the position estimations compared to the single system with more satellites and better spatial geometry.

Joint estimation of tiltmeter drift and volume variation during reservoir monitoring

Thu, 08/01/2019 - 00:00
Abstract

Borehole tiltmeters are widely used to continuously record small surface deformation of reservoirs and volcanoes. Because these instruments display unknown long-term drift, only short-term tilt signal can be used for monitoring purpose. We propose a method to invert long-term time series of tilt data induced by strain variations at depth. The assumption that tiltmeter drift is linear over time is on its own insufficient to remove the drift and uniquely determine the deformation source parameters. To overcome this problem, we first invert the data with no constrain on the drift to obtain one particular solution among all admissible. Then, using the linearity of the forward model, we use the statistical properties of the drift distributions to restore the uniqueness of the solution. We illustrate our approach with four synthetic cases simulating volume changes of a reservoir. We demonstrate the efficiency of our method and show that the accuracy of estimated volume variation dramatically improves if low drift tiltmeters are used.

The effect of observation correlations upon the basic characteristics of reliability matrix as oblique projection operator

Thu, 08/01/2019 - 00:00
Abstract

The reliability matrix, being an oblique projection operator, transforms correlated observations into the least squares residuals in Gauss–Markov models. It also allows to study model responses in individual observations to the assumed configurations of gross errors. The variability of the basic characteristics of the operator due to the increase in observation correlations is investigated by means of numerical tests and theoretical derivations. The characteristics such as diagonal elements and asymmetry indices have not that long ago been introduced as the response-based measures of internal reliability and subjected to the analysis. Here, additionally, the relationship between the asymmetry indices and the angles of non-orthogonality of projection is derived. The measures are compared in terms of the effect of observation correlations with the commonly used reliability measures obtained on the basis of statistical tests for detection and identification of outliers, such as generalized reliability numbers and minimal detectable biases. For the purposes of the present paper, the latter are termed the testing-based measures. The comparative analysis shows that both the types, when taken together, provide complete information about the behaviour of a GMM with correlated observations in the presence of a gross error in a particular observation and about its detectability. Hence, the conclusion is that the response-based measures can be a useful supplementation of the testing-based measures for the phase of network design.

Temporal and spatial movement characteristics of the Altyn Tagh fault inferred from 21 years of InSAR observations

Thu, 08/01/2019 - 00:00
Abstract

We use 21 years of interferometric synthetic aperture radar data obtained between 1996 and 2017 from three European Space Agency satellites, namely the European Remote Satellite (ERS), Environmental Satellite (ENVISAT) and Sentinel-1, to estimate the present-day strain accumulation rate within the central segment of the Altyn Tagh fault (ATF) along the northern boundary of the Tibetan Plateau. We obtain the first Sentinel-1 line-of-sight velocity map, thereby revealing a velocity gradient across the main ATF. Due to significant near-fault deformation, which cannot be modeled by utilizing only the classic Savage deep slip model, a dislocation extending to a certain depth representing strain release by shallow creep is combined with a deep screw dislocation to model the observations. The best-fitting results are a 9.0 mm/year slip rate with a 20.0 km locking depth and a 2.0 mm/year creep rate with a 2.0 km creep extent for the ERS data, a 7.4 mm/year slip rate with an 18.0 km locking depth and a 1.8 mm/year creep rate with a 2.8 km creep extent for the ENVISAT data, and a 6.0 mm/year slip rate with a 21.0 km locking depth and a 1.5 mm/year creep rate with a 1.0 km creep extent for the Sentinel-1 data based on a single fault inversion. The slip rate decreases with time along this section of the ATF, while the creep rate varies around 1.8 mm/year, which does not constitute an obvious variation. In the spatial domain, the fault was divided into three segments, only Sentinel-1 data were used in the inversion, and the best-fitting values for the slip rate, locking depth, creep rate and creep extent are [7.4, 21.0, 1.8, 1.5], [6.8, 23.0, 1.5, 1.0] and [5.3, 20.0, 2.0, 1.1] for the Kuyake segment, Kulukuole segment and Aqiang segment, respectively. We conclude that deep slip motion within the ATF was variable in both the time domain and the space domain, whereas shallow creep, which must be accounted for using the Fattahi model considering both deep and shallow motions, was stable at ~ 2.0 mm/year with a creep extent of 1.0–2.0 km. The seismic moment release rate is less than 9.0% of the geodetic moment accumulation rate, which indicates a high seismic risk within the ATF.

The Iranian height datum offset from the GBVP solution and spirit-leveling/gravimetry data

Thu, 08/01/2019 - 00:00
Abstract

The gravity potential of the zero point of the Iranian height datum (IRHD) is determined as well as the IRHD offset from a global geoid. For this purpose, the geodetic boundary value problem (GBVP) solution based on the remove–compute–restore (RCR) technique is used. In the RCR technique, a global geopotential model (GGM) is required as a reference to remove and restore the long wavelengths of the gravity field. Since the GGMs do not have adequate accuracy over Iran, the IRHD offset is not precisely estimated by the GBVP solution. In this study, aiming to improve the latter, a combination solution based on the GBVP approach and spirit-leveling/gravimetry (LG) data, called the GBVP_LG solution, is proposed. To obtain the GBVP_LG solution, gravity potential obtained from the GBVP solution and the gravity potential differences derived from the LG data are used as two types of observations in a least-squares adjustment. The proper relative weight matrices are determined using the variance component estimation method. To evaluate the proposed method, the gravity potential differences between the start and end points of several check-lines in the leveling network derived from the GBVP and GBVP_LG solutions are compared with those of the LG data. The results show that the dependency of the GBVP_LG solution on the reference model used is much less than that of the GBVP solution. In addition, the results indicate that the GBVP_LG solution has a 42% improvement with respect to the GBVP solution in terms of root-mean-square error. As a result of the GBVP_LG solution, the gravity potential of the IRHD zero point is estimated equal to \( W_{0}^{\text{IRHD}} = 62,636,855.89 \pm 0.16\,{\text{m}}^{2} / {\text{s}}^{2} \) . Therefore, the IRHD offset with respect to the geoid defined by \( W_{0} = 62,636,853.4\,{\text{m}}^{2} / {\text{s}}^{2} \) is obtained equal to \( - \,25.4 \pm 1.6\,{\text{cm}} \) , which means that the IRHD is 25.4 cm below the geoid.

Robust M–M unscented Kalman filtering for GPS/IMU navigation

Thu, 08/01/2019 - 00:00
Abstract

In this paper, a robust unscented Kalman filter (UKF) based on the generalized maximum likelihood estimation (M-estimation) is proposed to improve the robustness of the integrated navigation system of Global Navigation Satellite System and Inertial Measurement Unit. The UKF is a variation of Kalman filter by which the Jacobian matrix calculation in a nonlinear system state model is not necessary. The proposed robust M–M unscented Kalman filter (RMUKF) applies the M-estimation principle to both functional model errors and measurement errors. Hence, this robust filter attenuates the influences of disturbances in the dynamic model and of measurement outliers without linearizing the nonlinear state space model. In addition, an equivalent weight matrix, composed of the bi-factor shrink elements, is proposed in order to keep the original correlation coefficients of the predicted state unchanged. Furthermore, a nonlinear error model is used as the dynamic equation to verify the performance of the proposed RMUKF with a simulation and field test. Compared with the conventional UKF, the impacts of measurement outliers and system disturbances on the state estimation are both controlled by RMUKF.

Galileo and QZSS precise orbit and clock determination using new satellite metadata

Thu, 08/01/2019 - 00:00
Abstract

During 2016–2018, satellite metadata/information including antenna parameters, attitude laws and physical characteristics such as mass, dimensions and optical properties were released for Galileo and QZSS (except for the QZS-1 optical coefficients). These metadata are critical for improving the accuracy of precise orbit and clock determination. In this contribution, we evaluate the benefits of these new metadata to orbit and clock in three aspects: the phase center offsets and variations (PCO and PCV), the yaw-attitude model and solar radiation pressure (SRP) model. The updating of Galileo PCO and PCV corrections, from the values estimated by Deutsches Zentrum für Luft- und Raumfahrt and Deutsches GeoForschungsZentrum to the chamber calibrations disclosed by new metadata, has only a slight influence on Galileo orbits, with overlap differences within only 1 mm. By modeling the yaw attitude of Galileo satellites and QZS-2 spacecraft (SVN J002) according to new published attitude laws, the residuals of ionosphere-free carrier-phase combinations can be obviously decreased in yaw maneuver seasons. With the new attitude models, the 3D overlap RMS in eclipse seasons can be decreased from 12.3 cm, 14.7 cm, 16.8 cm and 34.7 cm to 11.7 cm, 13.4 cm, 15.8 cm and 32.9 cm for Galileo In-Orbit Validation (IOV), Full Operational Capability (FOC), FOC in elliptical orbits (FOCe) and QZS-2 satellites, respectively. By applying the a priori box-wing SRP model with new satellite dimensions and optical coefficients, the 3D overlap RMS are 5.3 cm, 6.2 cm, 5.3 cm and 16.6 cm for Galileo IOV, FOCe, FOC and QZS-2 satellites, with improvements of 11.0%, 14.7%, 14.0% and 13.8% when compared with the updated Extended CODE Orbit Model (ECOM2). The satellite laser ranging (SLR) validation reveals that the a priori box-wing model has smaller mean biases of − 0.4 cm, − 0.4 cm and 0.6 cm for Galileo FOCe, FOC and QZS-2 satellites, while a slightly larger mean bias of − 1.0 cm is observed for Galileo IOV satellites. Moreover, the SLR residual dependencies of Galileo IOV and FOC satellites on the elongation angle almost vanish when the a priori box-wing SRP model is applied. As for satellite clocks, a visible bump appears in the Modified Allan deviation at integration time of 20,000 s for Galileo Passive Hydrogen Maser with ECOM2, while it almost vanishes when the a priori box-wing SRP model and new metadata are applied. The standard deviations of clock overlap can also be significantly reduced by using new metadata.

IAG Newsletter

Thu, 08/01/2019 - 00:00

NKG2016LU: a new land uplift model for Fennoscandia and the Baltic Region

Thu, 07/18/2019 - 00:00
Abstract

We present the official land uplift model NKG2016LU of the Nordic Commission of Geodesy (NKG) for northern Europe. The model was released in 2016 and covers an area from 49° to 75° latitude and 0° to 50° longitude. It shows a maximum absolute uplift of 10.3 mm/a near the city of Umeå in northern Sweden and a zero-line that follows the shores of Germany and Poland. The model replaces the NKG2005LU model from 2005. Since then, we have collected more data in the core areas of NKG2005LU, specifically in Norway, Sweden, Denmark and Finland, and included observations from the Baltic countries as well. Additionally, we have derived an underlying geophysical glacial isostatic adjustment (GIA) model within NKG as an integrated part of the NKG2016LU project. A major challenge is to estimate a realistic uncertainty grid for the model. We show how the errors in the observations and the underlying GIA model propagate through the calculations to the final uplift model. We find a standard error better than 0.25 mm/a for most of the area covered by precise levelling or uplift rates from Continuously Operating Reference Stations and up to 0.7 mm/a outside this area. As a check, we show that two different methods give approximately the same uncertainty estimates. We also estimate changes in the geoid and derive an alternative uplift model referring to this rising geoid. Using this latter model, the maximum uplift in Umeå reduces from 10.3 to 9.6 mm/a and with a similar reduction ratio elsewhere. When we compare this new NKG2016LU with the former NKG2005LU, we find the largest differences where the GIA model has the strongest influence, i.e. outside the area of geodetic observation. Here, the new model gives from − 3 to 4 mm/a larger values. Within the observation area, similar differences reach − 1.5 mm/a at the northernmost part of Norway and − 1.0 mm/a at the north-western coast of Denmark, but generally within the range of − 0.5 to 0.5 mm/a.

Observed geocenter motion from precise orbit determination of GRACE satellites using GPS tracking and accelerometer data

Thu, 07/18/2019 - 00:00
Abstract

We present a method to estimate geocenter motion through single low-earth orbiter (LEO) precise orbit determination (POD) using global positioning system (GPS) tracking data and accelerometer data from the GRACE satellites. We fix the values of the GPS ephemerides and time-varying clock offsets to precise estimates from the definitive constellation product produced by the Jet Propulsion Laboratory. As part of the POD process of the LEOs, we estimate a translation of the reference coordinate system realized by the GPS orbit and clock product. Doing so accounts for the inconsistency between the Earth’s center of mass coordinate system used for the orbit integration of the LEOs, and the terrestrial reference frame produced by the GPS orbit and clock product. The resulting translation parameters estimated separately from the GRACE-A and GRACE-B satellites show very similar variations from day to day. They represent the geocenter motion, as realized from the difference between the origin of the terrestrial reference frame represented by the GPS orbit and clock product and the Earth’s instantaneous center of mass defined by the GRACE satellite dynamical orbital motion. Comparisons with geocenter motion observations from other techniques show that our daily estimates of geocenter motion agree well, when smoothed, in both the amplitude and phase of the annual signal. This validates both the high sensitivity of the GRACE GPS measurement type to geocenter motion and the high precision of the GRACE force model enabled by accelerometer measurements, the two essential components for estimation of geocenter motion with this technique.

Residual least-squares collocation: use of covariance matrices from high-resolution global geopotential models

Tue, 07/16/2019 - 00:00
Abstract

The paper presents a modified formulation of least-squares collocation. This residual least-squares collocation (RLSC) includes a remove–compute–restore procedure with a high-resolution global geopotential model (GGM) and a topographic gravitational potential model. In contrast to previous approaches, in RLSC, the remaining input residuals are modeled with error covariance matrices instead of signal covariance matrices. Therefore, we include the full variance–covariance information of a high-resolution GGM, namely the XGM2016, to the procedure. The included covariance matrices are anisotropic and location-dependent and enable a realistic error modeling of a target area. This fact represents an advantage over covariance matrices derived from signal degree variances or empirical covariance fitting. Additionally, due to the stochastic modeling of all involved components, RLSC provides realistic accuracy estimates. In a synthetic closed-loop test case with a realistic data distribution in the Andes we demonstrate the advantages of RLSC for regional geoid modeling and quantify the benefit which results mainly from a rigorously handled high-resolution GGM. In terms of root mean square deviations from the true reference solution, RLSC delivers an improvement of about 30% compared to a standard LSC approach, where the benefit is particularly pronounced in areas with a sparse data distribution. This improved performance, together with the fact that the resulting stochastic error estimates better reflect the true errors, might be an important aspect for the application of RLSC to derive gravity potential values and their uncertainties at reference stations of the international height reference system.

Regional ionospheric TEC modeling based on a two-layer spherical harmonic approximation for real-time single-frequency PPP

Mon, 07/15/2019 - 00:00
Abstract

The ionosphere has been considered as one of the major error sources in GNSS signal propagation, and it is still difficult to be modeled precisely, especially for real-time positioning applications. The commonly used ionospheric models are usually based on the one-layer approximation, which neglects the ionospheric variation in the vertical domain and limits the scope of improvement over one-layer models. A new ionospheric model based on the two-layer approximation and two spherical harmonic (SH) functions is proposed in this contribution, where a quasi-globe projection is designed to avoid the inherent ill-posed problem and retain the physical meaning of the SH when regional data are used. GPS and BDS data from the National Positioning Infrastructure of Australia and Crustal Movement Observation Network of China are used for validating the new model’s performance in different areas and different periods. Results show (1) the precision of ionospheric TEC estimates from the new model can be improved by about 26% and 31% in the cross-validation experiment compared to the traditional one-layer model in Australian and Chinese regions, respectively; (2) the positioning accuracy of kinematic single-frequency precise point positioning (SF-PPP) in the experimental regions using the new model reaches about 0.7 m and 0.8 m in the horizontal and vertical components, respectively, in comparison with the one-layer model’s 1.0 m (horizontal) and 1.4 m (vertical); (3) the convergence time of the SF-PPP using the new model is 5–10 min for achieving a sub-meter level of positioning accuracy in both horizontal and vertical components, whereas it needs 30–40 min in case the one-layer model is used.

Improving GNSS PPP accuracy through WVR PWV augmentation

Tue, 07/09/2019 - 00:00
Abstract

Using 5 months of observations at a Global Navigation Satellite System (GNSS) and a Water Vapor Radiometer (WVR) collocated station at Tongji University, Shanghai, a mid-latitude coastal city in China with high level of water vapor, we analyzed the precipitable water vapor (PWV) from different sources including WVR, GNSS, Numerical Weather Prediction model (NWP) and radiosonde (RS). The highest correlation coefficient of 99.8% between GNSS PWV and WVR PWV with a linear fitting root-mean-square (RMS) error of 1 mm was obtained. The WVR observations were further applied in GNSS Precise Point Positioning (PPP) to demonstrate its benefits compared to the traditional PPP where troposphere delay was estimated. Both the Global Positioning System (GPS) and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) observations were used, and the impact of estimating tropospheric gradients was also investigated. Experiments show that the WVR-constrained PPP improves the weekly repeatability, convergence time, and short-term precision in the vertical component for GPS + GLONASS and GPS-only PPP, in both static and kinematic cases. For the vertical component of daily static GPS + GLONASS PPP, the weekly repeatability of the daily static solutions was improved by ~ 5%; the convergence time was shortened by ~ 30–50%. The short-term static GPS + GLONASS PPP vertical precision was improved by 30–53% when the WVR PWV was used as a constraint and troposphere gradients were estimated. The kinematic GPS-only PPP solution showed 10–15% improvement in the vertical component when the WVR PWV was used as a constraint. However, the kinematic GPS + GLONASS PPP solution showed very limited improvement in the vertical precision when the WVR PWV was constrained. In general, the use of WVR PWV constraint did not improve the horizontal accuracy in either GPS-only or GPS + GLONASS PPP solutions, in either static or kinematic cases.

Attitude variometric approach using DGNSS/INS integration to detect deformation in railway track irregularity measuring

Tue, 07/09/2019 - 00:00
Abstract

Track irregularity measuring is of crucial importance to ensure the dynamic driving safety of the train as well as riding comfort of passengers. Navigation-grade inertial navigation system (INS) is capable of providing highly precise attitude solution with its accumulated error calibrated by global navigation positioning system (GNSS). Moreover, the application of post-mission smoothing method could further improve both relative and absolute accuracy of the attitude output. After investigating the variometric relationship between attitude of the measuring apparatus and track deformation, we proposed an attitude variometric approach (AVA) using double-differenced GNSS (DGNSS) and INS integration to detect deformation in railway track irregularity measuring. In this method, instead of using the position solutions directly from DGNSS/INS integration like previous researchers, the three-dimensional track is mainly reconstructed by integrating the high-accuracy attitude information. Prior to the track irregularity experiment, the error characteristics of the AVA are validated by evaluating the versine deviation and the analysis shows that the proposed method could achieve high repeatability in horizontal and vertical direction by reducing the error in the concerned lateral and up directions. Besides, the impact of misalignment could be limited with a rotation correction, and a preliminary analysis shows that the new method is robust and able to maintain high precision even with a GNSS outage of 2 min. A field test was conducted on a segment of real high-speed railway, and four sets of observation data over the same mission track were collected with a DGNSS/INS-based trolley. Apart from that, external reference data were gathered by a total station-based trolley. The results show that the proposed method, which demonstrates good repeatability and provides 0.3 mm and 0.4 mm external measuring accuracy in horizontal and vertical components for shortwave track alignment irregularity as well as 1.8 mm and 2.0 mm for longwave track alignment irregularity, respectively, can meet the requirements of China Specifications for Survey Engineering of High-speed Railway and be utilized in practical track irregularity measuring application.

High-precision Ocean navigation with single set of BeiDou short-message device

Tue, 07/09/2019 - 00:00
Abstract

The high-precision navigation and positioning with GNSS has become widely used in various applications with the development of new GNSS systems, such as BeiDou and Galileo. For marine applications, high-precision navigation and positioning with GNSS is still a challenge since the requirement of the communication link is a problem on sea. Both DGNSS and RTK require data communication between base station on land and the rover station on sea. The data communication can be performed by providers of the marine satellite communication service, such as Intelsat, Eutelsat, Telesat and the SpaceX. However, the cost is too high to be afforded by ordinary GNSS users. The BeiDou short-message service provides an efficient way for the data communication between reference station on land and rover station on sea. Each BeiDou message length is limited to 78 bytes, and the communication frequency is limited to 60 s. Based on the BeiDou short-message service, the high-precision positioning has been achieved in previous studies. However, multiple set of short-message devices are used and the cost is still high. In this research, based on dual-frequency GNSS data, we propose the high-precision navigation on sea with single set of short-message device. The space-relative and time-relative positioning methods are integrated to reduce the data requirements. That is, first, every minute precise position is acquired with traditional space-relative positioning method and then the position of the other epochs is derived with time-relative positioning method. The experimental results based on buoy observations on sea show that the navigation accuracy can reach up to cm level in both horizontal and vertical directions. The proposed method can meet the requirements of different marine applications such as tide monitoring and wave monitoring.

Combination of GRACE monthly gravity fields on the normal equation level

Tue, 07/02/2019 - 00:00
Abstract

A large number of time-series of monthly gravity fields derived from GRACE data provide users with a wealth of information on mass transport processes in the system Earth. The users are, however, left alone with the decision which time-series to analyze. Following the example of other well-known combination services provided by the geodetic community, the prototype of a combination service has been developed within the frame of the project EGSIEM (2015–2017) to combine the different time-series with the goal to provide a unique and superior product to the user community. Four associated analysis centers (ACs) of EGSIEM, namely AIUB, GFZ, GRGS and IfG, generated monthly gravity fields which were then combined using the different normal equations (NEQs). But the relative weights determined by variance component estimation (VCE) on the NEQ level do not lead to an optimal combined product due to the different processing strategies applied by the individual ACs. We therefore resort to VCE on the solution level to derive relative weights that are representative of the noise levels of the individual solutions. These weights are then applied in the combination on the NEQ level. Prior to combination, empirical scaling factors that are based on pairwise combinations of NEQs are derived to balance the impact of the NEQs on the combined solution. We compare the processing approaches of the different ACs and introduce quality measures derived either from the differences w.r.t. the monthly means of the individual gravity fields or w.r.t. a deterministic signal model. After combination, the gravity fields are validated by comparison to the official GRACE SDS RL05 time-series and the individual contributions of the associated ACs in the spectral and the spatial domain. While the combined gravity fields are comparable in signal strength to the individual time-series, they stand out by their low noise level. In terms of noise, they are in 90% of all months as good or better than the best individual contribution from IfG and significantly less noisy than the official GRACE SDS RL05 time-series.

PPP-RTK based on undifferenced and uncombined observations: theoretical and practical aspects

Mon, 07/01/2019 - 00:00
Abstract

A synthesis of two prevailing global navigation satellite system positioning technologies, namely the precise point positioning and the network-based real-time kinematic, results in the emergence of the PPP-RTK, enabling single-receiver users to achieve high positioning accuracy with reasonable timeliness through integer ambiguity resolution. The realization of PPP-RTK needs to accomplish two sequential tasks. The first task is to determine a class of corrections including, among others, the satellite phase biases (SPBs) at the network level. With these corrections, the second task, then, is to solve for the ambiguity-fixed, absolute position at the user level. In this contribution, we revisit three variants (geometry-free, geometry-fixed and geometry-plus-satellite-clock-fixed) of the undifferenced and uncombined PPP-RTK network model and then point out their implications for practical use. We also carry out a case study using multi-day, dual-frequency global positioning system data from the crustal movement observation network of China stations, aiming to figure out what are the most appropriate linear combinations of the SPBs to be transmitted to the users from the viewpoint of decorrelation, and to assess the static and kinematic positioning performance.

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