The topic of downward continuation (DWC) has been studied for many decades without very conclusive answers on how different methods compare with each other. On the other hand, there are vast amounts of airborne gravity data collected by the GRAV-D project at NGS NOAA of the United States and by many other groups around the world. These airborne gravity data are collected on flight lines where the height of the aircraft actually varies significantly, and this causes challenges for users of the data. A downward continued gravity grid either on the topography or on the geoid is still needed for many applications such as improving the resolution of a local geoid model. Four downward continuation methods, i.e., Residual Least Squares Collocation (RLSC), the Inverse Poisson Integral, Truncated Spherical Harmonic Analysis, and Radial Basis Functions (RBF), are tested on both simulated data sets and real GRAV-D airborne gravity data in a previous joint study between NGS NOAA and CGS NRCan. The study group is further expanded by adding the TU Delft group on RBF and the TUM group on RLSC to incorporate more updated knowledge in the theoretical background and more in-depth discussion on the numerical results. A formal study group will be established inside IAG for providing the best answers for downward continuing airborne gravity data for local gravity field improvement. In this presentation, we review and compare the four methods theoretically and numerically. Simulated and real airborne and terrestrial data are used for the numerical comparison over block MS05 of the GRAV-D project in Colorado, USA, where the 1cm geoid experiment was performed by 15 international teams. The conclusion drawn from this study will advance the use of GRAV-D data for the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

How to cite: Li, X., Huang, J., Slobbe, C., Klees, R., Willberg, M., and Pail, R.: On Downward Continuing Airborne Gravity Data for Local Geoid Modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3042, https://doi.org/10.5194/egusphere-egu2020-3042, 2020.

Geodetic boundary-value problems (BVPs) and their solutions are important tools for describing and modelling the Earth’s gravitational field. Many geodetic BVPs have been formulated based on gravitational observables measured by different sensors on the ground or moving platforms (i.e. aeroplanes, satellites). Solutions to spherical geodetic BVPs lead to spherical harmonic series or surface integrals with Green’s kernel functions. When solving this problem for higher-order derivatives of the gravitational potential as boundary conditions, more than one solution is obtained. Solutions to gravimetric, gradiometric and gravitational curvature BVPs (Martinec 2003; Šprlák and Novák 2016), respectively, lead to two, three and four formulas. From a theoretical point of view, all formulas should provide the same solution, but practically, when discrete noisy observations are exploited, they do not.

In this contribution we present combinations of solutions to the above mentioned geodetic BVPs in terms of surface integrals with Green’s kernel functions by a spectral combination method. We investigate an optimal combination of different orders and directional derivatives of potential. The spectral combination method is used to combine terrestrial data with global geopotential models in order to calculate geoid/quasigeoid surface. We consider that the first-, second- and third-order directional derivatives are measured at the satellite altitude and we continue them downward to the Earth’s surface and convert them to the disturbing gravitational potential, gravity disturbances and gravity anomalies. The spectral combination method thus serves in our numerical procedures as the downward continuation technique. This requires to derive the corresponding spectral weights for the n-component estimator (n = 1, 2, … 9) and to provide a generalized formula for evaluation of spectral weights for an arbitrary N-component estimator. Properties of the corresponding combinations are investigated in both, spatial and spectral domains.

How to cite: Pitoňák, M., Šprlák, M., Novák, P., and Tenzer, R.: Overview on the spectral combination of integral transformations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3403, https://doi.org/10.5194/egusphere-egu2020-3403, 2020.

When treating geodetic boundary value problems in gravity field studies, the geometry of the physical surface of the Earth may be seen in relation to the structure of the Laplace operator. Similarly as in other branches of engineering and mathematical physics a transformation of coordinates is used that offers a possibility to solve an alternative between the boundary complexity and the complexity of the coefficients of the partial differential equation governing the solution. The Laplace operator has a relatively simple structure in terms of spherical or ellipsoidal coordinates which are frequently used in geodesy. However, the physical surface of the Earth substantially differs from a sphere or an oblate ellipsoid of revolution, even if these are optimally fitted. The situation may be more convenient in a system of general curvilinear coordinates such that the physical surface of the Earth is imbedded in the family of coordinate surfaces. The structure of the Laplace operator, however, is more complicated in this case and in a sense it represents the topography of the physical surface of the Earth. The Green’s function method together with the method of successive approximations is used for the solution of geodetic boundary value problems expressed in terms of new coordinates. The structure of iteration steps is analyzed and if useful, it is modified by means of the integration by parts. Subsequently, the individual iteration steps are discussed and interpreted.

How to cite: Holota, P. and Nesvadba, O.: Laplacian structure, solution domain geometry and successive approximations in gravity field studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12839, https://doi.org/10.5194/egusphere-egu2020-12839, 2020.

We present a novel approach to the solution of the geodetic boundary value problem with an oblique derivative boundary condition by the finite element method. Namely, we propose and analyse a finite element approximation of a Laplace equation holding on a domain with an oblique derivative boundary condition given on a part of its boundary. The oblique vector in the boundary condition is split into one normal and two tangential components and derivatives in tangential directions are approximated as in the finite difference method. Then we apply the proposed numerical scheme to local gravity field modelling. For our two-dimensional testing numerical experiments, we use four nodes bilinear quadrilateral elements and for a three-dimensional problem, we use hexahedral elements with eight nodes. Practical numerical experiments are located in area of Slovakia that is given by grid points located on the Earth's surface with uniform spacing in horizontal directions. Heights of grid points are interpolated from the SRTM30PLUS topography model. An upper boundary is in the height of 240 km above a reference ellipsoid WGS84 corresponding to an average altitude of the GOCE satellite orbits. Obtained solutions are compared to DVRM05.

How to cite: Macák, M., Minarechová, Z., Čunderlík, R., and Mikula, K.: The finite element method for solving the oblique derivative boundary value problems in geodesy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13411, https://doi.org/10.5194/egusphere-egu2020-13411, 2020.

We present local gravity field modelling based on a numerical solution of the oblique derivative bondary value problem (BVP). We have developed a finite volume method (FVM) for the Laplace equation with the Dirichlet and oblique derivative boundary condition, which is considered on a 3D unstructured mesh about the real Earth’s topography. The oblique derivative boundary condition prescribed on the Earth’s surface as a bottom boundary is split into its normal and tangential components. The normal component directly appears in the flux balance on control volumes touching the domain boundary, and tangential components are managed as an advection term on the boundary. The advection term is stabilised using a vanishing boundary diffusion term. The convergence rate, analysis and theoretical rates of the method are presented in [1].

Using proposed method we present local gravity field modelling in the area of Slovakia using terrestrial gravimetric measurements. On the upper boundary, the FVM solution is fixed to the disturbing potential generated from the GO_CONS_GCF_2_DIR_R5 model while exploiting information from the GRACE and GOCE satellite missions. Precision of the obtained local quasigeoid model is tested by the GNSS/levelling test.

[1] Droniou J, Medľa M, Mikula K, Design and analysis of finite volume methods for elliptic equations with oblique derivatives; application to Earth gravity field modelling. Journal of Computational Physics, s. 2019

How to cite: Medľa, M., Mikula, K., and Čunderlík, R.: FVM approach for solving the oblique derivative BVP on unstructured meshes above the real Earth’s topography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13527, https://doi.org/10.5194/egusphere-egu2020-13527, 2020.

The notion of an equipotential surface of the Earth’s gravity potential is of key importance for vertical datum definition. The aim of this contribution is to focus on differential geometry properties of equipotential surfaces and their relation to parameters of Earth’s gravity field models. The discussion mainly rests on the use of Weingarten’s theorem that has an important role in the theory of surfaces and in parallel an essential tie to Brun’s equation (for gravity gradient) well known in physical geodesy. Also Christoffel’s theorem and its use will be mentioned. These considerations are of constructive nature and their content will be demonstrated for high degree and order gravity field models. The results will be interpreted globally and also in merging segments expressing regional and local features of the gravity field of the Earth. They may contribute to the knowledge important for the realization of the World Height System.

How to cite: Holota, P. and Nesvadba, O.: Differential geometry and curvatures of equipotential surfaces in the realization of the World Height System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13418, https://doi.org/10.5194/egusphere-egu2020-13418, 2020.

The equation of the ellipse in a meridian plan is well known to be which can be defined in its most generic form as where the variable (s) is the indicator value of the ellipsoidal height. For (h=0), the value of  (s) equals (1) , for negative  (h), the value  is lower than (1), and vice versa, for positive (h) , the corresponding  value of (s) is greater than (1). Hence (s) and (h) are highly-correlated. The main goal of this work is to exploit the  (s-h) correlation and to represent it both statistically and mathematically. Moreover, the essential role of (s) in the transformation of the Cartesian coordinates (x_p, y_p, z_p) of any generic point (P) into its geodetic coordinates (φ, λ, h) in reference to the geodetic ellipsoid is thoroughly studied.

How to cite: Eleiche, M. and Mansi, A.: Heuristic Algorithm to Compute Geodetic Height (h) from Ellipse Equation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10857, https://doi.org/10.5194/egusphere-egu2020-10857, 2020.

The errors-in-variables (EIV) model is applied to surveying and mapping fields such as empirical coordinate transformation, line/plane fitting and rigorous modelling of point clouds and so on as it takes the errors both in coefficient matrix and observation vector into account. In many cases, not all of the elements in coefficient matrix are random or some of the elements are functionally dependent. The partial EIV (PEIV) model is more suitable in dealing with such structured coefficient matrix. Furthermore, when some reliable prior information expressed by inequality constraints is considered, the adjustment result of inequality constrained PEIV (ICPEIV) model is expected to be improved. There are two kinds of algorithms to solve the ICPEIV model under the weighted total least squares (WTLS) criterion currently. On the one hand, one can linearize the PEIV model and transform it into a sequence of quadratic programming (QP) sub-problems. On the other hand, one can directly solve the nonlinear target function by common used programming algorithms.All the QP algorithms and nonlinear programming methods are complicated and not familiar to the geodesists, so the ICPEIV model is not widely used in geodesy.   

In this contribution, an algorithm based on standard least squares is proposed. First, the estimation of model parameters and random variables in coefficient matrix are separated according to the Karush-Kuhn-Tucker (KKT) conditions of the minimization problem. The model parameters are obtained by solving the QP sub-problems while the variables are determined by the functional relationship between them. Then the QP problem is transformed to a system of linear equations with nonnegative Lagrange multipliers which is solved by an improved Jacobi iterative algorithm. It is similar to the equality-constrained least squares problem. The algorithm is simple because the linearization process is not required and it has the same form of classical least squares adjustment. Finally, two empirical examples are presented. The linear approximation algorithm, the sequential quadratic programming algorithm and the standard least squares algorithm are used. The examples show that the new method is efficient in computation and easy to implement, so it is a beneficial extension of classical least squares theory.

How to cite: Jian, X. and Sichun, L.: Weighted total least squares problems with inequality constraints solved by standard least squares theory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6362, https://doi.org/10.5194/egusphere-egu2020-6362, 2020.

Model-based hydrodynamic leveling is an efficient and flexible alternative method to connect islands and offshore tide gauges with the height system on land. The method uses a regional, high-resolution hydrodynamic model that provides total water levels. From the model, we obtain the differences in mean water level (MWL) between tide gauges at the mainland and at the islands or offshore platforms, respectively. Adding them to the MWL relative to the national height system at the mainland’s tide gauges realizes a connection of the island and offshore platforms with the height system on the mainland. Usually, the geodetic leveling networks are based on spirit leveling. So, as we can not make the direct connections between coastal countries, due to the inability of the spirit leveling method to cross the water bodies, they are weak in these regions. In this study, we assessed the impact of using model-based hydrodynamic leveling connections among the North Sea countries on the quality at which the European Vertical Reference System can be realized. In doing so, we combined the model-based hydrodynamic leveling data with synthetic geopotential differences among the height markers of the Unified European Leveling Network (UELN) used to realize the European Vertical Reference Frame 2019. The uncertainties of the latter data set were provided by the BKG. The impact is assessed in terms of both precision and reliability. We will show that adding model-based hydrodynamic leveling connections lowers the standard deviations of the estimated heights in the North Sea countries significantly. In terms of reliability, no significant improvements are observed.

How to cite: Afrasteh, Y., Slobbe, C., Verlaan, M., Sacher, M., and Klees, R.: Model-based hydrodynamic leveling; a power full tool to enhance the quality of the geodetic networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19292, https://doi.org/10.5194/egusphere-egu2020-19292, 2020.

In this study, we assess the accuracy of deformation rates produced from GNSS campaign measurements sampled in different frequencies. The ideal frequency of the sampling seems to be 1 measurement per month however it is usually found to be cumbersome. Alternatively the sampling was performed 3 measurements per year and time series analyses were carried out. We used the continuous GPS time series of JPL, NASA from a global network of the IGS to decimate the data down to 4 monthly synthetic GNSS campaign time series. Minimum data period was taken to be 4 years following the suggestions from the literature. Furthermore, the effect of antenna set-up errors in campaign measurements on the estimated trend was taken into account. The accuracy of deformation rates were then determined taking the site velocities from ITRF14 solution as the truth. The RMS of monthly velocities agreed pretty well with the white noise error from global studies given previously in the literature. The RMS of four monthly deformation rates for horizontal positioning were obtained to be 0.45 and 0.50 mm/yr for north and east components respectively whereas the accuracy of vertical deformation rates was found to be 1.73 mm/yr. This is slightly greater than the average level of the white noise error from a global solution previously produced, in which antenna set up errors were out of consideration. Antenna set up errors in campaign measurements modified the above error level to 0.75 and 0.70 mm/yr for the horizontal components north and east respectively whereas the accuracy of the vertical component was slightly shifted to 1.79 mm/yr.

How to cite: Turen, Y. and Sanli, D. U.: Accuracy of GNSS campaign site velocities with respect to ITRF14 solution , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2101, https://doi.org/10.5194/egusphere-egu2020-2101, 2020.

Recovering the full underlying magnetization from geomagnetic potential field measurements is known to be highly nonunique. Localization constraints on the magnetization can improve this to a certain extent. We present some analytic background as well as some examples on what is meant by 'to a certain extent'. In particular, if no constraints other than spatial localization are imposed, only two out of three components of the vectorial magnetization can be reconstructed uniquely. If it is additionally assumed that the magnetization is induced by an (unknown) ambient dipole field, then the susceptibility and the direction of the ambient dipole can be reconstructed.

How to cite: Gerhards, C.: Hidden Magnetizations and Localization Constraints, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1891, https://doi.org/10.5194/egusphere-egu2020-1891, 2020.

We consider the quantitative inverse problem for the recovery of subsurface Earth's properties, which relies on an iterative minimization algorithm. Due to the scale of the domains and lack of apriori information, the problem is severely ill-posed. In this work, we reduce the ill-posedness by using the ``regularization by discretization'' approach: the wave speed is described by specific bases, which limits the number of coefficients in the representation. Those bases are associated with the eigenvectors of a diffusion equation, and we investigate several choices for the PDE, that are extracted from the field of image processing. We first compare the efficiency of these model descriptors to accurately capture the variation with a minimal number of coefficients. In the context of sub-surface reconstruction, we demonstrate that the method can be employed to overcome the lack of low-frequency contents in the data. We illustrate with two and three-dimensional acoustic experiments.

How to cite: Faucher, F., Scherzer, O., and Barucq, H.: Eigenvector Model Descriptors for the Seismic Inverse Problem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22224, https://doi.org/10.5194/egusphere-egu2020-22224, 2020.

Approaches based on Stokes coefficient filtering and « mass concentration » representations have been proposed for recovering changes of the surface water mass density from along-track accurate GRACE K-Band Range Rate (KBRR) measurements of geopotential change. The number of parameters, i.e. surface triangular tiles of water mass, to be determined remains large and the choice of the regularization strategy as the gravimetry inverse problem is non unique. In this study, we propose to use regional sets of orthogonal surface functions to image the structure of the surface water mass density variations. Since the number of coefficients of the development is largely smaller than the number of tiles, the computation of daily GRACE solutions for continental hydrology, e.g. obtained by Extended Kalman Filtering (EKF), is greatly fastened and eased by the matrix dimensions and conditioning. The proposed scheme of decomposition is applied to the African continent where it enables to very localized sources of (sub-)monthly water mass amplitudes.

How to cite: Ramillien, G. and Seoane, L.: Fast determination of surface water mass changes using regional orthogonal functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2692, https://doi.org/10.5194/egusphere-egu2020-2692, 2020.

Airborne gravimetry is capable to provide Earth’s gravity data of high accuracy and spatial resolution for any area of interest, in particular for hard-to-reach areas. An airborne gravimetry measuring system consists of a stable-platform or strapdown gravimeter, and GNSS receivers. In traditional (scalar) airborne gravimetry, the vertical component of the gravity disturbance vector is measured. In actively developing vector gravimetry, all three components of the gravity disturbance vector are measured.

In this research, we aim at developing new postprocessing algorithms for estimating gravity from airborne data taking into account a priori information about spatial behavior of the gravity field in the survey area. We propose two algorithms for solving the following two problems:

1) In scalar gravimetry:  Mapping gravity at the flight height using the gravity disturbances estimated along the flight lines (via low-pass or Kalman filtering), taking into account spatial correlation of the gravity field in the survey area and statistical information on the along-line gravity estimate errors.

2) In vector gravimetry:  Simultaneous determination of three components of the gravity disturbance vector from airborne measurements at the flight path.

Both developed algorithms use an a priori spatial gravity model based on parameterizing the disturbing potential in the survey area by three-dimensional harmonic spherical scaling functions (SSFs). The algorithm developed for solving Problem 1 provides estimates of the unknown coefficients of the a priori gravity model using a least squares technique. Due to the assumption that the along-line gravity estimate errors at any two lines are not correlated, the algorithm has a recursive (line-by-line) implementation. At the last step of the recursion, regularization is applied due to ill-conditioning of the least squares problem. Numerical results of processing the GT-2A airborne gravimeter data are presented and discussed.

To solve Problem 2, one need to separate the gravity horizontal component estimates from systematic errors of the inertial navigation system (INS) of a gravimeter (attitude errors, inertial sensor bias). The standard method of gravity estimation based on gravity modelling over time is not capable to provide accurate results, and additional corrections should be applied. The developed algorithm uses a spatial gravity model based on the SSFs. The coefficients of the gravity model and the INS systematic errors are estimated simultaneously from airborne measurements at the flight path via Kalman filtering with regularization at the last time moment. Results of simulation tests show a significant increase in accuracy of gravity vector estimation compared to the standard method.

This research was supported by RFBR (grant number 19-01-00179).

How to cite: Vyazmin, V. and Bolotin, Y.: Using spherical scaling functions in scalar and vector airborne gravimetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11745, https://doi.org/10.5194/egusphere-egu2020-11745, 2020.

 Uncertainties usually exist in the process of acquisition of measurement data, which affect the results of the parameter estimation. The solution of the uncertainty adjustment model can effectively improve the validity and reliability of parameter estimation. When the coefficient matrix of the observation equation has a singular value close to zero, i.e., the coefficient matrix is ill-posed, the ridge estimation can effectively suppress the influence of the ill-posed problem of the observation equation on the parameter estimation. When the uncertainty adjustment model is ill-posed, it is more seriously affected by the error of the coefficient matrix and observation vector. In this paper, the ridge estimation method is applied to ill-posed uncertainty adjustment model, deriving an iterative algorithm to improve the stability and reliability of the results. The derived algorithm is verified by two examples, and the results show that the new method is effective and feasible.

How to cite: lu, T.: Ridge estimation iterative algorithm to ill-posed uncertainty adjustment model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18906, https://doi.org/10.5194/egusphere-egu2020-18906, 2020.

The International GNSS service (IGS) has been providing precise reference products for the Global Navigation Satellite Systems (GNSS) GPS and (starting later) GLONASS since more than 25 years. These orbit, clock correction, coordinate reference frame, troposphere, ionosphere, and bias products are freely distributed and widely used by scientific, administrative, and commercial users from all over the world. The IGS facilities needed for data collection, product generation, product combination, as well as data and product dissemination, are well established. The Center for Orbit Determination in Europe (CODE) is one of the Analysis Centers (AC) contributing to the IGS from the beginning. It generates IGS products using the Bernese GNSS Software.

In the last decade new GNSS (European Galileo and Chinese BeiDou) and regional complementary systems to GPS (Japanese QZSS and Indian IRNSS/NAVIC) were deployed. The existing GNSS are constantly modernized, offering - among others - more stable satellite clocks and new signals. The exploitation of the new data and their integration into the existing IGS infrastructure was the goal of the Multi-GNSS EXtension (MGEX) when it was initiated in 2012. CODE has been participating in the MGEX with its own orbit and clock solution from the beginning. Since 2014 CODE’s MGEX (COM) contribution considers five GNSS, namely GPS, GLONASS, Galileo, BeiDou2 (BDS2), and QZSS. We provide an overview of the latest developments of the COM solution with respect to processing strategy, orbit modelling, attitude modelling, antenna calibrations, handling of code and phase biases, and ambiguity resolution. The impact of these changes on the COM products will be discussed.

Recent assessment showed that especially the Galileo analysis within the MGEX has reached a state of maturity, which is almost comparable to GPS and GLONASS. Based on this finding the IGS decided to consider Galileo in its third reprocessing campaign, which will contribute to the next ITRF. Recognizing the demands expressed by the GNSS community, CODE decided in 2019 to go a step further and consider Galileo also in its IGS RAPID and ULTRA-RAPID reference products. We summarize our experiences from the first months of triple-system (ULTRA)-RAPID analysis including GPS, GLONASS, and Galileo. Finally we provide an outlook of CODE’s IGS analysis with the focus on the new GNSS.

How to cite: Prange, L., Villiger, A., Schaer, S., Dach, R., Sidorov, D., Jäggi, A., and Beutler, G.: CODE IGS reference products including Galileo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18142, https://doi.org/10.5194/egusphere-egu2020-18142, 2020.

The year 2020 is going to mark the first time of four global navigation satellite systems (i.e., GPS, GLONASS, Galileo, and BeiDou) in full operational capability. Utilizing the various available observation types together in global multi-GNSS processing offers new opportunities, but also poses many challenges. The raw observation approach facilitates the incorporation of any undifferenced and uncombined code and phase observation on any frequency into a combined least squares adjustment. Due to the increased number of observation equations and unknown parameters, using raw observations directly is more computationally demanding than using, for example, ionosphere-free double-differenced observations. This is especially relevant for our contribution to the third reprocessing campaign of the International GNSS Service, where we process observations from up to 800 stations per day to three GNSS constellations at a 30-second sampling. For a single day, this results in more than 200 million raw observations, from which we estimate almost 5 million parameters.

Processing such a large number of raw observations together is computationally challenging and requires a highly optimized processing chain. In this contribution, we detail the key steps that make such a processing feasible in the context of a distributed computing environment (i.e., large computer clusters). Some of these steps are the efficient setup of observation equations, a suitable normal equation structure, a sophisticated integer ambiguity resolution scheme, automatic outlier downweighting based on variance component estimation, and considerations regarding the estimability of certain parameter groups.

How to cite: Strasser, S. and Mayer-Gürr, T.: Efficient multi-GNSS processing based on raw observations from large global station networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3014, https://doi.org/10.5194/egusphere-egu2020-3014, 2020.

Precise point positioning (PPP) technique is an effective tool for time and frequency applications. Using phase/code observations and precise products, the PPP time transfer allows an accuracy of sub-nanoseconds within a latency of several days. Although the PPP time transfer is usually implemented in the post-processing mode, using the real-time PPP (RT-PPP) technique for time transfer with the shorter latency remains attractive to time community. In 2012, the IGS (International GNSS Service) launched an open-access real-time service (RTS) project, broadcasting satellite orbit and clock corrections on the Internet, which enables PPP time transfer in the real-time mode. In this contribution, we apply the RT-PPP for high-precision time transfer and synchronization. The GNSS receiver is required to be equipped with an atomic clock as the external local clock. We use the RT-PPP technique to compute the receiver clock offset with respective to the GNSS time scale. On the basis of clock offsets, we steer the local clock by frequency adjustment method. In this way, all the local clocks are synchronized to the GNSS time scale, making local clocks synchronized with each other.

The time scales of the RTS products are evaluated at first. Six kinds of the RTS products (IGS01, CLK10, CLK53, CLK80 and CLK93) on DOY220-247, 2019 are pre-saved to compute the receiver clock offsets. The clock offset with respect to the GPST (GPS Time) obtained from the IGS final product is applied as the reference. The standard deviations (STDs) of the clock offsets with respect to the reference are 0.63, 1.76, 0.28, 0.27 and 1.28 ns for IGS01, CLK10, CLK53, CLK80 and CLK93, respectively.

Finally, we set up a hardware system to examine the validity of our time synchronization method. The baseline of the time synchronization experiment is about 5 m. The synchronization error of the 1 PPS outputs is precisely measured by the frequency counter. The STD of the 4-days results is about 0.48 ns. The peak-to-peak value of the synchronization error is about 2.5 ns.

How to cite: Lyu, D., Dong, T., Zeng, F., and Ouyang, X.: Time Synchronization Method Based on Real-Time Precise Point Positioning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12206, https://doi.org/10.5194/egusphere-egu2020-12206, 2020.

Strain rate fields within strike-slip regimes often possess complexity associated with along-strike  slip  rate  variations.  These along-strike  slip  rate  variations  produce dilatational  components  of  strain  rate  within  and  near  the  fault  zones  and  within  the adjacent block areas. These dilatation rates do not directly reflect the slip rate magnitude on the strike  slip  fault,  but  rather  the  relative  change in  along-strike  slip  rate.  Displacement rates measured  using  GPS  observations  reflect  the  full  deformation  gradient  field,  which may involve significant dilatational components and other off-fault deformation. Thus, using displacement rates  to  infer  slip  rate  and  locking  depth  of  major  strike-slip  faults  may introduce  errors  when  along-strike  slip  rate  variations  are  present.  On the  contrary,  true locking depth and slip rate can be obtained from the pure strike-slip component of shear strain rates. In this study we investigate the use of shear strain rates alone (obtained from the full displacement rate field of the SCEC 4.0 velocity field in southern California) to infer fault slip rate and locking depth parameters along the San Andreas (up to 37° N) as well as the San Jacinto fault zones. Such an  analysis is  critical  for  accurate  estimation  of  off-fault  strain  rates  outside  of  the major shear zones.

We conducted benchmarking tests to determine if accurate shear strain rates  can  be  obtained  from  a  synthetic  fault  slip  rate  field  possessing  the  same  station spacing as the SCEC 4.0 dataset. The synthetics were derived using Okada’s [1992] elastic dislocation routine (Coulomb  3.2).  These displacement  rates  were  interpolated  using  bi-cubic  Bessel  interpolation  to  infer the full  horizontal  velocity  gradient  tensor  field,  along with model uncertainties.  To test realistic conditions, along-strike slip  rates were put into the elastic dislocation model and model displacements were output at the true GPS station spacing in southern California from the SCEC 4.0 dataset. The modeled strain rate field shows negligible strain rate artifacts in most regions and both the shear strain and dilatation rates obtained  from  the  bi-cubic interpolation  were  well-resolved.  The inferred shear strain rate field was then  inverted, using a simple screw dislocation  forward model  for the best-fit  fault location,  fault locking depth,  and  fault  slip  rate.  Model parameter estimates were well resolved,  both  near  and away from fault slip rate transitions (± 1 km for fault locking depth; ± 1-2 mm/yr for fault slip rate). Test results to date show the method can resolve slip rates and locking depth within the zones of along-strike transition. Results to date from this methodology applied to southern California using the SCEC 4.0 GPS velocity field show  remarkably  well-resolved  and  prominent  shear  strain  rate  bands  that follow  both  the San Andreas and San  Jacinto  fault  systems.   The shear strain rates  reflect dramatic  along-strike  slip  rate  variations,  found  in  many  previous  studies.  However, fault locking depths are generally shallower than previously published results. Residual off-fault strain rates, not associated with the major strike-slip faults, appear to accommodate ~30% of the total Pacific-North American plate relative motion.

How to cite: Campbell, L., Holt, W., and Chen, Y.: Using GPS Derived Shear Strain Rates in Southern California to Constrain Fault Slip Rate, Locking Depth, and Residual Off-Fault Strain Rates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21179, https://doi.org/10.5194/egusphere-egu2020-21179, 2020.

It has been shown that the earth surface can be observed using Global Navigation Satellite System (GNSS) signals as signals of opportunity. An important advantage of GNSS in this regard is that it provides a global coverage of the earth thanks to dozens of satellites, projected to be 120 by 2030, distributed in various constellations.

GNSS signals parameters such as the carrier phase and the amplitude can be used for example for soil moisture estimation, sea ice detection or sea surface altimetry, which is an important indicator for studying climate evolution. As the sea level varies in centimeters, sea surface altimeters have to be very precise. This accuracy can be achieved using satellite altimeters, provided the availability of precise validation and calibration techniques and in-situ experiments. The objective of this study is the definition of an original GNSS buoy system for satellite altimeters calibration.

GNSS buoy systems are a cheap and light alternative solution to mareographs and can provide high rate measurements. In our approach using this system, we consider, as observable, the phase difference between incoming GPS-L1 signals at a reference, fixed antenna at ~10m height on the ground and at a buoy antenna on the sea. In an analogy with a GNSS reflectometry system, the buoy can be compared to a specular reflection point, but presents the advantage of collecting the data from all visible satellites at the same location. The signals sensed by both antennas are digitized before processing.

Assuming that the horizontal two-dimensions position of the buoy is accurately known by GNSS positioning (which is more efficient in these dimensions than for estimating the height of the buoy), a new phase observable evolving linearly in the [-π , π] interval as a function of the sine of the satellite elevation can be defined. The slope of this linear evolution is proportional to the height between the two antennas, which is the parameter to estimate. For accuracy and robustness purpose, the estimation of this slope is realized using a circular-linear regression technique that includes the fusion of the data from all visible satellites signals. Indeed, we can show that, using the full span of the sines of visible satellites elevations, centimeter accuracy can be reached for integration times as short as a few milliseconds. The GNSS buoy technique described in this work is evaluated on synthetic and real data.

How to cite: Kouassi, W., Stienne, G., and Reboul, S.: Fusion applied to phase altimetry in a GNSS buoy system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15474, https://doi.org/10.5194/egusphere-egu2020-15474, 2020.

The exploitation of GNSS signals for reflectometry opens several fields of application over the ocean, land and in the cryosphere. Coherence of the reflection allows precise measurements of the carrier phase and signal amplitude for accurate sea surface altimetry and sea ice characterisation. A coherence condition can be set by a threshold of the signal-to-noise power ratio (SNR). Previous simulations suggest that an SNR > 30 dB will ensure a coherent processing of the signal.

This paper presents reflectometry measurements that provide signal coherence information. The measurements have been conducted on two research vessels: R/V Lance and R/V Polarstern. The objective is to reveal the required conditions for coherent reflectometry depending on sea state and sea ice occurrence. Three data sets from expeditions of the two research vessels to Fram Strait, the Northern Atlantic and the Arctic Ocean are analysed.

On both ships a GORS (GNSS Occultation Reflectometry Scatterometry) receiver with three antenna links has been installed. A common up-looking link is dedicated to direct signal observations. Two additional side-looking links allow sampling the reflected signal with right- and left-handed polarization (RHCP and LHCP). The respective setups have suitable positions to observe grazing sea surface reflections (< 30 deg elevation angle). The antennas are mounted on Lance and Polarstern about 24 m and 22 m above sea level, respectively.

Reflection events are recorded continuously covering more than 70 days. Each event comprises a track of the satellite signal in the grazing angle elevation range. On average 2-3 reflection events were recorded in parallel. The results of the analysis show that in coastal waters (German Bight and Svalbard fjords) up to 44%, 37% (RHCP, LHCP) of the measurements meet the coherence condition. On the high sea it is rarely met, only <0.5% of RHCP and LHCP records fulfill the coherence condition there. The rate of coherent observations increases up to 14%, 13% (RHCP, LHCP) in case of sea ice occurrence.

It can be concluded that the sea state plays an important role for coherent reflectometry. Applications of coherent reflectometry over the ocean may concentrate on the retrieval of sea ice properties and altimetry in coastal waters. For the early data set, recorded in Fram Strait 2016, the estimation of sea concentration has been demonstrated. At present the Polarstern setup continues reflectometry measurements in the MOSAiC expedition with unique opportunities for sea ice observations in the central Arctic.

The limits of coherent reflectometry at high sea became clear. However, it is worth noting that the direct signal link meets the SNR condition also at high sea with an average rate of 55%. This result motivates further investigations to exploit the direct link of shipborne GNSS for atmospheric and ionospheric soundings on the sparsely covered ocean using coherent phase delay measurements.

How to cite: Semmling, M., Gerland, S., Gerber, T., Ramatschi, M., Dick, G., Wickert, J., and Hoque, M.: Coherent GNSS reflections over the sea surface - A classification for reflectometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18035, https://doi.org/10.5194/egusphere-egu2020-18035, 2020.

Commercial software are usually refered to in national surveying practice and local deformation studies. Since their working environment is user friendly and implementation is easy, they could be prefered by many surveying practitioners or even researchers. However their usage is usually limited to 20-30 km due mainly to their crude ambiguity resolution algorithms and  the fact that they usually use broadcast ephemeris and standard troposphere models.  Since usualy the tropospheric zenith delay is not estimated but obtained from a standard troposphere model, the accuracy of the vertical component would be affected as the height difference between baseline points grows. As the baseline length becomes >20-30 km the tropospheric error would be coupled with orbital errors. Results based on large height difference would affect positioning solutions as well as local geoid determination studies. Monitoring local deformation such as landslides would also be affected if there is large height difference between the crown and the toe. The level of baseline dependent error is usually well covered in surveying standards manual however the effect of large height difference is generally ignored. In this study, we made an attempt to quantify vertical positioning error levels both considering large height difference between baseline points and longer baseline lengths. We used the data of CORS stations in the western US for the simulation of the observing session duration. TOPCON’s software MAGNET (Ver 4.0.1) was used to process the GNSS data. It appears that every 10 km increase in the baseline length and every 100 m increase in the height difference would cause 2.59mm and 1.24 mm vertical positioning error respectively. 

How to cite: Erol, T. and Sanli, D. U.: Quantifying baseline length and height-difference dependent errors from commercially available software, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2089, https://doi.org/10.5194/egusphere-egu2020-2089, 2020.

The evaluation of the observation data obtained from the GPS system is performed with software. The software used today is divided into academic, web-based and commercial software. Researches generally focus on academic software and web-based services that have become widespread in recent years.Commercial software is often used by daily users, mostly in classical geodesy. These softwares differ from each other; users, their purpose of use, processing methods, accuracy, users knowledge level etc. In this study, we focused commercial software’s (Topcon Magnet version 4.0.1) accuracy of GPS positioning in single and multiple base solutions.

10 stations included in IGS network in California, USA, one base and 2, 3 and 4 network solution results in different session times (1h to 24h) positioning accuracy was achieved. In our study, it has been found that the accuracy obtained for the horizontal components North and East varies between 2 mm and 8 mm and vertical component Up varies between 3 mm and 54 mm.

In evaluations with a reference station distance of up to 100km, increasing the number of more than 2 reference stations (3 or 4) for horizontal compenents (North and East) did not make a significant contribution to accuracy. In the case of vertical component (Up) accuracy, it is determined that accuracy is affected by interstation distance and observation time more than the number of reference stations(1, 2, 3 or 4). it was found that it was meaningful to increase the accuracy of the vertical component to be observation time for as long as possible and reference base stations to be selected from the closest possible stations. Avoidance of short observation time (1 hour and less) for all three components was found to be important in terms of accuracy to be achieved.

Keywords: Commercial software, GPS, Multiple base solution.

How to cite: Tas, B., Erol, T., and Turen, Y.: The Effect of Multiple Baseline Evalution with Commercial Software on GPS Position Accuracy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3267, https://doi.org/10.5194/egusphere-egu2020-3267, 2020.

In recent years, one can notice a significant development of the PPP, which both increases accuracy and speeds up the convergence time of the receiver position. New or improved computational algorithms have been developed. This development can also be seen in real-time measurements made possible by the IGS RTS. Other new trends are the development of PPP-AR and the use of cheap receivers such as smartphones. However, the development of the PPP method can be particularly seen in multi‑GNSS measurements. This applies to the continuous development of existing GPS and GLONASS systems and the emergence of new Galileo and BDS systems that have a significant impact on PPP. The development of multi GNSS will increase the number of satellites observed, which improves geometry and PDOP, and increases product accuracy or increases the number of available signals and frequencies. The use of multi-GNSS is possible thanks to the IGS MGEX.

This research shows how the accuracy and convergence time by the PPP changes with the development of GPS, GLONASS and Galileo systems. We used the globally distributed MGEX stations for three different weeks, each one from 2017, 2018 and 2019. The analysis was made for different constellations: GPS, GLONAS, Galileo, GPS+GLONAS, GPS+Galileo, GLONASS+Galileo and GPS+GLONASS+Galileo for different cut-off elevation angles: 0⁰, 5⁰, 10⁰, 15⁰, 20⁰, 25⁰, 30⁰, 35⁰ and 40⁰.

Based on the analysis, we show a progressive improvement of accuracy and a shortening of convergence time in recent years. This is especially visible for calculations with multi-GNSS, obtaining the best results for GPS+GLONASS+Galileo for the last analysed period. Already in 2019 on average, about 22 satellites were observed using a total of three systems together. It has also been shown that in 2019, the Galileo system already allows for positioning with high accuracy anywhere on Earth. On average, around 7 Galileo satellites were observed in 2019, where in 2017 on average, fewer than 5 satellites were observed. It has also been shown that the GPS still provides the highest accuracy and has the greatest impact on multi-GNSS positioning accuracy. Even for the GLONASS+Galileo, poorer accuracy was obtained than for GPS‑only. However, for the GLONASS+Galileo solution, a smaller error distribution and lower standard deviation values were obtained than for GPS-only. This may indicate constant bias-related error values (IFB, ISB) and poorer product quality. In addition, for higher elevation angles, it was shown that better accuracy was obtained for Galileo‑only than for GLONASS-only, but only for the third period. It was also noted that for the joint of GLONASS+Galileo, it eliminated errors that occurred in the GLONASS-only, for which, in the second period, much larger errors were obtained than for the other periods. Finally, the influence of multi-GNSS positioning for positioning in constraint conditions was demonstrated by analysing the effect of the elevation angle. It has been shown that even for elevation angle of 40⁰, the use of GPS+GLONASS+Galileo allowed obtaining about 90% of the availability of solutions with accuracy in estimation the position of individual cm.

How to cite: Kiliszek, D., Araszkiewicz, A., and Kroszczynski, K.: Accuracy of PPP along with the development of GPS, GLONASS and Galileo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7614, https://doi.org/10.5194/egusphere-egu2020-7614, 2020.

Precise Point Positioning (PPP) is one of the most promising processing techniques for Global Navigation Satellite System (GNSS) data. By the use of precise satellite products (orbits, clocks and biases) and sophisticated algorithms applied on the observations of a multi-frequency receiver, coordinate accuracies at the decimetre/centimetre level for a float solution and at the centimetre/millimetre level for a fixed solution can be achieved. In contrast to relative positioning methods (e.g. RTK), PPP does not require nearby reference stations or a close-by reference network. On the other hand PPP has a non-negligible convergence time. To make PPP more competitive against other high-precision GNSS positioning techniques, scientific research focuses on reducing the convergence time of PPP.

In this contribution, we present results of PPP with focus on integer ambiguity resolution (PPP-AR) using satellite products from different analysis centers. The resulting coordinate accuracy and convergence behaviour are evaluated in various test scenarios. In these test cases we distinguish between the use of satellite products from Graz University of Technology, which are calculated using a raw observation approach, and nowadays publicly available satellite products of different analysis centers (e.g. CNES, CODE). All those products enable PPP-AR in different approaches. To shorten the convergence time, we investigate and compare different PPP processing approaches using GPS and Galileo observations. The use of 2+ frequencies and alternatives to the classical PPP model, which is based on two frequencies and the ionosphere-free linear combination are discussed (e.g. uncombined model with ionospheric constraint). The PPP calculations are performed with the in-house software raPPPid, which has been developed at the research division Higher Geodesy of TU Vienna and is part of the Vienna VLBI and Satellite Software (VieVS PPP).

How to cite: Glaner, M. F., Weber, R., and Strasser, S.: PPP-AR with GPS and Galileo: Assessing diverse approaches and satellite products to reduce convergence time, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7792, https://doi.org/10.5194/egusphere-egu2020-7792, 2020.

The Precise Point Positioning (PPP) technique using the GPS has become a popular alternative to the differential approach. Thanks to the MGEX pilot project of the IGS, precise orbit and clock products from other constellations like Galileo, Beidou, GLONASS are today available. This presentation focusses on GPS and Galileo systems and compares their individual performance to a combined processing.

Resolving GNSS phase observations biases to their correct integer values significantly improves the precision and the accuracy of the estimated parameters. However, PPP with ambiguity resolution (PPP-AR) requires to deal with “hardware” biases at the satellite and receiver level. Nevertheless, several Analysis Centers within the IGS community are providing these biases. Using the orbit, clock and bias products of the GPS and Galileo constellations provided by the CNES-CLS group, we were able to compare PPP and PPP-AR station coordinates repeatability from a network of around 50 stations during 6 months. For both static and cinematic solutions, the hybridized solution significantly exceeds both individual ones.

How to cite: Perosanz, F., Katsigianni, G., Loyer, S., Gupta, M., Santamaria, A., and Mercier, F.: Improving PPP static and cinematic positioning by combining GPS and Galileo data., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7645, https://doi.org/10.5194/egusphere-egu2020-7645, 2020.

Usually train positioning is realized via counting wheel rotations (Odometer), and correcting at fixed locations known as balises. A balise is an electronic beacon or transponder placed between the rails of a railway as part of an automatic train protection (ATP) system. Balises constitute an integral part of the European Train Control System, where they serve as “beacons” giving the exact location of a train. Unfortunately, balises are expensive sensors which need to be placed over about 250 000 km of train tracks in Europe.

Therefore, recently tremendous efforts aim on the development of satellite-based techniques in combination with further sensors to ensure precise train positioning. A fusion of GNSS receiver and Inertial Navigation Unit (IMU) observations processed within a Kalman Filter proved to be one of potential optimal solutions for train traction vehicles positioning.

Today several hundreds of trains in Austria are equipped with a single-frequency GPS/GLONASS unit. However, when the GNSS signal fails (e.g. tunnels and urban areas), we expect an outage or at least a limited positioning quality. To yet ensure availability of a reliable trajectory in these areas, the GNSS sensor is complemented by a strapdown IMU platform and a wheel speed sensor (odometer).

In this study a filtering algorithm based on the fusion of three sensors GPS, IMU and odometer is presented, which enables a reliable train positioning performance in post-processing. Odometer data are counts of impulses, which relate the wheel’s circumference to the velocity and the distance traveled by the train. This odometer data provides non-holonomic constraints as one-dimensional velocity updates and complements the basic IMU/GPS navigation system. These updates improve the velocity and attitude estimates of the train at high update rates while GPS data is used to provide accurate determination in position with low rates. In case of GNSS outages, the integrated system can switch to IMU/odometer mode. Using the exponentially weighted moving average method to estimate of measurement noise for odometer velocity helps to construct measurement covariance matrices. In the presented examples an IMU device, a GPS receiver and an Odometer provide the data input for the loosely coupled Kalman Filter integration algorithm. The quality of our solution was tested against trajectories obtained with the software iXCOM-CMD (iMAR) as reference.

How to cite: Li, Q. and Weber, R.: GNSS/IMU/Odometer based Train Positioning , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7856, https://doi.org/10.5194/egusphere-egu2020-7856, 2020.

Since 1998 Ionosphere Associate Analysis Centers (IAAC) of the International GNSS Service (IGS) routinely provide global ionosphere maps (GIMs). They are used for a wide range of geophysical applications, including supporting precise positioning and improving space weather analysis. These GIMs are generated by different analysis centers with the use of different modelling techniques. Therefore they have different accuracy levels, which has already been evaluated in several studies. Until 2014 all GIMs were provided with 2-hour temporal resolution, and since 2015 some of the IAACs have started to provide their products with higher resolutions, up to 30 - 60 minutes. Since GIMs have different temporal resolutions, we investigated whether map interval affected their accuracies.

In this study we carried out IAAC GIM accuracy analysis for years 2014 and 2018, corresponding to high and low solar activity periods, respectively. Since in 2014 IAAC GIMs had 2-hour resolution, we also evaluated UQRG maps supplied with 15-minute interval. For low solar activity period (2018) we evaluated 4 models: CASG, CODG, EMRG and  UQRG. In addition, we studied ionosphere map performance during two selected geomagnetic storms: on 19 February 2014 and 17 March 2015. Our accuracy evaluation was based on GIM-TEC comparisons to differential STEC derived from GNSS data and VTEC derived from altimetry measurements.

The results show that temporal interval has no significant impact on the overall, annual map RMS during both high and low solar activity periods. However, during geomagnetic storms, when reducing map interval, the map accuracy improves by almost 25%.

How to cite: Milanowska, B., Wielgosz, P., Krypiak-Gregorczyk, A., and Jarmołowski, W.: Accuracy analysis of global ionospheric maps in relation to their temporal resolution and solar activity level., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9715, https://doi.org/10.5194/egusphere-egu2020-9715, 2020.

Knowledge of the ionospheric electron density is essential for a wide range of applications, e.g., telecommunications, satellite positioning and navigation, and Earth observation from space. Therefore, considerable efforts have been concentrated on modeling this ionospheric parameter of interest. Ionospheric electron density is characterized by high complexity and is space−and time−varying, as it is highly dependent on local time, latitude, longitude, season, solar cycle and activity, and geomagnetic conditions. Daytime disturbances cause periodic changes in total electron content (diurnal variation) and additionally, there are multi-day periodicities, seasonal variations, latitudinal variations, or even ionospheric perturbations that cause fluctuations in signal transmission.

Because of its multiple band frequencies, the current Global Navigation Satellite Systems (GNSS) offer an excellent example of how we can infer ionosphere conditions from its effect on the radiosignals from different GNSS band frequencies. Thus, GNSS techniques provide a way of directly measuring the electron density in the ionosphere. The main advantage of such techniques is the provision of the integrated electron content measurements along the satellite-to-receiver line-of-sight at a large number of sites over a large geographic area.

Deep learning techniques are essential to reveal accurate ionospheric conditions and create representations at high levels of abstraction. These methods can successfully deal with non-linearity and complexity and are capable of identifying complex data patterns, achieving accurate ionosphere modeling. One application that has recently attracted considerable attention within the geodetic community is the possibility of applying these techniques in order to model the ionosphere delays based on GNSS satellite signals.

This paper deals with a modeling approach suitable for predicting the ionosphere delay at different locations of the IGS network stations using an adaptive Convolutional Neural Network (CNN). As experimental data we used actual GNSS observations from selected stations of the global IGS network which were participating in the still-ongoing MGEX project that provides various satellite signals from the currently available multiple navigation satellite systems. Slant TEC data (STEC) were obtained using the undifferenced and unconstrained PPP technique. The STEC data were provided by GAMP software and converted to VTEC data values. The proposed CNN uses the following basic information: GNSS signal azimuth and elevation angle, GNSS satellite position (x and y). Then, the adaptive CNN utilizes these data inputs along with the predicted VTEC values of the first CNN for the previous observation epochs. Topics to be discussed in the paper include the design of the CNN network structure, training strategy, data analysis, as well as preliminary testing results of the ionospheric delays predictions as compared with the IGS ionosphere products.   

How to cite: Kaselimi, M., Doulamis, N., and Delikaraoglou, D.: An adaptive convolutional neural network model for ionosphere prediction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9818, https://doi.org/10.5194/egusphere-egu2020-9818, 2020.

The main ionospheric trough represents a large scale depletion of plasma density elongated in longitude, which is typically observed at the boundary between high- and mid-latitude ionosphere. The trough is characterized  by a steep density gradient in a poleward direction and gradual on the equatorward site. According to the recent studies it begins in the late afternoon, moves equatorward during the night hours and rapidly retreats to higher latitudes at a dawn. Due to the dynamic of auroral oval, this ionospheric feature exhibits a high temporal variability and shifts equatorward during the geomagnetic activity. In this work we demonstrate the initial assessment of the ionospheric trough detection performed with GNSS-based relative STEC values. The basis of this indicator are time series of  geometry-free combination with removed background variations. The separation of these low-term effects is realized with a polynomial fitting applied to the particular arcs of data. Such processed data have an accuracy of phase measurements and provide an epoch-wise information on enhancement/depletion of plasma density. In order to evaluate the applicability of the proposed approach for the trough detection, we have analyzed the state of the ionosphere during different geomagnetic conditions. In our investigations we have used the data from several tens of stations located in the northern hemisphere, what makes possible to provide the comprehensive view of this ionospheric phenomenon. The results have confirmed that the network-derived relative STEC values can be successfully used for the monitoring ionospheric trough. Its signature is more pronounced for expanded auroral oval during increased geomagnetic activity and reach in such case a few TEC units.     

How to cite: Sieradzki, R. and Paziewski, J.: The detection of ionospheric trough with GNSS measurements., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10676, https://doi.org/10.5194/egusphere-egu2020-10676, 2020.

Receiver antenna calibration plays an important role in precise point positioning (PPP). The correct management of the phase center offset and variations (PCV) and multipath effects can drastically improve the estimation of tropospheric parameters and the stability of the position over short measurement sessions. Correction parameters, to compensate for PCV, are usually computed in laboratory on all the geodetic grade antennas, but they are not available for low-cost apparatus; multipath can be partially mitigated by a robot calibration on site but this is an expensive procedure that is rarely adopted. Multipath staking maps (MPS) using carrier phase observation residuals are a cheap and powerful tool to generate site-specific corrections, effective for reducing both near-field and far-field effects. These maps can be generated by gridding multiple residuals falling in a cell of a pre-determined size. In this work, we propose to compute a set of polynomial coefficients of a Zernike expansion from the residuals of a PPP uncombined least-squares adjustment performed by the open-source goGPS processing software; these coefficients can be later used to synthesize corrections of the observations for the next processing of the target station. In contrast with gridding techniques, this approach allows a to generate smoother corrections and allows a limited automatic extrapolation of the correction values in areas of the sky that were not covered by observations in the set of data used in the calibration phase. The results show that the technique is effective in the modelling of multipath and residual phase center variations allowing a drastic reduction of the undifferenced residuals. Zernike polynomials are a sequence of polynomials orthogonal on the unit disk, vastly used in optics but, to our knowledge, never considered for GNSS applications, their symmetric properties and the circular support area makes them an interesting object of investigation for other possible usages in GNSS processing.

How to cite: Gatti, A., Tagliaferro, G., and Realini, E.: Site Characterization and Multipath Maps Using Zernike Polynomials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15265, https://doi.org/10.5194/egusphere-egu2020-15265, 2020.

The International VLBI Service for Geodesy and Astrometry (IVS) is a globally operating service that coordinates and performs Very Long Baseline Interferometry (VLBI) activities through its constituent components. The VLBI activities are associated with the creation, provision, dissemination, and archiving of relevant VLBI data and products. The operational station network of the IVS currently consists of about 40 radio telescopes worldwide, subsets of which participate in regular 24-hour and 1-hour observing sessions. This legacy S/X observing network dates back in large part to the 1970s and 1980s. Because of highly demanding new scientific requirements such as sea-level change but also due to the aging infrastructure, the larger IVS community planned and started to implement a new VLBI system called VGOS (VLBI Global Observing System) at existing and new sites over the past several years. In 2020, a fledgling network of 8 VGOS stations started to observe in operational IVS sessions. We anticipate that the VGOS network will grow over the next couple of years to a global network of 25 stations and will eventually replace the legacy S/X system as the IVS production system. We will provide an overview of the recent developments and anticipated evolution of the geodetic VLBI station infrastructure.

How to cite: Behrend, D., Nothnagel, A., Böhm, J., Ruszczyk, C., and Elosegui, P.: Station Infrastructure Developments of the IVS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12847, https://doi.org/10.5194/egusphere-egu2020-12847, 2020.

Having modern and well-maintained geodetic infrastructure is critical for the development of an accurate and reliable Global Geodetic Reference Frame (GGRF). Geoscience Australia (GA) contributes to the development of the GGRF through a network of Global Navigation Satellite System (GNSS) reference stations positioned in key locations across Australia, Antarctica and the Pacific. Data from these reference stations contribute to the realisation of the GGRF, the development and maintenance of the Asia-Pacific Reference frame and the monitoring of deformation across the Australian continent. We are also seeing a rapid increase in the use of this data for location-based positioning applications, such as civil engineering, transport, agriculture and community safety. These applications bring with them a new suite of challenges for geodetic infrastructure operators, such as reduced data latency, denser networks and accessing the latest signals in the most modern formats. Through the Positioning Australia program, GA is addressing these challenges by developing a modern highly-available GNSS reference station design that will be deployed at over 200 sites across the region. This paper discusses the concept of highly-available infrastructure and presents a GNSS reference station design that is openly available for use by the global geodetic community.

How to cite: Ruddick, R., Peterson, A., Jacka, R., and Thomas, B.: Developing a highly-available GNSS reference station network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12593, https://doi.org/10.5194/egusphere-egu2020-12593, 2020.

An objective of the International Height Reference System (IHRS) and its realization (the IHRF) is to support the monitoring and analysis of Earth’s system changes. The more accurate the IHRS/IHRF is, the more phenomena can be identified and modelled. Thus, the IHRS/IHRF must provide vertical coordinates and their changes with time as accurately as possible. As many global change phenomena occur at different scales, the global frame should be extended to regional and local levels to guarantee consistency in the observation, detection, and modelling of their effects. From this perspective, an operational infrastructure is needed to ensure the long-term sustainability of the IHRS/IHRF. In this contribution, we discuss the possibility of establishing such as infrastructure within the International Association of Geodesy (IAG) and its International Gravity Field Service (IGFS). Some aspects to be considered are:

• - Improvements in the IHRS definition and realization following future developments in geodetic theory, observations and modelling.
• - Refinement of the IHRS/IHRF standards/conventions based on the unification of the standards/conventions used by the geometric and gravity IAG Services.
• - Development of strategies for collocation of IHRF stations with existing gravity and geometrical reference stations at different densification levels.
• - Identification of the geodetic products associated with the IHRF and description of the elements to be considered in the corresponding metadata.
• - Servicing the vertical datum needs of other geosciences such as, e.g., hydrography and oceanography.
• - Implementation of a registry containing the existing local/regional height systems and their connections to the global IHRS/IHRF.

How to cite: Barzaghi, R., Sanchez, L., and Vergos, G.: Operational infrastructure to ensure the long-term sustainability of the IHRS/IHRF, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7961, https://doi.org/10.5194/egusphere-egu2020-7961, 2020.

With the release of the combined GRACE monthly gravity field time-series COST-G RL01 the Combination Service for Time-variable Gravity fields (COST-G) of the International Association of Geodesy (IAG) became operational in July 2019. We present the COST-G RL01 time-series and provide validation in terms of orbit fit, ice mass trends, lake altimetry and sea level budget. We identify weak points in the combined monthly gravity fields and discuss possible improvements of the combination strategy for future combinations.

While COST-G RL01 is based on sets of re-processed GRACE monthly gravity fields, COST-G also provides combinations of monthly Swarm high-low satellite-to-satellite tracking (hl-SST) gravity fields on an operational basis with a latency of 3 months. Combinations of GRACE-FO monthly gravity fields are in the process of operationalization. We provide a status report and first results of GRACE-FO combinations. Combined GRACE, Swarm and GRACE-FO gravity fields complement each other to provide a long-term time-series of mass variation in the system Earth.

How to cite: Meyer, U., Jäggi, A., Flechtner, F., Dahle, C., Mayer-Gürr, T., Kvas, A., Lemoine, J.-M., Bourgogne, S., Groh, A., Eicker, A., and Meyssignac, B.: Combination Service for Time-variable Gravity Fields (COST-G) – operations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10380, https://doi.org/10.5194/egusphere-egu2020-10380, 2020.

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) provides the basis on which future advances in geosciences can be built. By considering the Earth system as a whole (including the geosphere, hydrosphere, cryosphere, atmosphere and biosphere), monitoring Earth system components and their interactions by geodetic techniques and studying them from the geodetic point of view, the geodetic community provides the global geosciences community with a powerful tool consisting mainly of high-quality services, standards and references, and theoretical and observational innovations. The mission of GGOS is: (a) to provide the observations needed to monitor, map and understand changes in the Earth’s shape, rotation and mass distribution; (b) to provide the global frame of reference that is the fundamental backbone for measuring and consistently interpreting key global change processes and for many other scientific and societal applications; and (c) to benefit science and society by providing the foundation upon which advances in Earth and planetary system science and applications are built. The goals of GGOS are: (1) to be the primary source for all global geodetic information and expertise serving society and Earth system science; (2) to actively promote, sustain, improve, and evolve the integrated global geodetic infrastructure needed to meet Earth science and societal requirements; (3) to coordinate with the international geodetic services that are the main source of key parameters and products needed to realize a stable global frame of reference and to observe and study changes in the dynamic Earth system; (4) to communicate and advocate the benefits of GGOS to user communities, policy makers, funding organizations, and society. In order to accomplish its mission and goals, GGOS depends on the IAG Services, Commissions, and Inter-Commission Committees. The Services provide the infrastructure and products on which all contributions of GGOS are based. The IAG Commissions and Inter-Commission Committees provide expertise and support for the scientific development within GGOS. In summary, GGOS is IAG’s central interface to the scientific community and to society in general. The whole figure of GGOS and its recent focus will be presented in the presentation.

How to cite: Miyahara, B.: Global Geodetic Observing System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13390, https://doi.org/10.5194/egusphere-egu2020-13390, 2020.

The International Association of Geodesy’s (IAG) Global Geodetic Observing System (GGOS) is managed administratively by the GGOS Coordinating Office, based at the Austrian Federal Office of Metrology and Surveying (BEV). The director of GGOS Coordinating Office (CO) supports the Executive Committee, the Coordinating Board and the Science Panel as well as he ensures coordination of the activities of the various GGOS components like the Bureau of Products and Standards (BPS), the Bureau of Networks and Observations (BNO) and the three GGOS Focus Areas.

The Coordinating Office ensures information flow, maintains documentation of GGOS activities, and manages specific assistance functions that enhance the coordination across all areas of GGOS, including coordination among IAG Services and support for workshops. The Coordinating Office, in its long‐term coordination role, ensures that the GGOS components contribute to GGOS and its stakeholder community in a consistent and continuous manner. The Coordinating Office also maintains, manages, and coordinates the GGOS web- and social media presence as well as outreach and external engagement. In 2020 the current GGOS website (www.ggos.org) will be refreshed and redesigned to optimize usability and ease of navigation. The website will serve as a source of information about GGOS, geodetic data, products, and services, as well as other non-technical resources for the IAG community.

On behalf of the GGOS community, the Coordinating Office manages external relations and engagement with stakeholder organizations such as the Group on Earth Observations (GEO), the Committee on Earth Observation Satellites (CEOS) and the International Science Council (ISC) World Data System (WDS). In this capacity, the Office also identifies opportunities to link geodesy with relevant United Nations frameworks and other instruments of engagement, such as the Sendai Framework for Disaster Risk Reduction and the UN-GGIM-World Bank Integrated Geospatial Information Framework.

The Coordinating Office also works to identify opportunities for improved coordination and advocacy within the geodetic community, establishing the Working Group on “Digital Object Identifiers (DOIs) for Geodetic Data Sets” in 2019. This working group consists of more than 20 members affiliated with IAG Services, working to establish usage parameters and advocate for the consistent implementation of DOIs across all IAG Services and in the greater geodetic community.

How to cite: Sehnal, M., Craddock, A., and Elger, K.: GGOS Coordinating Office – Recent Achievements and Activities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6540, https://doi.org/10.5194/egusphere-egu2020-6540, 2020.

The Bureau of Products and Standards (BPS) supports GGOS in its goal to obtain consistent products describing the geometry, rotation and gravity field of the Earth. A key objective of the BPS is to keep track of adopted geodetic standards and conventions across all IAG components as a fundamental basis for the generation of consistent geometric and gravimetric products. This poster gives an overview about the organizational structure, the objectives and activities of the BPS. In its present structure, the two Committees “Earth System Modeling” and “Essential Geodetic Variables” as well as the newly established Working Group “Towards a consistent set of parameters for the definition of a new GRS” are associated to the BPS. Recently the updated 2^nd version of the BPS inventory on standards and conventions used for the generation of IAG products has been compiled. Other activities of the Bureau include the integration of geometric and gravimetric observations towards the development of integrated products (e.g., GGRF, IHRF, atmosphere products) in cooperation with the IAG Services and the GGOS Focus Areas, the contribution to the re-writing of the IERS Conventions as Chapter Expert for Chapter 1 “General definitions and numerical standards”, the interaction with external stakeholders regarding standards and conventions (e.g., ISO, IAU, BIPM, CODATA) as well as contributions to the Working Group “Data Sharing and Development of Geodetic Standards” within the UN GGIM Subcommittee on Geodesy.

How to cite: Angermann, D., Gruber, T., Gerstl, M., Hugentobler, U., Sanchez, L., Heinkelmann, R., and Steigenberger, P.: The GGOS Bureau of Products and Standards, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7930, https://doi.org/10.5194/egusphere-egu2020-7930, 2020.

The GGOS Bureau of Networks and Observations works with the IAG Services (IVS, ILRS, IGS, IDS, IGFS, and PSMSL) to advocate for the expansion and upgrade of space geodesy networks for the maintenance and improvement of the reference frame and other applications, as well as for the integration with other techniques, including absolute gravity and sea level measurements from tide gauges. New sites are being established following the GGOS concept of “core” and co-location sites, and new technologies are being implemented to enhance performance in data yield as well as accuracy. The Bureau continues to meet with organizations to discuss possibilities, including partnerships, for new and expanded participation. The GGOS Network continues to grow as new stations join every year.

The Bureau holds meetings frequently, providing the opportunity for representatives from the services to meet and share progress and plans, and to discuss issues of common interest. It also monitors the status and projects the evolution of the network based on information from the current and expected future participants. Of particular interest at the moment is the integration of gravity and tide gauge networks and the forthcoming establishment of the new absolute gravity reference frame. 

The IAG Committees and Joint Working Groups play an essential role in the Bureau activity. The Standing Committee on Performance Simulations and Architectural Trade-offs (PLATO) uses simulation and analysis techniques to project future network capability and to examine trade-off options. The Committee on Data and Information is working on a strategy for a GGOS metadata system for data products and a more comprehensive long-term plan for an all-inclusive system. The Committee on Satellite Missions is working to enhance communication with the space missions, to advocate for missions that support GGOS goals and to enhance ground systems support. The IERS Working Group on Site Survey and Co-location (also participating in the Bureau) is working to enhance standardization in procedures, outreach and to encourage new survey groups to participate and improve procedures to determine systems’ reference points, a crucial aid in the detection of technique-specific systematic errors.
 
We will give a brief update on the status and projection of the network infrastructure of the next several years, and the progress and plans of the Committees/Working Group in their critical role in enhancing data product quality and accessibility to the users.   

How to cite: Noll, C., Pearlman, M., Behrend, D., Craddock, A., Pavlis, E., Saunier, J., Bradshaw, E., Barzaghi, R., Thaller, D., Maennel, B., Hippenstiel, R., Pail, R., and Brown, N.: GGOS Bureau of Networks and Observations: Network Infrastructure and Related Activities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22473, https://doi.org/10.5194/egusphere-egu2020-22473, 2020.

Since the establishment in 2013, GGOS Japan, formerly known as GGOS Working Group of Japan, has actively contributed to domestic and international space-geodetic activities.  Until it was established, six Japanese agencies with their own backgrounds and missions had individually conducted geodetic observations of GNSS, VLBI, SLR, DORIS and gravimetry in Japan and Antarctica.   GGOS Japan was established to strengthen the collaboration between the agencies and to get connected to international organizations such as IAG and GGOS.

Its core members consist of a chair, a secretary, a lead of the outreach working group, a lead of the DOI working group and representatives of five core techniques.  It is supported by tens of people in Japan and also by its parent entity, IAG subcommittee in Japan.

This presentation will cover our achievement such as assembling our site list, hosting various domestic/international meetings, planning a special issue in a domestic journal, producing its leaflet and website and so on.  Since 2017 it has been approved as an GGOS Affiliate and it is remarkable that Basara Miyahara was elected as GGOS President in 2019.

How to cite: Otsubo, T., Miyahara, B., Yokota, Y., Kurihara, S., Munekane, H., Watanabe, S., Miyazaki, T., Takiguchi, H., Aoyama, Y., Doi, K., Fukuda, Y., Matsuo, K., Jike, T., Matsumoto, T., and Ichikawa, R.: GGOS Japan: Uniting Space Geodetic Activities in Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3244, https://doi.org/10.5194/egusphere-egu2020-3244, 2020.

The Permanent Service for Mean Sea Level (PSMSL) is the global databank for long-term mean sea level data and is a member of the Global Geodetic Observing System (GGOS) Bureau of Networks and Observations. As well as curating long-term sea level change information from tide gauges, PSMSL is also involved in developing other products and services including the automatic quality control of near real-time sea level data, distributing Global Navigation Satellite System (GNSS) sea level data and advising on sea level metadata development.
At the GGOS Days meeting in November 2019, the GGOS Focus Area 3 on Sea Level Change, Variability and Forecasting was wrapped up, but there is still a requirement in 2020 for GGOS to integrate and support tide gauges and we will discuss how we will interact in the future. A recent paper (Ponte et al., 2019) identified that only “29% of the GLOSS [Global Sea Level Observing System] GNSS-co-located tide gauges have a geodetic tie available at SONEL [Système d'Observation du Niveau des Eaux Littorales]” and we as a community still need to improve the ties between the GNSS sensor and tide gauges. This may progress as new GNSS Interferometric Reflectometry (GNSS-IR) sensors are installed to provide an alternative method to observe sea level. As well as recording the sea level, these sensors will also provide vertical land movement information from one location. PSMSL are currently developing an online portal of uplift/subsidence land data and GNSS-IR sea level observation data. To distribute the data, we are creating/populating controlled vocabularies and generating discovery metadata.
We are working towards FAIR data management principles (data are findable, accessible, interoperable and reusable) which will improve the flow of quality controlled sea level data and in 2020 we will issue the PSMSL dataset with a Digital Object Identifier. We have been working on improving our discovery and descriptive metadata including creating a use case for the Research Data Alliance Persistent (RDA) Identification of Instruments Working Group to help improve the description of a time series where the sensor and platform may change and move many times. Representatives from PSMSL will sit on the GGOS DOIs for Data Working Group and would like to contribute help with controlled vocabularies, identifying metadata standards etc. We will also contribute to the next GGOS implementation plan.
Ponte, Rui M., et al. (2019) "Towards comprehensive observing and modeling systems for monitoring and predicting regional to coastal sea level." Frontiers in Marine Science 6(437).

How to cite: Bradshaw, E., Matthews, A., Gordon, K., Hibbert, A., Jevrejeva, S., Rickards, L., Williams, S., and Woodworth, P.: Sea level in the Global Geodetic Observing System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3054, https://doi.org/10.5194/egusphere-egu2020-3054, 2020.