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The scope of this paper is to perform an internal quality assessment, at frame parameter level, on the combined EPN weekly coordinate solutions. Specifically, the weekly SINEX solution files from the EPN Analysis Combination Centre are analyzed in order to infer the internal accuracy of the origin, orientation and scale of the respective epoch solutions. Our investigation covers almost the entire operational period of EPN starting from 1999 up to present time, and it uses both the routine and re-processed (repro2) series of combined weekly solutions. Through the results of this study we are able to assess the impact of various changes or updates that have been applied into the routine GNSS data processing and combination strategies on the quality of the estimated EPN weekly frames.

The objective of this paper is to introduce CyCLOPS, a novel strategic research infrastructure unit, and present its current progress of implementation, and integration in the National geodetic, geophysical and geotechnical infrastructure of the government-controlled areas of the Republic of Cyprus. CyCLOPS is co-funded by the European Regional Development Fund and the Republic of Cyprus through the Research and Innovation Foundation under the grant agreement RIF/INFRASTRUCTURES/1216/0050. CyCLOPS is developed via the collaboration of the Cyprus University of Technology (CUT) and the German Aerospace Center (DLR), and supported by the Cyprus Geological Survey Department and the Department of Lands and Surveys. The main objective of CyCLOPS is to establish an integrated infrastructure for space-based monitoring of geohazards using the most prominent earth observation technologies (EO), such as GNSS and InSAR. Furthermore, the infrastructure will densify and form the backbone for the definition of the next generation national datum of the Republic of Cyprus. Eleven Tier-1/2 state-of-the-art GNSS CORS, precise weather stations, tiltmeters and specifically designed InSAR triangular trihedral corner reflectors will be deployed, in a collocated fashion, at selected locations throughout the government-controlled areas of Cyprus. The collocated configuration will be established and installed to be compliant with the most stringent CORS monumentation specifications, support all current GNSS constellations and SAR missions. Finally, one of CyCLOPS’ fundamental aims is to actively contribute to the on-going efforts and growing demand for more precise positioning services and high-quality modern reference frames, in conformity with the recommendations of the UN-GGIM (and its Subcommittee of Geodesy) to establish and enhance national geodetic infrastructures to support the sustainable management of geospatial information on the changing Earth.

The aim of this paper is to present the results from the GNSS-based densification of the International Terrestrial Reference Frame (ITRF) in the region of Cyprus. The regional network used for this task consists of nine permanent GNSS Cypriot stations (eight stations of the existing CYPOS network plus the NICO station which belongs to the global IGS network) and 34 additional stations of the EUREF Permanent GNSS Network (EPN) which are mostly located in the continental part of Europe. The data used in this densification project include daily GNSS observations at the above stations within a period of 62 months (30/11/2011–28/01/2017). The present study was carried out using the resources of the CyCLOPS strategic research infrastructure unit. A robust procedure was designed for the daily data processing using the Bernese GNSS software v5.2 installed in the CyCLOPS operating center. The multi-year solution is computed by combining the constraint-free daily solutions using the normal equations stacking strategy. The reference frame of the multi-year solution is IGb08 and it was enforced through a no-net-translation condition on the positions and velocities of 24 EPN (Class A) stations. The quality of the computed solution is verified by comparing the estimated velocities with their official EPN_A_IGb08_C1934 values, and reveals minimal differences (

Permanent Global Navigation Satellite Systems (GNSS) reference stations are well established as a powerful tool for the estimation of deformation induced by man-made or physical processes. GNSS sensors are successfully used to determine positions and velocities over a specified time period, with unprecedented accuracy, promoting research in many safety-critical areas, such as geophysics and geo-tectonics, tackling problems that torment traditional equipment and providing deformation products with absolute accuracy. Cyprus, being located at the Mediterranean fault, exhibits a very interesting geodynamic regime, which has yet to be investigated thoroughly. Accordingly, this research revolves around the estimation of crustal deformation in Cyprus using GNSS receivers. CYPOS (CYprus POsitioning System), a network of seven permanent GNSS stations has been operating since 2008, under the responsibility of the Department of Lands and Surveys. The continuous flow of positioning data collected over this network, offers the required information to investigate the behavior of the crustal deformation field of Cyprus using GNSS sensors for the first time. This paper presents the results of a multi-year analysis (11/2011&ndash, 01/2017) of daily GNSS data and provides inferences of linear and nonlinear deforming signals into the position time series of the network stations. Specifically, 3D station velocities and seasonal periodic displacements are jointly estimated and presented via a data stacking approach with respect to the IGb08 reference frame.

New tools in the form of angular deficiency indices are developed for the physical interpretation of numerically detected rank deficiencies or weaknesses, in terms of defects or weaknesses in the definition of the reference system characteristics (origin, orientation and scale) to which coordinates and other unknown parameters refer. Within a least squares estimation procedure, the numerical detection is based on the subspace of the parameter space spanned by eigenvectors of the coefficient matrix of the normal equations, which correspond to zero or very small eigenvalues. The physical interpretation is based on the well-known theoretically derived basis of the null space of the design matrix, provided by the columns of the matrix appearing in the so-called inner constraints. This null space basis is elaborated for single-epoch (weekly) solutions of GNSS networks, for deformable networks with constant station velocities, as well as for the stacking problem of coordinate time series. The applicability of these new tools and their implementation strategy are demonstrated by simple numerical examples.

The singularity of input normal equations (NEQ) is a crucial element for their optimal handling in the context of terrestrial reference frame (TRF) estimation under the minimal-constraint framework. However, this element is often missing in the recovered NEQ from SINEX files after the usual deconstraining based on the stated information for the stored solutions. The same setback also occurs with the original NEQ that are formed by the least-squares processing of space geodetic data due to the datum information which is carried by various modeling choices and/or software-dependent procedures. In the absence of this datum-related singularity, it is not possible to obtain genuine minimally constrained solutions because of the interference between the input NEQ’s content and the external datum conditions, a fact that may alter the geometrical information of the original measurements and can cause unwanted distortions in the estimated solution. The main goal of this paper is the formulation of a filtering scheme to enforce the proper (or desired) singularity in the input NEQ with regard to datum parameters that will be handled by the minimal-constraint setting in TRF estimation problems. The importance of this task is extensively discussed and justified with the help of several numerical examples in different GNSS networks.

By considering a deformable geodetic network, deforming in a linear-in-time mode, according to a coordinate-invariant model, it becomes possible to get an insight into the rank deficiency of the stacking procedure, which is the standard method for estimating initial station coordinates and constant velocities, from coordinate time series. Comparing any two out of the infinitely many least squares estimates of stacking unknowns (initial station coordinates, velocity components and transformation parameters for the reference system in each data epoch), it is proven that the two solutions differ only by a linear-in-time trend in the transformation parameters. These pass over to the initial coordinates (the constant term) and to the velocity estimates (the time coefficient part). While the difference in initial coordinates is equivalent to a change of the reference system at the initial epoch, the differences in velocity components do not comply with those predicted by the same change of reference system for all epochs. Consequently, the different velocity component estimates, obtained by introducing different sets of minimal constraints, correspond to physically different station velocities, which are therefore non-estimable quantities. The theoretical findings are numerically verified for a global, a regional and a local network, by obtaining solutions based on four different types of minimal constraints, three usual algebraic ones (inner or partial inner) and the lately introduced kinematic constraints. Finally, by resorting to the basic ideas of Felix Tisserand, it is explained why the station velocities are non-estimable quantities in a very natural way. The problem of the optimal choice of minimal constraints and, hence, of the corresponding spatio-temporal reference system is shortly discussed.

The paper presents a detailed accuracy evaluation of the Hellenic Geodetic Reference System 1987 (HGRS87), a non-geocentric traditional local datum which was established thirty years ago and it is still used as the official coordinate system for surveying applications in Greece. This is the first time that a large-scale assessment of HGRS87 is performed, which aims to reveal its weakness by quantifying its consistency with a higher quality ITRF/GNSS-based frame over the entire country. A geodetic velocity model is employed in our analysis in an attempt to eliminate the accumulated geodynamical distortions of the (static) HGRS87 coordinates, and thus to provide a more judicious comparison between the underlying frames at a common epoch. Our results show that, even after removing the geodynamical distortions, the average accuracy of HGRS87 at national scale remains worse than 45 cm, yet significant variations exist in its regional accuracy level over different parts of Greece.

Coodinate time series from a regional GNSS network in Greece covering a period of 5 years are used in order to study the effect of the choice of the reference system on their linear, nonlinear and spectral characteristics. The standard solution where the reference system is defined by removing the translational rank defect in GNSS data by partial inner constraints is compared with two different solutions. The first is obtained by a posteriori aligning the network at each epoch to the IGS08 reference system through the coordinates of common points by means of a similarity (Helmert) transformation. The second solution is achieved by a regional stacking where the original standard coordinate time series are best fit to a linear in time coordinate model and the derived similarity transformation parameters are used to convert the standard solution into coordinate time series where variations due to reference system instability are removed. The analysis shows that the stacking solution leads to better results more suitable for regional geodynamic studies free from effects, which are not reflecting actual temporal variations in the shape of the network.

The Global Positioning System (GPS) has been used in the remote sensing of the atmosphere. A significant component of the atmosphere that affects the GPS signals is the zenith tropospheric delay (ZTD). The computation of ZTD estimates can directly or indirectly reflect weather variations. Through the analysis of ZTD values the hydrostatic and wet component of the total delay can be determined. For example, the wet tropospheric delay could be derived by subtracting the hydrostatic from the total delay. Hydrostatic delay can be estimated from surface or other meteorological data. The wet tropospheric delay can then be used in the derivation of the amount of precipitable water. Precipitable water plays a significant role in the physical and chemical processes of the atmosphere. It also greatly contributes to studies of weather forecasting and climate change. In this study GPS data from 12 permanent stations covering the broader area of the city of Athens, between February 18th and 24th, were used. This period was selected because of a heavy rainfall event on February 22nd. Data were processed using the GAMIT software and precipitable water (PW) estimates with 1 h time interval were derived. The PW values were analyzed in combination with meteorological data such as cloudiness, wind direction and precipitation obtained from the Hellenic Center for Marine Research and the Hydrological Observatory of Athens. The results indicate consistency between the estimated PW values and the related meteorological observations. This study suggests that a continuous record of PW estimates and meteorological variables is highly recommended for further studies on the behavior of the atmospheric water vapor and its contribution to the climate monitoring.

Minimum constraints (MCs) is a standard tool for the datum definition in geodetic network adjustment and they are regularly used for the alignment of regional GNSS networks to successive realizations of the International Terrestrial Reference System. Their implementation has been restricted to an unweighted setting without using any weighting for the reference stations that participate in the datum definition process. The aim of this paper is to discuss the optimal choice problem for the weight matrix of the reference stations within the context of MC network adjustment, and to expose its relevance for practical applications having particularly in mind network densification problems. Numerical examples are presented from the alignment of two regional GNSS networks to ITRF2008 using weighted and unweighted MCs. The results obtained from our analysis offer a preliminary view of the expected improvement in the estimation accuracy of MC solutions due to the optimal weighting of the used reference stations.