Volume 12, Issue 1

For determining precise sea surface heights, six marine GNSS (global navigation satellite system) survey campaigns were performed in the eastern Baltic Sea in 2021. Four GNSS antennas were installed on the vessel, the coordinates of which were computed relative to GNSS–CORS (continuously operating reference stations). The GNSS–CORS results are compared to the PPP (precise point positioning)-based results. Better accuracy is associated with the GNSS–CORS postprocessed points; however, the PPP approach provided more accurate results for longer than 40 km baselines. For instance, the a priori vertical accuracy of the PPP solution is, on average, 0.050 ± 0.006 m and more stable along the entire vessel’s survey route. Conversely, the accuracy of CORS-based solutions decreases significantly when the distances from the GNSS–CORS exceed 40 km, whereas the standard deviation between the CORS and PPP-based solutions is up to 0.075 m in these sections. Note that in the harbor (about 4 km from the nearest GNSS–CORS), the standard deviation of vertical differences between the two solutions remains between 0.013 and 0.024 m. In addition, the GNSS antennas situated in different positions on the vessel indicated different measurement accuracies. It is suggested for further studies that at least one GNSS antenna should be mounted above the mass center of the vessel to reduce the effects of the dominating pitch motion during the surveys.

Recently a number of geoid campaigns were performed to verify different types of geoid and quasigeoid modeling techniques. Typically, GNSS-leveling was employed as an independent method, but in some cases zenith camera astronomic deflection data were also used in astrogeodetic determinations of the geoid and/or quasigeoid. However, due to the uncertainty in the topographic density distribution data (and thereby in orthometric heights), we conclude that neither GNSS-leveling nor astrogeodetic techniques can reliably verify differences between gravimetric geoid models at several centimeter levels in rough mountainous regions. This is because much the same topographic data are used both in the gravimetric geoid models and in their verifications by geometric and/or astrogeodetic geoid models. On the contrary, this is not a problem in verifying gravimetric quasigeoid models, as they are independent of the topographic density distribution, and so is the related normal height used in GNSS-leveling.

Sub- and super-harmonics of luni-solar forcing are proxies for the natural variations in sea levels observed at tide gauge stations with long records as demonstrated in earlier studies. This study also identified their signatures in the noisy yearly misclosures of the global mean sea level budget for the period 1900–2018. The analyses of the yearly misclosures revealed a temporal linear systematic error of 0.08 ± 0.02 mm/year, which is not explained by the budget components. The estimate is statistically significant ( α = 0.05) but small in magnitude and accounts for only 11% (adjusted R 2 ) of the variations in the yearly misclosures. Meanwhile, the yearly misclosures have also a statistically significant constant bias as large as −12.2 ± 0.9 mm, which can be attributed to the lack of a common datum definition for the global mean sea level budget components. Modeling the low-frequency changes of luni-solar origin together with a trend and constant bias parameters reduces variability in the misclosures. Accounting for their effects explains 50% (adjusted R 2 ) of the fluctuations in the yearly misclosures compared to the 11% if they are not. In addition, unmodeled low-frequency variations in the yearly global budget closure assessments have the propensity of confounding the detection of a statistically significant recent uniform global sea level acceleration triggered by anthropogenic contributors.

The most common approaches for assigning weights to observations in minimum L1-norm (ML1) is to introduce weights of p or p \sqrt{p} , p being the weights vector of observations given by the inverse of variances. Hence, they do not take covariances into consideration, being appropriated only to independent observations. To work around this limitation, methods for decorrelation/unit-weight reduction of observations originally developed in the context of least squares (LS) have been applied for ML1, although this adaptation still requires further investigations. In this article, we presented a deeper investigation into the mentioned adaptation and proposed the new ML1 expressions that introduce weights for both independent and correlated observations; and compared their results with the usual approaches that ignore covariances. Experiments were performed in a leveling network geometry by means of Monte Carlo simulations considering three different scenarios: independent observations, observations with “weak” correlations, and observations with “strong” correlations. The main conclusions are: (1) in ML1 adjustment of independent observations, adaptation of LS techniques introduces weights proportional to p \sqrt{p} (but not p ); (2) proposed formulations allowed covariances to influence parameters estimation, which is unfeasible with usual ML1 formulations; (3) introducing weighs of p provided the closest ML1 parameters estimation compared to that of LS in networks free of outliers; (4) weighs of p \sqrt{p} provided the highest successful rate in outlier identification with ML1. Conclusions (3) and (4) imply that introducing covariances in ML1 may adversely affect its performance in these two practical applications.

Representing the Earth’s physical features onto a flat surface is a critical and challenging issue for geodesists to build topographic mappings at field scale in various applications. Artificial satellite positioning data are currently defined on a global geocentric frame, while terrestrial geodetic networks are determined on a local ellipsoid. Hence, coordinate transformations in three-dimensional space are required for data fusion involving different coordinate systems utilizing common points in two sets of coordinates. On the other hand, small companies in many developing countries have some data conversion difficulties due to the need for high-cost software and qualified persons. A low-cost automated tool is helpful in achieving this task and ensuring quality and positional accuracy. In this investigation, the problem was undertaken by establishing a software tool in the Microsoft Visual Studio environment for map-matching with global coordinates based on similarity transformations and a conformal polynomial approach. The tool’s performance was evaluated through a numerical example to assign transformation parameters and derive coordinates of checkpoints from the prediction surface.

Today, most of the aircrafts are navigated by global navigation satellite systems (GNSSs). Landing is a dangerous phase of a flight especially when an airport runway is not clearly seen from the aircrafts. In such cases, GNSSs are useful for a safe landing under the circumstances that healthy signals, free of any interference, reach to GNSSs receiver antennas mounted on the aircrafts. This shows the importance of establishing GNSS interference localisation security networks around airports. Designing a good configuration for the points with GNSS antennas at for receiving interference signals is important for a successful localisation of the interference device. Here, the time-difference of the arrivals of an interference signal to such points or anchor nodes (ANs), are used as observables, and a security network with four ANs is optimally designed along the runways of the Arlanda airport to reduce the dilution of precision (DOP) of the network. Our study showed that by such an optimisation, the maximum DOP value can reduce by 50% meaning a significant increase in the probability of a successful interference device localisation.

The precision and the reliability of very long baseline interferometry (VLBI) depend on several factors. Apart from fabrication discrepancies or meteorological effects, gravity-induced deformations of the receiving unit of VLBI radio telescopes are identified as a crucial error source biasing VLBI products and obtained results such as the scale of a realized global geodetic reference frame. Gravity-induced deformations are systematical errors and yield signal path variations (SPVs). In 1988, Clark and Thomsen derived a VLBI delay model, which was adopted by the International VLBI Service for Geodesy and Astrometry (IVS) to reduce these systematic errors. However, the model parametrizes the SPV by a linear substitute function and considers only deformations acting rotationally symmetrically. The aim of this investigation is to derive the signal path variations of a legacy radio telescope and a modern broadband VGOS-specified radio telescope and to study the effect of nonrotationally symmetric deformation patterns. For that purpose, SPVs are obtained from a nonlinear spatial ray tracing approach. For the first time, a tilt and a displacement of the subreflector perpendicular to the optical axis of the feed unit is taken into account. The results prove the commonly used VLBI delay model as a suitable first-order delay model to reduce gravity-induced deformations.

Airborne gravimetry is an efficient and reliable method to obtain information on the gravity field, fundamental to gravity field modelling, geoid determination, and flood risk mapping. In evaluation and utilization of gravity estimates, two measures are of fundamental importance, namely the accuracy and spatial resolution. These measures are related to one another through the filtering required to suppress observational noise. As strapdown inertial measurement units (IMUs) are increasingly deployed for airborne gravity surveys, the Kalman filter estimation method is routinely used for gravity determination. Since filtering is not applied directly to the observations in Kalman filtering, it is not straightforward to associate the derived gravity estimates with a measure of spatial resolution. This investigation presents a method for deriving spatial resolution by evaluating the transfer function formed after applying a delta function to the observed accelerations. The method is applied to Kalman-filter-derived gravity estimates from an airborne strapdown IMU system, yielding a full-wavelength spatial resolution of 5.5 km at an accuracy of 0.6 mGal. These results are consistent with a comparison with upward continued terrestrial gravity observations.

The result of surveys largely depends on the accuracy and measurement technique of the control network. This research work presents the application Metrica , dedicated for collection and navigation of geodetic control network points. The main goal of this study is to present the motivation, methodology, and issues with analysis of the database of vertical control points. Such analysis covers south part of Poland territory for the update of the existing set of points of the vertical control network on an ongoing basis. The application has been developed with the use of a test database of points. The status of the points was checked by means of a multi-stage analysis. The prepared database of points was verified taking into account the criteria defined by the authors. In order to optimise the scoring analysis process, two groups of criteria were distinguished: independent of each other and those whose analysis was justified only under the condition of prior verification of other criteria. Based on the analysis of the collected data, the main conclusion was the ongoing need to update the data on the matrix points. It was found that information on points is often incomplete and inaccurate, especially in terms of their horizontal position, which often prevents efficient localisation of points. One of the most significant findings was that only 55.6% of the points available in the database were identified in the field. It should be noted that despite the analysis of the state of the matrix carried out in 2019 on behalf of the GUGiK, there are still many points about which information is not fully up-to-date. It is now possible to extend the application with new attributes and validation rules and conduct updates for several types of geodetic networks. Due to the extensive character of the discussed subject, the study has been divided into two parts. The continuation of this study is the second research work, which presents the development of a mobile application for the update of the geodetic control network. It will present in detail the procedures and IT application development processes.

Ground-based augmentation system (GBAS) was developed to guide aircraft precision approach and landing, aiming to replace the instrument landing system (ILS), which is currently used in most airports worldwide. GBAS based on differential positioning with reference stations that provide differential corrections to the aircraft to improve its positioning accuracy and ensure other performance parameters such as integrity, continuity, and availability. However, using GBAS in low latitude regions such as Brazil, the occurrence of ionospheric irregularities can affect global navigation satellite system (GNSS) performance so that it does not meet the requirements for aviation. This article evaluates five vertical ionospheric gradient variability scenarios for a GNSS data set of four reference stations, one station simulating an aircraft with GBAS in a static model based on performance requirements for Category Approach Type – CAT I. The results showed that the increase in the variability of the ionosphere and the geometry of satellites used in positioning could affect the integrity and availability of GBAS. In the scenario of more significant variability of the ionosphere evaluated, there was a loss of 38.4% of the availability of GBAS for the CAT I approach.

The SIRGAS-CON network currently has more than 450 continuous GNSS stations, and it is used for geodetic purposes. In atmospheric studies, it is used for ionospheric monitoring and for the estimation of zenith tropospheric delays (ZTDs). From the Neutral Atmosphere Analysis Center of SIRGAS, Centro de Ingeniería Mendoza Argentina, the final tropospheric products of this network are generated after several stages of quality controls and filtering, in order to be published on a daily basis in the official website of SIRGAS, since 2014 (https://sirgas.ipgh.org/en/products/tropospheric-delays). These products arise from adjusting the solutions estimated by different SIRGAS analysis centers. Prior to the combination, a quality control of the individual solutions is carried out, based on the precision estimator of each parameter and an internal control of each solution with respect to the combined value. In this work, we show the quality control process of the inputs, the selected tolerance and its justification. The internal consistency analysis of tropospheric parameters for a period of 7 years was carried out. We also exposed the improvements in the estimation of tropospheric parameters implemented during 2021 and its impact in the generation of the final ZTD products (in 99% of the stations the mean standard deviation of ZTD is less than 1 mm).

The high-frequency effects of the gravity field can be obtained from residual terrain modeling (RTM) technique. However, for the International height reference system (IHRS), this is an open problem. Over the last decades, various approaches have been proposed for the practical solution of Newton’s Integral, such as point-mass, tesseroid, prism, and polyhedron. Harmonic correction (HC) and the use of more realistic mass density values have also been studied. In this work, in order to calculate RTM height anomalies at BRAZ and PPTE IHRS future stations in Brazil, such approaches and issues have been evaluated. Density values have been obtained from CRUST 1.0 global model. For the HC, the traditional mass condensation technique and Poisson’s equation have been used. The resulting values proposed by different approaches are very close to each other, with differences at the submillimeter-level. The RTM height anomaly values reached 1.5 and 1.4 mm at the BRAZ and PPTE stations, respectively. HC at BRAZ station can be disregarded without prejudice since its order of magnitude was 10 −5  m. The use of CRUST 1.0 density values has proposed slight improvements of 0.8 and 0. 4 mm at BRAZ and PPTE station, respectively, in relation to the use of Harkness constant density value.

The Deutsches Geodätisches Forschungsinstitut (DGFI) has been involved in the research activities of the Latin American Reference Frame SIRGAS since its establishment in 1993. DGFI coordinated the SIRGAS Global Positioning System campaigns of 1995 and 2000 and acted as an analysis centre of both campaigns contributing to the first two SIRGAS realisations known as SIRGAS95 and SIRGAS2000. In 1996, DGFI established the Regional Network Associate Analysis Centre for SIRGAS of the International GNSS (Global Navigation Satellite System) Service (IGS RNAAC SIRGAS) and took on responsibility for processing the SIRGAS continuously operating stations and generating weekly position solutions. Later followed the determination of cumulative (multi-year) solutions, consisting of station positions and constant velocities, providing accurate solutions for the SIRGAS reference frame. DGFI was integrated into the Technical University of Munich (TUM) in 2015, becoming DGFI–TUM, and based on the SIRGAS operational analyses, it continues investigating strategies to guarantee the reliability of the reference frame through time. This includes the estimation of the reference frame kinematics, evaluation, modelling, and reduction of seismic and post-seismic deformations on the reference frame, and modelling crustal kinematics in the SIRGAS region by continuous velocity models. This article summarises analysis strategies and science data products developed by DGFI–TUM as a SIRGAS analysis centre and as the IGS RNAAC SIRGAS. Special care is given to the determination of the most recent SIRGAS reference frame solution called SIRGAS2022, which is based on the second SIRGAS reprocessing campaign performed by DGFI–TUM to obtain homogeneously computed SIRGAS daily and weekly station position solutions referring to the IGS reference frame IGS14/IGb14 since January 2000.

The Geodetic Reference System for the Americas (Sistema de Referencia Geodésico para las Américas, SIRGAS) was initiated in 1993 for South America at an international conference organised by the International Association of Geodesy (IAG), the Pan-American Institute for Geography and History (PAIGH), the Deutsches Geodätisches Forschungsinstitut (DGFI), and the U.S. Defense Mapping Agency (DMA) in Asunción, Paraguay. The corresponding South American reference network was observed in 1995 by a ten-day GPS campaign at 58 stations. The network was extended to Central and North America in 2000 and immediately afterwards converted to a frame of continuously observing GNSS stations instead of short-term campaigns. The linear station position changes (velocities) were estimated by a multi-year least squares adjustment of weekly solutions, the first being published in 2002. The total set of station velocities served for the computation of continuous surface deformation models, the first over South America was published in 2005. Today, SIRGAS is accepted by most of the American states as the official geodetic reference frame. Besides the product generation (station positions, velocities, and surface deformation), SIRGAS is active in education and training offering schools and workshops for students, surveyors, and other stakeholders.

Following the definition and realization of the International Height Reference System (IHRS), the vertical coordinate of a given point at the Earth’s surface can be obtained from the computation of the geopotential value from a harmonic expansion of a Global Gravity Model of High-Resolution (GGM-HR) or based on the computation of a local or regional pure gravimetric geoid or quasigeoid. Therefore, an evaluation of the accuracy of GGMs-HR and the geoid model available is needed in order to assess its capability to infer IHRS coordinates. In this study, different GGMs-HR are evaluated against 2287 benchmarks in Argentina. Moreover, the most recent geoid model of Argentina is also evaluated. Geoid undulations at these benchmarks are obtained based on ellipsoidal and orthometric heights in the local vertical datum. Results suggest that among the evaluated GGMs-HR, XGM2019e provides the best agreement with the observed geoid heights, but none of them is accurate enough in order to infer vertical coordinates in the IHRS. Similar conclusions are obtained for the local geoid model for Argentina demonstrating the necessity for a more precise geoid model, following the standards and recommendations given for the IHRS.

Zenith Tropospheric Delays (ZTDs) are used to correct tropospheric effects that cause a delay in the signal measured by Global Navigation Satellite Systems (GNSS) receivers and obtain accurate measurements. ZTD can be estimated from GNSS processing, which means they may suffer from occasional or systematic errors. Therefore, it is necessary to assess the quality and stability of these data over time, since ZTDs are used in several applications that require centimeter precision. Within this context, this work aims to assess the available ZTD of the whole Geodetic Reference System for the Americas Continuously Operating Network (SIRGAS-CON), consisting of 467 stations, spanning the period from January 2014 to December 2020 using the most recent Numerical Weather Model ERA5 from the European Centre for Medium-Range Weather Forecasts and common stations to the International GNSS Service (IGS) for an intercomparison. Results show that 10% of the stations present some instability, such as periods of highly dispersed data or discontinuities, with more occurrence in stations located in Argentina, Uruguay and Colombia. The remaining 90% proved to have stable and reliable ZTD, both in comparison with ERA5 and IGS.

The wet tropospheric correction (WTC) is a required correction to satellite altimetry measurements, mainly due to the atmospheric water vapor delay. On-board microwave radiometers (MWR) provide information for WTC estimation but fail in coastal zones and inland waters. In view to recover the WTC in these areas, the Global Navigation Satellite System (GNSS)-derived Path Delay Plus (GPD+) method, developed by the University of Porto, uses Zenith Tropospheric Delays from GNSS global and regional networks’ stations combined with other sources of information, providing a WTC solution for all along-track altimeter points. To densify the existing dataset used by GPD+, it is necessary to add new GNSS stations, mainly in the southern hemisphere, in regions such as South America, Africa and Oceania. This work aims to exploit the SIRGAS-CON data and its potential for densification of the GPD+ input dataset in Latin America and to improve GPD+ performance. The results for the three analyzed satellites (Sentinel-3A, Sentinel-3B and CryoSat-2) show that, when compared with the WTC from GNSS and radiosondes, the densified GPD+ WTC leads to a reduction in the RMS of the WTC differences with respect to the non-densified GPD+ solution, up to 2 mm for the whole region and up to 5 mm in some locations.