Beyond 100: The Next Century in Geodesy

Establishing an International Height Reference Frame (IHRF) has been a major goal of the International Association of Geodesy (IAG) for a long time. One challenge is to obtain the vertical coordinates, i.e., geopotential numbers, of the reference stations with high precision and global consistency. A promising approach is using clock networks, which are powerful in precisely obtaining geopotential or height differences between distant sites through measuring the gravitational redshift effect by comparing clocks’ frequencies. We propose a hybrid clock network following a specific hierarchy. It includes stationary clocks as the backbone of the frame and transportable clocks for regional densifications. The vertical coordinates of the clock stations can be straightforwardly referenced to the unique benchmark by various long-distance frequency transfer techniques, like using optical fibers or free-space microwave and laser links via relay satellites. Another practical way towards an IHRF is to unify all local height systems around the world. Clock networks are considered as an alternative to classical geodetic methods. The idea was verified through closed-loop simulations. We found that the measurements acquired by a few 10⁻¹⁸ clocks, three or four in triangular or quadrangular distributions for each local system, are sufficient to adjust the discrepancies between local datums and the systematic slopes within local height networks.

This paper describes a practical implementation of the International Height Reference System (IHRS) in Argentina. The contribution deals with the determination of potential values W(P) at five Argentinean stations proposed to be included in the reference network of the International Height Reference Frame (IHRF). All sites are materialized with GNSS stations of the Argentine continuous satellite monitoring network and most of them are included in the SIRGAS Continuously Operating Network. Not all the stations are connected to the National Vertical Reference System 2016 and most of them are near to an absolute gravity station measured with an A10 gravimeter.

This paper also discusses the approach for the computation of W(P) at the IHRF stations using the Argentinean geoid model GEOIDE-Ar 16 developed by the Instituto Geográfico Nacional, Argentina together with the Royal Melbourne Institute of Technology (RMIT) University, Australia using the remove-compute-restore technique and the GOCO05s satellite-only Global Gravity Model. Then, geoid undulations (N) were transformed to height anomalies (ζ) in order to infer W(P) at the stations located on the Earth’s surface. The transformation from N to ζ must be consistent with the hypothesis used for the geoid determination. Special emphasis is made on the standards, conventions and constants applied.

We are using various models and analysis strategies, such as galactic aberration, ray-tracing etc., to create different Vienna celestial reference frame (CRF) solutions. These solutions are then compared against the Gaia reference frame (Gaia-CRF2). This is done using a degree 2 vector spherical harmonics approach. The estimated parameters are used to investigate the impact of the various analysis methods on the differences between Gaia and the Very Long Baseline Interferometry (VLBI) CRF. We find that correcting for galactic aberration reduces the difference between the Gaia-CRF2 and the VLBI CRF significantly (30 \( \mathop {\mu }\)as in D₂ and 13 \( \mathop {\mu }\)as in D₃). Furthermore, we find that using a priori ray-traced tropospheric delays in addition with low absolute constraints on tropospheric gradients reduces the \(a_{20}^e\) parameter by 20 \( \mathop {\mu }\)as. Using these analysis strategies we can explain almost all significant differences between the Gaia-CRF2 and the VLBI CRF. However, the vector spherical harmonic (VSH) parameter \(a_{20}^e\) is still highly significant and can not be explained by modeling and analysis choices from the VLBI technique.

Based on a large network of continuously operated GNSS tracking stations the International GNSS Service (IGS) has a valuable contribution for the realization of the International Terrestrial Reference System (ITRS). In order to contribute to its next realization, the IGS is preparing for a new reprocessing of the GNSS data from 1994 to 2020 including GPS, GLONASS, and – for the first time – Galileo. A first test campaign including single- and multi-system solutions for 2017 and 2018 was performed to derive consistent transmitter phase center corrections for all systems. Preliminary results of the test solutions derived at GFZ show well determined orbits with overlaps of 28 mm for GPS, 67 mm for GLONASS, and 40 mm for Galileo and an overall RMS of satellite laser ranging residuals for Galileo of 58 mm. Using multi-GNSS antenna calibrations (including also E5a and E5b calibrations) horizontal coordinate differences are almost zero between a GPS+GLONASS and a Galileo-only solutions. Due to the mixture of estimated (GPS, GLONASS) and measured (Galileo) transmitter phase center offsets a scale difference of 1.16 ± 0.27 ppb is found between both solutions which agrees nicely to results derived by other analysis centers.

Various signals of crustal deformation and mass loading deformation are contained in a GNSS position time series. However, a GNSS position time series is also polluted by outliers and various colored noise, which must be reasonably modelled before estimating deformation signals. Since temporal signals of the GNSS position time series are non-linear and complicated, we propose a wavelet-based approach for outlier detection, which first retrieves the temporal signals from the GNSS position time series by using wavelet analysis, and then detect outliers in the residual position time series by using the interquartile range. After the detected outliers are eliminated from the residual time series, the noise components, including white noise and flicker noise, are estimated by using MINQUE approach. Our proposed approach is used to process the real GNSS position time series of the Crustal Movement Observation Network of China (CMONOC) over the period spanning 1999–2018. The results demonstrate that our approach can detect the outliers more efficiently than the traditional approach, which retrieves the temporal signals by using a functional model with trend and periodic variations. As a result, the noise components estimated with our proposed approach are smaller than those with the traditional approach for the GNSS position time series of all CMONOC stations.

The International Combination Service for Time-variable Gravity Fields (COST-G) is a new Product Center of IAG’s International Gravity Field Service (IGFS). COST-G provides consolidated monthly global gravity fields in terms of spherical harmonic coefficients and thereof derived grids of surface mass changes by combining existing solutions or normal equations from COST-G analysis centers (ACs) and partner analysis centers (PCs). The COST-G ACs adopt different analysis methods but apply agreed-upon consistent processing standards to deliver time-variable gravity field models, e.g. from GRACE/GRACE-FO low-low satellite-to-satellite tracking (ll-SST), GPS high-low satellite-to-satellite tracking (hl-SST) and Satellite Laser Ranging (SLR). The organizational structure of COST-G and results from the first release of combined monthly GRACE solutions covering the entire GRACE time period are discussed in this article. It is shown that by combining solutions and normal equations from different sources COST-G is taking advantage of the statistical properties of the various solutions, which results in a reduced noise level compared to the individual input solutions.

In this contribution, we present the LUH-GRACE2018 time series of monthly gravity field solutions covering the period January 2003–March 2016. The solutions are obtained from GRACE K-Band Range Rate (KBRR) measurements as main observations. The monthly solutions are computed using the in-house developed GRACE-SIGMA software. The processing is based on dynamic orbit and gravity field determination using variational equations and consists of two main steps. In the first step, 3-hourly orbital arcs of the two satellites and the state transition and sensitivity matrices are dynamically integrated using a modified Gauss-Jackson integrator. In this step, initial state vectors and 3D accelerometer bias parameters are adjusted using GRACE Level-1B reduced-dynamic positions as observations. In the second step, normal equations are accumulated and the normalized spherical harmonic coefficients up to degree and order 80 are estimated along with arc-wise initial states, accelerometer biases and empirical KBRR parameters. Here KBRR measurements are used as main observations and reduced-dynamic positions are introduced to solve for the low frequency coefficients. In terms of error degree standard deviations as well as Equivalent Water Heights (EWH), our gravity field solutions agree well with RL05 solutions of CSR, GFZ and JPL.

In the framework of the IAG African Geoid Project, an attempt towards a precise geoid model for Africa is presented in this investigation. The available gravity data set suffers from significantly large data gaps. These data gaps are filled using the EIGEN-6C4 model on a 15′× 15′ grid prior to the gravity reduction scheme. The window remove-restore technique (Abd-Elmotaal and Kühtreiber, Phys Chem Earth Pt A 24(1):53–59, 1999; J Geod 77(1–2):77–85, 2003) has been used to generate reduced anomalies having a minimum variance to minimize the interpolation errors, especially at the large data gaps. The EIGEN-6C4 global model, complete to degree and order 2190, has served as the reference model. The reduced anomalies are gridded on a 5′× 5′ grid employing an un-equal weight least-squares prediction technique. The reduced gravity anomalies are then used to compute their contribution to the geoid undulation employing Stokes’ integral with Meissl (Preparation for the numerical evaluation of second order Molodensky-type formulas. Ohio State University, Department of Geodetic Science and Surveying, Rep 163, 1971) modified kernel for better combination of the different wavelengths of the earth’s gravity field. Finally the restore step within the window remove-restore technique took place generating the full gravimetric geoid. In the last step, the computed geoid is fitted to the DIR_R5 GOCE satellite-only model by applying an offset and two tilt parameters. The DIR_R5 model is used because it turned out that it represents the best available global geopotential model approximating the African gravity field. A comparison between the geoid computed within the current investigation and the existing former geoid model AGP2003 (Merry et al., A window on the future of geodesy. International Association of Geodesy Symposia, vol 128, pp 374–379, 2005) for Africa has been carried out.

The Earth precession-nutation model endorsed by resolutions of each the International Astronomical Union and the International Union of Geodesy and Geophysics is composed of two theories developed independently, namely IAU2006 precession and IAU2000A nutation. The IAU2006 precession was adopted to supersede the precession part of the IAU 2000A precession-nutation model and tried to get the new precession theory dynamically consistent with the IAU2000A nutation.

However, full consistency was not reached, and slight adjustments of the IAU2000A nutation amplitudes at the micro arcsecond level were required to ensure consistency. The first set of formulae for these corrections derived by Capitaine et al. (Astrophys 432(1):355–367, 2005), which was not included in IAU2006 but provided in some standards and software for computing nutations. Later, Escapa et al. showed that a few additional terms of the same order of magnitude have to be added to the 2005 expressions to get complete dynamical consistency between the official precession and nutation models. In 2018 Escapa and Capitaine made a joint review of the problem and proposed three alternative ways of nutation model and its parameters to achieve consistency to certain different extents, although no estimation of their respective effects could be worked out to illustrate the proposals. Here we present some preliminary results on the assessment of the effects of each of the three sets of corrections suggested by Escapa and Capitaine (Proceedings of the Journées, des Systémes de Référence et de la Rotation Terrestre: Furthering our Knowledge of Earth Rotation, Alicante, 2018) by testing them in conjunction with the conventional celestial pole offsets given in the IERS EOP14C04 time series.

This report focuses on some selected scientific outcomes of the activities developed by the IAU/IAG Joint Working Group on Theory of Earth rotation and validation along the term 2015–2019. It is based on its end-of-term report to the IAG Commission 3 published in the Travaux de l’IAG 2015–2019, which in its turn updates previous reports to the IAG and IAU, particularly the triennial report 2015–2018 to the IAU Commission A2, and the medium term report to the IAG Commission 3 (2015–2017). The content of the report has served as a basis for the IAG General Assembly to adopt Resolution 5 on Improvement of Earth rotation theories and models.

We present the activities and improvements of the International Geodynamics and Earth Tide Service (IGETS) over the last four years. IGETS collects, archives and distributes long time series from geodynamic sensor, in particular superconducting gravimeter data currently from more than 40 stations and 60 different sensors. In addition to the raw 1-s and 1-min gravity and atmospheric pressure data (Level 1), IGETS produces end-user products on different levels. These include gravity and atmospheric pressure data corrected for major instrumental perturbations and ready for tidal analysis (Level 2). Since 2019, IGETS provides gravity residuals corrected for most geophysical contributions (Level 3) which can be used directly for geophysical applications without any expert knowledge in the processing of gravimetric time series.

Vertical displacements and time-varying gravity fluctuations are representative of various deformation mechanisms of the Earth occurring at different spatial and temporal scales. The inter-comparison of ground-gravity measurements with vertical surface displacements enables to estimate the transfer function of the Earth at various time-scales related to the rheological properties of the Earth. In this paper, we estimate the gravity-to-height changes ratio at seasonal time-scales due mostly to hydrological mass variabilities. We investigate this ratio at nine sites where Global Navigation Satellite System (GNSS) and Superconducting Gravimeter continuous measurements are collocated. Predicted gravity-to-height change ratios for a hydrological model are around −2 nm/s²/mm when there is no local mass effect. This is in agreement with theoretical modeling for an elastic Earth’s model. Spectral analysis of vertical displacement and surface gravimetric time-series show a coherency larger than 50% at seasonal time-scales at most sites. The obtained gravity-to-height change ratios range between −5 and −2 nm/s²/mm for stations Lhasa, Metsahovi, Ny-Alesund, Onsala, Wettzell and Yebes. At Canberra and Sutherland, this ratio is close to zero. Finally, at Strasbourg site the coherency is low and the ratio is positive because of local mass effects affecting gravimetric records.

Localization in GNSS-denied/challenged indoor/outdoor and transitional environments represents a challenging research problem. As part of the joint IAG/FIG Working Groups 4.1.1 and 5.5 on Multi-sensor Systems, a benchmarking measurement campaign was conducted at The Ohio State University. Initial experiments have demonstrated that Cooperative Localization (CL) is extremely useful for positioning and navigation of platforms navigating in swarms or networks. In the data acquisition campaign, multiple sensor platforms, including vehicles, bicyclists and pedestrians were equipped with combinations of GNSS, Ultra-wide Band (UWB), Wireless Fidelity (Wi-Fi), Raspberry Pi units, cameras, Light Detection and Ranging (LiDAR) and inertial sensors for CL. Pedestrians wore a specially designed helmet equipped with some of these sensors. An overview of the experimental configurations, test scenarios, characteristics and sensor specifications is given. It has been demonstrated that all involved sensor platforms in the different test scenarios have gained a significant increase in positioning accuracy by using ubiquitous user localization. For example, in the indoor environment, success rates of approximately 97% were obtained using Wi-Fi fingerprinting for correctly detecting the room-level location of the user. Using UWB, decimeter-level positioning accuracy is demonstrable achievable under certain conditions. The full sets of data is being made available to the wider research community through the WG on request.

In the application of GNSS in the velocity determination, it is often the case that some GNSS receivers give an opposite sign for the raw Doppler observations which do not correspond to the real Doppler shift. This is caused by different methods of the GNSS signal processing in different GNSS receivers. If the velocities of kinematic platforms are calculated by using raw Doppler observations from the GNSS receiver directly, the directions of the estimated velocities may be reversed, and the value of the velocity is wrong with respect to the actual movement. This would lead to incorrect results, and unacceptable for research and applications. To overcome this problem, a new method of sign correction for raw Doppler observations is proposed in this study. This algorithm constructs a correction function based on the GNSS carrier-phase-derived Doppler observations. To test this approach, GNSS data of GEOHALO airborne gravimetric missions have been used. The results show that the proposed method, which is straightforward and practical, can produce the correct velocity for a kinematic platform in any case, independent of the internal hardware structure and the specific way of the signal processing of the GNSS receivers in question.

Wi-Fi derived positions have been used in the past few years as a complementary source of positioning information for GNSS and Inertial Systems (INS). Ubiquitous positioning that transitions from indoors to outdoors and vice-versa is currently a hot topic of research. In this context, this study aims to analyze the potential of directional antennas sequentially tracking Wi-Fi signals on the 11 channels around the 2.4 GHz frequency in order to serve as an integrated signal for GNSS and INS positioning. Considering, as an example, a single point positioning (SPP) strategy coupled with an INS, the use of directional antennas can be beneficial in order to provide absolute directions of travel by the means of a Support Vector Machine (SVM) lane matching. In order to test the given hypothesis, real-world experiments were performed in areas with and without obstruction in an urban environment. Using a post-processed, smoothed in both forward and backward modes, and finally edited post-processed kinematic (RTK) solution as a reference, the solution integrating SPP GNSS, INS and Wi-Fi was assessed in terms of accuracy. Preliminary results show that such a combination of the directional antennas along with GNSS and INS and their respective SVM and EKF filters, can provide sub-meter accuracy at all times without the need of precise orbits or differential corrections, increasing solution availability, reliability and accuracy on a scalable and cost-effective way.

The Global Navigation Satellite System (GNSS) Precise Point Positioning (PPP) technology benefits from not needing local ground infrastructure such as reference stations and accuracy attained is at the decimetre-level, which approaches real-time kinematic (RTK) performance. However, due to its long position solution initialization period and complete dependence on the receiver measurements, PPP finds limited utility. The emergence of low-cost, micro-electro-mechanical sensor (MEMS) inertial measurement units (IMUs) has prompted research in integrated navigation solutions with the PPP processing technique. This sensor fusion aids to achieve continuous positioning and navigation solution availability when there are insufficient numbers of navigation satellites visible. In the past, research has been conducted to integrate high-end (geodetic) GNSS receivers with PPP processing and MEMS IMUs, or low-cost, single-frequency GNSS receivers with point positioning processing and MEMS IMUs. The objective of this research is to investigate and analyze position solution availability and continuity by integrating low-cost, dual-frequency GNSS receivers using PPP processing with the latest low-cost, MEMS IMUs to offer a complete, low-cost navigation solution that will enable continuously available positioning and navigation solutions, even in obstructed environments. The horizontal accuracy of the developed low-cost, dual-frequency GNSS PPP with MEMS IMU integrated algorithm is approximately 20 cm. During half a minute of simulated GNSS signal outage, the integrated solution offers 40 cm horizontal accuracy. A low-cost, dual-frequency GNSS receiver PPP solution integrated with a MEMS IMU forms a unique combination of a total low-cost solution, that will open a significant new market window for modern-day applications such as autonomous vehicles, drones and augmented reality.

Population growth, coupled with the expansion of exploitation of groundwater resources for agricultural and industrial purposes, has led Iran to face the necessity of proper use and sustainable management of existing water resources. In this study we will use the existing valuable geodetic data (gravity, GNSS, precise levelling, InSAR) in Iran to better understand the surface deformation and gravity variations caused by underground water depletion attributed to drastic pumping. Based on repetition of first order precise leveling network of Iran, about 44 subsidence areas are identified and continuous data collected by the Iranian permanent GNSS and geodynamic network (IPGN), as well as InSAR data, indicate strong elevation changes in some parts of the country. GRACE satellite gravity solutions over Iran also show a general gravity decrease between 2002 and 2016. New absolute gravity campaigns were performed in Iran in 2017 and 2018 in the frame of the TRIGGER French-Iranian program. Several new absolute gravity stations were established and former stations, first measured between 2000 and 2007, were repeated showing that the gravity values of many stations have changed in time. Most of these changes indicate a gravity decrease mostly linked to mass deficit due to water depletion. On the contrary some stations show a large gravity increase that can be merely explained by land subsidence itself linked to water depletion by poroelastic effects.

We have studied the land uplift and relative sea level changes in the Baltic Sea in northern Europe. To observe the past changes and land uplift, we have used continuous GNSS time series, campaign-wise absolute gravity measurements and continuous tide gauge time series. To predict the future, we have used probabilistic future scenarios tuned for the Baltic Sea. The area we are interested in is Kvarken archipelago in Finland and High Coast in Sweden. These areas form a UNESCO World Heritage Site, where the land uplift process and how it demonstrates itself are the main values. We provide here the latest numbers of land uplift for the area, the current rates from geodetic observations, and probabilistic scenarios for future relative sea level rise. The maximum land uplift rates in Fennoscandia are in the Bothnian Bay of the Baltic Sea, where the maximum values are currently on the order of 10 mm/year with respect to the geoid. During the last 100 years, the land has risen from the sea by approximately 80 cm in this area. Estimates of future relative sea level change have considerable uncertainty, with values for the year 2100 ranging from 75 cm of sea level fall (land emergence) to 30 cm of sea-level rise.

Vertical land motion in insular areas is a crucial parameter to estimate the relative sea-level variations which impact coastal populations and activities. In subduction zones, it is also a relevant proxy to estimate the locking state of the plate interface. This motion can be measured using Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS). However, the influence of the processing software and the geodetic products (orbits and clock offsets) used for the solution remains barely considered for geophysics studies.

In this study, we process GNSS observations of Guadeloupe and Martinique network (Lesser Antilles). It consists of 40 stations over a period of 18 years for the oldest site. We provide an updated vertical velocity field determined with two different geodetic software, namely EPOS (Gendt et al, GFZ analysis center of IGS–Annual Report. IGS 1996 Annual Report, pp 169–181, 1998) and GINS (Marty et al, GINS: the CNES/GRGS GNSS scientific software. In: 3rd International colloquium scientific and fundamental aspects of the Galileo programme, ESA proceedings WPP326, vol 31, pp 8–10, 2011) using their Precise Point Positioning modes. We used the same input models and orbit and clock offset products to maintain a maximum of consistency, and then compared the obtained results to get an estimation of the time series accuracy and the software influence on the solutions. General consistency between the solutions is noted, but significant velocity differences exist (at the mm/yr level) for some stations.

We analyze the vertical component of GEONET GNSS measurements in Central Japan and clarify in some of the sites the origin of large annual time variations, as well as the secular variations. Many of these vertical movements may be attributable to the use of groundwater for agriculture, for snow melting, industrial, and hospital usages, etc. and the pumping up of the groundwater mining for refining natural gas and iodine at the production area of natural gas dissolved in water. For this reason, highly accurate monitoring of vertical variations by GNSS observations can provide new observation methods for understanding of not only geodynamics but also hydrology through monitoring groundwater fluctuation, and natural gas and oil resource development through monitoring ground movements caused by mining.

The geodetic observations of static deformations, including gravity perturbations and displacement fields due to huge earthquakes, are understood and explained using recent dislocation theories. Due to multiple possible mechanisms for the post-seismic phase of earthquakes, the dominant mechanism may change at different spatiotemporal ranges for different earthquake types. Accurate forward and inverse modeling of post-seismic deformations is valuable and needed information for geoscience communities. The existing methods for calculating gravitational viscoelastic relaxation can be improved or simplified to make them more suitable for more realistic Earth models and/or to overcome the poor convergence performance and/or overflow risks during numerical calculations. In this study, a simple and effective method for calculating the post-seismic relaxation deformations is proposed. This method is different from previous methods, such as the normal mode summation and rectangle integration methods. The proposed method consists of a rational functional approximation of the integral kernel and a transformation of the numerical inverse Laplace transform (NILT) into an alternating series summation using the residual theorem. Then the intrinsic oscillation and overflow risks are thoroughly suppressed. The accuracy of the calculated Green’s functions can be easily controlled by choosing a suitable parameter. In addition, the proposed method also has applicability in different Earth models with linear rheological profiles.

Based on 24 years of high-level GNSS data analysis, we present a sequence of crustal deformation models showing the varying surface kinematics in Latin America. The deformation models are inferred from GNSS station horizontal velocities using a least-squares collocation approach with empirically determined covariance functions. The main innovation of this study is the assumption of continuous surface deformation. We do not introduce rigid microplates, blocks or slivers which enforce constraints on the deformation model. Our results show that the only stable areas in Latin America are the Guiana, Brazilian and Atlantic shields; the other tectonic entities, like the Caribbean plate and the North Andes, Panama and Altiplano blocks are deforming. The present surface deformation is highly influenced by the effects of seven major earthquakes: Arequipa (Mw8.4, Jun 2001), Maule (Mw8.8, Feb 2010), Nicoya (Mw7.6, Sep 2012), Champerico (Mw7.4, Nov 2012), Pisagua (Mw8.2, Apr 2014), Illapel (Mw8.3, Sep 2015), and Pedernales (Mw7.8, Apr 2016). We see very significant kinematic variations: while the earthquakes in Champerico and Nicoya have modified the aseismic deformation regime in Central America by up to 5 and 12 mm/a, respectively, the earthquakes in the Andes have resulted in changes of up to 35 mm/a. Before the earthquakes, the deformation vectors are roughly in the direction of plate subduction. After the earthquakes, the deformation vectors describe a rotation counter-clockwise south of the epicentres and clockwise north of the epicentres. The deformation model series reveals that this kinematic pattern slowly disappears with post-seismic relaxation. The numerical results of this study are available at https://doi.pangaea.de/10.1594/PANGAEA.912349 and https://doi.pangaea.de/10.1594/PANGAEA.912350.

Since its early days, GNSS has been employed for the monitoring of sudden ground movements, such as earthquakes. Its use as a tool to enhance tsunami detection was boosted after analysis of data following the December 2004 Great Ocean Indian Tsunami. The contribution of GNSS towards tsunami warning systems is possible due to several factors, such as advances in the measurement of crustal displacement, developments in GNSS methodology, the growing availability of real-time data streams and advances in processing power and communication means. The paper focuses on the progress of Global Navigation Satellite System Tsunami Early Warning Systems (GTEWS) identifying current implementations and future directions and challenges. The discussion leads to the conclusion that the GNSS technology already satisfies requirements of tsunami early warning systems and that the major hurdles are with other aspects, such as optimal network configuration, real-time flow of data, communication infrastructure, and national and international collaboration. The paper ends highlighting the important role that the Global Geodetic Observing System (GGOS) can play to help overcoming those hurdles.

Upper-atmospheric processes under different space weather conditions are still not well understood, and the existing models are far away from the desired operational requirements due to the lack of in-situ measurements input. The ionospheric perturbation of electromagnetic signals affects the accuracy and reliability of Global Navigation Satellite Systems (GNSS), satellite communication infrastructures, and Earth observation techniques. Furthermore, the variable aerodynamic drag, due to variable thermospheric mass density, disturbs orbital tracking, collision analysis, and re-entry calculations of Low Earth Orbit (LEO) objects, including manned and unmanned artificial satellites. In this paper, we use the Principal Component Analysis (PCA) technique to study and compare the main driver-response relationships and spatial patterns of total electron content (TEC) estimates from 2003 to 2018, and total mass density (TMD) estimates at 475 km altitude from 2003 to 2015. Comparison of the first TEC and TMD PCA mode shows a very similar response to solar flux, but annual cycle shown by TEC is approximately one order of magnitude larger. A clear hemispheric asymmetry is shown in the global distribution of TMD, with higher values in the southern hemisphere than in the northern hemisphere. The hemispheric asymmetry is not visible in TEC. The persistent processes including a favorable solar wind input and particle precipitation over the southern magnetic dip may produce a higher thermospheric heating, which results in the hemispheric asymmetry in TMD.

The present geodetic reference frame in Latin America and the Caribbean is given by a network of about 400 continuously operating GNSS stations. These stations are routinely processed by ten Analysis Centres following the guidelines and standards set up by the International Earth Rotation and Reference Systems Service (IERS) and International GNSS Service (IGS). The Analysis Centres estimate daily and weekly station positions and station zenith tropospheric path delays (ZTD) with an hourly sampling rate. This contribution presents some attempts aiming at combining the individual ZTD estimations to generate consistent troposphere solutions over the entire region and to provide reliable time series of troposphere parameters, to be used as a reference. The study covers ZTD and IWV series for a time-span of 5 years (2014–2018). In addition to the combination of the individual solutions, some advances based on the precise point positioning technique using BNC software (BKG NTRIP Client) and Bernese GNSS Software V.5.2 are presented. Results are validated using the IGS ZTD products and radiosonde IWV data. The agreement was evaluated in terms of mean bias and rms of the ZTD differences w.r.t IGS products (mean bias −1.5 mm and mean rms 6.8 mm) and w.r.t ZTD from radiosonde data (mean bias −2 mm and mean rms 7.5 mm). IWV differences w.r.t radiosonde IWV data (mean bias 0.41 kg/m² and mean rms 3.5 kg/m²).

Vertical crustal displacements induced by atmospheric, hydrological, cryospheric, and oceanic load changes are detectable with sub-cm accuracy by precise continuous GPS measurements. Areas subjected to rapid load changes due to ice sheet melt, drought, massive groundwater extraction, or lake level drop, are characterized by a dominant non-linear vertical signal. Here, we investigate possible relations between vertical crustal movements and climate change by analyzing the relations between observed GPS vertical movements, predicted movements, and climatic indices, where we have long GPS time series (>20 years). Applying our analysis to GPS records from western and eastern North America indicates different load change characteristics. In the western US, the seasonal and climatic signals are dominated by hydrological load changes and, consequently, the GPS signal correlates well with the Palmer Severe Drought Index (PSDI) calculated for the same region. However, vertical crustal movements in eastern North America, as detected by long GPS time series, reveal poor correlation with PSDI and other climatic indices. Our results suggest that long continuous GPS observations of vertical crustal displacements primarily driven by climate related changes in water storage can serve as independent measures of regional-scale climate change in some cases, mainly in western north America.

Tropical cyclones are one of the most powerful severe weather events that produce devastating socioeconomic and environmental impacts in the areas they strike. Therefore, monitoring and tracking of the arrival times and path of the tropical cyclones are extremely valuable in providing early warning to the public and governments. Hurricane Florence struck the East cost of USA in 2018 and offers an outstanding case study. We employed Global Positioning System (GPS) derived precipitable water vapor (PWV) data to track and investigate the characteristics of storm occurrences in their spatial and temporal distribution using a dense ground network of permanent GPS stations. Our findings indicate that a rise in GPS-derived PWV occurred several hours before Florence’s manifestation. Also, we compared the temporal distribution of the GPS-derived PWV content with the precipitation value for days when the storm appeared in the area under influence. The study will contribute to quantitative assessment of the complementary GPS tropospheric products in hurricane monitoring and tracking using GPS-derived water vapor evolution from a dense network of permanent GPS stations.

In mountainous area, spring water constitutes the only drinking water resource and local economy is highly dependent on forest health and productivity. However, climate change is expected to make extreme water shortage episodes more and more frequent. Forest is therefore more and more exposed to water stress. It appears necessary to quantify the drought induced by water deficit to evaluate forest vulnerability and to plan the future of forest management. In this study we quantified the 2018 water deficit experienced by the forest in the Strengbach catchment, located in the French Vosges mountains. Three methods for estimating catchment water storage changes (WSC) have been compared. The first relies on superconducting gravimeter monitoring while the second relies on catchment water balance. The third one relies on global hydrological model MERRA2. We show that WSC estimated from measured gravity changes correlate well with WSC estimated from catchment water balance while WSC inferred from MERRA2 significantly differs. The Strengbach catchment water cycle is mostly annual but exhibits significant interannual variability associated with the 2018 drought episode: August 2018 has a water deficit of 37 mm (as inferred from catchment water balance) or 76 mm (as seen with superconducting gravimetry) compared to August 2017. We illustrate here the use of superconducting gravimeter monitoring as an independent proxy for WSC in a mountainous catchment while most of hydro-gravimetric studies have been conducted on relatively flat areas. We therefore contribute to expand the area of use of high precision gravity monitoring for the hydrological characterization of the critical zone in mountainous context. This innovative method may help to assess forest vulnerability to drought in the context of climate change.

Zenith Total Delay (ZTD) from ground-based Global Navigational Satellite Systems (GNSS) observations plays an important role in meteorology. It contains information about the troposphere due to the interactions that GNSS signals have with the atmosphere while traveling from satellites to ground receivers. Since almost all weather is formed in the troposphere, the analysis of a collection of ZTD time series would provide insight about the periodic characteristics of the weather of a place. It would also provide insight about the influences that meteorological parameters such as pressure, temperature and relative humidity have on the weather’s periodic nature. In this study, the least-squares spectral analysis approach is employed to determine the periodic oscillations in a 7-year time series of ZTD obtained from collocated GNSS and meteorological stations at the University of New Brunswick, Fredericton. Least-Squares Coherency Analysis of the time series spectra of the ZTD and its component hydrostatic and wet delays, and pressure, temperature and relative humidity is also performed. This is done to evaluate the level of contributions those parameters have in the periodicities inherent in the ZTD time series. Except for the zenith hydrostatic delay and pressure which show no annual periodic oscillation, the spectra of all the other time series show strong annual and semi-annual oscillations. Being the most dominant oscillation in the ZTD time series, the annual oscillation is largely driven by temperature, and this is maybe due to the high temperature variation characteristic of the climatic zone Fredericton falls under.