Monitoring and understanding crustal deformation by means of GPS and InSAR data

Monitoring deformation of the Earth’s crust by using data acquired by both the GNSS and SAR techniques allows describing crustal movements with high spatial and temporal resolution. This is a key contribution for achieving a deeper and better insight of geodynamic processes. Combination of the two techniques provides a very powerful means, however, before combing the different data sets it is important to properly understand their respective contribution. For this purpose, strictly simultaneous and long time series would be necessary. This is not, in general, a common case due to the relatively long SAR satellites revisit time. A positive exception is represented by the data set of COSMO SKYMed (CSK) images made available for this study by the Italian Space Agency (ASI). The flyover area encompass the city of Bologna and the smaller nearby town of Medicina where permanent GPS stations are operational. At the times of the CSK flyovers, we compared the GPS and SAR Up and East coordinates of a few stations as well as differential tropospheric delays derived by both techniques. The GPS time series were carefully screened and corrected for the presence of discontinuities by adopting a dedicated statistical procedure. The comparisons of both the estimated deformation and the tropospheric delays are encouraging and highlight the need for having available a more evenly sampled SAR data set

Space geodetic activities, from the early days to present, with focus on the northeastern Adriatic

It has become of greatest importance to monitor and understand how the dynamics of the System Earth works. Geodesy has gone far beyond the provision of a static knowledge of the Earth\u2019s geometric and physical parameters and with the development of high-accuracy satellite and space techniques and increased measurement capability, this discipline is now providing observations of changes in the Earth\u2019s shape, gravity field and rotation which are essential for Earth system science. In a short historical excursus of the main space geodetic techniques, we review achievements in the determination and study of Earth\u2019s crustal deformation and sea-level change/variations. Focus is given to geodetic research activities developed in northeastern Italy

Assessment of the possible contribution of space ties on-board GNSS satellites to the terrestrial reference frame

The realization of the international terrestrial reference frame (ITRF) is currently based on the data provided by four space geodetic techniques. The accuracy of the different technique-dependent materializations of the frame physical parameters (origin and scale) varies according to the nature of the relevant observables and to the impact of technique-specific errors. A reliable computation of the ITRF requires combining the different inputs, so that the strengths of each technique can compensate for the weaknesses of the others. This combination, however, can only be performed providing some additional information which allows tying together the independent technique networks. At present, the links used for that purpose are topometric surveys (local/terrestrial ties) available at ITRF sites hosting instruments of different techniques. In principle, a possible alternative could be offered by spacecrafts accommodating the positioning payloads of multiple geodetic techniques realizing their co-location in orbit (space ties). In this paper, the GNSS\ue2\u80\u93SLR space ties on-board GPS and GLONASS satellites are thoroughly examined in the framework of global reference frame computations. The investigation focuses on the quality of the realized physical frame parameters. According to the achieved results, the space ties on-board GNSS satellites cannot, at present, substitute terrestrial ties in the computation of the ITRF. The study is completed by a series of synthetic simulations investigating the impact that substantial improvements in the volume and quality of SLR observations to GNSS satellites would have on the precision of the GNSS frame parameters

Long-period sea-level variations in the Mediterranean

Since the beginning of its long-lasting lifetime, the Wegener initiative has devoted careful consideration to studying sea-level variations/changes across the Mediterranean Sea. Our study focuses on several long-period sea-level time series (from end of 1800 to 2012) acquired in the Mediterranean by tide gauge stations. In general, the analysis and interpretation of these data sets can provide an important contribution to research on climate change and its impacts. We have analyzed the centennial sea-level time series of six fairly well documented tide gauges. They are: Marseille, in France, Alicante in Spain, Genoa, Trieste, Venice and Marina di Ravenna (formerly Porto Corsini), in Italy. The data of the Italian stations of Marina di Ravenna and Venice clearly indicate that land subsidence is responsible for most of the observed rate of relative sea level rise. It is well known that, in the two areas, subsidence is caused by both natural processes and human activities. For these two stations, using levelling data of benchmarks at, and/or close to, the tide gauges, and for the recent years, also GPS and InSAR height time series, modelling of the long-period non-linear behavior of subsidence was successfully accomplished. After removing the land vertical motions, the estimate of the linear long-period sea-level rise at all six stations yielded remarkably consistent values, between +1,2 and +1,3 mm/yr, with associated errors ranging from ±0,2 to ±0,3 mm/yr (95% confidence interval), which also account for the statistical autocorrelation of the time series. These trends in the Mediterranean area are lower than the global mean rate of 1,7±0,2 mm/yr (1901-2010) presented by the IPCC in its 5th Assessment Report; however, they are in full agreement with a global mean sea-level rise estimate, over the period 1901-1990, recently published by Hay et al. (2015, doi:10.1038/nature14093) and obtained using probabilistic techniques that combine sea-level records with physics-based and model-derived geometries of the contributing processes. An EOF analysis (Empirical Orthogonal Functions) has also been carried out on the six sea-level time series to identify the dominant modes of variability

Sea-Level Change in the Northern Mediterranean Sea from Long-Period Tide Gauge Time Series

The oldest tide gauge observations date back to the 18th century. Although, globally, they are available in limited number, these centuries-old sea level time series are the only data records providing information on the long-period rates of change of the mean ocean surface. Knowledge of the past sea level behavior can contribute key insights to the understanding of climate change impacts. We highlight the greatest importance of monitoring sea-level changes at all spatial scales, from global to local, using terrestrial and space techniques and outline the physical processes, natural and man-induced, responsible for such changes. In general, tide gauge data are made available through different archiving facilities serving both international and national developments. Tide gauges measure local sea-level relative to a benchmark on land, hence, correctly interpreting these observations is challenging since it demands, among other requirements, a proper knowledge of vertical land motions at the stations. In general, it is not easy to find well documented historical data; moreover, benchmarks were not frequently leveled. For more than two decades, space geodetic techniques, such as GNSS (Global Navigation Satellite System) and InSAR (Interferometric Synthetic Aperture Radar), have provided the opportunity to accurately position points in the surroundings of tide gauge sites, potentially giving rise to a large amount of information. However, despite the availability of these techniques, the evolution of the international efforts aiming at realizing consistent observational infrastructures for sea level networks is undergoing only a slow development. In the Mediterranean area, there are a few centennial tide gauge records. Our study focuses on the time series of Alicante, in Spain, Marseille, in France, Genoa, Marina di Ravenna (formerly Porto Corsini), Venice and Trieste, in Italy. After briefly reviewing the gauge types presently in use for sea level measurements, a comprehensive historical description is given for each time series, which may assist understanding an assessment of the problems these stations have experienced over more than one century of operations. Two Italian stations, Marina di Ravenna and Venice, are affected by both natural and anthropogenic subsidence, the latter was particularly intense during a few decades in the 20th century because of ground fluid withdrawal. For these two stations, we have retrieved leveling data of benchmarks close to the tide gauges from the end of the 19th century and, for the last couple of decades, we have evaluated GPS and InSAR heights in close proximity to the stations. The GPS (Global Positioning System) and SAR results were carefully compared. Modeling of the long-period non-linear behavior of subsidence was successfully accomplished by using an ensemble of leveling, GPS and SAR data. After removing the vertical land motions in Venice and Marina di Ravenna, and the inverted barometer effect at all the sites, the linear long period sea-level rates were estimated. The results are in excellent agreement ranging between + 1.2 and + 1.3 mm/year for the overall period from the last decades of the 19th century till 2012. The associated errors, computed by accounting for serial autocorrelation, are of the order of 0.2\u20130.3 mm/year for all stations, except Alicante, for which the error turns out to be 0.5 mm/year. Our estimated rates for the northern Mediterranean, a relatively small regional sea, are slightly lower than the global mean rate, + 1.7 \ub1 0.2 mm/year, recently published in the IPCC AR5 (Intergovernmental Panel on Climate Change 5th Assessment Report) (Church et al., 2013), but close enough, if uncertainties are taken into account. It is known that Mediterranean stations had always had lower trends than the global-average ones. Our regional results, however, are in close agreement with the global mean rate, + 1.2 mm/year, published by Hay et al. (2015) which is currently being discussed by the oceanographic community (see, for example, Hamlington and Thompson, 2015). The six time series were also analyzed by means of the EOF (Empirical Orthogonal Functions) technique over the 1934\u20132012 common period. As a result, about 50% of the total variance is explained by the first mode, which is characterized by a coherent behavior of the six stations