Natural gas is an important source of energy used for domestic, industrial and transport purposes. Its presence is unevenly distributed in the world, and its consumption is subject to large seasonal fluctuations. In addressing this challenge, Underground Gas Storage (UGS) represents an important option to create national strategic reserves and helps ensure a steady and reliable supply of natural gas. However, gas storage activities, inducing continuous stress changes, can threaten caprock integrity, possibly resulting in gas leakages. The inherent risks necessitate the development of operational protocols for effectively characterizing and monitoring UGS activities. Interferometric Satellite Aperture Radar (InSAR) data offers a comprehensive site-specific perspective, which proves valuable for enhancing the management of withdrawal and injection rates, as well as assessing environmental impacts near UGS facilities. Its utilization contributes to higher levels of operational reliability and safety.

Satellite interferometric SAR (InSAR) has demonstrated to be a powerful technology to perform millimeter-scale precision measurements of ground motions typically caused by landslides, subsidence, earthquakes or volcanic activity, and to monitor the stability of slopes, mining areas, buildings, infrastructures, etc. This work presents the European Ground Motion Service (EGMS), funded by the European Commission as an essential element of the Copernicus Land Monitoring Service (CLMS). EGMS constitutes the first application of the interferometric SAR (InSAR) technology to high-resolution monitoring of ground deformations over an entire continent, based on full-resolution processing of all Sentinel-1 (S1) satellite images over most of Europe. EGMS employs advanced persistent scatterer (PS) and distributed scatterer (DS) InSAR processing techniques. Moreover, a global navigation satellite system (GNSS) model is realized to calibrate the InSAR ground motion products. To foster as wide usage as possible, EGMS also provide tools for visualization, exploration, analysis and download of the ground deformation products, as well as elements to promote best practice applications and user uptake.

This paper highlights the importance of data calibration in wide-area, multi-temporal InSAR analyses, for improving data usability at a regional, national, and continental scale. The calibration process, based on GNSS data, can help the generation of high-quality, accurate and reliable databases of displacement data, ready-to-use by different types of final users. Calibrated data are easier to integrate with other information sources and can help limit spurious low frequency components associated with atmospheric effects and orbit errors, as well as mitigate possible edge effects between different processing units, quite common in wide area processing (WAP). The key point determining the quality of the calibration step is the spatial density of permanent GNSS stations available over the area of interest.

Interferometric processing of a time series of acquisitions from synthetic aperture radar (SAR) satellites makes it possible to detect and measure ground motion phenomena, typically caused by landslides, subsidence, earthquakes or volcanic activity, with millimeter-scale precision. This enables, for example, monitoring of the stability of slopes, mining areas, buildings and infrastructures. This work presents the European Ground Motion Service (EGMS), funded by the European Commission as an essential element of the Copernicus Land Monitoring Service (CLMS). The EGMS constitutes the first application of the interferometric SAR (InSAR) technology to high-resolution monitoring of ground deformations over an entire continent, based on full-resolution processing of all Sentinel-1 (S1) satellite acquisitions over most of Europe (Copernicus Participating States). Upscaling from existing national precursor services to pan-European scale is challenging. The EGMS will employ the most advanced persistent scatterer (PS) and distributed scatterer (DS) InSAR processing techniques in combination with a high-quality Global Navigation Satellite System (GNSS) model to calibrate the ground motion products. To foster as wide usage as possible, the EGMS will also provide tools for visualization, exploration, analysis and download of the ground deformation products, as well as elements to promote best practice applications and user uptake.

A recent publication claims that closure phases in SAR interferometry bear no relationship to physical changes of the scatterer, but only to the statistical properties of the averaged pixels. We disprove this claim with a simple counterexample and remind the reader of cases in which closure phases indicate a clear physical content, including the exploitation of closure phases in other fields.

More than 1,100 ESA Sentinel-1 SAR images acquired over Japan since November 2014 have been processed by means of the SqueeSAR™ algorithm, creating a national InSAR database all over the country. This interferometric dataset allowed the generation of an unprecedented synoptic view of the areas affected by displacement phenomena. Special attention was paid to the estimation of vertical and horizontal (east-west) displacement data, by properly combining InSAR data obtained from both ascending and descending satellite tracks. This SAR data processing all over Japan allowed the generation of an easy-to-use geodetic dataset, largely complementary to the data available via GPS-GNSS receivers. A first preliminary analysis of the results made it possible the detection (or just the confirmation) of a number of deformation phenomena associated to different natural or anthropogenic causes: from secular creeping of seismic faults, to subsidence induced by fluid extraction, from slow landslides to inflation of volcanic areas.

The availability of long time series of interferometric data acquired all over the world from several synthetic aperture radar (SAR) satellite missions makes possible to perform a worldwide analysis of ground surface deformations and infrastructure stability by SAR interferometry. When this technology is applied to large areas, several problems have to be faced to handle huge amounts of data. In this work, we present the first example in the world of persistent scatterer (PS) SAR interferometry analysis at national scale (the whole Italian territory), performed with ERS, Envisat and COSMO-SkyMed SAR data acquired from 1992 till 2014. Moreover, we discuss the complementarity of high resolution SAR systems like COSMO-SkyMed, with the new Sentinel-1 SAR satellite, which has lower resolution but larger swath. Based on these characteristics, we explore the possible worldwide extension of our national experience.

The availability of long time series of interferometric data acquired all over the world from several synthetic aperture radar (SAR) satellite missions makes possible to perform a worldwide assessment of the terrain and infrastructure stability by persistent scatterer (PS) SAR interferometry techniques. This technology is computationally demanding, in particular because it requires a 3D processing. When applied to large areas, several problems have to be faced to handle huge amounts of data. In this work, we present a significant example of PS big data processing performed at national scale (the whole Italian territory) with ERS, Envisat and COSMO-SkyMed data. The main challenges and results related to this project are discussed, and the possible worldwide extension with Sentinel data is suggested.

This study presents the spatial and temporal ground-deformation pattern of the Nisyros-Yali volcanic field after the period of the seismic crisis (1996–1998) based on the local GPS network remeasurement and the analysis of Interferometric data (2003–2010) applying the SqueeSARTMtechnique. The GPS results indicated intense subsidence in the northern and central parts of Nisyros that caused the western and eastern flanks of the island to “collapse” towards its center. The observed LOS velocity field of the SqueeSARTManalysis revealed a nearly linear type of ground deformation, exhibiting values ranging between −3 to +3mm/yr with small standard deviations. The interferometric results have also shown subsidence along the northern and central parts of the island, and an inherent eastward horizontal component at its southeastern part. The overall pattern of the observed subsidence in the area after 2000 is consistent with decrease of pressure in the associated magma chambers and hydrothermal system.

Permanent Scatterer SAR Interferometry (PSInSAR) aims to identify coherent radar targets exhibiting high phase stability over the entire observation time period. These targets often correspond to point-wise, man-made objects widely available over a city, but less present in non-urban areas. To overcome the limits of PSInSAR, analysis of interferometric data-stacks should aim at extracting geophysical parameters not only from point-wise deterministic objects (i.e., PS), but also from distributed scatterers (DS). Rather than developing hybrid processing chains where two or more algorithms are applied to the same data-stack, and results are then combined, in this paper we introduce a new approach, SqueeSAR, to jointly process PS and DS, taking into account their different statistical behavior. As it will be shown, PS and DS can be jointly processed without the need for significant changes to the traditional PSInSAR processing chain and without the need to unwrap hundreds of interferograms, provided that the coherence matrix associated with each DS is properly “squeezed” to provide a vector of optimum (wrapped) phase values. Results on real SAR data, acquired over an Alpine area, challenging for any InSAR analysis, confirm the effectiveness of this new approach.

We propose a calibration method suitable for a set of repeated synthetic aperture radar (SAR) acquisitions that uses both absolute calibrated devices (such as corner reflectors) and stable targets identified in the scene [the permanent scatterers (PSs)]. Precisely, the role of the PS is to extend the initial calibration sequence by monitoring the radiometric stability of the system throughout the whole mission life span. At a first step, this paper approaches the problem of PS-based normalization by an iterative maximum-likelihood method that exploits the stack of complex interferometric SAR images. Two solutions are given based on different assumptions on the PS phases. As a second step, the merging of these estimates with the available calibration information is discussed. Results achieved by experimental acquisitions are shown in two different SAR systems: 1) a C-band spaceborne SAR and 2) a Ku-band ground-based SAR.

This paper presents the results of a blind experiment that is performed using two pairs of dihedral reflectors. The aim of the experiment was to demonstrate that interferometric synthetic aperture radar (InSAR) measurements can indeed allow a displacement time series estimation with submillimeter accuracy (both in horizontal and vertical directions), provided that the data are properly processed and the impact ofin situas well as atmospheric effects is minimized. One pair of dihedral reflectors was moved a few millimeters between SAR acquisitions, in the vertical and east-west (EW) directions, and the ground truth was compared with the InSAR data. The experiment was designed to allow a multiplatform and multigeometry analysis, i.e., each reflector was carefully pointed in order to be visible in both Envisat and Radarsat acquisitions. Moreover, two pairs of reflectors were used to allow the combination of data gathered along ascending and descending orbits. The standard deviation of the error is 0.75 mm in the vertical direction and 0.58 mm in the horizontal (EW) direction. GPS data were also collected during this experiment in order to cross-check the SAR results

The relative low resolution (~25 m times 5 m on the ground) of spaceborne C-band synthetic aperture radar (SAR) data as acquired, for example, by European Space Agency sensors ERS and Envisat, can be significantly increased (up to submeter precisions) by processing coherently long series of images. Moreover, by analyzing the amplitude of the radar signal and by exploiting polarization diversity, the main radar characteristics of urban targets can be estimated, and a system for automatic recognition of a set of scattering structures can be developed. In this paper, we study the variation of the amplitude of the received radar signal as a function of the acquisition geometry [normal baseline and Doppler centroid (DC)] to retrieve the extension of the targets in range and azimuth. The dependence of the radar amplitude on temperature at the time of acquisition has been discovered to be very useful to identify extended resonating targets. Dihedrals are discriminated from specular or trihedral reflectors through the phase of Envisat alternating polarization (AP) acquisitions. By means of all gathered radar measurements, the bases for the development of a system for the automatic recognition of six main typologies of urban SAR targets (ground-level and elevated backscatterers, simple and resonating dihedrals, poles and trihedrals) have been laid. Radar data are then combined with in situ surveys and aerial photos, allowing a first assessment of the methodology in urban area.

The application of the Permanent Scatterers (PS) Technique in multi-temporal data-sets, namely the identification and exploitation of sparse coherent targets, has shown that it is possible to estimate and remove interferometric phase components due to atmospheric effects and orbital fringes. So far, the application of the PS technique has been focused on the extraction of the motion field of the area of interest. However, it is also known that PS relative elevations can be estimated with sub-meter precision while smooth errors can be removed using a coarse resolution DEM or the data of the Shuttle Radar Topography Mission (SRTM). In this paper, we describe a new approach combining the PS Technique and standard interferometry to improve the quality of lnSAR DEM's. ERS Tandem interferograms are exploited to increase the number of coherent pixels, while atmospheric effects are estimated and subtracted by means of the sparse PS grid. Prior information and PS elevation are used to reduce the probability of phase-unwrapping errors. Preliminary results are reported and the key-factors for its successful application (e.g. the number of Tandem acquisitions available, PS density) are discussed.

The phase time series of a perfect point-wise Permanent Scatterer (PS) would show a phase jump passing from ERS to ENVISAT SAR images. A PS analysis has been carried out on a dataset including both ERS and ENVISAT images. For each PS, elevation, LOS velocity and phase jump have been jointly estimated and removed, allowing the identification of the atmospheric phase screen. The PS population so found represents the intersection of ERS and ENVISAT PS populations. The probability of natural targets to behave as PS both in ERS and ENVISAT has been analyzed as a function of their Radar Cross-Section, acquisition geometry and amplitude. Distributed targets with a narrow backscattering lobe pointed towards the first sensor (ERS) are not coherently observed by the second one (ENVISAT), whereas point-wise targets (e.g. natural corner reflectors, dihedrals and small mirrors) will remain coherent in both cases. Experimental results have been carried out on a data set of 69 ERS images and 7 ENVISAT images (ERS-like mode) on Milano over a 20 km side area.

The aim of this paper is to highlight some important issues that should be taken into account when designing and planning a SAR mission dedicated to differential SAR interferometry (DInSAR) and Permanent Scatterers (PS) analysis. Special attention is paid to the impact of different design parameters on the information that can be retrieved, namely: (1) different wavelengths; (2) repeat cycle; (3) polarization modes; (4) platform stability; (5) state vectors accuracy. The impact of satellite dead band at different frequencies is discussed as well, taking into account DEM estimation issues and the problem of atmospheric effects. It is shown how very small baselines are not always the best design solution at least for PS analysis. PS results obtained processing 3 multi-temporal data sets acquired by JERS, ERS and Radarsat over Tokyo are presented and discussed

In this paper we wish to show a few exemplificative results obtained in the framework of a wide area Permanent Scatterers analysis financed by Regione Lombardia. The core purpose was the identification and description of slope instability/failure phenomena in a relevant part of the Lombardy alpine territory (Northern Italy).

The Permanent Scatterers (PS) technique is an advanced tool for processing series of interferometric SAR data aiming at millimetric precision ground deformation mapping. The approach is based on a joint time-space-acquisition geometry analysis that is carried out at individual point-wise radar targets. The aim of this paper is twofold: (1) describe the main issues related to the precision of PS products; (2) show preliminary PS results obtained using, instead of ESA-ERS scenes, data acquired by other spaceborne SAR platforms characterized different acquisition parameters (namely RADARSAT and JERS).

The permanent scatterers (PS) technique (Ferretti et al, 2001, 2000, Colesanti et al., 2003), is a powerful and fully operational tool for monitoring ground deformations on a high spatial density grid of point-wise targets, exploiting long series of SAR data. The most attractive aspects of this approach is the capability of providing measurements relative to individual radar targets with unprecedented precision. Up to now, PS analyses have been carried out on ERS, RADARSAT, and JERS data sets. The purpose of this presentation is to discuss the feasibility of updating results obtained by means of a PS analysis on ERS interferometric data using ENVISAT ASAR images. In particular, the main goal is to stitch coherently the new ENVISAT ASAR measurements to the already available ERS displacement time series relative to individual PS. To this end, we will model the interferometric phase of point-wise targets taking account the different ERS and ENVISAT carrier frequencies. Then, we identify the main constraints to be met at individual permanent scatterers in order to guarantee the feasibility of coherent stitching.

We develop and assess a new orbital tuning approach by applying differential synthetic aperture radar (SAR) interferometry. We use interferograms of the same site for estimating across-track and radial orbit adjustments from fringe gradients caused by orbit uncertainties. Our approach eliminates these fringes by using the improved short-arc orbit estimates. Taking six estimates from the Delft Institute for Earth-Orientated Space Research (DEOS), the approach yields mean standard deviations of 2.4 cm for the across-track and 4.5 cm for the radial components. The interferograms calculated with our post-fit orbital estimates compare favorably with those corrected with a conventional orbital tuning approach. We can now distinguish between orbital and deformation contributions to interferometric SAR phase gradients and are able to measure surface deformation changes over an interseismic time interval longer than one year. Our new approach is limited, however, to well-correlated interferograms where it is possible to measure the fringe gradient. We have also applied Permanent Scatterers (PS) technique to 42 SAR images acquired by ERS between August 1992 and June 1998. This approach estimates the average range change rate of more than 3 million PS with a formal standard deviation of 0.3 mm/yr along the line of sight. We obtain a phase coherence factor greater than 0.8 and a standard deviation of 3 mm for a single line of sight measurement. This velocity field is much easier to interpret than the separate interferometric pairs. Interpreting these interesting features in terms of geophysical models of inter-seismic and post-seismic deformation, however, will require further research effort.

The necessity to identify and monitor slope movement is of paramount importance to reduce the socio-economic toll that every year is paid in developing as well as in developed countries. From the late 80's Italian and Swiss legislations regarding hydro-geological risks, have identified landslide hazard maps as essential tools to mitigate the risk associated to landslides. Several projects funded by the European Space Agency within the framework of its Data User Program (DUP), have investigated the feasibility and the operational applicability of spaceborne imagery to respond to the needs of governmental institutions that have a mandate in landslide analysis and prevention. Techniques based on SAR interferometry and on the combination of state of the art Remote Sensing observations with CIS modeling have been analysed in order to assess the contribution of Satellite Remote Sensing information in support to the practices of the Geological Risk Services Agencies.

Spaceborne differential radar interferometry has proven a remarkable potential for mapping ground deformation phenomena (e.g., urban subsidence, volcano dynamics, coseismic and postseismic displacements along faults, as well as slope instability). However, a full operational capability has not been achieved yet due to atmospheric disturbances and phase decorrelation phenomena. These drawbacks can often be-at least partially-overcome by carrying out measurements on a subset of image pixels corresponding to natural or artificial stable reflectors [permanent scatterers (PS)] and exploiting long temporal series of interferometric data. This approach allows one to push the measurement precision very close to its theoretical limit (in the order of /spl sim/1 mm for C-band European Remote Sensing (ERS)-like sensors). In this paper, the detection of both time-uniform and seasonal deformation phenomena is addressed, and a first assessment of the precision achievable by means of the PS Technique is discussed. Results highlighting deformation phenomena occurring in two test sites in California are reported (Fremont in the Southern Bay Area and San Jose in the Santa Clara Valley).

In this paper we start addressing the main advantages and possible applications of carrying out permanent scatterers analyses involving more independent data sets covering the same test area (i.e. data acquired along parallel orbits relative to adjacent tracks as well as data gathered by ascending and descending passes). First interesting results are provided.

Temporal and geometrical decorrelation often prevents SAR interferometry from being an operational tool for surface deformation monitoring and topographic profile reconstruction. Moreover, atmospheric disturbances can strongly compromise the accuracy of the results. The authors present a complete procedure for the identification and exploitation of stable natural reflectors or permanent scatterers (PSs) starting from long temporal series of interferometric SAR images. When, as it often happens, the dimension of the PS is smaller than the resolution cell, the coherence is good even for interferograms with baselines larger than the decorrelation one, and all the available images of the ESA ERS data set can be successfully exploited. On these pixels, submeter DEM accuracy and millimetric terrain motion detection can be achieved, since atmospheric phase screen (APS) contributions can be estimated and removed. Examples are then shown of small motion measurements, DEM refinement, and APS estimation and removal in the case of a sliding area in Ancona, Italy. ERS data have been used.

Discrete and temporarily stable natural reflectors or permanent scatterers (PS) can be identified from long temporal series of interferometric SAR images even with baselines larger than the so-called critical baseline. This subset of image pixels can be exploited successfully for high accuracy differential measurements. The authors discuss the use of PS in urban areas, like Pomona, CA, showing subsidence and absidence effects. A new approach to the estimation of the atmospheric phase contributions, and the local displacement field is proposed based on simple statistical assumptions. New solutions are presented in order to cope with nonlinear motion of the targets.

Multibaseline synthetic aperture radar (SAR) interferometry can be exploited successfully for high-quality digital elevation model (DEM) reconstruction, provided that both noise and atmospheric effects are taken into account. A weighted combination of many uncorrelated topographic profiles strongly reduces the impact of phase artifacts on the final DEM. The key issue is weights selection. In the present article a wavelet domain approach is proposed. Taking advantage of the particular frequency trend of the atmospheric distortion, it is possible to estimate, directly from the data, noise and atmospheric distortion power for each interferogram. The available DEMs are then combined by means of a weighted average, carried out in the wavelet domain. This new approach provides a valuable tool not only for DEMs combination (improving accuracy), but for data evaluation and selection, since the phase error power is estimated for each interferogram. Results obtained using simulated and real data (ERS-1/2 TANDEM data of a test area around the Etna volcano, Sicily) are presented.