We apply a Bayesian Network (BN) paradigm to the problem of monitoring flood events through synthetic aperture radar (SAR) and interferometric SAR (InSAR) data. BNs are well-founded statistical tools which help formalizing the information coming from heterogeneous sources, such as remotely sensed images, LiDAR data, and topography. The approach is tested on the fluvial floodplains of the Basilicata region (southern Italy), which have been subject to recurrent flooding events in the last years. Results show maps efficiently representing the different scattering/coherence classes with high accuracy, and also allowing separating the multitemporal dimension of the data, where available. The BN approach proves thus helpful to gain insight into the complex phenomena related to floods, possibly also with respect to comparisons with modeling data.

We focus on the use of advanced multi-temporal interferometry (MTI) for mapping and monitoring of ground deformations caused by open-cast mining and hydrocarbon production. We also show how MTI can be exploited to monitor the stability of infrastructure in adjacent areas. Open-cast mines represent a good target for MTI, because they are (1) often very large (from few to tens of km(2)); (2) free of or covered by sparse vegetation; (3) require long-term regular monitoring. The operational deformation monitoring via MTI can now rely on free of charge medium-resolution Sentinel-1 data, consistently and regularly acquired by the European Space Agency (ESA) since 2014. To illustrate the application potential of MTI based on Sentinel-1 data, we present the case study of the Belchatow mine (Poland), one of the largest open-cast mines in Europe. We stress that thanks to wide-area coverage; space-borne MTI represents a cost-effective approach to monitoring ground/slope instability hazards in large open pits, as well as the stability of the associated engineering structures and facilities. On-land oil and gas fields are also often huge and ground deformations induced by their exploitations can be profitably targeted by MTI. This is illustrated through an example of MTI application from the Middle East that relies on high-resolution (3m) radar data. The example highlights the possibility of obtaining extremely dense (spatially continuous) information, which is important for monitoring complex ground deformations caused by oil field exploitation.

The local magnitude (ML) scale 6.22 earthquake struck eastern Taiwan on 6 February 2018. The epicenter was on the coastline near Hualien and caused a maximum shaking intensity of 7 in Hualien City, which was the most severely affected area. The crushed buildings and main surface cracks are distributed along the Milun fault, which is a oblique-slip fault. The earthquake is the largest of a sequence of events that affected the area over a period of days. In order to identify the surface deformation from Milun fault to coastal mountain range of Taiwan, we processed C-band Sentinel-1 and L-band ALOS-2 interferometric pairs acquired across the earthquake date, from both ascending and descending orbits. The Generic Mapping Tools Synthetic Aperture Radar (GMTSAR), and SNAPHU tools were used. According to the interferometric line-of-sight (LOS) displacement maps, the northwestern Milun fault moved of about 40 cm in the descending geometry, and about 0-10 cm in the ascending one, while the southwestern part of the fault moved 15-20 cm in the ascending geometry and is stable in the descending one. By combining interferometric LOS displacements derived from both ascending and descending acquisition geometries, just E-W and vertical displacement components can be derived. However, GPS and field survey results show northwards movements ranging from 50 and 70 cm over the Milun tableland, which is located on the eastern side of Milun fault. In order to provide SAR-based displacement measurements along north-south direction, we adopted the pixel-off set technique. The offsets along the azimuth direction were derived by computing the maximum of the amplitude cross-correlation of two SAR images, by using estimation windows of 128 x 128 pixel size. The 3D displacement was then computed by combining through least-square method the interferometric LOS displacements from ascending and descending acquisition geometry, and azimuth displacement component derived though pixel offset. Results indicate that, in the northern part of Milun tableland the maximum of E-W displacement reaches 60 cm, and that the whole Milun tableland moves northwards (up to about 60 cm) and upwards (up to about 40 cm). Moreover, the western side of Milun fault shows westwards movement, while Hualien city is affected by subsidence. The correlation coefficients in three directions between GPS data and 3D displacement are 0.81 (E-W), 0.89 (N-S), and 0.91 (U-D).

With the increasing quantity and quality of the imagery available from a growing number of SAR satellites and the improved processing algorithms, multi-temporal interferometry (MTI) is expected to be commonly applied in landslide studies. MTI can now provide long-term (years), regular (weekly-monthly), precise (mm) measurements of ground displacements over large areas (thousands of km(2)), at medium (similar to 20 m) to high (up to 1-3 m) spatial resolutions, combined with the possibility of multi-scale (regional to local) investigations, using the same series of radar images. We focus on the benefits as well as challenges of multi-sensor and multi-scale investigations by discussing MTI results regarding two landslide prone regions with distinctly different topographic, climatic and vegetation conditions (mountains in Central Albania and Southern Gansu, China), for which C-band (ERS or ENVISAT) and X-band COSMO-SkyMed (CSK) imagery was available (all in Stripmap descending mode). In both cases X-band MTI outperformed C-band MTI by providing more valuable information for the regional to local scale detection of slope deformations and landslide hazard assessment. This is related to the better spatial-temporal resolutions and more suitable incidence angles (40 degrees-30 degrees versus 23 degrees) of CSK data While the use of medium resolution imagery may be appropriate and more cost-effective in reconnaissance or regional scale investigations, high resolution data could be preferentially exploited when focusing on urbanized landslides or potentially unstable slopes in urban/peri-urban areas, and slopes traversed by lifelines and other engineering structures.

Classical applications of the MTInSAR techniques have been carried out in the past on medium resolution data acquired by the ERS, Envisat (ENV) and Radarsat sensors. The new generation of high-resolution X-Band SAR sensors, such as TerraSAR-X (TSX) and the COSMO-SkyMed (CSK) constellation allows acquiring data with spatial resolution reaching metric/submetric values. Thanks to the finer spatial resolution with respect to C-band data, X-band InSAR applications result very promising for monitoring single man-made structures (buildings, bridges, railways and highways), as well as landslides. This is particularly relevant where C-band data show low density of coherent scatterers. Moreover, thanks again to the higher resolution, it is possible to infer reliable estimates of the displacement rates with a number of SAR scenes significantly lower than in C-band within the same time span or by using more images acquired in a narrower time span. We present examples of the application of a Persistent Scatterers Interferometry technique, namely the SPINUA algorithm, to data acquired by ENV, TSX and CSK on selected number of sites. Different cases are considered concerning monitoring of both instable slopes and infrastructure. Results are compared and commented with particular attention paid to the advantages provided by the new generation of X-band high resolution space-borne SAR sensors.

In the Northern part of the Apulia Region, between the Lesina Lagoon and the mouth of the Fortore River, isplaced the Pietre Nere (black stones) Point, a very interesting site and the only outcrop of magmatic rocks in thePuglia region and in the whole Adriatic coast of Italy. The excavation of a canal through this area exposed greymicro- and meso-crystalline gypsum with intercalations of black limestones and marls of Upper Triassic age,mantled by loose sandy Quaternary deposits.The gypsum bedrock shows a high density of cavities, either dissolutional conduits or voids related to gravitationalcollapse processes. Starting from about 1970, a wide new touristic settlement has been built over the area, whichin turn, starting from about 1990, began to suffer for the increasing formation of sinkholes.Several interpretations have been proposed to explain the outcrops of Triassic evaporites that occur in the PietreNere Point area, previously buried by a sequence several kilometres thick of Mesozoic rocks: diapirism; pushingupwards by compressional tectonics; both halokinesis and tectonic deformation.The presence of the sinkholes and of the karst reactivation leads to the demand of a monitoring system, for thedetection of vertical displacements over a large area.Thanks to the ability of radar systems to operate in all weather conditions, day or night, and the possibilityof accurately measuring small surface deformations (changes in altitude of a few millimeters), SAR (syntheticaperture radar) differential interferometry (DInSAR) is an ideal technique for detecting and monitoring grounddeformation phenomena (subsidence, faults, landslides, etc.) over vast areas. As an evolution of DInSAR,persistent scatterers interferometry (PSI) [1] allows to follow millimetric movements of stable objects (mainlybuilding and man-made features) present on the Earth surface through time, studying the interferometric responseof such objects along series of SAR acquisitions.We used data from the ASAR sensor onboard the European Space Agency's ENVISAT satellite, from bothascending (34 acquisitions) and descending (28 acquisitions) geometry, covering a total time interval from May2003 to December 2009.Data were processed with a combination of open-source and in-house developed software [2] in order to extractrelevant information about mean velocities of stable points located on the Lesina Marina area.Relying on the relatively smooth nature of the investigated phenomenon, and assuming negligible north-southmovements, as justified by the overall geometry of the site geomorphological units, information coming fromascending and descending geometries was interpolated over a common georeferenced grid, then combined toobtain vertical and horizontal (east-west) velocity components.We observe PS objects undergoing uplift displacements in both ascending and descending data, with uplift ratesdecreasing in a roughly W-SW direction

The shorter repeat cycle (6 days since October 2016) and regularity of acquisitions of Sentinel-1A/B with respectto earlier European Space Agency (ESA) satellites with C-band sensors (ERS1/2, ENVISAT) represent the keyadvantages for the research-oriented and practical applications of multi-temporal interferometry (MTI). Theapplicability of the Interferometric Wide Swath acquisition mode of Sentinel-1 (images covering a 250 kmswath on the ground) to regional scale slope instability detection through MTI has already been demonstrated,e.g., via studies of landslide-prone areas in Italy. Here we focus on the potential of Sentinel-1 data for local(site-specific), MTI-based monitoring and capturing pre-failure signs of slope instability, by exploiting thePersistent and Distributed Scatterers processing capability of the SPINUA algorithm. In particular, we presentan example of a retrospective study of a large (over 2 km long) landslide, which took place in 2016 in an activeopen-cast coal mine in central Europe. This seemingly sudden failure caused destruction of the mining equipment,blocked the mining operations thereby resulting in significant economic losses. For the study, we exploited over60 Sentinel-1A/B images acquired since November 2014. The MTI results furnished a valuable overview of theground instability/stability conditions within and around the active mine, even though considerable spatial gaps ininformation were encountered due to surface disturbance by mining operations. Significantly, the ground surfacedisplacement time series revealed that the 2016 slope failure was preceded by very slow (generally 1-3 cm/yr)creep-like deformations, already present in 2014. The MTI results also indicated that the slope experienced a phaseof accelerated movement several weeks prior to the landslide event. Furthermore, the spatio-temporal analysis ofinterferometric coherence changes in the unstable area (mapped on Sentinel-2 Bottom Of Atmosphere reflectanceimages processed by using the ESA Sen2Cor processor), indicated a sharp coherence loss in the last few weeksbefore the slope collapse. The availability of more frequent measurements represents a key improvement forMTI-based ground surface displacement monitoring and this will better support research on slope destabilizationprocesses over time and, ultimately, on slope failure forecasting.

Multi-temporal InSAR (MTI) applications pose challenges related to the availability of coherent scattering from the ground surface, the complexity of the ground deformations, the atmospheric artifacts, the visibility problems related to the ground elevation. Nowadays, several satellite missions are available providing interferometric SAR data at different wavelengths, spatial resolutions, and revisit time. A new interesting opportunity is provided by Sentinel-1 mission, which has a spatial resolution comparable to previous ESA C-band missions, and revisit times reduced to up to 6 days. It is envisioned that, by offering regular, global-scale coverage, improved temporal resolution and freely available imagery, Sentinel-1 will guarantee an increasing use of MTI for ground displacement investigations. According to these different SAR space-borne missions, the present work discusses current and future opportunities of MTI applications to ground instability monitoring. Issues related to coherent target detection and mean velocity precision will be addressed through a simple theoretical model assuming backscattering mechanisms related to point scatterers. The paper also presents an example of multi-sensor ground instability investigation over the site of Marina di Lesina, Southern Italy, a village lying over a gypsum diapir, where a hydration process, involving the underlying anhydride, causes a smooth uplift pattern affecting the entire village area, and the formation of scattered sinkholes. More than 20 years of MTI SAR data have been used, coming from both legacy ERS and ENVISAT missions, and last-generation Radarsat-2, COSMO-SkyMed, and Sentinel-1A sensors

Space-borne SAR Differential Interferometry (DInSAR) techniques are attractive for landslide investigations because of their capability to provide regional scale coverage and, under favourable conditions, spatially dense information on small ground surface deformations. In particular, advanced multi-temporal InSAR techniques such as Persistent Scatterer Interferometry (PSI) allow detecting and monitoring, with millimetre precision, displacements occurring on selected radar targets (PS) exhibiting coherent radar backscattering properties. PS targets correspond mainly to man-made structures or to rock outcrops, and their spatial density depends on the ground coverage, and it is maximum over urban areas. The application of multi-temporal InSAR analysis to slope instability monitoring poses challenges related to the complex kinematics of the phenomenon, as well as to the unfavourable settings of the area affected by landslides, often occurring on sites of limited extension, characterized by steep topography and variable vegetation cover. This is the case of the Daunia region, located in the Southern Italian Apennine Mountains, which is characterised by scarce urbanisation (mainly small hill-top towns) and dense vegetation cover. The SPINUA (Stable Point INterferometry over Un-urbanised Areas) PSI multi-temporal processing technique was used in the past years to detect and measure ground displacements over this region. Both C-band medium resolution SAR data from ERS-1/2 and ENVISAT ESA satellites, and X-band high resolution SAR data from the TerraSAR-X (TSX) satellite were used. Results indicate that PSI can be profitably used to investigate slope instability, mainly over the urban and peri-urban areas, and that, on these sites, TSX data result very promising for monitoring areas where ERS/ENVISAT PS density is too low. Nevertheless, the application of PSI for slope instability monitoring still remain problematic or impossible in rural and mountainous areas. This is the case, for instance, of the Municipality of Carlantino, where PS targets detected by both C- and X-band data correspond to urban structures or peri-urban walls and guard rails, while a large landslide, extending for about 2 km from the hilltop down to the valley, is lacking stable coherent targets, due to the vegetation cover. In order to allow stability monitoring through spaceborne SAR interferometry, a network of passive reflectors was designed and deployed on the area of interest. The Corner Reflectors (CR) were designed for TerraSAR-X stripmap acquisitions, and consist of three triangular metal panels welded perpendicular to each others to form a trihedral shape which ensures that the radar signal is scattered back to the sensor. A small size is preferred to minimize the curvature of the side panels, the effect of wind, the exposition to vandalism, and to allow easier transportation and deployment in the harsh terrain setting. To design the CR network, different factors were taken

The application of Persistent Scatterer Interferometry (PSI) to slope instability monitoring poses challenges related to the complex kinematics of the phenomenon, as well as to the unfavourable settings of the area affected by landslides, often occurring on sites of limited extension, characterized by steep topography and variable vegetation cover. New-generation SAR sensors, such as TerraSAR-X (TSX) thanks to their higher spatial resolution, make PSI applications very promising for monitoring areas with low density man-made. Nevertheless, the application of techniques still remains problematic or impossible in rural and mountainous areas. This is the case, for instance, for the Municipality of Carlantino, in Southern Italy. Both C-band medium resolution SAR data from ESA satellites, and X-band high resolution SAR data from the TSX satellite, were processed through the PSI algorithm SPINUA. Despite the higher spatial density of PS from TSX, the landslide body is lacking coherent targets, due to vegetation and variable land cover. To allow stability monitoring, a network of six CRs was designed and deployed over the landslide test site. Twenty-six TSX stripmap images were processed by using both PSI and an ad hoc procedure based on double-difference analysis of DInSAR phase values on the CR pixels, constrained by the accurate CR height measurements provided by DGPS. Despite the residual noise due to the sub-optimal CR network and the strong atmospheric signal, displacement estimation on the CRs allows to propagate the PSI results downslope, proving the stability of the landslide area subjected to consolidation works.

On March 11th, 2011, 05:46 UTC, a giant earthquake of magnitude Mw=9.0 occurred off the Pacific coast of Tohoku, Honshu Island, Japan. Massive damage has been reported, mainly related to the subsequent tsunami. In this work we present first results concerning the displacement induced by the Tohoku-oki earthquake by using ASAR ENVISAT data acquired few days before and after the event. The work points out the reliability of ASAR ENVISAT data to provide interferometric-based displacement field even though the satellite is recently came into a new operative phase with degraded orbital control and SAR data were processed in an emergency framework soon after the acquisition without precise orbital records. After removing artefacts due to strong orbital errors, the interferometric deformation pattern is mainly related to the coseismic displacement and it results comparable to that provided by the GEONET GPS network which has an unique density of in situ measurements found elsewhere in the world.

Indonesia is periodically affected by severe volcanic eruptions and earthquakes, which are geologically coupled to the convergence of the Australian tectonic plate beneath the Sunda Plate. Multi-temporal SAR interferometry (MTI) can be used to support studying and modelling of terrain movements. This work is aimed at performing an analysis of ground displacements over Indonesian sites through MTI techniques. Two test sites in Sumatra and Java have been selected according to the availability of archived SAR data, GNSS networks, and geological data. Both COSMO-SkyMed (CSK) and Senitnel-1 data-sets have been processed through MTI algorithms. The derived displacement maps have been interpreted according to the available geological and geophysical information.

Thanks to the technological maturity as well as to the wide availability of SAR data, Multi-temporal SAR Interferometry (MTInSAR) can be used to support systems devoted to environmental monitoring and risk management. In particular, high resolution X-band MTInSAR applications are also suitable for monitoring single man-made structures (buildings, bridges, railways and highways). The paper presents examples concerning the application of MTInSAR techniques and COSMO-SkyMed constellation for instability monitoring of infrastructures and, in particular, harbor docks and railways.

We present a case study of a long-term integrated monitoring of a flood event which affected part of the Strymonas dammed river basin, a transboundary river with source in Bulgaria, which flows then through Greece to the Aegean Sea. The event, which affected the floodplain downstream the Kerkini dam, started at the beginning of April 2015, due to heavy rain upstream of the monitored area, and lasted for several months, with some water pools still present at the beginning ofSeptember, due to the peculiar geomorphological conditions of the watershed. We collected a multi-temporal dataset consisting of a high-resolution, X-band COSMO- SkyMed, and several C-band Sentinel-1 SAR and optical Landsat-8 images ofthe area. The results allow following the event in time, sketching amulti-temporal map ofthe post-flood evolution, with relatively high temporal res- olution. We then use hydrological modeling to mimic the dynamics of the flooded area against post event weather patterns and thus explain the observed flood extent evolution. We show how integrating remote sensing-derived maps offlooded areas, geomorphological analyses of the landscape and simplified hydrological modeling allows accurate inference about long-termdynamics offlooded areas, very important in the post event in anthropogenic highlymodified areas, where recovery time after the flood event is considerable, and long term water persistence may lead to large consequences, carrying economic damages and medical emergencies.

In the present work we describe a new and alternative repeat-pass interferometry algorithm designed and developed with the aim to: i) ncrease the robustness wrt to noise by increasing the number of differential interferograms and consequently the information redundancy; ii) guarantee high performances in the detection of non linear deformation without the need of specifying in input a particular cinematic model.The starting point is a previous paper dedicated to the optimization of the InSAR coregistration by finding an ad hoc path between the images which minimizes the expected total decorrelation as in the SABS-like approaches. The main difference wrt the PS-like algorithms is the use of couples of images which potentially can show high spatial coherence and, which are neglected by the standard PSI processing. The present work presents a detailed description of the algorithm processing steps as well as the results obtained by processing simulated InSAR data with the aim to evaluate the algorithm performances. Moreover, the algorithm has been also applied on a real test case in Poland, to study the subsidence affecting the Wieliczka Salt Mine. A cross validation wrt SPINUA PSI-like algorithm has been carried out by comparing theresultant displacement fields.

Monitoring represents the main tool for carrying out evaluation procedures and criteria for spatial and temporallandslide forecast. The forecast of landslide behaviour depends on the possibility to identify either evidences ofactivity (displacement, velocity, volume of unstable mass, direction of displacement, and their temporal variation)or triggering parameters (rainfalls).Generally, traditional geotechnical landslide monitoring technologies permit to define, if correctly positionedand with adequate accuracy, the critical value of displacement and/or acceleration into landslide body. Inmost cases, they do not allow real time warning signs to be generated, due to environmental induced errors, andthe information is related to few points on unstable area. Remote-sensing monitoring instruments are capableof inspecting an unstable slope with high spatial and temporal frequency, but allow solely measurements ofsuperficial displacements and deformations.Among these latest technologies, the satellite Persistent Scatterer SAR Interferometry (PSInSAR) is veryuseful to investigate the unstable area both in terms of space and time. Indeed, this technique allows to analysewide areas, individuate critical unstable areas, not identifiable by means detailed in situ surveys, and study thephenomenon evolution in a long time-scale.Although this technique usually adopts, as first approximation, a linear model to describe the displacementof the detected targets, also non-linear models can be used. However, the satellite revisit time, which defines thetime sampling of the detected displacement signal, limits the maximum measurable velocity and acceleration.This makes it difficult to assess in the short time any acceleration indicating a loss of equilibrium and,therefore, a probable reactivation of the landslide.The recent Sentinel-1 mission from the European Space Agency (ESA), provides a spatial resolution comparableto the previous ESA missions, but a nominal revisit time reduced to 6 days. By offering regularglobal-scale coverage, better temporal resolution and freely available imagery, Sentinel-1 improves the performanceof PSInSAR for ground displacement investigations.In particular, the short revisit time allows a better time series analysis by improving the temporal samplingand the chances to catch pre-failure signals characterised by high rate and non-linear behaviour signals. Moreover,it allows collecting large data stacks in a short time period, thus improving the PSInSAR performance inemergency (post-event) scenarios.In the present work, we propose to match satellite data with numerical analysis techniques appropriate toevidence unsteady kinematics and, thanks to the high resolution of satellite data and improved temporal sampling,to detect early stages of land instability phenomena.The test area is situated in a small town in the Southern Apennine, Basilicata region, affected by old andnew huge landslides, now close to a live

The recent availability of large amounts of remotely sensed data requires setting up efficient paradigms for the extraction of information from long series of multi-temporal, often multi-sensor, datasets. In this field, monitoring of terrain instabilities is currently performed through algorithms which estimate millimetric displacements of stable (coherent) objects, through analysis of stacks of SAR images acquired in interferometric mode. The result is generally a decomposition of at least part of the complete complex covariance matrix obtained from all possible pairwise combinations of the images in the stack, separating its spatially- and temporally-correlated parts.The same SAR temporal data stacks can be used to apply change detection algorithms, to reveal, over potentially huge spatial scales and with high resolution, terrain surface changes due to e.g. environmental hazards (floods, fires, earthquakes). In this case, again, the temporal covariance matrix contains in practice all the information related to the environmental changes.The covariance matrix, or its normalized version, known as coherence matrix, expresses thus all the information content related to a time series of remotely sensed, coherent data. In the case of SAR data, this kind of representation offers a unified framework for the study of phenomena linked either to the presence of "periods" of persistent scattering characteristics, or to changes of backscattering patterns, hinting to variations in the terrain characteristics.The average operation, involved in the definition of the above-mentioned covariance and coherence matrices, has to be performed necessarily over "homogeneous" pixel sets. This homogeneity criterion can be intended in various ways, including the one connected to the covariance definition itself, thus leading to a sort of recursive estimation process.Moreover, such homogeneity measures are often used as a substitute for the classical Euclidean distance in nonlocal estimate implementation frameworks, used for instance in the design of effective SAR speckle filters.The coherence matrix highlights the role of the interferometric phase. After having suitably modeled various phase contributions, due to topography, atmosphere, etc., it is possible to detect periods in which a target remains stable, and can thus be used as a benchmark for estimating ground deformations or other effects related to the variations of the signal optical path.From the above discussion, it appears that a thorough, physically based modeling of the coherence over such long times series of SAR data constitutes a priority for efficient data exploitation.We illustrate some of the inference which can be made starting from a time series of more than a hundred COSMO-SkyMed (CSK) images acquired in InSAR mode over the Haiti capital of Port-Au-Prince, spanning a period of almost 3 years with short repeat times. Such tight acquisition schedule can be obtained nowadays with latest-generation

In this work we explored a dataset made by more than 100 images acquired by COSMO-SkyMed (CSK) constellation over the Port-au-Prince (Haiti) metropolitan and surrounding areas that were severely hit by the January 12th, 2010 earthquake. The images were acquired along ascending pass by all the four sensors of the constellation with a mean rate of 1 acquisition/week. This consistent CSK dataset was fully exploited by using the Persistent Scatterer Interferometry algorithm SPINUA with the aim of: i) providing a displacement map of the area; ii) assessing the use of CSK and PSI for ground elevation measurements; iii) exploring the CSK satellite orbital tube in terms of both precision and size. In particular, significant subsidence phenomena were detected affecting river deltas and coastal areas of the Port-au-Prince and Carrefour region, as well as very slow slope movements and local ground instabilities. Ground elevation was also measured on PS targets with resolution of 3m. The density of these measurable targets depends on the ground coverage, and reaches values higher than 4000 PS/km2 over urban areas, while it drops over vegetated areas or along slopes affected by layover and shadow. Heights values were compared with LIDAR data at 1m of resolution collected soon after the 2010 earthquake. Furthermore, by using geocoding procedures and the precise LIDAR data as reference, the orbital errors affecting CSK records were investigated. The results are in line with other recent studies.

This study explores the potential of Synthetic Aperture Radar (SAR) to aid Unmanned Aerial Vehicle (UAV) navigation when Inertial Navigation System (INS) measurements are not accurate enough to eliminate drifts from a planned trajectory. This problem can affect medium-altitude long-endurance (MALE) UAV class, which permits heavy and wide payloads (as required by SAR) and flights for thousands of kilometres accumulating large drifts. The basic idea is to infer position and attitude of an aerial platform by inspecting both amplitude and phase of SAR images acquired onboard. For the amplitude-based approach, the system navigation corrections are obtained by matching the actual coordinates of ground landmarks with those automatically extracted from the SAR image. When the use of SAR amplitude is unfeasible, the phase content can be exploited through SAR interferometry by using a reference Digital Terrain Model (DTM). A feasibility analysis was carried out to derive system requirements by exploring both radiometric and geometric parameters of the acquisition setting. We showed that MALE UAV, specific commercial navigation sensors and SAR systems, typical landmark position accuracy and classes, and available DTMs lead to estimated UAV coordinates with errors bounded within ±12 m, thus making feasible the proposed SAR-based backup system.

The Multi-Chromatic Analysis (MCA) uses interferometric pairs of SAR images processed at range sub-bands and explores the phase trend of each pixel as a function of the different central carrier frequencies. The MCA technique introduces the concept of targets exhibiting stable radar returns across the frequency domain (PSfd). In this work we compare this stability along frequencies with the temporal stability which is at the base of persistent scatterers interferometry (PSI) techniques. Different populations of PSfd and "temporal" PS were derived by using COSMOSkyMed SAR data. An ad hoc processing scheme was developed to derive PSI products by processing the same range sub-bandwidth used by the MCA in order to guarantee the same scattering conditions. The populations of PSfd and "temporal" PS were compared and preliminary considerations provided concerning the scattering properties of the targets selected by the two criteria.

This paper reports on the application of radar satellite data and Persistent Scatterer Inter- ferometry (PS-InSAR) techniques for the detection of ground deformation in the semi-arid loess region of Lanzhou, northwestern China. Compared with Synthetic Aperture Radar Interferometry (InSAR), PS- InSAR overcomes the problems of temporal and geometric de-correlation and atmospheric heteroge- neities by identifying persistent radar targets (PS) in a series of interferograms. The SPINUA algorithm was used to process 40 ENVISAT ASAR images for the study period 2003-2010. The analysis resulted in the identification of over 140000 PS in the greater Lanzhou area covering some 300 km2. The spatial distribution of moving radar targets was checked during a field campaign and highlights the range of ground instability problems that the Lanzhou area faces as urban expansion continues to accelerate. The PS-InSAR application detected ground deformations with rates up to 10mma-1; it resulted in the detection of previously unknown unstable slopes and two areas of subsidence.

The paper presents results of SPINUA (Stable Point Interferometry overUnurbanised Areas) Persistent Scatterers Interferometry (PSI) processing chain to studyEarth surface deformations along the SW coast of the Gulf of Gdañsk, along the SE part ofthe Baltic Sea. As the input for SPINUA techniques 40 descending ERS-1/2 SLC (Frame =251, Track = 36) images from the period 1995-2001 has been used. The area of interest(AOI) includes few cities and several towns, villages and harbors. The low lying coastalareas of the SW part of the Gulf of Gdañsk are at risk of floods and marine erosion. The PSIresults, however, did not reveal the presence of a regional scale, spatially consistent patternof displacements. It is likely that any crustal deformations in the AOI simply do not exceed ±2mm/year, which is the velocity threshold we assumed to distinguish between moving and non-moving persistent scatterers (PS). Importantly,for the most part the urban areas of the main cities (Gdañsk, Gdynia and Sopot) results show ground stability. Nevertheless, significantdownward movements up to several mm/year, are locally noticed in the Vistula river delta - alluvial plain system located in thecoastal zone east of Gdañsk as well as in the inland area west of the Gdañsk city. Indeed, the highest subsidence rates (-12 mm/year)was observed in the Gdañsk petroleum refinery constructed on alluvial sediments. Thus the anthropogenic loading and consolidationof the recent deposits can locally be an important factor causing ground subsidence.

he new multi-temporal interferometry (MTI) techniques can be profitably used to study land instability hazards. The potential appears great, but the MTI-derived results are yet to be fully explored, especially those based on high spatio-temporal resolution data. Here we focus on the use of MTI for mapping and monitoring of slope and ground instabilities in open-cast mines. They represent a good target for MTI, because they are i) often very large (from few to tens of km2); ii) free of or covered by sparse vegetation; iii) require long-term (years-decades) regular monitoring. Furthermore, given the often large extent of areas affected by surface mining and life span of mines (tens of years), long-term monitoring via traditional in-situ methods can be impractical (economically and technically). However, a review of the recent literature suggests that in comparison to applications to underground mines, MTI has been relatively little exploited to investigate ground instabilities related to surface mining. One reason for this is that some portions of open-cast mines can lack measurable radar targets due to rapid changes of ground surface caused by mining operations (e.g., overburden stripping, waste material damping). We argue that this limitation can now be (in part) mitigated by the higher frequency and regularity of acquisitions provided by the European Space Agency Sentinel-1 (S-1) mission (nominally every 6 days since Oct 2016). Indeed, S-1 medium resolution C-band SAR data acquired at 6 days imply improved time coherence with respect to previous C-band missions, leading to better coverage. We also argue that the initial reconnaissance approaches using S-1 data can be suitably integrated with high spatial resolution MTI (based on e.g., COSMO-SkeyMed or TerraSAR-X imagery) to provide most valuable information for the spatial and temporal analyses of slope deformation and a sound basis for derived products ranging from early slope instability hazard identification to individual landslide monitoring. To illustrate the potential and limitations of MTI for detecting and monitoring ground instabilities induced by surface mining, we present case study examples from two large coal open-cast operations in Europe.

With the increasing number of radar satellites and improved data processing tools, multi-temporal interferometry (MTI) can considerably enhance our capabilities of monitoring landslide and subsidence hazards. MTI provides long-term (years), regular (weekly-monthly), precise (mm) measurements of ground displacements over large areas (thousands of km2), combined with high spatial resolution (up to 1-3 m) and possibility of multi-scale (regional to site-specific) investigations using the same series of radar images. To highlight the great potential of high resolution MTI we discuss application examples from two seismically active regions prone to land instability: i) Albania, including the large plain area occupied by the city of Tirana and nearby scarcely populated mountains, and ii) Haiti, including the Port-au-Prince metropolitan area, with coastal and mountain zones destabilized by the 2010 Mw 7.0 earthquake. It is shown that MTI can provide very useful results in a wide range of geomorphic, climatic and vegetation environments.

Among a number of advanced satellite-based remote sensing techniques, synthetic aperture radar (SAR), multi-temporal interferometry (MTI) appears the most promising for fostering new research opportunities in landslide and subsidence hazards detection and assessment. MTI is attractive to those concerned with terrain instability hazards because it can provide very precise quantitative information on slow displacements of the ground surface over huge areas with limited vegetation cover. Although MTI is a mature technique, we are only beginning to realize the benefits of the high-resolution imagery that is currently acquired by the new generation radar satellites (e.g. COSMO-SkyMed, TerraSAR-X). In this work we demonstrate the great potential of high resolution MTI for regular, wide-area detection of ground instability hazards by presenting results from two regions characterized by different geomorphic, climatic and vegetation conditions: densely populated metropolitan area of Port-au-Prince (Haiti), with the coastal areas and local slopes destabilized by the 2010 Mw 7.0 earthquake, and the remote high mountain region of Southern Gansu Province (China) prone to large slope failures. The interpretation and widespread exploitation of high spatio-temporal resolution MTI data can be facilitated by web-based applications (e.g. Google EarthTM tools with the associated high resolution optical imagery).

Advanced remote sensing techniques are now capable of delivering more rapidly high quality information that is sufficiently detailed (and cost-effective) for many engineering applications. Here we focus on synthetic aperture radar (SAR), multi-temporal interferometry (MTI). With radar satellites periodically re-visiting the same area, MTI provides information on distance changes between the on-board radar sensor and the targets on the ground (e.g., human-made structures such as buildings, roads and other infrastructure). The detected distance changes are thus interpreted as evidence of ground and/or structure instability. In settings with limited vegetation cover, MTI can deliver very precise (mm resolution), spatially dense information (from hundreds to thousands measurement points/km 2) on slow (mm-cm/year) deformations affecting the ground and engineering structures. Radar satellites offer wide-area coverage (thousands km 2) and, with the sensors that actively emit electromagnetic radiation and thus can "see'' through the clouds, one can obtain deformation measurements even under bad weather conditions. We illustrate the potential of high resolution MTI and explain what this technique can deliver to assist in infrastructure instability hazard assessment. This is done by presenting selected examples of MTI applied to monitor post-construction behavior of engineering structures. The examples are from Italy and include: an earthfill dam, an off-shore vertical breakwater built to protect an oil terminal, city buildings and a highway. We also stress that the current approach to the assessment of instability hazard can be transformed by capitalizing more on the presently underexploited advantage of the MTI technique, i.e., the capability to provide regularly spatially dense quantitative information for large areas where engineering infrastructure may currently be unaffected by instability, but where the terrain and infrastructure history (e.g., aging) may indicate potential for future failures.

Image alignment is a crucial step in synthetic aperture radar (SAR) interferometry. Interferogram formation requires images to be coregistered with an accuracy of better than a few tenths of a resolution cell to avoid significant loss of phase coherence. In conventional interferometric precise coregistration methods for full-resolution SAR data, a 2-D polynomial of low degree is usually chosen as warp function, and the polynomial parameters are estimated through least squares fit from the shifts measured on image windows. In case of rough topography or long baselines, the polynomial approximation may become inaccurate, leading to local misregistrations. These effects increase with spatial resolution of the sensor. An improved elevation-assisted image-coregistration procedure can be adopted to provide better prediction of the offset vectors. This approach computes pixel by pixel the correspondence between master and slave acquisitions by using the orbital data and a reference digital elevation model (DEM). This paper aims to assess the performance of this procedure w.r.t. the standard one based on polynomial approximation. Analytical relationships and simulations are used to evaluate the improvement of the DEM-assisted procedure w.r.t. the polynomial approximation as well as the impact of the finite vertical accuracy of the DEM on the final coregistration precision for different resolutions and baselines. The two approaches are then evaluated experimentally by processing high-resolution SAR data provided by the COnstellation of small Satellites for the Mediterranean basin Observation (COSMO/SkyMed) and TerraSAR-X missions, acquired over mountainous areas in Italy and Tanzania, respectively. Residual-range pixel offsets and interferometric coherence are used as quality figure. © 2006 IEEE.

Many applications of synthetic aperture radar differential interferometry (DInSAR) lead to a set of sparse phase measurements, e.g. in the processing of long multitemporal stacks of SAR differential interferograms through persistent scatterers interferometry (PSI) techniques. Often, sparse phase data have to be unwrapped, and then interpolated on a regular grid to be useful for subsequent processing steps. This step is necessary for instance in the reconstruction of the so-called APS (Atmospheric Phase Screen). Atmospheric artifacts superimposed on DInSAR measurements have the potential of hindering the accurate estimation of deformation signals. Indeed, sometimes the spatial frequencies of the atmospheric phase contributions can overlap those of deformation signals, so that such artifacts can be misinterpreted as deformation features.For the phase unwrapping stage, the solutions are directly dependent on the PS network density; moreover, phase aliasing, which appears when the signal sampling does not satisfy the Nyquist condition, especially in presence of noise, increases when passing from regular-grid to sparse data. This is because the phase sampling conditions get usually worse.An improvement of the APS estimation step has been proposed, by investigating from the empirical point of view an alternative procedure, which involves an interpolation of the complex field derived from the sparse phase measurements. Unlike traditional approaches, the proposed method allows to bypass the PU step and obtain a regular-grid complex field, from which a wrapped phase field can be extracted. Under general conditions, this smooth phase field can be shown to be a good approximation of the original phase without noise. Moreover, the interpolated, wrapped phase field can be fed to state of the art, regular grid PU algorithms, to obtain a smoother absolute phase field.The performances of this empirical approach are evaluated here over a real dataset, that is composed by 30 ascending SAR X-band COSMO-SkyMed images. The images cover the urban area and outskirts of the capital of Haiti, Port-au-Prince.The accuracy of the reconstructed phase fields is analyzed by the local value of the final inter-image phase coherence (?int), a quality figure related to the residual phase noise after subtraction of all modeled contributions. Its values are taken on points (PS) not used in the interpolation, using different spatial densities and random subsampling patterns in a test area characterized by a strong subsidence bowl.The obtained results may be applied into a broader context than the one specific to the PSI technique, considering the few assumptions on the initial phase field, i.e. its smoothness and good sampling conditions.

A relevant part of the Assisi urban area (central Italy), built up after 1950 and located outside of the ancient town center, is interested by a landslide characterized by a slow rate of movement, still causing important damages for an accumulation effect in time.For the monitoring of the landslide behaviour, the determination of the motion field and its evolution in time, a precision GNSS network has been established over the area since 1995, connecting by means of a baseline network the moving area with stable geologic formations. Further (1999), a leveling network has been added to improve the definition of the vertical component of the motion field and the density of controlled points. Surveys have been carried out until the actuality. Time series of coordinates and heights spanning along the observation period (1995 or 1999 to 2010) are hence available for the network points.The Assisi landslide area has also been investigated by means of InSAR: the data here presented derive from the analysis of Envisat data spanning in time from 2003 to 2010, thus with a 7-years overlapping with the GNSS and leveling surveys, which make possible a comparison.SAR data refer to scatter points which are numerous and well spread over the landslide area but almost never coinciding with GNSS and leveling markers. Moreover, the type of movements which can be put into evidence are different: along an assigned direction (LOS) from the SAR data, in 3 dimensions from GNSS, along the vertical direction from leveling. This paper presents a comparison of the InSAR results with the GNSS data, from which the LOS component has been derived. The comparison has been made for each GNSS marker with the surrounding SAR scatters, trying to take into account local topological effects when possible. A comparison between InSAR and leveling data requires a different approach, considering the different one-dimensional components of the movement, vertical for leveling and oblique (along the LOS) for InSAR.

We provide new data and insights into a 6 February 2018 Mw 6.4 earthquake that shook the city of Hualien in eastern Taiwan at the leading edge of a modern arc-continent collision. Fatalities and damages were concentrated near the Milun fault and extended south to the northern Longitudinal Valley fault.Although the Hualien area has one of the highest rates of seismicity in Taiwan, the geologic structures responsible for active deformation were not well understood before this event. We analyzed Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data and produced a 3Ddisplacement model with InSAR and azimuth offset of radar images to document surface deformation induced by this earthquake. The 3D displacement model was inverted to estimate slip on the Milun fault.We find that models assuming a single fault are inconsistent with observations of coseismic deformation and regional strain patterns, providing evidence for linked slip on a little-studied offshore thrust belt. Based on data presented here and elsewhere, we propose a odel for transpressive deformation in a zone of oblique convergence and left-lateral wrench tectonics to explain this and a prior 1951 M 7.3 earthquake.

We present an example of integration of persistent scatterer interferometry (PSI) and in situ measurements over a landslide in the Bovino hilltop town, in Southern Italy. First, a wide-area analysis of PSI data, derived from legacy ERS and ENVISAT SAR image time series, highlighted the presence of ongoing surface displacements over the known limits of the Pianello landslide, located at the outskirts of the Bovino municipality, in the periods 1995-1999 and 2003-2008, respectively. This prompted local authorities to install borehole inclinometers on suitable locations. Ground data collected by these sensors during the following years were then compared and integrated with more recent PSI datafrom a series of Sentinel-1 images, acquired from March 2014 to October 2016. The integration allows sketching a consistent qualitative model of the landslide spatial and subsurface structure, leading to a coherent interpretation of remotely sensed and ground measurements. The results were possible thanks to the synergistic operation of local authorities and remote sensing specialists, and could represent an example for best practices in environmental management and protection at the regional scale.

The Multi-Chromatic Analysis can be applied to interferometric pairs of SAR images processed at range sub-bands, and consists of exploring the phase trend of each pixel as a function of the different central carrier frequencies. The phase of stable scatterers evolves linearly with the sub-band central wavelength, with a slope proportional to the absolute e. m. path difference. The technique appears optimally suited for the new generation of satellite sensors, which operate with larger bandwidths than previously available instruments, generally limited to few tens of MHz. A first experiment on satellite data was carried out by processing a spotlight interferometric pair of images acquired by TerraSAR-X on the well-known Uluru monolith in Australia. In the present work, we illustrate MCA processing on SAR data acquired over the same site by the COSMO-SkyMed constellation. The topographic profile of the monolith is successfully reconstructed. Furthermore, the results are also compared with those previously derived by processing TerraSAR-X data.

The present study is aimed at investigating the potentialities of the COSMO/SkyMed (CSK) constellation for ground elevation measurement with particular attention devoted to the impact of the improved spatial resolution wrt the previous SAR sensors.Assuming no movement and successful orbital error removal, the main problem in height computation derives from the atmospheric artifacts (APS). Different strategies can be adopted to filter out this signal:1.By processing stack of images and through advanced multi-temporal interferometric analysis, it is possible to infer with sub-metric precision the height of targets which behave coherently in time (persistent scatterers). These techniques allow to filter out the atmospheric signal thanks to its decorrelation in time and correlation in space. The main drawback is related to the availability of coherent scatterers on the scene. High resolution sensors (as CSK) allow to increase the density of the measurable targets.2.By using tandem-like high resolution interferometric pairs and a reference low resolution DEM, it is possible to filter the differential phase field in order to remove the atmospheric artifacts. The filtered InSAR phase may hence improve the accuracy of the original DEM. 3.Through Numerical Weather Models (NWM), it is possible to estimate the InSAR phase related to the interaction between microwave and atmosphere.In the present work we explored all the mentioned strategies for APS mitigation. The selected test site was Parkfield (California, USA), where a consistent number of both Stripmap HIMAGE and Enhanced Spotlight right-descending HH acquisitions are available with very close incidence angles. Both dataset were hence used to experiment the reliability of multi temporal analysis for height computation (strategy 1). Our results are in line with indications found in recent literature, proving the potential of PSI to provide sub-metric precision of height measurements. Satisfactory performances may be achieved also by properly filtering the differential interferometric phase derived by using CSK tandem-like pairs (strategy 2). In particular, when normal the baselines exceed 300 m, it is possible to derive DEMs fulfilling the HRTI Level 3 specifications on the relative vertical accuracy. On the contrary, the mitigation of atmospheric artifacts through NWM (strategy 3) is still uncertain especially for X-band InSAR. Our preliminary results confirm the indications coming from other similar studies: NWM are effective for the long wavelengths (>20 km) and for vertical stratification which depends on the hydrostatic component of the troposphere, while, when dealing with the turbulent component of the low troposphere, this approach is unfeasible. SAR processing indeed requires millimetric accuracy in the zenith atmospheric delay while the outcomes of the NWM are in the order of centimeters, as confirmed by the validation performed through independent RAOBS and GPS ZTD data.

The Multi-Chromatic Analysis (MCA) consists of performing sub-bands splitting in range frequency domain, thus generating chromatic views of lower range resolution, centered at different carrier frequencies. Multi-chromatic interferograms can be then generated by coupling chromatic views coming from an interferometric pair of SAR images. The interferometric phase of spectrally-stable scatterers evolves linearly with the sub-band central frequency, the slope being proportional to the absolute optical path difference. Unlike the standard "monochromatic" InSAR approach, this new technique allows performing spatially independent and absolute phase unwrapping (PU). Potential applications for the study of spectrally-stable targets include topographic measurements, atmospheric research or urban monitoring.The technique appears optimally suited for new-generation, wide-band, high-resolution satellite SAR sensors. This work presents first successful applications of the technique using both TerraSAR-X (TSX) and COSMO/SkyMed (CSK) spotlight data. In particular, we provide results concerning the use of MCA for performing absolute PU as well as for height measurement on a pixel-by-pixel basis. Moreover, the impact of coregistration procedure on the MCA-based inference is investigated.

Applications such as SAR interferometry [1] are increasingly used in "sparse" contexts, in which information about some geophysical parameters (e.g. millimetric terrain deformations) are only available over some of the imaged pixels, corresponding to stable objects [2]. In such cases, it is often necessary to adapt processing algorithms, developed and optimized for regular data grids, to work on sparse samples. One of such algorithms, at the basis of several InSAR processing chains, is the so-called phase unwrapping (PU), consisting of obtaining absolute phase values (i.e. defined over the whole real interval) from the corresponding principal values, i.e. limited to the interval [??, ?[.Recently, a method to reduce the unwrapping problem of a sparse-grid field to one corresponding to a regular grid, has been proposed [3], based on a preliminary nearest-neighbor interpolation step. The solution to the sparse problem is shown to be mathematically equivalent to that of a corresponding regular grid problem, properly derived from the former. The approach allows to employ existing algorithms for regular-grid PU, such as those based on network theory (e.g. the so-called Minimum Cost Flow, or MCF).In this work, stemming from an analysis of the above-mentioned methodology, giving as a solution an absolute phase significant only over the sampled pixels, we propose an alternate procedure, in which the principal phase interpolation step is based on algorithms more advanced than the simple nearest-neighbor scheme. Such interpolation can be performed over the unit-magnitude complex field obtained from the wrapped phase. In this way, the obtained wrapped phase field results more similar to the original, "physical" regular field from which the sparse samples have been obtained.In the case in which this latter field can be assumed to satisfy general conditions of smoothness and homogeneity [4], this allows to exploit at best such characteristics, and to have finally an absolute phase regular matrix more representative of the real data, and then more effective to use in the subsequent processing steps [5].In the paper, several interpolators are considered, such as radial basis functions (RBF), as well as, more generally, Kriging [6], and their performances and application limits are evaluated in simulation, as a function of both the regularity conditions of the original sampled surface, and the sampling density.

Indonesia is periodically affected by severe volcanic eruptions and earthquakes, which are geologically coupled to theconvergence of the Australian tectonic plate beneath the Sunda Plate. Multi-temporal SAR interferometry (MTI) can beused to support studying and modelling of terrain movements. This work is aimed at performing an analysis of grounddisplacements over Indonesian sites through MTI techniques. Test sites have been selected according to the availabilityof archived SAR data, GNSS networks, and geological data. A stack of COSMO-SkyMed data, acquired in stripmapmode between 2011 and 2015, has been selected over the Banda Aceh region in Sumatra island. Geological maps of thetest sites are available, and several GNSS stations from the Continuously Operating Reference Stations Indonesiannetwork are found in the area of interest. Both the SPINUA and the StaMPS MTI algorithms have been used forprocessing the data, and deriving displacement maps. The ground deformations detected on the area are interpretedaccording to the available geological and geophysical information. The MTI results seem to confirm the inactivity of theAceh fault segment, while the lack of coherent targets hinders reliable displacement measurements along the Seulineumsegment. MTI data additionally allowed to identify local, non-tectonic ground instabilities: several areas are affected bysubsidence due to unconsolidated coastal and alluvial sediments, deserving more investigations by local authorities.Finally, MTI results could be useful to integrate and update data from the existing GPS network.

Multi Temporal Interferometry (MTI) stands for advanced synthetic aperture radar differential interferometry (DInSAR) techniques, which include Permanent/Persistent Scatterers Interferometry -- PSInSAR(TM)/PSI and similar methods, as well as Small Baseline Subset -- SBAS and related/hybrid approaches. These techniques are capable to provide wide-area coverage (thousands of km2) and precise (mm-cm resolution), spatially dense information (from hundreds to thousands of measurement points/km2) on ground surface deformations. New MTI application opportunities are emerging thanks to i) greater data availability from radar satellites, and ii) improved capabilities of the new space radar sensors (X-band Cosmo-SkyMed, C-band RADARSAT-2, TerraSAR-X) in terms of resolution (from 3 to 1 m) and revisit time (from 11 to 4 days for X-band acquisitions). This implies greater quantity and quality information about ground surface displacements and hence improved landslide detection and monitoring capabilities. Even though the applicability of MTI to regional and local-scale investigations of slow landslides has already been demonstrated, the awareness of the MTI utility and its technical limitations among landslide scientists and practitioners is still rather low. By referring to recent works on radar remote sensing, many regional and local scale MTI application examples from the geoscience literature and our own studies, we present an up-to-date overview of current opportunities and challenges in this field. We discuss relevant technical constraints and data interpretation issues that hamper the use of MTI in landslide assessment. Then guidelines on how to mitigate MTI technical limitations and avoid erroneous interpretations of radar-derived slope surface deformations are presented for the benefit of users lacking advanced knowledge in SAR applications. Finally, in view of the upcoming radar satellite launches, future perspectives on MTI applications are outlined and recommendations for applied research priorities are suggested. We foresee that with regular globe-scale coverage, improved temporal resolution (weekly or better) and freely available imagery, new radar satellite background missions such as the European Space Agency's Sentinel-1 will guarantee ever increasing and more efficient use of MTI in landslide investigations. Furthermore, thanks to the improved temporal and spatial resolutions of the new generation radar sensors, significant breakthroughs are expected in detailed slope instability process modeling (e.g. kinematic and geotechnical models), as well as in the understanding of spatial and temporal patterns of landslide movement/activity and their relationships to causative or triggering factors (e.g. precipitation, seismic loading).

Indonesia is periodically affected by severe volcanic eruptions and earthquakes, which are geologically coupledto the convergence of the Australian tectonic plate beneath the Sunda Plate. This work is aimed at performing ananalysis of ground displacements over Indonesian sites through Multi-temporal SAR interferometry (MTI). Twotest sites, in Sumatra and Java, have been selected according to the following requirements: presence of groundinstabilities, possibly related to onshore active faults or volcanoes; good expected interferometric coherence,availability of reliable archived interferometric SAR datasets, availability of ancillary geophysical data.Displacement maps have been obtained by processing COSMO-SkyMed and Sentinel-1 datasets available on thearea, through SPINUA algorithm, which performs Persistent Scattering (PS) analysis. The use of datasets comingfrom two datasets, allows cross-validating final results. The processing of Sentinel-1 data has been more complexw.r.t. that of COSMO-SkyMed data, as standard MTI displacement maps showed strong artifacts, likely due toresidual atmospheric contributions and orbital errors. In order to overcome this problem, an alternative processingscheme has been experimented.The tectonic analysis in Indonesia is difficult because the vegetation cover in the area causes lack of PS along andacross the faults. Our MTI results provided useful information about the ground stability/instability within theselected test sites. In particular, concerning the tectonic activity in Sumatra, the MTI displacement analysis seemsto confirm the inactivity of the Aceh fault segment, as foreseen by geodetic studies. Also, in the Java test site nodisplacement signal was detected related to possible activity of the faults present in the area.Besides the tectonic activity, ground displacements were also identified basically reflecting local effects. Thecauses of these displacements were investigated by using ancillary geological data, and in situ inspections.The subsidence phenomena are mainly related to the presence of unconsolidated coastal/alluvial sediments andgroundwater pumping.An interesting example concerns a coastal area in Banda Aceh, which was overrun and completely destroyedby the 2004 tsunami. Most subsiding PS targets are positioned on port facilities structures and embankments.Extensive rebuilding and new constructions in the area add weight to the unconsolidated sediments. There is alsoan extensive presence of seasonally flooded crops and salt production flats. This suggests that the subsidenceoccurring in the area is probably related to compaction of sediments and/or recent artificial fill.A subsidence has been also revealed in Java over the Yogyakarta urban area. This local displacement is inducedby groundwater exploitation and soft sediment compaction, a result of major urban expansion and human activityduring the last years. The high resolution of COSMO-SkyMed data allows catching

We apply persistent scatterer interferometry (PSI) techniques to synthetic aperture radar (SAR) data from ERS and ENVISAT satellites on the Lesina Marina area, a coastal tourist village in Apulia, Southern Italy, where the excavation of a canal exposed grey micro- and meso-crystalline gypsum which is now showing a high density of cavities and sinkholes due to gravitational collapse processes. We observe PS objects undergoing displacements, along the sensor line of sight, forming the same relatively smooth pattern in all the processed data stacks. Vertical displacement rates, derived through integration of ascending and descending geometries, reach about 4 mm/year on locations adjacent to the canal, gently decreasing towards the western end of the built-up area. High-precision leveling measurements, performed in 1999 and 2010, reveal a substantial agreement with the ENVISAT PSI data, taking into account a small bias due to the choice of the leveling reference point. The dataset, thus validated, suggests the presence of an uplift phenomenon going on steadily for the entire timespan covered by the SAR observations (1992--2009). These observations, supported by petrographic data and in situ investigations, seem only in part compatible with a residual diapirism, and hint instead to more complex processes, such as a combination of diapirism and the hydration of the residual anhydrite in the core of the gypsum mass. These results confirm the importance of the integration between in situ, geologic and geophysical, remotely sensed investigations, as the latter often represent an essential tool to infer whether a given phenomenon, which can be hypothesized by the former, is presently under development.

The use of satellite Synthetic Aperture Radar Interferometry (InSAR) for monitoring ground instability due to landslide events, although advantageous over large spatial scales, still poses challenges related to the recurrently complex kinematics of the phenomena or to the unfavorable settings of the examined areaswith respect to steep topography and vegetated land cover. This paper presents results obtained by usingMulti-temporal InSAR techniques with high resolution TerraSARX (TSX) data formonitoring the Carlantino landslide, located in theDaunian Subapennine (Apulia region, southern Italy) on a slope overlooking a water reservoir, and subjected to several investigations and consolidationworks. The targets detected by using Persistent Scatterer Interferometry (PSI) correspond to urban structures or peri- urban walls and guard rails, while the landslide body is almost completely devoid of stable targets, due to the widespread vegetation and variable land cover. To allowstability monitoring, a network of six Corner Reflectors (CR)was designed and deployed over the landslide test site. The TSX imageswere analyzed by using both the PSI processing and a procedure, based on the double difference analysis of InSAR phase values on the CR pixels. De- spite residual noise and the loss of 2 CRs due to vandalism, the processing allowed verifying the stability of the upper and central part of the landslide body, and relating indirectly the movements at the toe of the landslide to the water level fluctuations of the reservoir. Finally, this experiment suggests some recommendations and guidelines in planning CR deployment in complex landslide sites.©

A relevant part of the Assisi urban area (central Italy), built up after 1950 and located outside of the ancient town center, is interested by a landslide characterized by a rather slow rate of movement, which has caused important damages to buildings for an accumulation effect in the years. The movements of the soil surface have both a horizontal and a vertical component. For the monitoring of the landslide behaviour, the determination of the motion field and its evolution in time, a precision GNSS network has been established over the area since 1995, connecting by means of a baseline network the moving area with stable geologic formations located well outside (up to some km distance) of the landslide body. Further (1999), a leveling network has been added to improve the definition of the vertical component of the motion field. Surveys of both GNSS and leveling networks have been carried out in time, with an approximately annual cadence, until the actuality. Time series of coordinates and heights spanning along the observation period (1995-2010) are hence available for the network points.The Assisi landslide area has also been investigated by means of SAR: the data here presented derive from the analysis of Envisat data spanning in time from 2003 to 2010, thus with a 7-years overlapping with the GNSS and leveling surveys, which make possible a comparison.SAR data refer to scatter points which are numerous and well spread over the landslide area but almost never coinciding with the GNSS and leveling markers. Moreover, the type of movements which can be put into evidence are different: along an assigned direction (the LOS, Line Of Sight) from the SAR data, in 3 dimensions from GNSS, along the vertical direction from leveling. This paper presents a comparison of the SAR results with the GNSS data, from which the LOS component has been derived. The comparison has been made for each GNSS marker with the surrounding SAR scatters, trying to take into account local topological effects when possible. A good correspondence between the results of the different techniques has been found in many cases, and a deeper analysis of the movement field has been possible.

L'osservazione della Terra da piattaforme spaziali, integrata con misure in situ e con acquisizione da piattaforme aeree, è una tecnologia di riferimento per il monitoraggio di ampie zone del Territorio con una elevata frequenza spaziale. Queste ca-ratteristiche sono essenziali per investigare, da un lato, l'effetto di modifiche indotte da cambiamenti climatici, dall'altro la presenza di situazioni che siano precursori di cambiamenti, in un'ottica di previsione ed allerta. Nel presente lavoro vengono presentati alcuni esempi applicativi in questo scenario.

The Multi-Chromatic Analysis (MCA) uses interferometric pairs of SAR images processed at range sub-bands located at different spectrum positions, and explores the phase trend of each pixel in the frequency domain. By exploring the linear phase trend along the sub-bands frequencies, both phase unwrapping as well as height computation can be performed on a pixel by pixel basis without spatial integration. This work is aimed at experimenting a further application of MCA consisting in processing a single SAR acquisition for measuring the optical path between the SAR sensor and the imaged scene. We used TerraSAR-X SAR data acquired in Spotlight mode over the Venice Lagoon, and took advantage of corner reflectors already deployed for other purposes which guarantee phase stability as well as precise geographical positioning. We computed the delay induced by the atmosphere on the SAR signal, then compared this measure with that derived by analytical models and in situ records.

The multichromatic analysis (MCA) uses interferometric pairs of SAR images processed at range subbands and explores the phase trend of each pixel as a function of the dif- ferent central carrier frequencies to infer absolute optical path difference. This approach allows retrieving unambiguous height information on selected pixels, potentially solving the problem of spatial phase unwrapping, which is instead critical in the stan- dard monochromatic processing. The method, based on concepts originally introduced by Madsen and Zebker, has been developed in previous work both theoretically and through simulations. This paper presents the first MCA experimental validation of the procedure, through application to a wideband SAR single-pass interferometric data set acquired by the AES-1 airborne sensor. An evaluation of the impact of theMCA processing parameters on the height estimation performances is obtained through a para- metric analysis. The results confirm the indications derived by the theoretical analysis, demonstrating the feasibility of the MCA absolute phase measurement, provided that a sufficient bandwidth is available.

The present paper presents the results of the application of Multi Chromatic Analysis (MCA) for height retrieval by processing both AES-1 airborne data and satellite TerraSAR-X data. In particular, a test of the robustness of the MCA technique with respect to total processed bandwidth has been performed through comparison of results from datasets with bandwidths spanning form 100 to 400 MHz.A first validation of the mentioned technique has been carried out by comparing the retrieved heights w.r.t. ground elevation from external SRTM DEM, as well as by verifying the reliability of the fringe classifications based on the integer number of phase cycles computed through MCA.Results are presented and commented by addressing potential and limitation of the technique.

The Multi-Chromatic Analysis, as introduced in [1], uses interferometric pairs of SAR images processed at range sub-bands and explores the phase trend of each pixel as a function of the different central carrier frequencies. The phase of stable scatterers evolves linearly with the sub-band central wavelength, the slope being proportional to the absolute optical path difference. Unlike the standard "monochromatic" InSAR approach, this technique allows performing spatially independent and absolute topographic measurements, if the attention is focused on single targets exhibiting stable phase behaviour across the frequency domain. Potential applications for the study of frequency-stable targets include topographic measurement, atmospheric research, and urban monitoring. Through a simplified model [2], we obtained a first evaluation of the impact of the MCA processing parameters on the height estimation performances. A total bandwidth of at least 300 MHz seems to be required to provide reliable results. Thus, the technique appears optimally suited for the new generation of satellite sensors, which operate with larger bandwidths than previously available instruments, generally limited to few tens of MHz. SAR sensors such as those mounted on TerraSAR-X (TSX) or COSMO-SkyMed (CSK) spacecraft, all pose great expectations on the potential use of multi-chromatic methods.The practical feasibility of the technique was demonstrated in [2] by using a set of SAR data collected by the airborne AES-1 radar interferometer, operating at X-band by multi-channel electronics, which provides a total radar bandwidth of 400 MHz. A first successful application of the technique to satellite data was also shown in [3] by using a spotlight interferometric pair of images acquired by TSX on the well-known Uluru monolith in Australia. In the present work, we illustrate results obtained through MCA processing on spaceborne SAR data acquired by the CSK constellation both on Parkfield in California (USA), and on the Uuluru monolith. A CSK tandem pair acquired on the Uluru test site was used to validate the MCA-based height measurements by using a digital surface model (DSM) derived from optical stereo imagery captured at 15 cm resolution. The same dataset was also processed to validate the theoretical analysis [2] developed to assess the performance of the MCA with respect to the radiometric parameters involved in the processing (total bandwidth, sub-bandwidth, number of sub bands).The MCA technique introduces the concept of frequency-stable targets (PSfd), i.e. objects exhibiting stable radar returns across the frequency domain. This concept is complementary to that of temporal stability, which is at the base of persistent scatterers interferometry (PSI) techniques [4]. In PSI applications, stable targets (PS) are recognized as those exhibiting temporal stability through a stack of tens of SAR images. It is then natural to try to compare the two concepts, examining t

Multi-Chromatic Analysis (MCA) of SAR images relays on exploring sub-band images obtained by processing portions of range spectrum located at different frequency positions. It has been applied to interferometric pairs for phase uwrapping and height computation. This work investigates two promising applications: the comparison between the frequency-persistent scatterers (PSfd) and the temporal-persistent scatterers (PS), and the use of inter-band coherence of a single SAR image for vessel detection. The MCA technique introduces the concept of frequency-stable targets, i.e. objects exhibiting stable radar returns across the frequency domain which is complementary to that of temporal stability at the base of PS interferometry. Both spotlight and stripmap TerraSAR-X images acquired on the Venice Lagoon have been processed to identify PSfd and PS. Different populations have been analyzed to evaluate the respective characteristics and the physical nature of PSfd and PS. Concerning the spectral coherence, it is derived by computing the coherence between sub-images of a single SAR acquisition. In the presence of a random distribution of surface scatterers, spectral coherence must be proportional to sub-band intersection of sub-images. This model is fully verified when observing measured spectral coherence on open see areas. If scatterers distribution departs from this distribution, as for manmade structures, spectral coherence is preserved. We investigated the spectral coherence to perform vessel detection on sea background by using spotlight images acquired on Venice Lagoon. Sea background tends to lead to verylow spectral coherence while this latter is preserved on the targeted vessels, even for very small ones. A first analysis shows that all vessels observable in intensity images are easily detected in the spectral coherence images which can be used as a complementary information channel to constrain vessel detection.

This work investigates the possibility of performing target analysis through the Multi-Chromatic Analysis (MCA), a technique that basically explores the information content of sub-band images obtained by processing portions of the range spectrum of a synthetic aperture radar (SAR) image. According to the behavior of the SAR signal at the different sub-bands, MCA allows target classification. Two strategies have been experimented by processing TerraSAR-X images acquired over the Venice Lagoon, Italy: one exploiting the phase of interferometric sub-band pairs, the other using the spectral coherence derived by computing the coherence between sub-band images of a single SAR acquisition. The first approach introduces the concept of frequency-persistent scatterers (FPS), which is complementary to that of the time-persistent scatterers (PS). FPS and PS populations have been derived and analyzed to evaluate the respective characteristics and the physical nature of the targets. Spectral coherence analysis has been applied to vessel detection, according to the property that, in presence of a random distribution of surface scatterers, as for open sea surfaces, spectral coherence is expected to be proportional to sub-band intersection, while in presence of manmade structures it is preserved anyhow. First results show that spectral coherence is well preserved even for very small vessels, and can be used as a complementary information channel to constrain vessel detection in addition to classical Constant False Alarm Rate techniques based on the sole intensity channel.

The multichromatic analysis (MCA) can be applied to interferometric pairs of synthetic aperture radar (SAR) images processed at range subbands and consists of exploring the phase trend of each pixel as a function of the different central carrier frequencies. The phase of stable scatterers linearly evolves with the subband central frequency, with a slope proportional to the absolute electromagnetic path difference that can be estimated and used for both phase unwrapping and height computation. MCA has been theoretically evaluated and tested on airborne wideband SAR data, appearing optimally suited for the new generation of satellite sensors, which operate with larger bandwidths than previously available instruments, generally limited to few tens of megahertzs. In this letter, we illustrate MCA application to satellite SAR data acquired in spotlight mode over the Uluru monolith in Australia. The topographic measurements derived through MCA on the monolith are compared with those provided by a high-resolution digital elevation model from optical stereo imagery. The theoretical parametric model describing the MCA performances according to the processing parameters is also validated.

Marina di Lesina is a peculiar geological site, affected by sinkhole phenomena, causing instabilities and failures of infrastructures. This tourist village, lying not far from Punta delle Pietre Nere, the only outcrop of magmatic rock in the Mediterranean basin, sits on a diapir made of Triassic gypsum, mantled by Quaternary sandy deposits. The cutting of the artificial Acquarotta canal in 1930, connecting the nearby Lesina lagoon to the Mediterranean Sea, exposed this grey micro and meso-crystalline gypsum with intercalations of black limestones and marls. This event is a likely cause for the formation of dissolutional conduits and cavities, found in the area, leading to the formation of the sinkholes which have been plaguing the site in the last years [1]. This peculiar geological setting, coupled with its relatively high value as a local touristic resort, led to its selection as a test site for precise InSAR displacement monitoring techniques. The monitoring, started with legacy ERS and ENVISAT sensors, is continuing through analysis of higher-resolution data.

The recent availability of wide-bandwidth, high-frequency, high-resolution SAR data is contributing to improved moni-toring capabilities of spaceborne remote sensing instruments. In particular, the new COSMO/SkyMed (CSK) and Ter-raSAR-X (TSX) X-band sensors allow better performances in multitemporal DInSAR and PSI applications than legacy C-band sensors such as ENVISAT ASAR, with respect to both target detection and terrain displacement monitoring ca-pabilities. In this paper we investigate about the possibility of achieving performances of PSI displacement detection comparable to those of C-band sensors, by use of reduced numbers of high-resolution X-band acquisitions. To this end, we develop a simple model for phase and displacement rate measurement accuracies taking into account both target characteristics and sensors acquisition schedule. The model predicts that the generally better resolution and repeat-time characteristics of new-generation X-band sensors allow reaching accuracies comparable to C-band data with a significantly smaller number of X-band acquisitions, provided that the total time span of the acquisitions remains the same. This allows in principle to contain the costs of monitoring campaigns, by using less scenes. Indications are more variable in the case of short-time acquisition schedules, such as those involved in the generation of so-called "rush products" for emergency applications. In this case, the higher uncertainty given by shorter total time spans lowers X-band performances to levels mostly comparable to those of the legacy medium-resolution C-band sensors, so that no significant gain in image number budget are foreseen. These theoretical results are confirmed by comparison of three PSI datasets, acquired by ENVISAT ASAR, CSK and TSX sensors over Assisi (central Italy) and Venice.

Accurate geolocation of SAR data is nowadays strongly required because of the increasing number of high resolution SAR sensors available as for instance from TerraSAR-X/TanDEM-X and COSMO-SkyMed space-borne missions. Both stripmap and spotlight acquisition modes provide from metric to sub metric spatial resolution which demands the ability to ensure a geolocation accuracy of the same order of magnitude. Geocoding quality depends on several factors and in particular on the knowledge of the actual values of the satellite position along the orbit, and the delay introduced by the additional path induced by changes in the refractivity index due to the presence of the atmosphere (the so called Atmospheric Path Delay or APD). No definitive results are reported yet in the scientific literature, concerning the best performances achievable by the COSMO-SkyMed constellation in terms of geolocation accuracy. Preliminary studies have shown that sub-pixel geolocation accuracies are hardly achievable with COSMO-SkyMed data. The present work aims at inspecting the origin of the geolocation error sources in COSMO-SkyMed Single-look Complex Slant (SCS) products, and to investigate possible strategies for their compensation or mitigation. Five different test sites have been selected in Italy and Argentina, where up to 30 corner reflectors are installed, pointing towards ascending or descending passes. Experimental results are presented and discussed.

Sparse phase measurements often need to be interpolated on regular grids, to extend the information to unsampled locations. Typical cases involve the removal of atmospheric phase screen information from Interferometric Synthetic Aperture Radar (InSAR) stacks, or the retrieval of displacement information over extended areas in Persistent Scatterers Interferometry (PSI) applications, when sufficient point densities are available. This operation is usually done after a phase unwrapping (PU) of the sparse measurements to remove the sharp phase discontinuities due to the wrap operation. PU is a difficult and error-prone operation, especially for sparse data. In this work, we investigate from the empirical point of view an alternative procedure, which involves an interpolation of the complex field derived from the sparse phase measurements. Unlike traditional approaches, our method allows to bypass the PU step and obtain a regular-grid complex field from which a wrapped phase field can be extracted. Under general conditions, this can be shown to be a good approximation of the original phase without noise. Moreover, the interpolated, wrapped phase field can be fed to state-of-the-art, regular-grid PU algorithms, to obtain an improved absolute phase field, compared to the canonical method consisting of first unwrapping the sparse-grid data. We evaluate the performance of the method in simulation, comparing it to the classical methodology described above, as well as to an alternative procedure, recently proposed, to reduce a sparse PU problem to a regular-grid one, through a nearest-neighbor interpolation step. Results confirm the increased robustness of the proposed method with respect to the effects of noise and undersampling.

The work experiments the potentialities of COSMO/SkyMed (CSK) data in providing interferometric Digital Elevation Model (DEM). We processed a stack of CSK data for measuring with meter accuracy the ground elevation on the available coherent targets, and used these values to check the accuracy of DEMs derived from 5 tandem-like CSK pairs. In order to suppress the atmospheric signal we experimented a classical spatial filtering of the differential phase as well as the use of weather prediction (NWP) model RAMS. Tandem-like pairs with normal baselines higher than 300 m allows to derive DEMs fulfilling the HRTI Level 3 specifications on the relative vertical accuracy, while the use of NWP models still seems unfeasible especially for X-band.

The present study is aimed at investigating the potentialities of the COSMO/SkyMed (CSK) constellation for ground elevation measurement with particular attention devoted to the impact of the improved spatial resolution wrt the previous SAR sensors. Assuming no movement and successful orbital error removal, the main problem in height computation through InSAR techniques derives from the interferometric phase artifacts related to the interaction between microwave and the lower layers of the atmosphere (APS, Atmospheric Phase Screen). Different strategies can be adopted to filter out this signal, ranging from the exploitation of the well-known spatial and temporal statistics of the APS to the estimation of independent APS measurements through Numerical Weather Prediction (NWP) models. Their feasibility and the achievable accuracies are discussed here.

Stochastic models are often used to describe the spatial structure of atmospheric phase delays in differential interferometric synthetic aperture radar (DInSAR) data. Synthetic aperture radar interferograms often exhibit anisotropic atmospheric signals. In view of this, the use of anisotropic models for atmospheric phase estimation is increasingly advocated. However, anisotropic models lead to increased computational complexity in estimating the correlation function parameters with respect to the isotropic case. Moreover, the performance is degraded when dealing with DInSAR techniques involving only a few sparse points usable for computations, as in the case of persistent scatterer interferometry applications, particularly when this estimation has to be done in an automated way on many interferograms. In the present work, we propose some observations about the actual advantage given by anisotropic modeling of atmospheric phase in the case of sparse-grid point-target DInSAR applications. Through analysis of simulated data, we observe that an improvement in the performances of kriging reconstruction approaches can be obtained only when sufficient sampling densities are available. In critical sampling conditions, automated methods with reasonable computational cost may improve their performance if external information on the atmospheric phase screen field is available. © 2006 IEEE.

In order to ensure sub-pixel accuracy of geocoded SAR products, precise estimation and correction of theAtmospheric Path Delay (APD) is needed, in particular for the new generation of high resolution satellite SARsensors (TerraSAR-X, COSMO-SkyMed). The goal of the present study is to assess the performances ofoperational Numerical Weather Models (NWM) as tools for APD mitigation. The Regional AtmosphericModeling System (RAMS) has been selected for this purpose. In order to guarantee an accurate knowledge ofboth the satellite orbit and the target position, TerraSAR-X data and corner reflectors have been used for theexperiment. Differential GPS measurements confirm that NWM are able to estimate APD with an accuracy offew decimeters. Therefore, NWM can be also exploited as tools for providing preliminary indications on theamount of orbital or timing errors. The analysis has been hence extended to COSMO-SkyMed data.

High resolution numerical weather models (NWM) are being to play a role of increasing importance foratmospheric phase screen (APS) mitigation. Here we present preliminary investigations concerning the estimationof the atmospheric contribution to X-band InSAR phase fields through numerical weather modeling. We selectedtandem-like pairs of Stripmap COSMO/SkyMED images acquired over Parkfield (California, USA) with shortnormal baselines, thus ensuring low sensitivity to elevation. By using a 30m SRTM DEM available for the area ofinterest, we generated differential phase fields, mainly related to the difference between atmospheric conditions atthe times of the two acquisitions.The interferometric artifacts have been hence compared to independent estimates of the atmospheric phase delayintroduced by both wet and dry the components of the troposphere, obtained through Regional AtmosphericModeling System (RAMS), a finite-difference, primitive equation, three-dimensional mesoscale NWM originallydeveloped at Colorado State University. RAMS is a prognostic model capable of simulating a wide range ofatmospheric motions due to the use of a nested grid system. Incorporation of topographic features occurs throughthe use of a terrain-following vertical coordinate system, while turbulence is parameterized using Mellor andYamada's level 2.5 scheme, as modified by Helfand and Labraga for growing turbulence.In order to assess the impact of the boundary conditions, numerical simulations have been repeated by usingGFS, ECMWF and NAM data (resolution: 0.5 deg, 0.25 deg and 12km respectively). A spin-up time exceeding24h was necessary for ensuring a realistic computation of the atmospheric boundary layer depth. Finally, the 3Dcomputation of the scaled-up refractive index and its integration along the Line-Of-Sight (LOS) of the SAR sensorwas performed in order to estimate the two-way radar phase delay.The preliminary results confirm the indications coming from recent similar studies: weather models are good forthe long wavelengths (>20 km) and for vertical stratification which depends on the hydrostatic component of thetroposphere, while they cannot actually ensure a sub-centimetric accuracy in the estimation of the wet component,as instead required in X-band interferometry.Finally, we used the GPS daily RINEX available on the Parkfield area to infer the atmospheric Zenith Total Delay(ZTD) and validate the outcomes of the NWM. GPS data were processed at ASI/CGS by using the NASA/JPLGIPSY-OASIS II for data reduction. The Precise Point Positioning approach was applied fixing JPL fiducial-freesatellite orbits, clocks and earth orientation parameters, IGS absolute phase center variations and estimating, witha cut-off angle of 7deg, site coordinates, station clock, phase ambiguities, ZTD and tropospheric gradients. ZTDand tropospheric gradients are modeled as random walk processes and estimated with a sampling rate of 5

Multi-temporal InSAR (MTI) applications pose challenges related to the availability of coherent scattering from the ground surface, the complexity of the ground deformations, presence of atmospheric artifacts, and visibility problems related to the ground elevation. Nowadays, several satellite missions are available, providing interferometric SAR data at different wavelengths, spatial resolutions, and revisit times.High-resolution X-Band SAR sensors, such as the COSMO-SkyMed constellation, acquire data with spatial resolution reaching metric values, and revisit time up to a few days, leading to an increase in the density of usable targets, as well as to an improved detection of non linear movements. Medium resolution C-band SAR data have been thoroughly exploited in the last two decades, thanks to the ERS-1/2 and ENVISAT-ASAR missions, and Radarsat-1/2. A new interesting opportunity is provided by the Sentinel-1 mission, which has a spatial resolution comparable to previous ESA C-band missions, and a revisit time reduced to 12 and 6 days, by considering, respectively, one or two satellites. It is envisioned that, by offering regular, global scale coverage, improved temporal resolution and freely available imagery, Sentinel-1 will guarantee an increasing use of MTI for ground displacement investigations.The present work discusses opportunities of MTI applications to ground instability monitoring by assessing the performance of the different available satellite missions, according to acquisition parameters such as wavelength, spatial resolution, revisit time and orbital tube size. This performance analysis allows to foresee the quality of displacement maps estimated through MTI according to mission characteristics, and thus to support SAR data selection. In particular, a comparative analysis is carried out, aimed at addressing specific advantages of different satellite missions in L-, C- and X-band. For instance, high resolution data increase the density of coherent targets, thus improving the monitoring of local scale events. Short (X-band) wavelengths improve the sensitivity to displacements. Short revisit times allow collecting large data stacks in short times, and improve the temporal sampling, thus increasing the chances to catch pre-failure signals (high-rate, nonlinear signals). The precision of the displacement rate detection depends on the number of images and on the phase noise, while the precision of the residual height error estimation depends also on the orbital tube size. Sentinel-1 will provide data for the next years with short revisit time, and it is thus likely to provide reliable displacement estimations at large scale, and in quite limited observation time spans. However, due to its narrow orbital tube size, it has a limited height precision, which leads to poor geo-location quality.An example of multi-sensor ground instability investigation is also presented concerning the site of Marina di Lesina, in Southern Italy, where s

Multi-temporal InSAR (MTI) applications pose challenges related to the availability of coherent scattering from the ground surface, the complexity of the ground deformations, atmospheric artifacts, and visibility problems related to ground elevation. Nowadays, several satellite missions are available providing interferometric SAR data at different wavelengths, spatial resolutions, and revisit time. A new and interesting opportunity is provided by Sentinel-1, which has a spatial resolution comparable to that of previous ESA C-band sensors, and revisit times improved by up to 6 days. According to these different SAR space-borne missions, the present work discusses current and future opportunities of MTI applications in terms of ground instability monitoring. Issues related to coherent target detection, mean velocity precision, and product geo-location are addressed through a simple theoretical model assuming backscattering mechanisms related to point scatterers. The paper also presents an example of a multi-sensor ground instability investigation over Lesina Marina, a village in Southern Italy lying over a gypsum diapir, where a hydration process, involving the underlying anhydride, causes a smooth uplift and the formation of scattered sinkholes. More than 20 years of MTI SAR data have been processed, coming from both legacy ERS and ENVISAT missions, and latest-generation RADARSAT-2, COSMO-SkyMed, and Sentinel-1A sensors. Results confirm the presence of a rather steady uplift process, with limited to variations throughout the whole monitored time-period.

COSMO/SKYMED is currently the unique constellationof synthetic aperture radar (SAR) sensors operative, whichis also for civilian use. On April 6, 2009, an Mw 6.3 earthquakestruck the city of l’Aquila in Central Italy. The constellation acquireddata stacks over the hit area at an unprecedented temporalrate. In this letter, the results obtained by processing several dataset via two independent multitemporal differential interferometricSAR techniques are presented to demonstrate the capability of thisconstellation in postseismic deformations monitoring.

Rheticus® is an innovative cloud-based data and services hub able to deliver Earth Observation added-value productsthrough automatic complex processes and, if appropriate, a minimum interaction with human operators. In this paper, weoutlines the capabilities of the "Rheticus® Displacement" service, designed for geohazard and infrastructure monitoringthrough Multi-Temporal SAR Interferometry techniques.

The Rheticus (R) cloud-based platform provides continuous monitoring services of the Earth's surface. One of the services provided by Rheticus (R) is the Displacement Geo-information Service, which offers monthly monitoring of millimetric displacements of the ground surface, landslide areas, the stability of infrastructures, and subsidence due to groundwater withdrawal/entry or from the excavation of mines and tunnels. To provide this information, the Rheticus (R) platform processes a large amount of Geospatial Big Data. In particular, Rheticus (R) is capable to process Synthetic Aperture Radar images acquired by the X-band COSMO-SkyMed constellation, as well as satellite Open Data provided by Copernicus Sentinels, and it is capable to integrate local INSPIRE data sources. In this paper, we summarize the main features of the Rheticus (R) services and we provide examples of the detection and monitoring of geohazard and infrastructure instabilities through Multi-temporal InSAR techniques. Furthermore, we outline the porting activity and the efficient implementation of the most time-consuming algorithmic kernels in the GPGPU environment.

We apply high-resolution, X-band, stripmap COSMO-SkyMed data to the monitoring of flood events in the Basilicata region (Southern Italy), where multitemporal datasets are available with short spatial and temporal baselines, allowing interferometric (InSAR) processing. We show how the use of the interferometric coherence information can help to detect more precisely the areas affected by the flood, reducing false alarms and missed identifications which affect algorithms based on SAR intensity alone. The effectiveness of using the additional InSAR information layer is illustrated by RGB composites of various combinations of intensity and coherence data. Analysis of multitemporal SAR intensity and coherence trends reveals complex behavior of various field types, which we interpret through a Bayesian inference approach, based on a manual identification of representative scattering and coherence signatures of selected homogeneous fields. The approach allows to integrate external, ancillary information to derive a posteriori probabilistic maps of flood inundation accounting for different scattering responses to the presence of water. First results of this semiautomated methodology, using simple assumptions for the SAR signatures and a priori information based on the distance from river courses, show encouraging results, and open a path to improvement through use of more complex hydrologic and topo-hydrographic information.

Advanced multi-temporal interferometry (MTI) techniques are being increasingly used in landslide assessment, as they can provide precise (mm-cm resolution) measurements of very slow ground surface displacements for huge areas with limited vegetation cover. We illustrate the potential of high resolution MTI for wide-area and local-scale detection of slope and associated infrastructure instability hazards in the peri-Adriatic region. This is done by presenting MTI applications to two landslide-prone mountainous areas characterized by different geomorphic, climatic and vegetation conditions, and hence by variable density and distribution of potential radar targets: the eastern-most part of the Southern Apennines and the mountains in central Albania. The results demonstrate that even in such scarcely urbanized areas MTI can provide valuable information on the presence of slope movements that locally affect small human settlements and road network. The gaps in satellite-derived information, especially evident in the more forested Albanian mountains, suggest that MTI could be most profitably exploited in the reconnaissance stage of a slope hazard assessment, to be followed by more detailed investigation and monitoring of sites at risk.

We apply high-resolution, X-band, stripmap COSMO/SkyMed data to the monitoring of a flood event in Southern Basilicata region (Italy), where a multi-temporal dataset is available, allowing interferometric processing. We show how the use of the interferometric phase information can actually help to detect precisely the areas affected by the flood, using e.g. RGB composites of various information layers derived from the data. We also present results of unsupervised clustering of the multi-temporal data, which allow to shed some light on the physical interpretation of some of the identified clusters.

The regularity and higher frequency of acquisitions of Sentinel-1A/B (S-1) with respect to earlier ESA's satellite C-band sensors (ERS1/2, ENVISAT) represent clear advantages for users of multi-temporal interferometry (MTI) products. The utility of the IW acquisition mode of S-1 for regional scale slope instability detection through MTI has already been demonstrated, e.g., via studies of landslide-prone areas in Italy. In this work, we explore the potential of S-1 data for local (site-specific) scale landslide monitoring and capturing pre-failure signs of slope instability. This is done by using examples of two unstable slopes from different environmental settings and MTI through the Persistent and Distributed Scatterers processing capability of the SPINUA algorithm.The first case regards a hilltop town in the Apennine Mts., whose stability is threatened by a large (~600 x 300 m2), slow moving deep landslide. We processed over 50 S-1A images acquired since October 2014. The comparison of the MTI results with those based on ERS and ENVISAT imagery shows that a much higher number of radar targets is obtained from S-1A data (e.g., from ~2 to 5 times higher, respectively on the landslide and in the overall area of interest, including also the town and peri-urban areas). With more targets, we can better depict the spatial movement pattern and local velocity gradients, which is important for geotechnical assessment. Furthermore, the lateral boundaries of the landslide can now be delimited in more detail, overcoming the mapping uncertainties typical in cases of very slow, deep landslides affecting urbanized areas. This offers invaluable information for local inhabitants/property owners and for engineering scale hazard assessment. Importantly, the MTI from S-1A data also revealed an accelerating trend with a nearly doubled velocity of the displacements with respect to those in the earlier period covered by ERS-ENVISAT data. The higher frequency of S-1A acquisitions (about 30/year in this case) helped highlighting the non-linearity of surface deformations within the faster displacement phase, whose timing was consistent with the increase in landslide movements detected through subsurface inclinometer monitoring and field observations. The latter demonstrated that this faster movement phase coincided with (or was preceded by) a failure of the landslide toe, which occurred in the inhabited area.The second case represents an example of a retrospective investigation of a huge (over 2 km long, few tens of m deep) landslide, which occurred in 2016 in an important open-cast coal mine in central Europe. The apparently sudden failure disrupted the mining operations, destroyed in part the mining machinery and resulted in high economic losses. In this case, we exploited over 60 S-1A/B images acquired since November 2015. Despite the presence of spatial gaps in information (due to intensive surface disturbance by mining operations), the MTI results provided a good

Landslides and potentially unstable slopes are present in almost every country of the globe. Moreover, thepopulation growth, with increasing impact of humans on the environment and the urbanization of areas susceptibleto slope failures implies that landslide hazard mitigation only via traditional engineering stabilizationworks and in situ monitoring is no longer considered economically feasible. Given the global dimension of theproblem of slope instability, a sustainable way towards landslide hazard reduction seems to be via increasedexploitation of affordable remote-sensing systems, with focus on early detection, long-term monitoring, andpossibly early warning. In particular, satellite-based remote sensing, and especially the synthetic aperture radar(SAR), multi-temporal interferometry (MTI), has great potential thanks to the wide-area coverage of space-bornesensors, day/night image acquisitions and the capability to provide high precision (mm-cm), spatially dense(from hundreds to thousands points per km2) measurements of slow displacements of the ground surface. In thiscontext, Sentinel-1 A/B (S-1) twin satellites of the European Space Agency (ESA), launched in 2014 and 2016,are now providing truly global capacity for innovative, research-oriented and practical MTI applications, suchas mapping, characterizing and monitoring of landslides. The regularity of S-1 acquisitions, timeliness of datadelivery, increased revisit frequency (days) and the resulting high coherence, as well as the availability of freeimagery, facilitate a more effective and innovative use of MTI.The main aim of this work is to compare and assess the potential of MTI based on S-1 data in slope instabilityinvestigations with respect to MTI relying on the earlier C-band sensors (ERS and ENVISAT), as well as thehigh resolution X-band sensors (COSMO-SkyMed, TerraSAR-X). This is done by considering different areascharacterized by a wide range of geomorphic, climatic, and vegetation conditions, with case study examples oflocal to regional scale MTI applications comprising hill slopes in the Apennine Mts. (Italy) and in the EuropeanAlps, and unstable slopes in two large open-cast mines of Central Europe. The results show that, by using S-1data, MTI can now be more effective and affordable in long-term slope/landslide monitoring, early detection ofslope instability hazard, and (in some cases) in slope failure early warning.

The PSI (Persistent Scatterer Interferometry) processing of ENVISAT ASAR data (period 2003-2010) provided spatially dense information (more than 400 PS/km2) on ground surface displacements in Lanzhou, capital of Gansu Province, NW China. The geomorphological and geological context of the local Yellow River valley indicate that the lower, flat areas with floodplain and valley-fill deposits (Holocene terraces with mainly reworked loess at the surface) are stable, whereas some higher, gently sloping valley sides appear locally unstable, particularly where the Late Pleistocene terraces are covered by young aeolian (Malan) loess. The PS velocity data suggest that the relative susceptibility to ground and slope instabilities is the highest on the 4th and 5th order river terraces. This is consistent with the presence of collapsible Malan loess and recent land use of these terraces involving irrigation and construction.

Persistent scatterer synthetic aperture radar interferometry (PS-InSAR) is a remote sensing method that can be used to detect surface deformation, which is an indicator of potential geohazards. By capturing such deformations over time, it is possible to obtain valuable information regarding geohazards such as landslides. This study focused on the use of PS-InSAR to investigate the distribution and causes of surface deformation in the Lanzhou region of Gansu Province in China. Between 2003 and 2010, 41 advanced synthetic aperture radar images were captured by the Envisat satellite and analyzed using PS-InSAR, and the correlation between the observed surface deformation and topographic, geologic, and anthropogenic factors was derived based on a geographic information system platform. It was found that the largest number and highest density of surface deformations occurred at elevations of 1486-1686 m. It was also established that slope ranges of 25°-30° and 35°-40° are threshold values at which surface deformation changes abruptly, and that slopes with north and northwest aspects are most prone to surface deformation. The lithologies most susceptible to surface deformation are clay, sandy soil, and loess. The normalized difference vegetation index indicated that surface deformation occurred most often in areas with sparse vegetation. Anthropogenic activities, e.g., construction and wastewater discharge, could be inferred as causal mechanisms of surface deformation. Comparison of the distributions of geohazards and surface deformation showed considerable consistency, which proves surface deformation can induce geohazards. These results could help governments improve urban planning and geohazard mitigation.

We report on the InSAR-related results of the National Research Project (PRIN) entitled "Advanced technologiesin the assessment and mitigation of the landslide risk: precursors detection, previsional models and thematicmapping", funded by the Italian Ministry for Scientific Research. In the framework of the project, multi-temporalinterferometric techniques were applied to time series of SAR data, from legacy ERS and ENVISAT, as well ashigh-resolution TerraSAR-X sensors. We report on the final outcomes of the project, which concentrate on twosites of the Apulia Region, representative of terrain instability problems widespread in the area.The fist one is the coastal area near the Lesina Marina tourist village, at the north of the Region, close to theGargano promontory, where the excavation of a canal exposed grey micro- and meso-crystalline gypsum which isnow showing a high density of cavities and sinkholes due to gravitational collapse processes. A slow but steady upliftphenomenon has been detected by processing through persistent scatterers interferometry (PSI) methodologiesERS and ENVISAT data, acquired in both ascending and descending geometries, and spanning a total time intervalfrom 1995 to 2010. The displacement data were validated by comparison with leveling measurements performedin 2000 and 2010. Derived vertical displacement rates exceed 3-4 mm/y on locations adjacent to the canal, gentlydecreasing towards the western end of the built up area. These observations, supported by ancillary data and insitu investigations performed in the past, seem compatible with processes such as diapirism or the hydration of theresidual anhydrite in the core of the gypsum mass.The second site is an inland landslide area close to the municipality of Carlantino, in the Daunia mountains. Here,a relatively large landslide affects the slopes spreading from the town outskirts to the banks of the Occhito lake, anartificial basin formed by a dam on the Fortore river. PS targets detected by both C- and X-band data correspond tourban structures or peri-urban walls and guard rails, while the landslide body is almost completely devoid of stabletargets, due to the vegetation cover. In order to allow stability monitoring through spaceborne SAR interferometry,a network of passive reflectors was designed and deployed on the area of interest. To design the corner reflector(CR) network, different factors were taken into account: the visibility of the CR by the satellite in terms of geometryand radiometry, the accessibility of the location on the ground, and the relative distance between CR. Resultsof the comparison of phase data over the CR with that of surrounding objects are presented.Work supported by the Italian Ministry of Research in the framework of PRIN 2008 research grant "Advancedtechnologies in the assessment and mitigation of the landslide risk: precursors detection, previsional models andthematic mapping". Te

The COSMO-SkyMed (CSK) constellation acquires data from its four SAR X-band satellites in several imaging modes, providing in particular different view angles. The present work investigates the potential of CSK constellation for ground elevation measurement through SAR radargrammetry. We selected an area around Parkfield (California), where several CSK acquisitions are available. We used for radargrammetric processing 2 CSK spotlight image pairs acquired at 1 day of separation, in Same Side Viewing configuration, with baselines of 350 km. Furthermore, a dataset of 33 spotlight images were selected to derive height measurements through both persistent scatterers interferometry(PSI) and interferometric processing of 5 1-day separated pairs included in the dataset. We first predict how the errors in the geometrical parameters and the correlation level between the images impact on the height accuracy. Then, two DEMs were derived by processing the radargrammetric CSK pairs. According to the outcomes of the feasibility analysis, processing parameters were chosen in order to guarantee nominal values of height accuracy within the HRTI Level 3 specifications. The products have a final resolution of 3 m. In order to assess the accuracy of these radargrammetric DEMs, we used the height values provided by the PSI, and an interferometric DEM derived from the CSK tandem-like pairs. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

In multi-temporal applications of synthetic aperture radar (SAR) interferometry, differential phase contributions due to atmospheric inhomogeneities, estimated over sparse points, have to be interpolated and removed from the regular-grid interferograms in order to highlight the phase stability of more image pixels, which then add to the available data to infer useful information about terrain displacements or other phenomena of interest. Interpolation is usually done on the phase data after a phase unwrapping (PU) operation. In a previous work, we considered the alternative interpolation step applied directly to the complex phasor derived from the wrapped phase, thus bypassing the error-prone sparse PU operation. In this article, the performances of the proposed methodology are evaluated over atmospheric phase screen (APS) data estimated from a previous processing through persistent scatterers interferometry (PSI) methods. The original persistent scatterer (PS) population is reduced by thresholding their inter-image coherence values, and then further subsampled randomly in a rectangle inside a detected subsidence bowl. Both the classical and the proposed interpolation procedures are applied to the subsampled APS phase values. The interpolated fields are then removed from the rest of the PS, and the residual phase values are compared in terms of inter-image coherence. Results confirm that interpolating complex phasors, thus avoiding PU, gives results equivalent to the standard procedure in good sampling conditions. Moreover, when point sparsity induces phase aliasing, thus hindering the PU operation, the proposed method allows to better recover phase information over unsampled pixels, improving the final results of the PSI processing.

This work presents an analysis of the applicability of synthetic aperture radar (SAR) interferometry to landslide monitoring. This analysis was carried out by using differ- ent interferometric approaches, different spaceborne SAR data (both in the C-band and in the X-band), and in situ global navigation satellite system (GNSS) measurements. In particular, we investigated both the reliability of displacement monitoring and the issues of the cross-comparison and validation of the interferometric synthetic aperture radar (InSAR) results. The work was focused on the slow-moving landslide that affects a relevant part of the urban area of the historical town of Assisi (Italy).A C-band ENVISAT advanced synthetic aperture radar (ENVISAT ASAR) dataset acquired between 2003 and 2010 was processed by using two different interferometric techniques, to allow cross-comparison of the obtained displacement maps. Good corre- spondence between the results was found, and a deeper analysis of the movement field was possible. Results were further compared to a set of GNSS measurements with a 7 year overlap with SAR data. A comparison was made for each GNSS marker with the surrounding SAR scatterers, trying to take into account local topological effects, when possible.Further, the high-resolution X-band acquired on both ascending and descending tracks by the COSMO-SkyMed (CSK) constellation was processed. The resultant dis- placement fields show good agreement with C-band and GNSS measurements and a sensible increase in the density of measurements.

The application of multi-temporal differential SAR interferometric analysis to slope instability monitoring poses challenges related to the complex kinematics of the phenomenon, as well as to the unfavourable settings of the area affected by landslides, often occurring on sites of limited extension, characterized by steep topography and variable vegetation cover. The use of passive reflectors allows to extend the stability monitoring on areas lacking natural coherent reflectors. The present work discusses the problematic aspects of design and deployment of a corner reflector network, and presents preliminary results obtained on the Carlantino village in the Daunia Apennines (Italy) by using X-band TerraSAR-X data. © 2012 IEEE.

We explore new possibilities offered by the recently available X-band satellite radar sensors for landslidehazard assessments on a detailed scale, with particular reference to the exploitation of Persistent ScatterersInterferometry (PSI) techniques. Special attention is paid to the impact of the improved resolution of new Xbandradar imagery on the PSI results, in terms of quality and quantity of useful information. This evaluationis supported by theoretical modelling as well as by the comparison of results from X-band (CSK) and C-band(ENVISAT) PSI for two areas of interest: one in Italy and the other in Switzerland. It is demonstrated that withrespect to medium resolution ENVISAT PS processing, fewer CSK high resolution images are sufficient toachieve comparable precision of the mean displacement velocity estimates. This, together with the shorterrevisit times provided by the CSK constellation, can be very important when dealing with emergency situations.Furthermore, it is quantified that from about 3 to 11 times greater PS densities are obtained with thehigher resolution X-band data. This implies more information about ground surface displacements as wellas improved landslide monitoring and slope instability investigation capabilities. Furthermore, ground displacementmeasurements can be interpreted without local knowledge of the focus area or in situ controls,and, nonetheless, guide single hillslope instability assessments with support of Google Earth and its high resolutionoptical imagery. This "blind" approach will allow one to monitor remote and poorly known regions athigh risk of potentially disastrous slope failures.