The increasing importance of water supply systems, particularly in drought-prone regions, and the construction of linear structures, like reservoir dams, have led to the need for adapting these structures to changing conditions. Proper management of dam risks is crucial, as their failure can have severe economic and social consequences. Aging dams, such as those in Spain, require effective monitoring strategies to identify structural issues before they become critical threats. In-person inspections are conducted for maintenance, but modern dams integrate monitoring devices. Satellite radar interferometry (InSAR) technology, specifically the European Ground Motion Service (EGMS), enables precise and non-invasive monitoring of surface deformations using Sentinel-1 satellite data. EGMS provides millimeter-scale precision data, offering insights into subsidence, landslides, and stability issues impacting infrastructure. It is a valuable tool for civilian users and dam managers, providing monitoring data without requiring technical expertise. The SIAGUA project in Spain utilizes EGMS to monitor dams and surrounding areas, providing surveillance information to dam managers.

Several techniques have been proposed to observe long-term land surface deformation using phase information of synthetic aperture radar (SAR), for example, interferometric SAR (InSAR), differential interferometric SAR (DInSAR), permanent scatterer interferometric SAR (PS-InSAR), and small baseline subset (SBAS) using images of JERS-1, ALOS-1/2, ENVISAT, ERS-1/2, RADARSAT-1/2, TerraSAR-X, SENTINEL-1A/B, and so on. These methods have also been employed to investigate land deformation and the impact caused by the hot mudflow accident at the study area of regency of Sidoarjo, East Java province, Indonesia, where this disaster happened on May 29, 2006, and has been flowing until now. But these methods only derived the land surface deformation and did not figure geological situation in the Earth, especially the current of geological material outflow. Therefore, this research proposed the continuity equation of the law of conservation of material to estimate the current of material outflow to investigate the impact of the land surface disaster in the study area. The information of vertical deformation or subsidence is derived by the consecutive DInSAR using multispaceborne SAR and substituted into the proposed equation to estimate the volume and current of hot mud outflow. The differential global positioning system (GPS) data collected since 2006 were employed to validate the analysis result of land surface deformation, and the root means square (rms) error is 0.46 m. The result obtained the current at the center of the study area at the beginning of the mudflow period was 6800 m3/day and decreased from 2018 until now that matched well with the local reports. The proposed equation could also be applied for observation of land deformation, volcano, fault activity, underground water, and so on, using remote sensing technologies.

After the great activity of infrastructure development experienced in the 20thcentury, there are thousands of engineering infrastructures that require safety monitoring worldwide. Rigorous inspection programs and the monitoring of these infrastructures, such as reservoir dams, are essential for the safety of citizens and properties. These interventions are usually costly and time-consuming, making in many cases, virtually impossible to monitor individually each dam which may represent a potential security risk. Multi-temporal InSAR has been successfully applied in the monitoring of infrastructures affected by tunneling works, aquifer extraction, and natural phenomena (tectonics, volcanism, seism, …), among others. The great advantage of MT-InSAR is that it provides measurement uncertainties of the order of 1 mm/year, interpreting time series of interferometric phases of coherent reflectors present in the area, called Persistent Scatterers, without the need for field work or any special equipment. To demonstrate the potential and reliability of this technique, in this paper, we present the adaptation and application of MT-InSAR technique to monitor embankment dams, obtaining vertical displacements, characterizing their consolidation rates, and allowing the identification of potential problems that require further field investigation. This study is part of the ReMoDams project, a Spanish research initiative developed for monitoring dam structural stability from space using satellite radar interferometry.

Several techniques have proposed to observe long-term land deformation using phase information of synthetic aperture radar (SAR), for example, InSAR, DInSAR, PS-InSAR, SBAS using spaceborne SAR images. But every mission of spaceborne SAR has an orbit duty period less than ten years and other problems of discontinuity or blank period of observation using the similar specification of sensors. Hence land deformation with an observed time of more than ten years is not available to be monitored continuously using a single mission. This research proposed a method called Bridging Consecutive DInSAR (BC-DInSAR) to connect Consecutive DInSAR. These methods were employed to investigate land deformation and the impact caused by hot mudflow accident at Regency of Sidoarjo, Indonesia, where this disaster happened on 29 May 2006 and is flowing until now. The differential GPS data since 2006 was employed to validate the analysis result of BC-DInSAR, which obtained 0.46 m RMS error.

This work focuses on a reservoir with water leaks since its construction, the Benínar reservoir. The purpose of this reservoir was to regulate the Adra River basin, lying between the provinces of Almería and Granada, and located south of Sierra Nevada Mountains (in the Inner Zones of the Betic Cordilleras, SE Spain). This basin extends over 746 km2, at an altitude of 2780 m, with a very rough terrain and frequent torrential water flow. Due to the continuous extension of greenhouses in the east and west parts of Almería, the water demand for agriculture and urban consumption increases day by day. As a consequence, aquifers are being overexploited, causing the current system to not be sustainable for a long time, that is, the storage capacity of the underground media and their possible contributions to an efficient management of resources have not been adequately taken into account. The Benínar dam has always had problems with water leaks. The dam was built even knowing that the land was not the most suitable, due to the frequent earth movements that took place in the town of Benínar, which was submerged beneath the waters of the reservoir. In this work, we process multi-temporal SAR datasets coming from the C-band satellites ERS-1/2, Envisat, and Sentinel-1A/B using MT-InSAR techniques, being able to monitor the deformation behavior of this dam for a long time period of more than 25 years, from 1992 to 2018.

Coherent point (including persistent and distributed scatterers) selection and phase ambiguity treatment (or parameter estimation) are the key tasks involved in multitemporal InSAR (MTInSAR) algorithms, which are usually conducted separately with empirical thresholds. It is not rare to see that due to the discrepancies on threshold setting, even for the same MTInSAR technique with the same data sets, it will raise different (sometimes quite notable) results and affect the applicability of InSAR techniques. We propose here an integrated MTInSAR estimator that combines the coherent point selection and phase vector unwrapping into a single step. Essentially, the estimator aims to recover the unwrapped phase vector at coherent points. Therefore, it could serve as an alternative solution of spatial-temporal phase unwrapping problem. In the estimator, wrapped phase at all pixels in short baseline interferograms are taken as observations. Starting from the phase differences at arcs of a fully connected network of pixels, based on the residual analysis and spatial closure of phase triangularity, the estimator can detect and delete the arcs having unacceptable phase noise and phase ambiguities. By integrating the phase differences at the remained arcs, the unwrapped phase at coherent points in consecutive acquisition intervals can be obtained. Impressively, the estimator is immune to the bias raised by improper deformation model. The performance of the proposed estimator is evaluated via semisynthetic and real data tests. Considering that the phase enhancement algorithms (e.g., phase-linking and Extended Minimum Cost Flow-Small BAseline Subset) that can reconstruct high-quality wrapped phases are gaining popularity, the proposed estimator can also be implemented as a postprocessing module of these algorithms for retrieval of unwrapped phase vectors at coherent points.

Conventional synthetic aperture radar (SAR) image formation requires uniform sampling along the azimuth direction. These image formation relies on frequency domain algorithms including the Range-Doppler Algorithm (RDA), Chirp-Scaling Algorithm (CSA) [1] etc, that make use of azimuth Fast Fourier Transform, hence the requirement of uniform sampling. While uniform sampling in space is not always achievable, the samples can be interpolated onto a uniform grid given the sampling rate is Nyquist. This paper studies the effect of SAR image formation when the azimuth sampling is not uniform, and not Nyquist in some interval. The result showed that even if there is a wide interval in the middle of the synthetic aperture with no samples, an image can still be formed. This result could be useful when the SAR antenna is a phased array, or when the hardware is generating bursts of pulses followed by a quiet window.

Permanent scatterer interferometry is a multitemporal interferometric synthetic aperture radar technique that produces high-accuracy ground deformation measurement. A high density of permanent scatterer (PS) is required to provide accurate results. In natural environments with low PS density, distributed scatterers (DSs) could serve as additional coherent observations. This paper introduces a polarimetric scattering property-based adaptive filtering method that preserves PS candidates and filters DS candidates. To further increase the coherence estimate of DS candidates, the technique includes a complex coherence decomposition that adaptively selects the most stable scattering mechanisms, thus improving pixel coherence estimation. The proposed method was evaluated on 11 quad-polarized ALOS PALSAR images and 21 dual-polarized Sentinel-1 images acquired over San Fernando Valley, CA, USA, and Groningen, The Netherlands, respectively. The application of this method increased the number of coherent pixels by almost a factor of eight compared with a single-polarization channel. This paper concludes that a coherence estimate can be significantly improved by applying scattering property-based adaptive filtering and coherence matrix decomposition and accurate displacement measurements can be achieved.

An experiment using the Vector Network Analyzer, modified from LiSALAB Ground-Based Synthetic Aperture Radar (GBSAR) system has been performed to evaluate the feasibility of constructing a mobile SAR system operating at L-band. Data was collected in a parking lot in Purdue University, West Lafayette. The system utilizes a GNSS-INS unit to acquire position information for assisting azimuth signal compression, and the acquired data was azimuth compressed with Range-Doppler Algorithm (RDA) [1]. We have demonstrated the feasibility of using a GNSS-INS system to correct the irregular motion of the Radar platform to improve accuracy in signal compression, resultant image with and without position correction has been shown as a comparison.

This paper presents the Hybrid Analysis for Synthetic Aperture Radar (HASAR) framework for time series change detection (TSCD). The HASAR framework utilizes spatiotemporal observations to identify abrupt changes in SAR data stacks. Full resolution is used and no filtering processes are involved. Moreover, no thresholds or prior knowledge of the changes are required. The HASAR framework includes two steps: 1) extraction of spatiotemporal observations and 2) automatic change detection. First, it extracts the time series of statistically homogeneous pixels (SHP) to acquire the spatial temporal observations. Second, it performs the automatic change detection on the spatiotemporal observations to target significant changes in time. This hybrid analysis integrates pixel-/object-based processing techniques with spatial/temporal observations. It preserves details while mitigating the speckle effect. The temporal observations also help justify the significance of the changes. In our experiment, we apply this technique on 30 ERS-2 images. The results show that the HASAR system performs well and possesses the characteristics of generalization. A vast amount of change analysis can be carried out using the same framework. Therefore, the proposed method reveals high potentials for continuous Earth monitoring.

We present a detailed survey on the ongoing destabilization process of the Mosul dam. The dam is located on the Tigris river and is the biggest hydraulic structure in Iraq. From a geological point of view the dam foundation is unstable due to the underlying geology that is formed by alternate and variable strata of highly soluble materials such as gypsum, anhydrite, marl and limestone. Here we present the first comprehensive multi-sensor cumulative deformation map for the dam generated from space-based synthetic aperture radar (SAR) measurements from the Italian constellation COSMO-SkyMed and the European Sentinel-1a satellite. We compared 2014-2016 data to an historic dataset spanning 2004-2010 acquired with the Envisat ASAR sensor. We found that deformation was rapid during 2004-2010, slowed down in 2012-2014, and restarted in August 2014 when grouting operations stopped due to the temporary capture of the dam by the self proclaimed Islamic State in Iraq and Syria (ISIS). We took advantage of the availability of data from multiple SAR satellites to infer the deformation at the dam in great spatial and temporal detail and shed new light on the processes of the ongoing destabilization. This study highlights how new constellations of SAR sensors together with the availability of historical datasets are leading to important advances in deformation monitoring of small scale geologic and manmade features.

The launch of Sentinel-1 constellation raise a new era for InSAR applications and time series analysis. The wide coverage, fine temporal and spatial resolution, precise control of orbits and the free distribution policy bring more new chances than ever to Earth observations applications. In this experiment we use Sentinel-1A data that covers a period of one year and a half (from November 2014 to June 2016) to study the ground movement of California Central Valley. Two different datasets were used and persistent scatterers interferometry technique was adopted for the time series analysis. The result indicates severe ground subsidence inside the study area, reaching a maximum rate of approximately 250 mm/yr at some locations.

Center for Environmental Remote Sensing, Chiba University develops L Band circularly polarized synthetic aperture radar (SAR) sensor for microsatellite (150 kg class). This paper explains the project, specification, antenna deployment, RF system, and experiment of circularly polarized SAR in anechoic chamber. In the future, this sensor will be employed to monitor global land deformation.

Thresholding on coherence is a common practice for identifying the surface scatterers that are less affected by decorrelation noise during post-processing and visualisation of the results from multi-temporal InSAR techniques. Simple selection of the points with coherence greater than a specific value is, however, challenged by the presence of spatial dependence among observations. If the discrepancies in the areas of moderate coherence share similar behaviour, it appears important to take into account their spatial correlation for correct inference. Low coherence areas thus could serve as clear indicators of measurement noise or imperfections in mathematical models. Once exhibiting properties of statistical similarity, they allow for detection of observations that could be considered as outliers and trimmed from the dataset. In this paper we propose an approach based on renowned data mining and exploratory data analysis procedures for mitigating the impact of outlying observations in the final results.

Air or satellite borne radar systems has shown millimeter accuracy in change detection by realizing the phase of the returned signal, which has put the radar system a useful piece of tool in structural monitoring and disaster prevention [1]. However the temporal availability of data is very limited due to long revisiting time. This problem could be solved by using a ground based radar system, which provide continuous monitoring of the area of interest. When operated in synthetic aperture mode, the data frequency is in order of minutes, while when operated in real aperture mode, the data frequency is a fraction of second, allowing not only long term displacement but also vibration mode analysis. The ground based synthetic aperture radar system by LiSALAB was used in this study to demonstrate monitoring of a railway bridge in real aperture and synthetic aperture mode.

Comparing with its former C-band fellows ERS and Envisat, Sentinel data is favored for its short revisit days (12 days) and free distribution policy. Moreover, TOPS mode can cover a much larger monitoring area (250 km in azimuth) while maintaining a relative high range/ azimuth resolution (4m*20m in general). The advantages of TOPS mode made it become the default mode for Sentinel-1. This paper describes the general processing chain for sentinel-1A TOPS mode from importing, coregistration to generating interferograms and its application in monitoring ground subsidence over the Dead Sea region. The data that comes from TOPS's steering antenna will require some additional processing steps comparing with the stripmap mode. In this paper we apply the TOPS data over the Dead Sea region. A number of interferograms are generated to monitor the local ground subsidence and sinkholes. Distinct ground subsidence is clearly visible on TOPS interferograms.

The Jakarta province proposed the Jakarta Giant Sea Wall as the waterfront city for the new urban settlement zone and the deep seaport for the new economic zone along the coastal areas at northern Jakarta. This letter investigated land deformation at 11 watersheds of the West Java Mega Urban Region using the persistent scatterer interferometry technique of the Advanced Land Observing Satellite phased-array-type L-band synthetic aperture radar data. The result shows that land deformation at the study area, particularly the Bandung city area gives a significant impact to sedimentation velocity along the eastern Jakarta strait, particularly the deep seaport for 43 years later. This letter recommends to evaluate land conservation at upland watersheds and the well management of artificial canals to reduce the impact of sedimentation at the Jakarta strait, particularly the new depth seaport.

Much attention was paid on monitoring of the subsidence along large-scale man-made linear features (LMLFs). The high resolution SAR data offers impressive details on these infrastructures with multi-temporal INSAR analysis. This paper validates the INSAR results of time series analysis along a certain railway in China using a high density leveling campaign. The Leveling points are distributed along the railway line and each two leveling points are 60 m apart at a time interval of nearly one month. The study area is located in the west of Tianjin downtown, covering an area of 1.5 km · 7.46 km. The validation results show that the average subsidence rate and time series of PS results are agreed well with that of these leveling points.

Over the last three decades interferometric synthetic aperture radar (InSAR) techniques have attained a fundamental role in surface deformation monitoring. The main importance is given by two key factors: the exploitement of rich SAR data archives and the development of processing techniques able to extract information from these datasets. InSAR time series techniques such as permanent scatterers (PS) or SBAS have proven capable of estimating displacements to with 1 mm precision for targets that show a stable electromagnetic signature. At the same time the availability of SAR constellations with a reduced revisit time, such as the X band COSMO-SkyMed (CSK) and TerraSAR-X/PAZ satellites, has reduced the minimum detectable deformation gradient between two neighboring points and improved the theoretical precision given for a network of selected stable pixels. Independently from the applied processing methodology, the main drawback at X-band is the shorter time interval to temporal decorrelation. This effect can be partially mitigated by a short repeat time acquisitions plan. A technique able to deal with distributed scatterers is fundamental for extending the spatial coverage of the coherent area. In this paper we analyze the potential of X-band COSMO-SkyMed short repeat pass interferometry over a rural area in the southern part of Italy for assessing the capabilities and limitations of the Quasi-PS (QPS) technique in an X-band non-optimal scenario.

In the state of Indiana over 1500 million tons of coal were extracted from underground in the past 150 years. As a consequence, today many of the long-abandoned mines pose a serious risk to the local. When the abandoned mine shafts collapse, the ground above can drop as much as several feet along with everything on it. Thus it is essential for a long-term-wide-range monitoring technique to map the ground subsidence over the local mining area. In this study, we try to evaluate the mining area ground subsidence with interferometric synthetic aperture radar (InSAR) technique, which is famous for its capability of wide-area and high-precision monitoring. Furthermore, for better understanding the ground motion with respect to difference RADAR frequencies, we use data with different wavelengths in this project, C-band and L-band respectively. The result shows that overall the monitored area were stable but we can still detect ground movements at the location of a few mining spots.

Change detection is an application to which SAR is particularly well suited since SARs can consistently produce high quality fine resolution imagery from multiple repeat pass collections. The aim of this paper is to extract all possible information from SAR-InSAR data and to asses to which extent this information can be exploited for change detection purposes in Hong Kong. We collaborate with Planning Department of Hong Kong and choose two small areas as experimental cases. A time series of high resolution SAR images acquired by the German satellite TerraSAR-X are used. High resolution optical images are used for comparison and comprehension.

The persistent scatterers (PS) technique is a powerful remote sensing technology that exploits a long series of synthetic aperture radar data for monitoring ground deformations with millimeter accuracy on a high spatial density of ground targets. One of the major limitations of this technique is due to atmospheric effects, and in particular to high Water Vapor (WV) variability. As a consequence, to successfully apply interferometric techniques, the atmospheric WV delay must be estimated and removed. On the contrary, PS technique could also be used to study high-resolution spatial-temporal water vapor characteristics. In this work, we investigate the atmosphere effects with a series of high-resolution TerraSAR-X (TSX) data and PS technique in Hong Kong. We will present some of the preliminary results as well as the discussions over the stratification and turbulence estimation.

Using high-resolution SAR data stacks, a large amount of persistent scatterers (PS) can be found in urban areas, which are used for very detailed surface deformation monitoring. However, in dense urban areas, many of these points consist of more than one dominant scatterer in elevation direction. Using tomographic SAR, points with more than one scatterer are detected.

Permanent Scatterers (PS) based on Synthetic Aperture Radar (SAR) data has been approved to be a feasible way to detect and monitor wide area ground subsidence at a low cost. Classical PS algorithms are focused on exploring the point-like radar targets while Quasi-PS (QPS) technique was proposed to manage the detection of both distributed and decorrelating targets. In this work, we explore the potential ability of PS and QPS analysis for subsidence monitoring with L- and X-band. A case study was conducted in Tianjin suburbs and the exploited SAR datasets were composed of 23 ALOS images acquired from 2009/4/24 to 2010/10/28 and 37 TerraSAR-X images from 2009/4/29 to 2010/11/11. The average subsidence velocity map and displacement history have been retrieved by PS and QPS analysis and the validation indicates good agreement between INSAR time series results and leveling data.

TerraSAR X-band (TSX) data can provide a high resolution of 1m, short revisit period of 11 days, and sensitive ground subsidence information. Permanent Scatterers (PS) technology was developed as a powerful tool for extracting the ground displacement information from TSX images. In this work, we exploit the potential of TSX for monitoring ground subsiding for the urban area in Hong Kong. A total of 57 TSX images and 11 TanDEM-X (TDX) images acquired between 25thOctober 2008 and 4thMay 2012 were used in the InSAR time series analysis for retrieving subsidence in the study area. A Corner Reflector (CR) validation test is conducted in the area to quantitatively analyze the performance of PS analysis, and the result indicates that the analysis can achieve millimetric accuracy for its monitoring the ground subsidence.

Cosmo SkyMed is a constellation of Satellites launched by the Italian Space Agency providing Synthetic Aperture Radar images in interferometric mode in X band with a resolution of 3m and revisit time of a few days. In this work, two Cosmo SkyMed datasets of 29 images each, acquired along ascending and descending orbits are exploited for studying the stability of the area of the Three Gorges Dam in China. The short observation time allowed by the short revisit time can generate troubles when distinguishing between seasonal dilation and displacement. Nonetheless, the preliminary analysis clearly highlights movements of the riverbanks around the dam.

By means of the permanent scatterer (PS) technique, repeated spaceborne synthetic aperture radar (SAR) images with relatively low resolution (about 25 m × 5 m for the European Remote Sensing (ERS) and Envisat satellites) can be used to estimate the displacement (1-mm precision) and 3-D location (1-m precision) of targets that show an unchanged electromagnetic signature. The main drawback of the PS technique is the limited spatial density of targets that behave coherently during the whole observation span (hundreds of PSs per square kilometer in urban site and up to few points in vegetated areas). In this paper, we describe a new approach for multitemporal analysis of SAR images that also allows extracting information from partially coherent targets. The basic idea is to loosen the restrictive conditions imposed by the PS technique. The results obtained in different test sites allowed to increase significantly the spatial coverage of the estimate of height and deformation trend, particularly in extraurban areas.

In the paper we proposed a comparison methodology between GPS delay and SAR Atmospheric Phase Screen (APS) in both differential and pseudo-absolute mode. ENVISAT ASAR and synchronous GPS campaign data in Como, Italy were collected and processed. APS from PSInSAR has been divided into even groups according to their height for analysis of stratification sensitivity. Then the stratification and assumed turbulent terms from SAR APS and GPS were compared. The stratified ratio from GPS and SAR APS in differential mode is in agreement of 7.7 mm/km with bias of 3.4 mm/km, with a correlation coefficient higher than 0.7 in the ascending case. The atmospheric total delay coincides with STD of differences smaller than 4 mm (~0.65 mmPWV) with correlation coefficients higher than 0.6. The results predicted the extent on which atmospheric measurements from GPS and InSAR are comparable.

Both the resolution SAR image and acquisition period are significantly improved with the dramatically development in astronautics and microelectronic techniques, providing new opportunities for SAR specialists to analyze data whose information has never been as abundant as nowadays. In this work, we introduce two algorithms to handle high resolution SAR images: for spatial SAR comparison, we employ a normalized difference algorithm and a Wiener filter; for time domain amplitude analysis, we conduct analysis of variance, or ANOVA. The validation of both algorithms is verified by our campus experiment and successfully applied in the administration work of Hong Kong government.

TerraSAR X-band provides high resolution of 1m, short revisit period of 11 days, and sensitive subsidence information, while it cannot afford relatively so high temporal and spatial coherence and wide area coverage as ALOS L-band data. In this work, we exploited the potential of combining Land X-band for subsidence monitoring. A case study was conducted in Tianjin induced by water withdrawal. A total of 9 ALOS images acquired from 2009/4/24 to 2010/10/28 and 37 TerraSAR-X images acquired from 2009/4/29 to 2010/11/11 were used in INSAR time series analysis for retrieving subsidence in the study area. DINSAR analysis based on Minimum Spanning Tree (MST) was applied into L-band to identify potential deformation area and X-band Permanent Scatterers (PS) analysis for this area was carried out for subsidence time series study.

In this work we report the main conclusions of the European Space Agency (ESA) Metawave project (Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects) for what concerns the Synthetic Aperture Radar Interferometry (InSAR) viewpoint. The Atmospheric Phase Screen (APS) estimated with the Permanent Scatterers (PS) technique in the test sites of Roma and Como has been compared with MM5 simulations, Meris Water Vapor (WV) data and GPS Zenith Wet Delays (ZWD). The experiment shows that, even though MM5, Meris and GPS data detect the similar absolute amount of WV, their accuracy is not enough to compensate the InSAR sensitivity to the WV spatial variability. In particular, Numerical Weather Prediction models look promising for correcting long WV spatial wavelengths in presence of topography; GPS measurements reveal the best performances toward short WV spatial wavelengths, while Meris data in the analyzed area show generally poor reliability.

Spaceborne Interferometric Synthetic Aperture Radar (InSAR) is a well established technique useful in many land applications, such as tectonic movements, landslide monitoring and digital elevation model extraction. One of its major limitation is the atmospheric effect, and in particular the high water vapour spatial and temporal variability which introduces an unknown delay in the signal propagation. This paper describes the general approach and some results achieved in the framework of an ESA funded project devoted to the mapping of the water vapour with the aim to mitigate its effect in InSAR applications. Ground based (microwave radiometers, radiosoundings, GPS) and spaceborne observations (AMSR-E, MERIS, MODIS) of columnar water vapour were compared with Numerical Weather Prediction model runs in Central Italy during a 15-day experiment. A dense network of GPS receivers was deployed close to Como, in Northern Italy, to complement the operational network in order to derive Zenith Total Delay as well as Slant Delay which can support InSAR processing. A comparison with Atmospheric Phase Screens (APS) derived from a sequence of Envisat multi pass interferometric acquisitions processed using the Permanent Scatters technique on the two test sites has been also performed. The acquired experimental data and their comparison give a valuable idea of what can be done to gather information on water vapour, which, besides InSAR applications, plays a fundamental role in weather prediction and radio propagation studies. The work has been carried out in the framework of an ESA funded project, named "Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects" (METAWAVE). This paper presents the general approach an the various methodologies exploited in the project, together with the overall intercomparison of the results. In deep details on the comparison with the InSAR APS maps derived by the PS technique, as well as on GPS receiver processing and water vapour tomography are reported in two companion papers.

Synthetic Aperture Radar Interferometry (InSAR) is an alternative technique to obtain measurements of surface displacement providing better spatial resolution and comparable accuracy at an extremely lower cost per area than conventional surveying methods. InSAR is becoming more and more popular in monitoring urban deformations, however, the technique requires advanced tools and high level competence to be successfully applied. In this paper we report important results obtained by analyzing new high resolution SAR data in urban areas. Using the SARPROZ InSAR tool and integrating the results with Google Earth, we discovered the (not public) precise tracks of new subways lines in Shanghai by detecting the millimetric surface subsidence caused by tunnel excavation. The data used in this work have been acquired by the Italian sensor Cosmo-SkyMed. About 1.2 million of individual and independent targets have been detected in 600 sqkm, revealing impressive details of the ground surface deformation. Through the high spatial resolution (about 2 m) and millimetric sensitivity in the Line Of Sight (LOS), Cosmo is able to highlight very localized subsidence with remarkable accuracy.

The phase coherence in synthetic aperture radar interferometry is often used in classification algorithms to detect possible temporal changes of the imaged terrain. However, in mountain areas, the interferometric coherence is also sensitive to the slight variations of the acquisition geometry. In this letter, we propose a very simple but effective method to separate the temporal decorrelation from the geometrical one. Assuming the imaged terrain can be modeled as a distributed target, the geometrical coherence can be estimated by exploiting a topographic model and the sensor acquisition parameters. The discrepancy between the geometrical coherence and the observed one can then be ascribed to temporal changes. Moreover, in presence of pointlike targets, the hypothesis of distributed terrain is no longer valid, and higher values of the observed coherence with respect to the synthetic geometrical one can be used to detect such targets. The proposed approach allows then in mountain areas the following conditions: (1) a simple and very fast rough estimation of the temporal coherence, and (2) the identification of pointlike targets using just two images. The method has been applied and tested in the Badong (China) site using European Remote Sensing satellite tandem data.

Spaceborne Interferometric Synthetic Aperture Radar (InSAR) is a well established technique useful in many land applications, such as monitoring tectonic movements and landslides or extracting digital elevation models. One of its major limitations is the atmospheric variability, and in particular the high water vapor spatial and temporal variability, which introduces an unknown delay in the signal propagation. On the other hand, these effects might be exploited, so as InSAR could become a tool for highresolution water vapor mapping. This paper describes the approach and some preliminary results achieved in the framework of an ESA funded project devoted to the mitigation of the water vapor effects in InSAR applications. Although very preliminary, the acquired experimental data and their comparison give a first idea of what can be done to gather valuable information on water vapor, which play a fundamental role in weather prediction and radio propagation studies.

The permanent scatterers (PS) technique is able to estimate atmospheric artifacts that delay the SAR interferometric phase over pointwise targets. In this work, for the first time, the atmospheric phase screen (APS) estimated with ERS data is compared to the APS retrieved with ENVISAT data at 30' time delay. The goal of the comparison is to add new insights to the phase delay that affects InSAR measurements. Preliminary but interesting real data results have been obtained over the city of Milan.

After the Three Gorges Dam began to function in China in 2003, the water level of the Yangtze River in Three Gorges area rose more than 100 meters. The impact of the man-made reservoir caused by the dam on the surroundings is becoming the object of several studies. In this paper we make use of two long term D-InSAR techniques, the quasi-permanent scatterer technique (QPS) and the Stanford Method for Persistent Scatterer (StaMPS), to measure the deformation trends in Badong, Three Gorges area, China. The results obtained by the two processing tools with the same focused and co-registered data sets are analyzed and compared. Two subsidence areas are identified by both techniques. However, since the QPS analysis is able to process partially coherent targets, many more points are extracted than in StaMPS, and more information can be retrieved.

The permanent scatterers (PSs) technique is an operational tool in the context of spaceborne synthetic aperture radar interferometry for monitoring the displacement of radar targets with millimetric accuracy. Recently, the target localization capability of the PS technique has been subject of study, and the possibility of generating digital elevation models (DEMs) and digital terrain models (DTMs) by means of the height of a sparse set of points has been evaluated. In this letter, for the first time, the PS height estimate has been validated by exploiting about 250,000 spot heights at street level derived from photogrammetric techniques in the urban area around Milan, Italy. The very high correlation between the two independent measurements confirms the theoretical submetric accuracy of vertical positioning. A multitrack PS DTM has then been generated and compared to the spot heights together with the corresponding Shuttle Radar Topography Mission (SRTM) DEM, showing the very high improvement given by the PS technique to the freely available topographic data. The results have been obtained by processing about 300 European Space Agency (ESA) European Remote Sensing (ERS) satellite and Envisat images acquired from two descending tracks and an ascending one over Milan.

In this work we present a methodology for developing a Permanent Scatterers (PS) analysis jointly exploiting data acquired from parallel orbits to estimate height and deformation trend of multi-angle urban targets. The methodology allows applying the PS technique also in areas where the number of ASAR acquisitions per single track would prevent to get reliable estimates. Preliminary experimental results achieved in the Shanghai test site confirm the promising potential of the proposed methodology.

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

The present paper provides the first example of 3D imaging using the ground based parasitic SAR instrument developed in the past years at Politecnico di Milano.

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

Continuity of the European Remote Sensing Satellite Synthetic Aperture Radar (ERS SAR) archive by means of Envisat Advanced SAR (ASAR) data acquired from March 2002 has introduced the problem of the coherent combination of images coming from sensors with slightly different frequencies. The spectral shift principle states that in case of extended distributed targets, the frequency shift is equivalent to a change of looking angle. In this paper, the same principle is exploited to analyze the behavior of permanent scatterers (PSs) with an extension that is smaller than the ground resolution cell. The conditions under which the PSs identified by ERS can be continued by Envisat are then theoretically determined and experimentally validated. Moreover, this analysis shows that acquisitions characterized by different frequencies can be used to identify the slant-range position of scatterers with high subcell accuracy (tens of centimeters). From the processing side, a very precise images coregistration step is required to get the results described in this paper

The permanent scatterers (PS) technique is a powerful operational tool that exploits a long series of synthetic aperture radar data for monitoring ground deformations with millimeter accuracy on a high spatial density grid of pointwise targets. The technique has been applied successfully to a number of applications, from subsidence and volcano monitoring to slow-landslide detection. This paper aims to analyze and demonstrate the positioning capability of the PS technique applied to the generation of urban elevation maps. The problem of the univocal identification of the PS position (discarding pixel-dependent sidelobes, both far and near) is addressed, and an easy and efficient solution is proposed. The results obtained in the Milan site allow the appreciation of the very high quality of an urban digital elevation model retrieved with the PS technique. The ground level of the city is identified with submeter accuracy, and elevated targets, where present, reveal building profiles. The estimated city street level (ranging plusmn3m in 16times18 km2) is then compared to those obtained with the same technique using a descending parallel track and an ascending one. Furthermore, the estimated PS elevation with respect to the ground has been connected to temperature-dependent elongations of high structures

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

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