Federico Minati

A New Method for Identification and Analysis of Persistent Scatterers in Series of SAR Images

Synthetic aperture radar (SAR) interferometry is a powerful technology for measuring slow terrain movements. The extraction of this information is a complex task, because the phase of the signal is measured only modulo 2pi and is affected by noise and systematic terms. The persistent scatterer (PS) approach brought important advances in the solution of this problem. In this work, we present a new method, named persistent scatterer pairs (PSP) method, for the identification and the analysis of PS in series of full resolution SAR images. The problems coming from orbital and atmosphere phase artifacts are effectively overcome by exploiting their spatial correlation, without using model based interpolations or fits, which can be advantageous when the atmospheric artifacts or the displacement to be retrieved are not very well described by the models used in the standard PS approach. Moreover, the proposed method does not need a preprocessing to calibrate the data and is insensitive to the density of PS candidates, it is able to identify PS in natural terrains and PS characterized by non linear movements, is computationally efficient and highly parallelizable. The results obtained on real ERS SAR data confirm the validity of the proposed approach.

A three-dimensional phase unwrapping algorithm for processing of multitemporal SAR interferometric measurements

Phase unwrapping is the problem of reconstructing a function on a grid given its values modulo 2/spl pi/. This is a key problem in SAR interferometry and in other fields. The typical availability of multiple 2D SAR interferograms of the same scene suggest the possibility of considering the data as samples of a function in a 3D space-time. This helps better reconstructing the right solution, in the same way as 2D phase unwrapping provides more reliable solutions than the 1D (quite trivial) algorithm. However, computational needs result increased in the 3D case. In this work we describe the proposed algorithm for 3D phase unwrapping, and show the results obtained on simulated and real SAR images.

A General Formulation for Redundant Integration of Finite Differences and Phase Unwrapping on a Sparse Multidimensional Domain

Phase unwrapping and integration of finite differences are key problems in several technical fields, among which is synthetic aperture radar (SAR) interferometry. In this paper, we propose a general formulation for robust and efficient integration of finite differences and for phase unwrapping, which includes standard techniques (e.g., minimum cost flow and least squares phase unwrapping) as subcases. The proposed approach allows obtaining more reliable and accurate solutions by exploiting redundant differential estimates (not only between nearest neighboring points) and multidimensional information (e.g., multitemporal). In addition, a model of the signal (e.g., multibaseline or multifrequency) or external data (e.g., GPS or leveling measurements) can be integrated. The method requires the solution of linear or quadratic programming problems, for which computationally efficient algorithms exist. The validation tests performed on real and simulated SAR data confirm the validity of the method, which was integrated in our production chain and successfully used also in massive productions.

Persistent Scatterer Pair Interferometry: Approach and Application to COSMO-SkyMed SAR Data

Persistent scatterer interferometry is a widely used technique to detect and monitor slow terrain movements, with millimetric accuracy, from satellite synthetic aperture radar (SAR) data. We have recently proposed a method, named persistent scatterer pair (PSP), aimed at overcoming some limitations of standard techniques. The PSP method is characterized by the fact of exploiting only the relative properties of neighboring pairs of points for both detection and analysis of persistent scatterers (PSs), intended in the general sense of scatterers that exhibit interferometric coherence for the time period and baseline span of the acquisitions, including both point-like and distributed scatterers. Thanks to the pair-of-point approach, the PSP technique is intrinsically not affected by artifacts slowly variable in space, like those depending on atmosphere or orbits. Moreover, by exploiting a very redundant set of pair-of-point connections, the PSP approach guarantees extremely dense and accurate displacement and elevation measurements, both in correspondence of structures and when the backscattering is weak or distributed as in the case of natural terrains. In all cases, the measurements keep the full resolution of the input SAR images. In this work, the qualifying characteristics of the PSP technique are described, and several application examples and validation tests based on COSMO-SkyMed data are reported, which demonstrate the validity of the proposed approach.

Multi-scale and block decomposition methods for finite difference integration and phase unwrapping of very large datasets in high resolution SAR interferometry

In the last few years high and very high resolution SAR data have become available, opening new possibilities in the field of SAR interferometry. However, the huge amount of data poses new challenges in terms of computational and memory requirements, in particular to those processing steps that require a global approach to obtain good results, such as elevation or velocity finite difference integration and phase unwrapping. In this paper we propose two approaches to overcome this problem. In the first approach the data to be processed are divided in blocks of smaller size, and different strategies are suggested to make the solutions of the different blocks consistent. The second approach is based on a decomposition of the problem at different scales according to a pyramidal scheme, which makes possible to exploit the available information at a global level (thus guaranteeing optimal quality results) with scalable computational demand. The proposed approaches were successfully tested on high resolution COSMO-SkyMed full frame data.

Sentinel-1 assessment of the interferometric wide-swath mode

This contribution reports on the performance investigations of the interferometric wide swath (IW) mode of Sentinel-1, which is implemented using the terrain observation by progressive scans (TOPS) mode. The key aspects of the TOPS mode that need to be considered for accurate interferometric processing will be presented, and first analyses with Sentinel-1 time series will be shown. The results focus on the pilot sites of Campi Flegrei/Vesuvius and Mexico City, as well as Greenland glaciers. Other aspects related to the interferometric performance are also presented, like the burst synchronization, the pointing accuracy, or the considerations when evaluating non-stationary scenes.

Enhanced PSP SAR interferometry for analysis of weak scatterers and high definition monitoring of deformations over structures and natural terrains

Synthetic aperture radar (SAR) interferometry is an effective technology for detection and monitoring of slow terrain movements with millimetric precision. This information is extracted by means of complex techniques from the phase of the signal. Building on the ideas of the persistent scatterer pair (PSP) method, we present an enhanced method aimed at fully extracting the coherent information even from low intensity SAR signals. The proposed method is very effective at extracting the available information from each single pixel of the SAR image where you could expect a coherent signal, even when there are not strongly scattering structures and the sensed signal is weak, as in the case of rather smooth surfaces or natural terrains. Several examples obtained from the processing COSMO-SkyMed data show that unprecedentedly dense ground deformation measurements can be obtained with the enhanced PSP method, not only corresponding to structures but also in natural terrains.

Analysis and correction of artifacts on differential SAR interferometry for the study of subsidence phenomena

This paper deals with the analysis and correction of artifacts on differential synthetic aperture radar (SAR) interferometry. Particularly, we concentrate on those artifacts arising from the use in interferometric processing of low accuracy acquisition geometry data. To remove these artifacts, we propose a new algorithm able to estimate the acquisition geometry parameters with high accuracy directly from the SAR data. The application of the proposed technique to the study of the subsidence phenomena in Bologna (Italy) and the surrounding area is also presented to validate the new algorithm.

Method of persistent scatterer pairs (PSP) and high resolution SAR interferometry

Synthetic aperture radar (SAR) interferometry is an effective technology for detection and monitoring of slow terrain movements with millimetric resolution. This information is extracted by means of complex techniques from the phase of the signal, which is wrapped modulo 2π and affected by noise and systematic terms. We have recently proposed a new method, named persistent scatterer pairs (PSP), aimed at overcoming some limitations of standard persistent scatter interferometry (PSI) techniques. The method is characterized in that it works only with pairs of nearby pixels both for selecting and analyzing the persistent scatterers (PS), thus being intrinsically not affected by artifacts slowly variable in space, like those depending on atmosphere or orbits. Moreover, the method does not require an initial selection of PS based on the radar backscattered amplitude. In this work, after resuming the main ideas of the PSP method, we show some results obtained in extensive applications with ERS/ENVISAT data, and the first results obtained with high resolution COSMO-SkyMed images.

Nationwide ground deformation monitoring by persistent scatterer interferometry

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