IEEE Geoscience and Remote Sensing Magazine - March 2020 - 98

phase correlation (in the complex domain) with a model-
implies compensation for both the APD contribution and
significant temporal NLD components but enables the localization and "tracking" of, generally, dominant scatterers.
Therefore, PSI techniques are far more suitable for the
monitoring of isolated scatterers, such as infrastructure
and, in general, the built environment, both of which are
characterized by long-term
stability (persistency) of the
backscattering. This feature
PSI TECHNIQUES ARE FAR
provided important perspecMORE SUITABLE FOR THE
tives for the specific monitorMONITORING OF ISOLATED
ing of single buildings in risk
SCATTERERS, SUCH AS
situations, which was further
INFRASTRUCTURE AND, IN
enhanced by the advent of
GENERAL, THE BUILT
VHR X-band data, offering a
ENVIRONMENT.
major advance in risk assessment in landslide areas.
In line with the development of different approaches based on PS full-resolution
analysis, the SAR tomography technique, which switches
the concept of phase matching to a model into the imaging
domain, was proposed [9], [101], [102]. Similar to the azimuth direction, where a synthetic aperture is constructed,
thereby providing the ability to easily achieve high alongtrack resolution, the availability of multiple baselines also
allows an antenna to be synthesized along the height direction. In addition to offering applications for the monitoring of media with distributed scattering in the vertical
direction (forest, ice, and so on), SAR tomography also represents one of the most advanced processing techniques for
the monitoring of built environments. It provides a unique
tool for accurate 3D reconstruction and long-term slowdisplacement monitoring of even single built structures,
which can be useful for security and long-term structural
health-monitoring issues, particularly in urban areas and
critical infrastructure [11], [12], [103].
SAR tomography expands PSI by focusing the signal
backscattered from ground objects in the third dimension
(height), hence, the name 3D imaging. Backscattering can be

TABLE 1. THE A-DInSAR PROCESSING CATEGORIES.
STACKING OF COHERENT
INTERFEROGRAMS

PERSISTENT SCATTERING
INTERFEROMETRY

Assumption of distributed scattering Assumption of concentrated,
dominant scattering
Spatial averaging to reduce noise

Analysis at full-resolution data
via a proper phase model

Use of only limited (spatial/temporal) baselines, with redundancy

Uses all baselines

Prevalence of sequential (spatial)
Relies on joint (multi-image)
processing of (single) interferograms processing
Monitoring of wide areas at small
scale (coarse resolution)

98

Monitoring of the built environment at full resolution

expanded into harmonics along time (scaled up by the inverse of the wavelength) to detect the presence of a slow deformation. A space/velocity (4D) imaging model is derived
by extending the 3D imaging concept into the time domain
to measure the deformation parameters of any temporal coherent PS in the focused 3D space [9], [102], [104].
Since SAR measures distances, due to its side-looking
geometry, it may frequently happen that scatterers sharing
the same distance from the SAR sensor (i.e., on equirange
curves) are focused into the same range resolution cell, thus
leading to a characteristic geometric distortion known as
layover. Complex scenarios, such as urban areas and infrastructure, are typically characterized by the dense presence
of vertical structures (i.e., buildings and walls). As discussed
in the "Landslide-Induced Damage to Buildings at Risk"
section, such elements generate interference between the
responses of scatterers located at different heights (for example, on roofs, facades, and ground), which may share the
same distance from the sensor and, therefore, be imaged in
the same pixel. The layover phenomenon is extremely evident on VHR SAR data, such as those acquired by X-band
SAR systems (e.g., TerraSAR-X and COSMO-SkyMed), due to
their increased spatial resolution [9], [11]-[13].
With respect to the layover problem, SAR tomography
allows the limitations of classical MT-DInSAR techniques
(such as PSI [4]) to be overcome by avoiding the assumption of the presence of a single (dominant) scatterer per resolution cell. The strength of the tomographic processing lies
in the direct access to the (focused) scene backscattering,
thus allowing separate interferences corresponding to different peaks [9]-[12]. Moreover, as discussed in the following sections, the exploitation of both amplitude and phase
information allows improvement, with respect to classical
PSI methods, in the identification (or, better, the detection)
of single PS per resolution and in the estimation of their
characteristic parameters (RT and DMV). In fact, SAR tomography essentially performs the detection of single scatterers based on the generalized likelihood ratio test (GLRT)
[106]. Approaches for the extension of the GLRT test to detect multiple scatterers per resolution cell have been proposed [107], [108]; the "Sequential Analysis and Decision
Strategies" section provides further details on this issue.
MULTISCALE APPROACHES IN MIXED SCENARIOS
SCI and PSI approaches are complementary in terms of
the spatial resolution of the processing and the nature of
the scatterers (see Table 1). In landslide areas, the fullresolution information is of particular interest for the risk
analysis of the built environment. Nevertheless, access
to the spatially distributed information of landslides is
mandatory for a complete kinematic characterization of
the phenomena.
To widen the applicability of the data-processing algorithms, each approach took inspiration from the main
features of the others. For example, inspired by the advantages offered by the multilook operation performed in the
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MARCH 2020



IEEE Geoscience and Remote Sensing Magazine - March 2020

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