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. Test sites have been selected according to the availability
of archived SAR data, GNSS networks, and geological data. A stack of COSMO-SkyMed data, acquired in stripmap
mode between 2011 and 2015, has been selected over the Banda Aceh region in Sumatra island. Geological maps of the
test sites are available, and several GNSS stations from the Continuously Operating Reference Stations Indonesian
network are found in the area of interest. Both the SPINUA and the StaMPS MTI algorithms have been used for
processing the data, and deriving displacement maps. The ground deformations detected on the area are interpreted
according to the available geological and geophysical information. The MTI results seem to confirm the inactivity of the
Aceh fault segment, while the lack of coherent targets hinders reliable displacement measurements along the Seulineum
segment. MTI data additionally allowed to identify local, non-tectonic ground instabilities: several areas are affected by
subsidence 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.
Ground Control Points (GCP), automatically extracted from Synthetic Aperture Radar (SAR) images through 3D stereo analysis, can be effectively exploited for an automatic orthorectification of optical imagery if they can be robustly located in the basic optical images. The present study outlines a SAR/Optical cross-matching procedure that allows a robust alignment of radar and optical images, and consequently to derive automatically the corresponding sub-pixel position of the GCPs in the optical image in input, expressed as fractional pixel/line image coordinates. The cross-matching in performed in two subsequent steps, in order to gradually gather a better precision. The first step is based on the Mutual Information (MI) maximization between optical and SAR chips while the last one uses the Normalized Cross-Correlation as similarity metric. This work outlines the designed algorithmic solution and discusses the results derived over the urban area of Pisa (Italy), where more than ten COSMO-SkyMed Enhanced Spotlight stereo images with different beams and passes are available. The experimental analysis involves different satellite images, in order to evaluate the performances of the algorithm w.r.t. the optical spatial resolution. An assessment of the performances of the algorithm has been carried out, and errors are computed by measuring the distance between the GCP pixel/line position in the optical image, automatically estimated by the tool, and the “true” position of the GCP, visually identified by an expert user in the optical images.
High-resolution Synthetic Aperture Radar (SAR) data represent an essential resource for the extraction of Ground Control Points (GCP) with sub-metric accuracy without in situ measurement campaigns. Conceptually, SAR-based GCP extraction consists of the following two steps: (i) identification of the same local feature on more SAR images and determination of their range/azimuth coordinates; (ii) spatial 3D positioning retrieval from the 2D radar coordinates, through spatial triangulation (stereo analysis) and inversion methods. In order to boost the geolocation accuracy, SAR images must be acquired from different line of sights, with intersection angles typically wider than 10 degrees, or even in opposite looking directions. In the present study, we present an algorithm specifically designed for ensuring robustness and accuracy in the fully automatic detection of bright isolated targets (steel light poles or towers) even when dealing with opposite looking data takes. In particular, the popular Harris algorithm has been selected as detector because it is the most stable and robust-to-noise algorithm for corners detection on SAR images. We outline the designed algorithmic solution and discusses the results derived over the urban area of Pisa (Italy), where more than ten COSMO-SkyMed Enhanced Spotlight (ES) stereo images are available, thus resulting an optimal test site for an assessment of the performances of the processing chain. The experimental analysis proofs that, assumed timing has been properly recalibrated, we are capable to automatically extract GCP from CSK ES data takes consisting in a very limited number of images.
Rheticus® is an innovative cloud-based data and services hub able to deliver Earth Observation added-value products through automatic complex processes and, if appropriate, a minimum interaction with human operators. In this paper, we outlines the capabilities of the “Rheticus® Displacement” service, designed for geohazard and infrastructure monitoring through Multi-Temporal SAR Interferometry techniques.
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.
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
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.
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/km<sup>2</sup> 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.
The paper investigates the potentialities of the COSMO/SkyMed (CSK) constellation for ground elevation measurement
through conventional and multi-temporal SAR Interferometry (InSAR), with particular attention devoted to the impact of
the improved spatial resolution with respect to the previous SAR sensors. The Atmospheric Phase Screen (APS) is wellknown
to be the main source of errors for accurate topographic mapping through SAR interferometry, in case of
monostatic sensors. Different strategies can be adopted to filter out this signal, ranging from the exploitation of the wellknown
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.
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.
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. The phase of stable
scatterers evolves linearly with the sub-band central wavelength, the slope being proportional to the absolute optical path difference. Consequently, both phase uwrapping and height computation can be performed on a pixel by pixel basis without spatial integration. Recently the technique has been used to derive ground elevation by processing
interferometric pairs acquired in Spotlight mode by both TerraSAR-X and COSMO-SkyMed satellite missions. However, further potential applications are possible. In particular, this work is aimed at experimenting the use of MCA for measuring the optical path between the SAR sensor and the scene by processing a single SAR acquisition. In this configuration, the slope of the phase trend along frequencies depends on the full optical path. In order avoid liasing, we adopted a processing scheme which consists in subtracting from the SAR image phase a term proportional to the distance computed through inverse geocoding. Assuming negligible the positioning errors, the validation of this approach can be performed by comparing the distance measured by MCA with the atmospheric delay computed through analytical models. We carried out a feasibility study aimed at evaluating the maximum value for the errors in satellite and target positions, allowed to perform the reliable validation. Then, in order to reduce the error in the target positions and to guarantee good phase stability, we selected SAR acquisitions which include artificial corner reflectors to be used for MCA processing and the following validation procedure. We present results obtained by exploiting two corner reflectors visible within two TerraSAR-X images acquired in Spotlight mode over Venice Lagoon.
This study is aimed at exploring the potentials of SAR Interferometry (InSAR) to aid Unmanned Aerial Vehicles (UAV)
navigation. The basic idea is to infer both position and attitude of an aerial platform by inspecting the InSAR phase
derived by a real time SAR interferometer mounted onboard the platform. Thanks to the expected favorable conditions in
terms of geometrical sensitivity as well as signal coherence, the InSAR phase field can be used to derive the terrain
elevation. By using both approximated position and attitude values of the platform as well as a reference Digital Terrain
Model (DTM) from a mission database available onboard, it is possible to generate a synthetic InSAR phase model to be
compared w.r.t. that derived by SAR observations. The geometrical transformation needed to match these two terrain
models depends on the difference between position and attitude values derived by the instruments available on board and
their actual values. Hence, this matching provides a feedback to be used for adjusting position and attitude. In order to
assess the reliability of the proposed approach, we evaluated the interferometric sensitivity to changes in position and
attitude. This analysis defines the limits of applicability of the InSAR-based approach and provides indications and
requirements on geometric and radiometric parameters.
The recent availability of wide-bandwidth, high-frequency, high-resolution SAR data is contributing to improved monitoring
capabilities of spaceborne remote sensing instruments. In particular, the new COSMO/SkyMed (CSK) and TerraSAR-
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
capabilities. 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 is large enough. 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.
In the present work we present first results of ground deformation measurements inferred through repeat-pass Synthetic Aperture Radar (SAR) Interferometry (InSAR) in C- and X-band over an Italian Alpine area, in Lombardia region. The activity was carried out in the framework of the MORFEO (MOnitoraggio e Rischio da Frana mediante dati EO) project founded by the Italian Spatial Agency (ASI) and dedicated to landslide risk assessment.
A number of areas affected by hydrogeological instabilities have been selected and studied in detail by processing both C- and X-band SAR data through SPINUA, a Persistent Scatterer like algorithm. In particular, two stacks of 30 Ascending ENVISAT SAR images (October 2004 - January 2009) and 32 Descending ENVISAT SAR images (December 2004 - January 2009) have been independently processed to ensures the detection of movements occurring along both west and east facing slopes. Moreover, further deformation measurements have been obtained by processing a set of 12 COSMO-SkyMED ascending HIMAGE interferometric acquisitions (Satellite CSKS1; Beam HI-03; POL: HH; Incidence Angle: 29°) provided by ASI. After a proper tuning of the interferometric algorithmic solutions, even with very high normal baselines, we are able to appreciate the potentials of X-band interferometry. Although the number of COSMO-SkyMED acquisitions is quite limited, spanning a period of only 10 months (August 2008 - June 2009), SPINUA was capable to retrieve preliminary ground displacement patterns that are in good agreement with those previously estimated in C-band. Quite impressive is to realize that, because of the tenfold improved resolution of X-band images, multi-temporal InSAR techniques may be successfully applied for the estimation of the displacement maps with a number of acquisitions much lower than in C-band.
InSAR-derived displacements provided on areas of hydrogeological interest are going to be validated in the framework of MORFEO project by the geological partnership thanks to the availability of ground truths. In the present work, we present the results obtained on the towns of Garzeno, Catasco and Germasino which are affected by landslide phenomena. We provide a first validation by comparing the deformation maps derived from ENVISAT C-band data, from COSMO-SkyMED X-band data as well as from ERS and RADARSAT C-band data freely available on the GeoIFFI web-catalogue.
The TerraSAR-X (copyright) mission, launched in 2007, carries a new X-band Synthetic Aperture Radar (SAR) sensor
optimally suited for SAR interferometry (InSAR), thus allowing very promising application of InSAR techniques
for the risk assessment on areas with hydrogeological instability and especially for multi-temporal analysis, such
as Persistent Scatterer Interferometry (PSI) techniques, originally developed at Politecnico di Milano. The
SPINUA (Stable Point INterferometry over Unurbanised Areas) technique is a PSI processing methodology
which has originally been developed with the aim of detection and monitoring of coherent PS targets in non
or scarcely-urbanized areas. The main goal of the present work is to describe successful applications of the
SPINUA PSI technique in processing X-band data. Venice has been selected as test site since it is in favorable
settings for PSI investigations (urban area containing many potential coherent targets such as buildings) and
in view of the availability of a long temporal series of TerraSAR-X stripmap acquisitions (27 scenes in all).
The Venice Lagoon is affected by land sinking phenomena, whose origins are both natural and man-induced.
The subsidence of Venice has been intensively studied for decades by determining land displacements through
traditional monitoring techniques (leveling and GPS) and, recently, by processing stacks of ERS/ENVISAT SAR
data. The present work is focused on an independent assessment of application of PSI techniques to TerraSAR-X
stripmap data for monitoring the stability of the Venice area. Thanks to its orbital repeat cycle of only 11 days,
less than a third of ERS/ENVISAT C-band missions, the maximum displacement rate that can be unambiguously
detected along the Line-of-Sight (LOS) with TerraSAR-X SAR data through PSI techniques is expected to be
about twice the corresponding value of ESA C-band missions, being directly proportional to the sensor wavelength
and inversely proportional to the revisit time. When monitoring displacement phenomena which are known to
be within the C-band rate limits, the increased repeat cycle of TerraSAR-X offers the opportunity to decimate
the stack of TerraSAR-X data, e.g. by doubling the temporal baseline between subsequent acquisitions. This
strategy can be adopted for reducing both economic and computational processing costs. In the present work,
the displacement rate maps obtained through SPINUA with and without decimation of the number of Single
Look Complex (SLC) acquisitions are compared. In particular, it is shown that with high spatial resolution SAR
data, reliable displacement maps could be estimated through PSI techniques with a number of SLCs much lower
than in C-band.
Image alignment is without doubt the most crucial step in SAR Interferometry. Interferogram formation requires
images to be coregistered with an accuracy of better than 1/8 pixel to avoid significant loss of phase coherence.
Conventional interferometric precise coregistration methods for
full-resolution SAR data (Single-Look Complex
imagery, or SLC) are based on the cross-correlation of the SLC data, either in the original complex form or
as squared amplitudes. Offset vectors in slant range and azimuth directions are computed on a large number
of windows, according to the estimated correlation peaks. Then, a two-dimensional polynomial of a certain
degree is usually chosen as warp function and the polynomial parameters are estimated through LMS fit from
the shifts measured on the image windows. In case of rough topography and long baselines, the polynomial
approximation for the warp function becomes inaccurate, leading to local misregistrations. Moreover, these
effects increase with the spatial resolution and then with the sampling frequency of the sensor, as first results on
TerraSAR-X interferometry confirm. An improved, DEM-assisted image coregistration procedure can be adopted
for providing higher-order prediction of the offset vectors. Instead of estimating the shifts on a limited number of
patches and using a polynomial approximation for the transformation, this approach computes pixel by pixel the
correspondence between master and slave by using the orbital data and a reference DEM. This study assesses the
performance of this approach with respect to the standard procedure. In particular, both analytical relationships
and simulations will evaluate the impact of the finite vertical accuracy of the DEM on the final coregistration
precision for different radar postings and relative positions of satellites. The two approaches are compared by
processing real data at different carrier frequencies and using the interferometric coherence as quality figure.