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We scrutinize the reliability of multilooked interferograms for deformation analysis. Designing a simple approach in the evaluation of the accuracy of the estimated deformation signals, we reveal a prominent bias in the deformation velocity maps. The bias is the result of propagation of small phase error of multilooked interferograms through the time series and can sum up to 6.5 mm/yr in case of using the error prone short temporal baseline interferograms. We further discuss the role of the phase estimation algorithms in reduction of the bias and put recommend a unified intermediate InSAR product for achieving high-precision deformation monitoring.

This paper deals with precise absolute geolocation of point targets by means of a pair of high-resolution synthetic aperture radar (SAR) acquisitions, acquired from a satellite. Even though a single SAR image is a 2-D projection of the backscatter, some 3-D information can be extracted from a defocussing analysis, depending on the resolution, thanks to orbital curvature. A second acquisition, observing the same scene under a different look angle, adds stereogrammetric capability and can achieve geolocation accuracy at decimeter level. However, for the stereogrammetric analysis to work, it is necessary to match targets correctly in the two images. This task is particularly difficult if it has to be automatic and Targets are dense. Unfortunately, the defocussing-based geolocation is not sufficient for reliable target matching: the limiting factor is the unknown tropospheric delay that can cause geolocation errors of several meters in the elevation direction. However, observing that the tropospheric phase screen displays a low-pass character, this paper shows how to identify statistically the local atmospheric disturbances, therefore dramatically improving the score of successful matching. All steps involved exploit peculiar radar image characteristics and, thanks to this, avoid generic point cloud matching algorithms. The proposed algorithm is shown at work on a pair of TerraSAR-X staring spotlight images.

SAR interferometry is increasingly accurate in retrieving the surface deformation history over large areas of the Earth. Advancements have occurred in atmosphere delay compensation, both ionospheric and tropospheric: the remaining contributions of atmospheric propagation to the error budget are a few cm for deformation or a few mm/yr for deformation rates, for a typical Sentinel-1 case. We think that today a crucial aspect that needs consideration is the choice of interferograms to be processed to estimate the deformation. Many groups are limiting the processing to high-coherent pairs with short temporal separation: however, short temporal interferograms tend to be biased. This can be understood observing the presence of closure phases. Examining three subsequent images and building the closure phase, one typically founds inconsistencies in the order of a few degrees: scaling the error to the time span of a year the final deformation rate bias can easily reach several mm/yr. A possible physical interpretation of non-zero closure phase is related to moisture variations in semi-transparent media. The short term interferograms are biased because of the asymmetry of the moisture cycles combined with a non-linearity in the phase variations w.r.t. moisture. In our experiments with Sentinel-1 with a limitation of the temporal baseline at 72 days, we have observed biases of ~7 mm/yr on distributed targets. Here permanent scatterers served as the gold standard since their closure phase is always zero. We consider two main approaches to the closure phase problem in InSAR time series: using full-covariance estimators, based on the temporal covariance matrix, and modelling the underlying moisture process. Our preliminary experiences with full covariance matrices indicate a small bias (< 1 mm/yr) with respect to PS processing. Given the low-resolution of available moisture products, we are working on the inversion of moisture levels directly from closure phases. First L-band results are encouraging, compared to ground truth. If models and inversions are correct, the compensation of the modelled phase should reduce the path-dependency in the temporal integration and consequently the biases.

This letter aims to discuss a general framework that allows the direct interpretation of the wrapped DInSAR phase in terms of surface strain S and rotation R components. The methodology is demonstrated showing the estimation of strain and rotation components of a glacier flow using three TerraSAR-X interferometric geometries (ascending right-looking, descending right-looking and descending left-looking. Finally since the left looking geometry can be difficult to obtain on a regular basis, the surface parallel flow assumption is extended to the phase gradients inversion in order to reduce the amount of necessary geometries from three to two.

The effect of the choice of polarization for Distributed Scatterers interferometry has been investigated taking in considerations standard linear polarizations (HH,VV and HV), the Hybrid polarizations (RH and RV) and the circular polarizations (RL,LL and RR. The distribution of the DSs in the different polarizations is discussed and compared with PSs distributions analyzed in [1]. The approach is purely experimental and it has the aim to give an idea of the impact that the different polarimetric bases can provide for DInSAR.

SAR Interferometry with stacks has already shown its potential in identifying permanent scatterers, in processing decorrelating targets, mitigating atmospheric delays, etc., but we believe that there is still potential for retrieving information on the scattering environment which has not been extensively studied yet. In particular interferometric stacks can reveal systematic phase inconsistencies which are not detectable in single interferograms, challenging any simple interpretation of the interferometric phase and associated coherence. The explanation of such inconsistencies requires more complex propagation models than the one based on a simple delay.

Interferometric synthetic aperture radar (InSAR) measurements are merely sensitive to the deformation along the Line of Sight (LOS) direction of the sensor. To improve the geometrical sensitivity and retrieve the three-dimensional deformation, the integration of InSAR from non-coplanar acquisitions as well as fusion with resolution-scale SAR image shift measurements has become a standard approach. Using different statistical measures, we assess and compare the influence of different image acquisition strategies as well as data fusion on the performance of InSAR in 3D deformation retrieval. Integrating nominal InSAR acquisitions, i.e. a set of measurements from ascending and descending tracks acquired from right-looking geometry, a strong correlation between the retrieved 3D parameters in the local vertical-north plane is observable. This correlation is sought to be decreased by non-nominal acquisitions; i.e. left-looking or squinted observations. These acquisition strategies are discussed for consideration in the future L-band mission Tandem-L.

Since more than 20 years Differential SAR Interferometry (DInSAR) is successfully exploited from the geoscientists to model geophysical phenomena. Its capability to detect and measures movements has been largely employed in the field of tectonics, glaciology and geology in general. The estimation of large scale deformation maps from the SAR acquisitions, without any in-situ deployment of instrumentation, is the main advantage of this technique. Many outstanding results have been produced using DInSAR deformation estimations as an input for geophysical modeling. However several source of errors such as geometric and temporal de-correlation or atmospheric delays can spoil the generation of accurate deformation fields limiting the usability of the interferometric data. The accuracy of the phase unwrapping in particular is hampered by the limited quality of the phase, as far as the noise induced errors in solving the ambiguities propagate trough the gradient integration up to the final estimations. People working in the field of SAR interferometry invested lot of efforts in finding methods that could avoid the problem of spatial phase unwrapping. The main objective of this study is to propose a way to reduce the impact of the phase unwrapping moving the attention from the interferometric phase to the local fringe frequencies. The idea is that the local fringe frequencies have also a geophysical meaning. Therefore instead of having the deformation relative to a reference point, as is normally done in DInSAR, an absolute estimation of the deformation gradient can be used. Then the measures of the estimated frequencies can be transformed and compared to the differential of the deformation model avoiding any spatial integration. This should extend the range of exploitation of the SAR interferograms as far as it removes the constrain that impose the interferogram pixels to be spatially connected, permitting also isolated but coherent areas to take part to the model inversion. These features can be particularly interesting in the case of the burst modes such SCANSAR and TOPSAR. In presence of large displacements the phase between different burst can be spatially inconsistent due to the change of LOS direction. This will make the unwrapping and the modeling of the phase considerably more difficult. Therefore the possibility to have estimations that can be independently handled in space would be strongly recommendable. In the first part of the paper the rationale of the method will be presented, showing first the link between the fringe frequencies and the derivatives tensor. Finally the feasibility of the technique is tested using some simulated examples. The full reconstruction of the deformation gradient tensor is shown using different observation geometries. Then two largely employed geophysical models, have been used for ”model-based”’ case studies. The Mogi source, used to describe volcanoes deformations and the Okada source used to model the movements of tectonics faults.