Phase statistics of interferograms with applications to synthetic aperture radar (original) (raw)

Phase Inconsistencies and Multiple Scattering in SAR Interferometry

IEEE Transactions on Geoscience and Remote Sensing, 2015

With three coherent SAR images it is possible to form three interferograms. In some cases the phases of the three averaged interferograms will be significantly inconsistent and indicate a sort of phase excess or deficit (which we call lack of triangularity or inconsistency). In this paper we illustrate theoretically which models can explain such phenomenon and provide some real-data examples. It is also shown that two or more independent scattering mechanisms are necessary to explain phase inconsistencies. The observation of lack of consistency might be useful to derive information on the target and also as a warning that the scatterer presents a temporal covariance matrix which is not intrinsically real, with consequences for the processing of interferometric stacks.

Phase gradient approach to stacking interferograms(SAR topography)

The phase gradient approach is used to construct averages and differences of interferograms without phase unwrapping. Our objectives for change detection are to increase fringe clarity and decrease errors due to tropospheric and ionospheric delay by averaging many interferograms. The standard approach requires phase unwrapping, scaling the phase according to the ratio of the perpendicular baseline, and finally forming the average or difference; however, unique phase unwrapping is usually not possible. Since the phase gradient due to topography is proportional to the perpendicular baseline, phase unwrapping is unnecessary prior to averaging or differencing. Phase unwrapping may be needed to interpret the results, but it is delayed until all of the largest topographic signals are removed. We demonstrate the method by averaging and differencing six interferograms having a suite of perpendicular baselines ranging from 18 to 406 m. Cross-spectral analysis of the difference between two Tandem interferograms provides estimates of spatial resolution, which are used to design prestack filters. A wide range of perpendicular baselines provides the best topographic recovery in terms of accuracy and coverage. Outside of mountainous areas the topography has a relative accuracy of better than 2 m. Residual interferograms (single interferogram minus stack) have tilts across the unwrapped phase that are typically 50 mm in both range and azimuth, reflecting both orbit error and atmospheric delay. Smaller-scale waves with amplitudes of 15 mm are interpreted as atmospheric lee waves. A few Global Positioning System (GPS) control points within a frame could increase the precision to -20 mm for a single interferogram; further improvements may be achieved by stacking residual interferograms. 1993; Massonnet et al., 1993; Zebker et al., 1994a; Massonnet and Feigl, 1995a; Dixon et al., 1993; Meade and Sandwell, 1996]. Sums of interferograms could be used to generate high-resolution topographic maps [Zebker and Goldstein, 1986; Werner et al., 1992; Madsen et al., 1993; Zebker et al., 1994b, 1997], while differences may reveal tectonic deformations and atmospheric-ionospheric disturbances [Afraimovich et al.We present a new approach to the analysis of interferograms based on the gradient of the phase rather than the phase itself. Because this method is largely untested, we attempt to address the following questions: What is the best mathematical model for relating phase and phase gradients given uncertainties in the data? What are the main limitations of InSAR measurements derived from ERS data for both line of sight (LOS) accuracy and horizontal resolution? How can InSAR data be improved for both topographic recovery and change detection? What is the best design for an InSAR processing system in order to achieve near optimal results and be efficient? Of course, many Paper number 1998JB900008. 0 ! 48-0227/98/! 998 JB900008509.00 of these questions have been adequately addressed in previous publications, and there already exist tested and efficient InSAR codes. Nevertheless, we hope our answers will help clarify the literature in several areas.

Multi-baseline SAR Interferometry using Elaboration of Amplitude and Phase Data

Universal Journal of Electrical and Electronic Engineering, 2015

Aperture Radar (In-SAR) systems can be exploited to estimate the Digital Elevation Model (DEM) of the observed scene without ambiguities and with an increased accuracy, even in the case of high sloped ground regions. The techniques usually used exploit only the interferometric phase information and they are based on Maximum Likelihood (ML) estimation. An important problem to be taken into account is the mutual correlation of the (complex) interferometric images which impedes the closed form evaluation of the interferometric phases likelihood function. Moreover the statistical independence approximation of the phase interferograms is usually adopted. In this paper we present a method exploiting both amplitude and phase of the interferometric images, with the purpose of expressing the multi-baseline likelihood function in closed form, and we show that, when the number of baselines increases, to achieve an higher estimation accuracy the images mutual correlation cannot be neglected. We also show that to obtain a full resolution speckle reduced intensity image from several full resolution multi-baseline interferometric (complex) images, a phase compensation and a whitening operation have to be performed before averaging the data intensities.

Synthetic aperture radar interferometry

Proceedings of the IEEE, 2000

Synthetic aperture radar (SAR) is a coherent active microwave imaging method. In remote sensing it is used for mapping the scattering properties of the Earth's surface in the respective wavelength domain. Many physical and geometric parameters of the imaged scene contribute to the grey value of a SAR image pixel. Scene inversion suffers from this high ambiguity and requires SAR data taken at different wavelength, polarization, time, incidence angle, etc.

Estimation and improvement of coherence in SAR interferograms

Proceedings of IGARSS '94 - 1994 IEEE International Geoscience and Remote Sensing Symposium, 1994

SAR Interferometric surveys are affected by several sources of decorrelation noise. An improvement of the phase accuracy can be achieved by minimizing that noise: this could require a careful "tuning" of some processing dependent parameters, beside a phase preserving focusing. The sensitivity of coherence with respect of processing parameters is studied, and techniques are presented t o minimize the effect of different decorrelation sources.

Multiresolution phase unwrapping for SAR interferometry

IEEE Transactions on Geoscience and Remote Sensing, 1999

An approach to two-dimensional (2-D) phase unwrapping for synthetic aperture radar (SAR) interferometry is presented, based on separate steps of coarse phase and fine phase estimation. A technique called adaptive multiresolution is introduced for local fringe frequency estimation, in which difference frequencies between resolution levels are estimated and summed such that a sufficiently conservative phase gradient field is maintained. A coarse unwrapped phase of the full terrain height is then constructed using weighted least-squares based on coherence weighting. This coarse phase is used in a novel approach to slope-adaptive spectral shift filtering and to reduce the phase variation of the interferogram. The resulting interferogram can be more accurately multilooked and unwrapped with any algorithm. In this paper, fine phase construction is done with weighted least-squares and with weights determined by simple morphological operations on residues. The approach is verified on a simulated complex interferogram and real SAR data.

EFFECTS OF LOCAL PHASE ERRORS IN MULTI-LOOK SAR IMAGES

The synthetic aperture radar (SAR) is a widely used instrument for high-resolution imaging from aircraft or satellite platforms. In the paper, the problem of the defocusing of multilook SAR images by uncompensated phase errors presented in the received data is analyzed. It is shown that the phase errors on a multi-look processing interval can be effectively described in terms of local quadratic and local linear phase errors. Approximate analytical expressions are derived to describe the azimuth resolution degradation. Criteria for acceptable phase errors are given. The obtained results are verified by numerical simulations. The approach is illustrated by two typical motion errors: slow deflections of a SAR platform trajectory from a reference flight line and periodic trajectory deviations.

On the importance of path for phase unwrapping in synthetic aperture radar interferometry

Applied Optics, 2011

Phase unwrapping is a key procedure in interferometric synthetic aperture radar studies, translating ambiguous phase observations to topography, and surface deformation estimates. Some unwrapping algorithms are conducted along specific paths based on different selection criteria. In this study, we analyze six unwrapping paths: line scan, maximum coherence, phase derivative variance, phase derivative variance with branch-cut, second-derivative reliability, and the Fisher distance. The latter is a new path algorithm based on Fisher information theory, which combines the phase derivative with the expected variance to get a more robust path, potentially performing better than others in the case of low image quality. In order to compare only the performance of the paths, the same unwrapping function (phase derivative integral) is used. Results indicate that the Fisher distance algorithm gives better results in most cases.

Phase gradient approach to stacking interferograms

Journal of Geophysical Research, 1998

Phase statistics of interferograms with applications to synthetic aperture radar (original) (raw)

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