Discusses the use of modern 2D spectral estimation algorithms for synthetic aperture radar (SAR) imaging. The motivation for applying power spectrum estimation methods to SAR imaging is to improve resolution, remove sidelobe artifacts, and reduce speckle compared to what is possible with conventional Fourier transform SAR imaging techniques. This paper makes two principal contributions to the field of adaptive SAR imaging. First, it is a comprehensive comparison of 2D spectral estimation methods for SAR imaging. It provides a synopsis of the algorithms available, discusses their relative merits for SAR imaging, and illustrates their performance on simulated and collected SAR imagery. Some of the algorithms presented or their derivations are new, as are some of the insights into or analyses of the algorithms. Second, this work develops multichannel variants of four related algorithms, minimum variance method (MVM), reduced-rank MVM (RRMVM), adaptive sidelobe reduction (ASR) and space variant apodization (SVA) to estimate both reflectivity intensity and interferometric height from polarimetric displaced-aperture interferometric data. All of these interferometric variants are new. In the interferometric contest, adaptive spectral estimation can improve the height estimates through a combination of adaptive nulling and averaging. Examples illustrate that MVM, ASR, and SVA offer significant advantages over Fourier methods for estimating both scattering intensity and interferometric height, and allow empirical comparison of the accuracies of Fourier, MVM, ASR, and SVA interferometric height estimates.

SAR imaging via modern 2-D spectral estimation methods

Gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation. The azimuthally isotropic coherence function be- tween the Bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber, a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof. In this study we investigate the isostatic response of continental Australia. We consider the effects of directionally anisotropic plate strength on the coherelce. The anisotropic coherence function is calculated for regions of Australia that have distinctive geological and geophysical properties. The coherence estimation is performed by the Thomson multiple-Slepian-taper spectral analysis method extended to two-dimensional fields. Our analysis reveals the existence of flexural anisotropy in central Australia, indicative of a weaker N-S direction of lower Te. This observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them. It can. also be related to the N-S direction of maximum stress and possibly the presence of E-W running zones weakened due to differential sediment burial rates. We also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function. The ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods, particularly at low wavenumbers. Our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness Te of the Australian lithosphere. In isotropic inversions for Te, we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using Bouguer gravity and topography but greater than those obtained by inversions that utilize free-air rather than Bouguer gravity and ignore the presence of subsurface loads. However, owing to the low spectral power of the Australian topography, the uncertainty on any estimate of Te is substantial.

/pdf/isostatic-response-of-the-australian-lithosphere-estimation-15kp58r89n.pdf

Isostatic response of the Australian lithosphere: Estimation of effective elastic thickness and anisotropy using multitaper spectral analysis

An analysis method for unfolding the spatially resolved wave-number spectrum and phase velocity from the 2D CO2 laser phase contrast imaging system on the large helical device is described. This is based on the magnetic shear technique which identifies propagation direction from 2D spatial Fourier analysis of images detected by a 6 × 8 detector array. Because the strongest modes have wave-number at the lower end of the instrumental k range, high resolution spectral techniques are necessary to clearly resolve the propagation direction and hence the spatial distribution of fluctuations along the probing laser beam. Multiple-spatial point cross-correlation averaging is applied before calculating the spatial power spectrum. Different methods are compared, and it is found that the maximum entropy method (MEM) gives best results. The possible generation of artifacts from the over-narrowing of spectra are investigated and found not to be a significant problem. The spatial resolution Δρ (normalized radius) around the peak wave-number, for conventional Fourier analysis, is ∼0.5, making physical interpretation difficult, while for MEM, Δρ ∼ 0.1.

/pdf/two-dimensional-wave-number-spectral-analysis-techniques-for-4zkcl0cl96.pdf

Two-dimensional wave-number spectral analysis techniques for phase contrast imaging turbulence imaging data on large helical device

The 2-D spectrum estimation is divided into the two consecutive 1-D spectrum estimations: a multichannel spectrum estimation in the "time" domain, and a scalar spectrum estimation in the "frequency" domain. The first is a purely parametric estimation, while the second is a mixture of parametric and nonparametric estimation. The maximum entropy method is used in each estimation in order to utilize the high resolution performance of the maximum entropy method, on the one hand, and to avoid the highly nonlinear nature of the maximum entropy method for 2-D problems, on the other hand. The estimate obtained by this procedure satisfies the covariance matching property and exhibits remarkably high resolution. A simplified version of the multichannel Levinson-Durbin algorithm is derived based on the special structure of the covariance data of homogeneous random fields. The related extendibility of the covariance array is discussed. It is shown that this method is particularly suitable for a class of random fields having some shift invariance.

A hybrid approach to high resolution two-dimensional spectrum analysis

The disagreement of the scaling of the correlation length xi between experiment and the Ginzburg-Landau (GL) model for domain chaos was resolved. The Swift-Hohenberg (SH) domain chaos model was integrated numerically to acquire test images to study the effect of a finite image size on the extraction of from the structure factor (SF). The finite image size had a significant effect on the SF determined with the Fourier-transform (FT) method. The maximum entropy method (MEM) was able to overcome this finite image-size problem and produced fairly accurate SFs for the relatively small image sizes provided by experiments. Correlation lengths often have been determined from the second moment of the SF of chaotic patterns because the functional form of the SF is not known. Integration of several test functions provided analytic results indicating that this may not be a reliable method of extracting xi. For both a Gaussian and a squared SH form, the correlation length xi[triple bond]1/sigma, determined from the variance sigma2 of the SF, has the same dependence on the control parameter epsilon as the length xi contained explicitly in the functional forms. However, for the SH and the Lorentzian forms we find xi approximately xi (1/2). Results for xi determined from new experimental data by fitting the functional forms directly to the experimental SF yielded xi approximately epsilon(-v) with v approximately equal to 1/4 for all four functions in the case of the FT method, but v approximately equal to 1/2, in agreement with the GL prediction, in the case of the MEM. Over a wide range of epsilon and wave number k, the experimental SFs collapsed onto a unique curve when appropriately scaled by xi.

/pdf/domain-chaos-puzzle-and-the-calculation-of-the-structure-52zgb3mztw.pdf

Domain chaos puzzle and the calculation of the structure factor and its half-width.

In this paper we present the results of a number of experiments that have been performed to study the properties of two-dimensional maximum entropy spectral estimates. The results presented include studies on the resolution differences for real and complex data, resolution properties of the spectral estimates, and the determination of the relative power of the sinusoids from the spectral estimates. The results also include the effects of signal-to-noise ratio, size and shape of the known autocorrelation region, and data length and initial phase of a sinusoid, on the spectral estimates. In most cases studied, the properties of two-dimensional maximum entropy spectral estimates can be viewed as simple extensions of their one-dimensional counterparts.

Properties of two-dimensional maximum entropy power spectrum estimates

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Properties of two-dimensional maximum entropy power spectrum estimates