The ionospheric propagation path delay is a major error source in synthetic aperture radar (SAR) interferograms and, therefore, has to be estimated and corrected. Various methods can be used to extract different kinds of information about the ionosphere from SAR images, with different accuracies. This paper presents a general technique, based on a Bayesian inverse problem, that combines various information sources in order to increase the estimation accuracy, and thus the correction. A physically realistic fractal modeling of the ionosphere turbulence and a data-based estimation of the model parameters allow the avoidance of arbitrary filtering windows and coefficients. To test the technique, the differential ionospheric phase screen was estimated by combining the split-spectrum method with the azimuth mutual shifts between interferometric pair images. This combination is convenient since it can benefit from the strengths of both sources: range and azimuth variations from the split-spectrum method and small-scale azimuth variations from more sensitive azimuth shifts. Therefore, the two methods can recover the long and short wavelength components of the ionospheric phase screen, respectively. A theoretical comparison between the Faraday rotation method and the split-spectrum method is also reported. For the use in the combination, precedence was then given to the split-spectrum method because of the comparable precision level, lower susceptibility to biases, and wider applicability. Finally, Advanced Land Observing Satellite Phased Array type L-band SAR L-band images are used to show how the combined result is more accurate than that obtained with the simple split-spectrum method.

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Bayesian Data Combination for the Estimation of Ionospheric Effects in SAR Interferograms

We describe the SAR Scintillation Simulator (SAR-SS), a new phase screen model for simulating the impact of small-scale ionospheric structure on SAR image formation and interferometry. We compare simulated and observed PALSAR imagery over Brazil, and our preliminary findings show that SAR-SS can reproduce the essential features of azimuthal streaking and contrast degradation caused by small-scale structure in the ionosphere.

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A phase screen simulator for predicting the impact of small-scale ionospheric structure on SAR image formation and interferometry

Electromagnetic backscatter characteristics of conducting polyhedral spheres, constructed of 12 pentagons and a large number of hexagons, were investigated. The polyhedral structures were classified by their number of vertices and by the width of the conducting edges in their frames. Polyhedral spheres with 60, 80, 180, 240, 320, 500, 540, 960, and 1500 vertices were evaluated for their ability to scatter electromagnetic waves back to a source. The edge widths were varied between the limits of (1) nearly zero, to produce a wire-frame sphere; and (2) full surface closure of the polygon holes in the sphere, to form solid conducting structures. Using Method-of-Moments simulations with the WIPL-D code, the monostatic scattering properties of these spheres were studied over a wide range of structural and frequency parameters. The spherical wire frames had up to a 10 dB larger radar cross section than the completely solid spheres. This result provides models for new light-weight radar cross section targets used as satellite-calibration spheres with enhanced radar cross section for easier detection, low mass for easier launch, and low drag cross section for long orbit life. With selected edge widths, the monostatic radar cross section becomes vanishingly small at specific anti-backscatter frequencies, for conducting spherical shells with a regular pattern of polygon holes. These shells can be placed around solid spheres to nearly eliminate radar reflection from the composite object at specific frequencies. The polyhedral conducting-mesh (PCM) spheres with vanishing radar cross section at a desired frequency have applications for isotropic scattering sources that reflect no energy back to the electromagnetic source. the anti-backscatter polyhedral conducting-mesh spheres may provide insight for future stealth applications with spherical targets.

Radar Backscatter from Conducting Polyhedral Spheres

In this paper, methods to separate the ionospheric and the scattering contributions on SAR data are proposed, and the dynamic components of the ionosphere and the lithosphere are quantitatively retrieved. These methods exploit changes of squint and time in the azimuth sub-bands.

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On the Separation of Dynamic Scattering and Ionospheric Effects in SAR Data

An apparatus for interfering with and canceling electromagnetic reflections from a conducting object such as a solid or hollow conducting sphere. The reflection of an incident electromagnetic wave from a conducting sphere can be reduced or cancelled by placing a polyhedral conducting mesh (PCM) around the conducting sphere. The incident EM wave induces electric fields in the region between the conducting mesh and the internal conducting sphere. These induced electric fields may interfere with and in some cases may completely cancel the incident EM wave and thus reduce reflection. The PCM can be tuned to reduce or cancel reflection of incident EM waves at one or more specific frequencies.

Apparatus for radar target cloaking using polyhedral conducting mesh

Numerical simulations of low-latitude ionospheric instabilities show the formation of plasma density structures can be detected by synthetic aperture radar (SAR) radio signals. At L-band, the phase front distortions produced by propagation through plasma "bubbles" should provide measurable changes in both the complex-amplitude and polarization waves from orbiting satellite radars. The diffraction pattern from scattered ground SAR signals can be detected by orbiting receivers. Based on reciprocity, ground radars at Kwajalein Atoll are being used to determine the feasibility of space-based detection of ionospheric density structures. This new measurement technique can provide a global data base of ionospheric data for space-weather models that predict the effects of the ionosphere on radio systems.

P. A. Bernhardt

Papers

Detection of Ionospheric Structures with L-Band Synthetic Aperture Radars