Octavio Ponce

Fully Polarimetric High-Resolution 3-D Imaging With Circular SAR at L-Band

This paper presents the first fully polarimetric high-resolution circular synthetic aperture radar (CSAR) images at L-band (1.3 GHz). The circular data were acquired in 2008 by the Experimental SAR (E-SAR) airborne system of the German Aerospace Center (DLR) over the airport of Kaufbeuren, Germany. The obtained images resulting from the coherent integration of the whole circular flight are investigated and discussed in terms of two of the main CSAR properties, namely, the theoretical subwavelength resolution in the horizontal plane (x, y) and the 3-D imaging capabilities. The 3-D imaging capabilities are of special interest due to the penetration of L-band in vegetated areas. These results were compared with images processed by the incoherent addition of the full synthetic aperture. The coherent approach showed a better performance since scatterers are focused at their maximum resolution. Due to the nonlinearity of the tracks and the high-computational burden, an efficient fast factorized back-projection (FFBP) has been developed. Unlike frequencydomain processors, it accommodates azimuthal variances and topography changes. Limits and considerations of the proposed algorithm are described and discussed. To further accelerate this process, the FFBP was also implemented in a graphics processing unit (GPU). Processing performance has been assessed with the direct BP (DBP) as a reference, obtaining speedup factors up to 1800. Residual motion errors have been estimated with a new frequency-based autofocus approach for CSAR configurations based on low signal-to-clutter ratio (SCR) isotropic scatterers. High-resolution images of man-made and distributed scatterers have been analyzed and compared with a stripmap SAR, both concerning anisotropic and isotropic-like scatterers. Results include a single-channel tomogram of a Luneburg lens and a fully polarimetric tomogram of a tree.

Processing of Circular SAR trajectories with Fast Factorized Back-Projection

This paper describes the implementation of an efficient implementation of a Fast Factorized Back Projection (FFBP) algorithm for Circular SAR (CSAR) trajectories for real airborne data. Unlike Fourier-domain based focusing processors, this approach considers the azimuth variance and topography changes with high accuracy, while improving significantly the computational time factor in comparison with the direct Back Projection (BP). To further accelerate the focusing, the circular FFBP was implemented also on a Graphics Processor Unit (GPU). In the second part of the document is shown a fully polarimetric image of the region of Kaufbeuren, Germany, (acquired by the DLRs E-SAR system) focused with this method to the theoretical limit of ∼ λ over 4. Thus, the efficiency, accuracy and performance of the circular FFBP is demonstrated, as well as the potential of CSAR when focusing over 360 °.

Polarimetric 3-D reconstruction from multicircular SAR at P-band

Multicircular synthetic aperture radar (SAR) (MCSAR) is an extension of circular SAR (CSAR) characterized by the formation of a synthetic aperture in elevation with several circular flights. This imaging mode allows an improved resolution in the plane perpendicular to the line of sight ( LOS⊥), thus suppressing the 3-D cone-shaped sidelobes that are formed when focusing with CSAR. This letter presents the first polarimetric MCSAR airborne experiment acquired at P-band by the German Aerospace Center (DLR)s F-SAR system over a forested area in Vordemwald, Switzerland. This letter also includes a phase calibration method based on the singular value decomposition (SVD) using ground signatures to estimate constant phase offsets within a stack of 2-D images. Focusing methods, such as fast-factorized back projection (FFBP), beamforming (BF), and compressive sensing (CS), described in previous publications are used to solve the complex reflectivity in the (x, y, z) space.

First Airborne Demonstration of Holographic SAR Tomography with Fully Polarimetric Multicircular Acquisitions at L-band

In the last few years, interest in multicircular synthetic aperture radar (SAR) acquisitions has arisen as a consequence of the potential achievement of full 3-D reconstructions at very high resolution over 360° azimuth angle variation. In particular, SAR systems at low frequencies are sensitive to volumetric backscattering of semi-transparent media, and they allow the imaging of internal structures, such as forests. To achieve a full 3-D reconstruction, a 2-D synthetic aperture is required, consisting of a circular (azimuthal) and a vertical component. This 3-D capability can be understood as the result of the combination of holographic and tomographic techniques. In this paper, both techniques will be presented to establish the concept of holographic SAR tomography (HoloSAR). As a further investigation, this paper also presents an analytical expression of the 3-D impulse response function (IRF) of targets in and off the center of the illuminated area. The IRF is characterized by its spatial resolution and sidelobe power, both being a function of the radar resolution capabilities and the geometric acquisition. The second part of this paper presents a polarimetric analysis of HoloSAR tomograms. In particular, the polarimetric signature of scatterers in forested areas is investigated for three different focusing approaches, namely coherent imaging, incoherent imaging, and the generalized likelihood ratio test (GLRT). The three algorithms use the fast-factorized back-projection (FFBP) for individual circular trajectories, and the latter two use in addition compressive sensing (CS) to retrieve the complex reflectivity in elevation. The IRF is validated using a polarimetric L-band HoloSAR survey, which consists of 19 circular passes conducted by the German Aerospace Centers airborne F-SAR sensor over a test site in Kaufbeuren, Germany. The same data set is used for the analysis of the backscattering of forests. Results show a significant horizontal resolution improvement for distributed targets with the coherent imaging approach, whereas a better sidelobe suppression in the direction perpendicular to the line of sight is achieved for the incoherent imaging and the GLRT.

Analysis and optimization of multi-circular SAR for fully polarimetric holographic tomography over forested areas

This paper presents an analytical analysis of the impulse response function (IRF) of multi-circular SAR (MCSAR) as a function of the system bandwidth and the number of tracks, thereby optimizing the acquisition geometry to get the best 3-D resolution and sidelobe suppression. Moreover, the wide range of spectrum measurements provided by MCSAR allows the information retrieval over 360 °, thus understanding the anisotropic signature of scatterers, e.g. by their polarimetric signature. The second part of this paper presents a polarimetric analysis of the resulting holographic SAR tomograms of forested areas by means of the generalized likelihood ratio test (GLRT) algorithm, as well as by coherent and incoherent imaging. The IRF and the anisotropic analysis are validated with a polarimetric MCSAR campaign conducted at L-band by the DLRs F-SAR sensor over a forested region in Kauf-beuren, Germany.

First demonstration of 3-D holographic tomography with fully polarimetric multi-circular SAR at L-band

This paper presents the first 3-D polarimetric holographic tomograms at L-band using fully polarimetric multi-circular SAR (MCSAR) over forested areas. This concept offers an innovative way of analyzing the Earth, in particular volume scatterers such as forested areas, ice and dry soils. Holographic tomograms are formed by low-frequency radar in L-band and by the acquisition of multi-angular measurements with MCSAR over 360°. MCSAR consists of the combination of CSAR with an additional synthetic aperture in height (z). The analysis of the impulse response function (IRF) of MCSAR shows 3-D sidelobe reduction and an improvement in the resolution along the z direction. A fully polarimetric MCSAR campaign using the DLRs F-SAR airborne system at L-band over the region of Kaufbeuren, Germany was carried out. Images in 3-D were focused with beamforming (BF) and compressive sensing (CS). Results depict advantages of MCSAR such as shadow reduction, low temporal decorrelation and a better understanding of the coherent 3-D radar backscattering.

A Survey of novel airborne SAR signal processing techniques and applications for DLR's F-SAR sensor

DLRs Microwave and Radar Institute has been operating the F-SAR airborne SAR instrument since 2007. This contribution presents first the SAR imaging capabilities of the system followed by a discussion on recent developments in SAR signal processing using F-SAR data acquired in diverse special modes, each suitable for a specific application: large baseline, dual-frequency SAR interferometry for precise DEM generation, high resolution circular SAR imaging, advanced filtering and polarimetric change detection. The paper concludes with an outlook on future developments.

Polarimetric 3-D imaging with airborne holographic SAR tomography over glaciers

This paper presents an assessment of the backscattering of glaciers in the holographic SAR tomography (HoloSAR) mode. The motivation of HoloSAR in the cryosphere is mainly driven by recent investigations in the biosphere that have shown the capabilities of this mode to reconstruct volumes with 3-D imaging reconstructions over 360 ° at very high resolution. To that end, an HoloSAR campaign was conducted at L-band by the DLRs F-SAR sensor over the Findel glacier, Monte Rosa, Switzerland. The first part of this study shows a polarimetric analysis of the resulting images of a single circular flight, where the arc- and circular patterns of the 2-D images in the (x, y) plane already indicate wave penetration. The second part presents 3-D images obtained by the combination of the circular and vertical synthetic apertures, which enable an estimation of the vertical profile of snow, ice sheets and bedrock.

Multi-baseline imaging: A vision for spaceborne SAR

This paper provides a vision for the future development of spaceborne SAR systems with multi-baseline imaging capability like polarimetric SAR interferometry (PolinSAR), tomography (TomoSAR) and holography (HoloSAR). The goal is to fill the multi-dimensional data space with additional information from acquisitions having different spatial or temporal baselines. Multi-baseline imaging opens the door for a new class of image products in spaceborne SAR. Well-known examples are across-track and along-track interferometry, which allow the measurement of surface topography, ground deformation, ocean currents as well as glacier movements. While across-track and along-track interferometry are well established techniques and have been widely used by current spaceborne SAR systems, PolinSAR, TomoSAR and HoloSAR are emerging techniques which are shaping the future development of spaceborne SAR. New mission concepts for multistatic SAR configurations with distributed and sparse arrays will pave the way for this development.

Multiple-input multiple-output circular SAR

Interest on circular and multicircular SAR acquisitions has been increased in the past few years due to the inherent potentials they provide, e.g., full 3-D reconstruction and subwavelength resolution. However, one main limitation is the size of the spotlighted region, which is basically defined by the range ambiguities, the pulse repetition frequency (PRF), and the minimum half-power beamwidth (HPBW) in the range and azimuth directions. This paper introduces multiple-input multiple-output (MIMO) techniques in order to tackle this limitation, thereby being able to extend the applications of CSAR to much wider areas. The basic principle of the proposed solution is based on three methods, namely scan-on-receive (SCORE), multi-channel reconstruction in azimuth (MCRA) and the use of orthogonal waveforms for quad-polarized systems. The impact and dependency of the PRF, HPBW and the range ambiguities is also investigated.