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H. Hellsten

Bio: H. Hellsten is an academic researcher from Swedish Defence Research Agency. The author has contributed to research in topics: Synthetic aperture radar & Radar imaging. The author has an hindex of 10, co-authored 19 publications receiving 886 citations.

Papers
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Journal ArticleDOI
TL;DR: It is shown that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization is constructed and the number of operations is drastically reduced and can be made to approach that of fast transform algorithms.
Abstract: Exact synthetic aperture radar (SAR) inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, back-projection integration in time domain can also be used. This technique has the benefit of handling a general aperture geometry. In the past, however, back-projection has seldom been used due to heavy computational burden. We show that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization. By representing images in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms. The algorithm is applied to data from the airborne ultra-wideband CARABAS SAR and shown to give a reduction in processing time of two to three orders of magnitude.

669 citations

Journal ArticleDOI
TL;DR: The results show that the dynamic range of the backscattering coefficient among the forest stands is higher than what has been found with conventional SAR using microwave frequencies, and shows an advantage compared to higher frequencies for stem volume estimation in dense forests.
Abstract: The ability to retrieve forest stem volume using CARABAS (coherent all radio band sensing) SAR images (28-60 MHz) has been investigated. The test site is a deciduous mixed forest on the island of Oland in southern Sweden. The images have been radiometrically calibrated using an array of horizontal dipoles. The images exhibit a clear discrimination between the forest and open fields. The results show that the dynamic range of the backscattering coefficient among the forest stands is higher than what has been found with conventional SAR using microwave frequencies. The backscatter increases with increasing radar frequency. This work shows an advantage compared to higher frequencies for stem volume estimation in dense forests.

66 citations

Patent
01 Nov 2001
TL;DR: In this article, a Synthetic Aperture Radar (SAR) system capable of detecting moving targets is presented, which consists of a platform which moves over a number of objects, e.g. in the form of a ground surface, and supports radar equipment which reproduces the objects by means of a fast backprojection synthetic aperture technique via at least two antennas.
Abstract: The present invention relates to a Synthetic Aperture Radar (SAR) System capable of detecting moving targets. It comprises a platform which moves over a number of objects, e.g. in the form of a ground surface, and supports radar equipment which reproduces the objects by means of a fast backprojection synthetic aperture technique via at least two antennas without requirement as to directivity or fractional bandwidth. The system comprises a signal-processing device in which the imaging process is divided into three steps which are carried out in a determined order, the steps and the order being formation of sub-aperture beams at one speed, performing clutter suppression, and detection of moving targets.

62 citations

Proceedings ArticleDOI
20 Jul 1999
TL;DR: Time-domain signal processing is emphasized, and time-domain SAR algorithms to simultaneously focus both stationary and moving targets are formulated, and the computational effort is dramatically reduced by factorization into a number of recursive stages.
Abstract: The paper discusses the necessity and feasibility of designing a SAR system for detection of low-signature stationary and moving targets, e.g. concealed by foliage. In particular, we investigate the basic requirements, measurement principles and signal processing algorithms. We show that the problem can be formulated using the same principles as ultra-wideband SAR by adding multiple antenna elements. Time-domain signal processing is emphasized, and we formulate such SAR algorithms to simultaneously focus both stationary and moving targets. The computational effort to the basic algorithm is dramatically reduced by factorization into a number of recursive stages. In this manner, the time-domain algorithm has essentially the same numerical efficiency as FFT-based methods. We also include a brief description of the present development of an airborne SAR system with an antenna array and operating in the 200 - 800 MHz band called LORA.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

42 citations

Proceedings ArticleDOI
18 Aug 1993
TL;DR: FOA has designed, built and tested a new airborne synthetic aperture radar system, CARABAS, which operates in the lower part of the VHF-band, which gives the system a good ability to penetrate vegetation and also to some extent ground.
Abstract: FOA has designed, built and tested a new airborne synthetic aperture radar system, CARABAS, which operates in the lower part of the VHF-band. This frequency region gives the system a good ability to penetrate vegetation and also to some extent ground. Furthermore, the resolution cell and the smallest scatterers influenced by the frequencies are comparable in size and a reduced speckle level in the final SAR image is obtained. Critical parts in the development have been the antenna, the receiver and the signal processing algorithms. A number of test flights have been carried out during 1992. The first major SAR campaign was conducted in October 1992 at two different test sites and included deployment of different reference targets in parallel with a ground truth programme. >

34 citations


Cited by
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Journal ArticleDOI
TL;DR: It is shown that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization is constructed and the number of operations is drastically reduced and can be made to approach that of fast transform algorithms.
Abstract: Exact synthetic aperture radar (SAR) inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, back-projection integration in time domain can also be used. This technique has the benefit of handling a general aperture geometry. In the past, however, back-projection has seldom been used due to heavy computational burden. We show that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization. By representing images in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms. The algorithm is applied to data from the airborne ultra-wideband CARABAS SAR and shown to give a reduction in processing time of two to three orders of magnitude.

669 citations

Journal ArticleDOI
TL;DR: With the availability of more channels combined with the powerful digital signal processing (DSP) capabilities of modern computers, the performance of mm-wave imaging systems is advancing rapidly.
Abstract: Due to the enormous advances made in semiconductor technology over the last few years, high integration densities with moderate costs are achievable even in the millimeter-wave (mm-wave) range and beyond, which encourage the development of imaging systems with a high number of channels. The mm-wave range lies between 30 and 300 GHz, with corresponding wavelengths between 10 and 1 mm. While imaging objects with signals of a few millimeters in wavelength, many optically opaque objects appear transparent, making mm-wave imaging attractive for a wide variety of commercial and scientific applications like nondestructive testing (NDT), material characterization, security scanning, and medical screening. The spatial resolution in lateral and range directions as well as the image dynamic range offered by an imaging system are considered the main measures of performance. With the availability of more channels combined with the powerful digital signal processing (DSP) capabilities of modern computers, the performance of mm-wave imaging systems is advancing rapidly.

258 citations

Journal ArticleDOI
TL;DR: In this article, a review of active synthetic aperture sonar (SAS) is presented, covering the early historical development of SAS with its provenance in synthetic aperture radar (SAR) and flows through into what work has been published in the open literature up to early 2007.
Abstract: This is a review paper that surveys past work in, and the recent status of, active synthetic aperture sonar (SAS). It covers the early historical development of SAS with its provenance in synthetic aperture radar (SAR) and flows through into what work has been published in the open literature up to early 2007. The list of references is sufficiently complete to include most past and recent SAS publications in the open refereed literature.

258 citations

Journal ArticleDOI
01 Mar 2013
TL;DR: A review about the abilities and needs of today's very high-resolution airborne SAR sensors is given, based on and summarizing the longtime experience of the German Aerospace Center (DLR) with airborne SAR technology and its applications.
Abstract: During the last decade, synthetic aperture radar (SAR) became an indispensable source of information in Earth observation. This has been possible mainly due to the current trend toward higher spatial resolution and novel imaging modes. A major driver for this development has been and still is the airborne SAR technology, which is usually ahead of the capabilities of spaceborne sensors by several years. Today's airborne sensors are capable of delivering high-quality SAR data with decimeter resolution and allow the development of novel approaches in data analysis and information extraction from SAR. In this paper, a review about the abilities and needs of today's very high-resolution airborne SAR sensors is given, based on and summarizing the longtime experience of the German Aerospace Center (DLR) with airborne SAR technology and its applications. A description of the specific requirements of high-resolution airborne data processing is presented, followed by an extensive overview of emerging applications of high-resolution SAR. In many cases, information extraction from high-resolution airborne SAR imagery has achieved a mature level, turning SAR technology more and more into an operational tool. Such abilities, which are today mostly limited to airborne SAR, might become typical in the next generation of spaceborne SAR missions.

233 citations

Journal ArticleDOI
TL;DR: Two novel image processing techniques have been developed to refocus a moving target image from its smeared response in the synthetic aperture radar (SAR) image which is focused on the stationary ground.
Abstract: Two novel image processing techniques have been developed to refocus a moving target image from its smeared response in the synthetic aperture radar (SAR) image which is focused on the stationary ground. Both approaches may be implemented with efficient fast Fourier transform (FFT) routines to process the Fourier spatial spectrum of the image data. The first approach utilizes a matched target filter that is derived from the signal history along the range-Doppler migration path mapped onto the SAR image from the moving target trajectory in real space. The coherent spatial filter is specified by the apparent target range in the image and the magnitude of the relative target-to-radar velocity. The second approach eliminates the range-dependence by reconstructing the moving target image from a spectral function that is obtained from the SAR image data spectrum via a spatial frequency coordinate transformation.

227 citations