Torleif Martin

Bio: Torleif Martin is an academic researcher from Lund University. The author has contributed to research in topics: Finite-difference time-domain method & Scattering. The author has an hindex of 11, co-authored 33 publications receiving 387 citations. Previous affiliations of Torleif Martin include Saab AB & Swedish Defence Research Agency.

An improved near- to far-zone transformation for the finite-difference time-domain method

Abstract: Near- to far-zone transformation for the finite-difference time-domain (FDTD) method can be performed by integration of the equivalent electric and magnetic currents originating from scattered electric and magnetic fields on a surface enclosing the object. Normally, when calculating the surface integrals, either the electric or magnetic fields are averaged since the electric and magnetic fields are spatially shifted in the FDTD grid. It is shown that this interpolation is unnecessary and also less accurate than if an integration is performed on two different surfaces. It is also shown that the accuracy of the far-zone transformation can be further improved if the phase is compensated with respect to a second-order dispersion corrected wavenumber. For validation, scattering results for an empty volume, a circular disk, and a sphere are compared with analytical solutions.

Fringing fields in a liquid crystal spatial light modulator for beam steering

Abstract: Phase modulating spatial light modulators (SLMs) can be used to alter the shape of a laser wavefront to achieve a deflection or change in the shape of a laser beam. This paper reports the results of characterization, simulation and optimization of a one-dimensional liquid crystal (LC) SLM. The device has a large ratio between LC layer thickness and pixel pitch that results in a fringing field between pixels. In effect, the applied phase patterns will be lowpass filtered and the loss of high frequency components limits, for instance, the usable steering range. A method is presented where intensity measurements in the far field are used to determine how the phase modulation at the SLM is distorted. The inhomogeneous optical anisotropy of the device was determined by modelling the liquid crystal director distribution within the electrode-pixel structure. Finite-difference time-domain (FDTD) simulations were used to calculate the light propagation through the LC. The simulated phase distortion was co...

A coherent scattering model to determine forest backscattering in the VHF-band

Abstract: A coherent scattering model to determine the forest radar backscattering at VHF frequencies (20-90 MHz) has been developed. The motivation for studying this frequency band is the recent development of the CARABAS Synthetic Aperture Radar (SAR). In order to model the scattering from branches and trunks, homogeneous dielectric cylinders placed above a semi-infinite di-electric ground have been analyzed. An analytical approach, where the theoretically exact currents induced in an infinite cylinder are truncated, has been compared to a numerical solution using the finite difference time domain (FDTD) method. If the first-order coherent ray tracing is included in the analytical approach, the results match well with the numerically exact FDTD solution. The results show that, in order to determine the VHF-backscattering from a forest stand, the coherent ground interaction is an important part and has to be considered. In this paper, modeling results are in good agreement with CARABAS measurements.

Bistatic ultra-wideband SAR for imaging of ground targets under foliage

Abstract: Bistatic SAR is compared with monostatic SAR concerning, e.g. coherent integration time and spatial resolution. We also suggest that bistatic SAR may be used to suppress background clutter, e.g. when the clutter scattering is dominated by dihedral or trihedral scattering mechanisms. This idea is applied to the problem of detecting concealed vehicles in foliage using VHF-band SAR. Electromagnetic simulations show that the vehicle-to-clutter ratio can dramatically increase by choosing different incidence angles for the transmitter and receiver in a bistatic SAR.

Semi-empirical modeling of apertures for shielding effectiveness simulations

Abstract: A novel method to include models of complex apertures into the finite-difference time-domain (FDTD) method is presented. Instead of resolving the geometrical details of the aperture, the aperture is treated as a magnetic dipole. The properties of the magnetic dipole moment are determined using the measured transmission cross section of the aperture. This semi-empirical model permits inclusion of complex apertures in FDTD simulations where the smallest dimension of the aperture is only a fraction of the FDTD cell size. Two types of complex apertures positioned in the top panel of a realistic enclosure have been modeled using the semi-empirical model in FDTD. Comparisons between simulated and measured shielding effectiveness of the enclosure are presented, and the results show that complex apertures can indeed be represented by simple magnetic dipoles in shielding effectiveness simulations.

Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

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Synthetic-aperture radar processing using fast factorized back-projection

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.

Spherical near-field antenna measurements

Abstract: From the material presented here, it is clear that the theory underlying the SNF approach is complex and involved to implement. However, it is also very elegant and provides one with many measurement options and powerful capabilities. The numerical implementation of the theory can be efficiently deployed through the use of the fast Fourier transform (FFT) enabling transforms of even electrically large antennas to be accomplished in a matter of a few seconds on a modern powerful digital computer. With the advent of commercially available SNF test systems, the user can exploit these techniques, largely unimpeded by the burden of the theory or the implementation thereof. The material presented here highlighted some of the fundamental concepts and limitations the user needs to be aware of in order to use these test systems with confidence.

Susceptibility of electronic systems to high-power microwaves: summary of test experience

Abstract: For more than a decade, the Swedish Defence Authorities have, in cooperation with Swedish industry and other countries, studied the effects of high-power microwave (RPM) radiation on electronic systems. Testing at high-field levels has been carried out on military equipment as well as on civil equipment, such as cars, computers, and security systems. From these studies, it is concluded that the distance for RPM sabotage can reach about a kilometer. Experience from system testing has, besides giving information about system susceptibility, also demonstrated the need for a careful pre-analysis of the system under test. This is due to the fact that high-level testing, in most cases, includes only a small fraction of the threat parameter space, such as test frequencies and irradiation angles.

Very-High-Resolution Airborne Synthetic Aperture Radar Imaging: Signal Processing and 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.