| 1. |
- Höök, Lars Josef, 1979-
(författare)
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Variance reduction methods for numerical solution of plasma kinetic diffusion
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Performing detailed simulations of plasma kinetic diffusion is a challenging task and currently requires the largest computational facilities in the world. The reason for this is that, the physics in a confined heated plasma occur on a broad range of temporal and spatial scales. It is therefore of interest to improve the computational algorithms together with the development of more powerful computational resources. Kinetic diffusion processes in plasmas are commonly simulated with the Monte Carlo method, where a discrete set of particles are sampled from a distribution function and advanced in a Lagrangian frame according to a set of stochastic differential equations. The Monte Carlo method introduces computational error in the form of statistical random noise produced by a finite number of particles (or markers) N and the error scales as αN−β where β = 1/2 for the standard Monte Carlo method. This requires a large number of simulated particles in order to obtain a sufficiently low numerical noise level. Therefore it is essential to use techniques that reduce the numerical noise. Such methods are commonly called variance reduction methods. In this thesis, we have developed new variance reduction methods with application to plasma kinetic diffusion. The methods are suitable for simulation of RF-heating and transport, but are not limited to these types of problems. We have derived a novel variance reduction method that minimizes the number of required particles from an optimization model. This implicitly reduces the variance when calculating the expected value of the distribution, since for a fixed error the optimization model ensures that a minimal number of particles are needed. Techniques that reduce the noise by improving the order of convergence, have also been considered. Two different methods have been tested on a neutral beam injection scenario. The methods are the scrambled Brownian bridge method and a method here called the sorting and mixing method of L´ecot and Khettabi[1999]. Both methods converge faster than the standard Monte Carlo method for modest number of time steps, but fail to converge correctly for large number of time steps, a range required for detailed plasma kinetic simulations. Different techniques are discussed that have the potential of improving the convergence to this range of time steps.
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| 2. |
- Akselsson, Cecilia, et al.
(författare)
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Assessing the risk of N leaching from forest soils across a steep N deposition gradient in Sweden.
- 2010
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Ingår i: Environmental Pollution. - : Elsevier BV. - 0269-7491 .- 1873-6424. ; 158:12, s. 3588-3595
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Tidskriftsartikel (refereegranskat)abstract
- Nitrogen leaching from boreal and temporal forests, where normally most of the nitrogen is retained, has the potential to increase acidification of soil and water and eutrophication of the Baltic Sea. In parts of Sweden, where the nitrogen deposition has been intermediate to high during recent decades, there are indications that the soils are close to nitrogen saturation. In this study, four different approaches were used to assess the risk of nitrogen leaching from forest soils in different parts of Sweden. Nitrate concentrations in soil water and C:N ratios in the humus layer where interpreted, together with model results from mass balance calculations and detailed dynamic modelling. All four approaches pointed at a risk of nitrogen leaching from forest soils in southern Sweden. However, there was a substantial variation on a local scale. Basing the assessment on four different approaches makes the assessment robust.
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| 3. |
- Frennby, Bo, et al.
(författare)
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Determination of the relative glomerular filtration rate of each kidney in man : Comparison between iohexol CT and 99mTc-DTPA scintigraphy
- 1995
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Ingår i: Acta Radiologica. - 0284-1851 .- 1600-0455. ; 36:4, s. 410-417
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Tidskriftsartikel (refereegranskat)abstract
- Iohexol and 99mTc-DTPA were used in 43 patients to determine the relative glomerular filtration rate (GFR), i.e., the GFR of each kidney in percent of total GFR. The amount of any GFR marker accumulating in Bowman's space, tubuli and renal pelvis within a few minutes after i.v. injection, before any marker had left the kidney via the ureter, was defined as proportional to the GFR of that kidney. The renal accumulation of iohexol was determined by CT using 10 slices of 8-mm thickness 1 to 4 minutes after injection. The renal accumulation of 99mTc-DTPA was determined with a gamma camera within 2 minutes after injection. The correlation coefficient between the two methods was 0.98. Due to the higher radiation dose from CT than from 99mTc-DTPA injection, relative GFR determination with CT should be performed when there is also a diagnostic need to reveal morphology.
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| 4. |
- Fröbert, Ole, et al.
(författare)
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Thrombus Aspiration during ST-Segment Elevation Myocardial Infarction
- 2013
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Ingår i: New England Journal of Medicine. - : Massachusetts Medical Society. - 0028-4793 .- 1533-4406. ; 369:17, s. 1587-1597
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Tidskriftsartikel (refereegranskat)abstract
- BackgroundThe clinical effect of routine intracoronary thrombus aspiration before primary percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI) is uncertain. We aimed to evaluate whether thrombus aspiration reduces mortality. MethodsWe conducted a multicenter, prospective, randomized, controlled, open-label clinical trial, with enrollment of patients from the national comprehensive Swedish Coronary Angiography and Angioplasty Registry (SCAAR) and end points evaluated through national registries. A total of 7244 patients with STEMI undergoing PCI were randomly assigned to manual thrombus aspiration followed by PCI or to PCI only. The primary end point was all-cause mortality at 30 days. ResultsNo patients were lost to follow-up. Death from any cause occurred in 2.8% of the patients in the thrombus-aspiration group (103 of 3621), as compared with 3.0% in the PCI-only group (110 of 3623) (hazard ratio, 0.94; 95% confidence interval [CI], 0.72 to 1.22; P=0.63). The rates of hospitalization for recurrent myocardial infarction at 30 days were 0.5% and 0.9% in the two groups, respectively (hazard ratio, 0.61; 95% CI, 0.34 to 1.07; P=0.09), and the rates of stent thrombosis were 0.2% and 0.5%, respectively (hazard ratio, 0.47; 95% CI, 0.20 to 1.02; P=0.06). There were no significant differences between the groups with respect to the rate of stroke or neurologic complications at the time of discharge (P=0.87). The results were consistent across all major prespecified subgroups, including subgroups defined according to thrombus burden and coronary flow before PCI. ConclusionsRoutine thrombus aspiration before PCI as compared with PCI alone did not reduce 30-day mortality among patients with STEMI. (Funded by the Swedish Research Council and others; ClinicalTrials.gov number, NCT01093404.)
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| 5. |
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| 6. |
- Hellsten, Hans, et al.
(författare)
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Evaluation of a geometrical autofocus algorithm within the framework of Fast Factorized Back-Projection
- 2012
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Ingår i: Proceedings of IET RADAR 2012, 22-25 October, Glasgow, Scotland. - : Institution of Engineering and Technology. - 9781849196765 ; 2012:603 CP
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Konferensbidrag (refereegranskat)abstract
- This paper introduces a new autofocus algorithm, specifically suitable for UWB SAR systems. The strategy is integrated in FFBP and relies on varying track parameters stage by stage to obtain a sharp image. Focus measures are provided by an object function (intensity correlation). The algorithm has been tried out successfully on a synthetic data set.
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| 7. |
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| 8. |
- Hellsten, Torbjörn, et al.
(författare)
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A dielectric response model for FEM solutions of ICRF wave fields
- 2012
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 401:1, s. 012009-
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Tidskriftsartikel (refereegranskat)abstract
- Modelling of fast wave ICRF heating in large machines with high density such as DEMO is challenging because of the short wave lengths. Therefore, fast, efficient global wave solvers are necessary. A major difficulty with calculating the wave field in a spatial dispersive medium is that the dielectric tensor becomes a function of the local wave vector, which in its turn depends on the solution. Furthermore, the solution may consist of several waves co-existing at the same location subjected to separate response functions. In order to model upshift of the parallel wave vector, higher order FLR-effects on the cyclotron absorption and TTMP damping for the electron absorption methods based on iteration, suitable for FEM codes, are proposed.
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| 9. |
- Hellsten, Torbjörn, et al.
(författare)
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A model for self-consistent simulation of ICRH suitable for integrating modelling
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:9, s. 093004-
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Tidskriftsartikel (refereegranskat)abstract
- A self-consistent modelling of ion cyclotron resonance heating (ICRH) is reviewed with the aim of obtaining a fast robust scheme suitable for routine simulation for transport codes and data analysis. Due to the complexity of calculating the wave field and the distribution function self-consistently simplifications are necessary. To improve modelling of the wave field, methods are developed to include higher order finite Larmor radius terms, up-and downshifts of the parallel wave number and to improve calculations of damping due to the transit time magnetic pumping in finite element wave codes without decomposing the wave locally into planar waves. A new code, SELFO-light, for self-consistent modelling of ion cyclotron heating suitable for routine calculations is developed. The code is based on coupling the global wave code LION with a simple one-dimensional time-dependent Fokker-Planck code. Both the wave and the Fokker-Planck codes use finite element representations. The importance of self-consistent modelling of ion cyclotron heating is illustrated by studying the effect on the power partition for a fast wave current drive scenario at lower harmonic resonances in a deuterium plasma. It is found that the fraction of the power absorbed on the deuterium and the time to reach the steady state vary strongly depending on the position of the resonances. It is found that the deuterium absorption becomes strongly localized to regions where the resonances are tangential to the magnetic flux surfaces.
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| 10. |
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| 11. |
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| 12. |
- Höök, Lars Josef, 1979-
(författare)
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Numerical solution of quasilinear kinetic diffusion equations in toroidal plasmas
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the main challenges for the realization of a working fusion power plant is an increased detailed understanding of kinetic phenomena in toroidal plasmas. The tokamak is a toroidal, magnetically confined plasma device and is currently the main line towards a power plant. The spatial and temporal scales in a tokamak plasma are extreme and the only tractable path for quantitative studies is to rely on computer simulations. Present day simulation codes can resolve only some of these scales. Nevertheless they still require the largest high performance computing (HPC) resources available in the world. In combination with the increase of computational performance, it is therefore necessary to improve the numerical algorithms used in the simulations.In this thesis we have developed new numerical methods designed for Monte Carlo simulation of plasma kinetic diffusion. Examples are simulation of fast-ion thermalization and radio-frequency heating. The aim has been to reduce the statistical random noise in particle codes, produced by a finite number of particles (or markers). Traditionally the statistical noise is improved by increasing the number of particles (N) or by simulating the perturbation of the distribution (with particles) from a known distribution function. This is the well known δf method. In this thesis we have developed a new type of δf method, which minimizes the number of particles used in a simulation. The computational speedup of the new method is substantial. In this thesis, we have further benchmarked quasi-Monte Carlo techniques that improve the convergence rate from N−1/2 to N−1 for some cases.In Monte Carlo simulations, error appears also from the time step discretization. Based on the mathematics of operator splitting, a new scheme for the pitch-angle scattering diffusion process has been developed that outperforms the standard methods. Finally this thesis also presents a new code, SELFO-light, for self-consistent simulations of ion cyclotron resonance heating, suitable for routine calculations, which couples a one dimensional Fokker-Planck model with the finite element wave solver LION.
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| 13. |
- Höök, Lars Josef, 1979-, et al.
(författare)
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Randomized quasi-Monte Carlo simulation of fast-ion thermalization
- 2012
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Ingår i: Computational Science & Discovery. - : Institute of Physics (IOP). - 1749-4680 .- 1749-4699. ; 5:1, s. 014010-
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates the applicability of the randomized quasi-Monte Carlo method for simulation of fast-ion thermalization processes in fusion plasmas, e.g. for simulation of neutral beam injection and radio frequency heating. In contrast to the standard Monte Carlo method, the quasi-Monte Carlo method uses deterministic numbers instead of pseudo-random numbers and has a statistical weak convergence close to O(N -1), where N is the number of markers. We have compared different quasi-Monte Carlo methods for a neutral beam injection scenario, which is solved by many realizations of the associated stochastic differential equation, discretized with the Euler-Maruyama scheme. The statistical convergence of the methods is measured for time steps up to 2 14.
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| 14. |
- Johnson, Thomas, et al.
(författare)
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Analysis of a quasilinear model for ion-cyclotron interactions in tokamaks
- 2005
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Ingår i: Radio Frequency Power in Plasmas. - MELVILLE : AMER INST PHYSICS. - 0735402760 ; , s. 54-57
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Konferensbidrag (refereegranskat)abstract
- The quasilinear diffusion coefficient deviates significantly from the lowest order Larmor radius scaling D proportional to nu(2n)(perpendicular to). This is not only caused by the finite Larmor radius effects, but also by the inhomogeneous electric field polarisation and the changes of the guiding centre orbits. The regions with strong interaction and the boundaries for resonant interaction are identified. At these boundaries the quasi linear diffusion coefficient becomes discontinuous. Anew Monte Carlo scheme has been developed to treat problems with discontinuous diffusion coefficients.
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| 15. |
- Karlsson, Per Erik, et al.
(författare)
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Biomass burning in eastern Europe during spring 2006 caused high deposition of ammonium in northern Fennoscandia
- 2013
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Ingår i: Environmental Pollution. - : Elsevier BV. - 0269-7491. ; 176, s. 71-79
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Tidskriftsartikel (refereegranskat)abstract
- High air concentrations of ammonium were detected at low and high altitude sites in Sweden, Finland and Norway during the spring 2006, coinciding with polluted air from biomass burning in eastern Europe passing over central and northern Fennoscandia. Unusually high values for throughfall deposition of ammonium were detected at one low altitude site and several high altitude sites in north Sweden. The occurrence of the high ammonium in throughfall differed between the summer months 2006, most likely related to the timing of precipitation events. The ammonia dry deposition may have contributed to unusual visible injuries on the tree vegetation in northern Fennoscandia that occurred during 2006, in combination with high ozone concentrations. It is concluded that long-range transport of ammonium from large-scale biomass burning may contribute substantially to the nitrogen load at northern latitudes. (C) 2013 Elsevier Ltd. All rights reserved.
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| 16. |
- Kosonen, Petteri, et al.
(författare)
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Intravascular ultrasound assessed incomplete stent apposition and stent fracture in stent thrombosis after bare metal versus drug-eluting stent treatment the Nordic Intravascular Ultrasound Study (NIVUS)
- 2013
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Ingår i: International Journal of Cardiology. - : Elsevier BV. - 0167-5273. ; 168:2, s. 1010-1016
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Tidskriftsartikel (refereegranskat)abstract
- Background: This prospective multicenter registry used intravascular ultrasound (IVUS) in patients with definite stent thrombosis (ST) to compare rates of incomplete stent apposition (ISA), stent fracture and stent expansion in patients treated with drug-eluting (DES) versus bare metal (BMS) stents. ST is a rare, but potential life threatening event after coronary stent implantation. The etiology seems to be multifactorial. Methods: 124 patients with definite ST were assessed by IVUS during the acute ST event. The study was conducted in 15 high-volume percutaneous coronary intervention -centers in the Nordic-Baltic countries. Results: In early or late ST there were no differences in ISA between DES and BMS. In very late ST, ISA was a more frequent finding in DES than in BMS (52% vs. 16%; p=0.005) and the maximum ISA area was larger in DES compared to BMS(1.1 +/- 2.3 mm(2) vs. 0.1 +/- 0.5 mm(2); p=0.004). Further, ISA was more prevalent in sirolimus-eluting than in paclitaxel-eluting stents (58% vs. 37%; p-0.02). Stent fractures were found both in DES (16%) and BMS (24%); p=0.28, and not related to time of stent thrombosis occurrence. For stents with nominal diameters >= 2.75 mm, 38% of the DES and 22% of the BMS had a minimum stent area of less than 5 mm(2); p=0.14. Conclusions: Very late stent thrombosis was more prevalent and associated with more extensive ISA in DES than in BMS treated patients. Stent fracture was a common finding in ST after DES and BMS implantation. (C) 2012 Elsevier Ireland Ltd. All rights reserved.
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| 17. |
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| 18. |
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| 19. |
- Tholerus, Emmi, 1986-
(författare)
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The dynamics of Alfvén eigenmodes excited by energetic ions in toroidal plasmas
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Experiments for the development of fusion power that are based on magnetic confinement deal with plasmas that inevitably contain energetic (non-thermal) particles. These particles come e.g. from fusion reactions or from external heating of the plasma. Ensembles of energetic ions can excite plasma waves in the Alfvén frequency range to such an extent that the resulting wave fields redistribute the energetic ions, and potentially eject them from the plasma. The redistribution of ions may cause a substantial reduction heating efficiency, and it may damage the inner walls and other components of the vessel. Understanding the dynamics of such instabilities is necessary to optimise the operation of fusion experiments and of future fusion power plants.A Monte Carlo model that describes the nonlinear wave-particle dynamics in a toroidal plasma has been developed to study the excitation of the abovementioned instabilities. A decorrelation of the wave-particle phase is added in order to model stochasticity of the system (e.g. due to collisions between particles). Based on the nonlinear description with added phase decorrelation, a quasilinear version of the model has been developed, where the phase decorrelation has been replaced by a quasilinear diffusion coefficient in particle energy. When the characteristic time scale for macroscopic phase decorrelation becomes similar to or shorter than the time scales of nonlinear wave-particle dynamics, the two descriptions quantitatively agree on a macroscopic level. The quasilinear model is typically less computationally demanding than the nonlinear model, since it has a lower dimensionality of phase space.In the presented studies, several effects on the macroscopic wave-particle dynamics by the presence of phase decorrelation have been theoretically and numerically analysed, e.g. effects on the growth and saturation of the wave amplitude, and on the so called frequency chirping events with associated hole-clump pair formation in particle phase space. Several effects coming from structures of the energy distribution of particles around the wave-particle resonance has also been studied.
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| 20. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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An Efficient Solution to the Factorized Geometrical Autofocus Problem
- 2016
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 54:8, s. 4732-4748
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a new search strategy within the scope of factorized geometrical autofocus (FGA) and synthetic-aperture-radar processing. The FGA algorithm is a fast factorized back-projection formulation with six adjustable geometry parameters. By tuning the flight track step by step and maximizing focus quality by means of an object function, a sharp image is formed. We propose an efficient two-stage approach for the geometrical variation. The first stage is a low-order (few parameters) parallel search procedure involving small image areas. The second stage then combines the local hypotheses into one global autofocus solution, without the use of images. This method has been applied successfully on ultrawideband CARABAS II data. Errors due to a constant acceleration are superposed on the measured track prior to processing, giving a 6-D autofocus problem. Image results, including resolution, peak-to-sidelobe ratio and magnitude values for point-like targets, finally confirm the validity of the strategy. The results also verify the prediction that there are several satisfying autofocus solutions for the same radar data.
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| 21. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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Autofocus and analysis of geometrical errors within the framework of fast factorized back-projection
- 2014
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781628410303 ; 9093, s. Article number 909303-
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Konferensbidrag (refereegranskat)abstract
- This paper describes a Fast Factorized Back-Projection (FFBP) formulation that includes a fully integrated autofocus algorithm, i.e. the Factorized Geometrical Autofocus (FGA) algorithm. The base-two factorization is executed in a horizontal plane, using a Merging (M) and a Range History Preserving (RHP) transform. Six parameters are adopted for each sub-aperture pair, i.e. to establish the geometry stage-by-stage via triangles in 3-dimensional space. If the parameters are derived from navigation data, the algorithm is used as a conventional processing chain. If the parameters on the other hand are varied from a certain factorization step and forward, the algorithm is used as a joint image formation and autofocus strategy. By regulating the geometry at multiple resolution levels, challenging defocusing effects, e.g. residual space-variant Range Cell Migration (RCM), can be corrected. The new formulation also serves another important purpose, i.e. as a parameter characterization scheme. By using the FGA algorithm and its inverse, relations between two arbitrary geometries can be studied, in consequence, this makes it feasible to analyze how errors in navigation data, and topography, affect image focus. The versatility of the factorization procedure is demonstrated successfully on simulated Synthetic Aperture Radar (SAR) data. This is achieved by introducing different GPS/IMU errors and Focus Target Plane (FTP) deviations prior to processing. The characterization scheme is then employed to evaluate the sensitivity, to determine at what step the autofocus function should be activated, and to decide the number of necessary parameters at each step. Resulting FGA images are also compared to a reference image (processed without errors and autofocus) and to a defocused image (processed without autofocus), i.e. to validate the novel approach further.
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| 22. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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Factorized Geometrical Autofocus for Synthetic Aperture Radar Processing
- 2014
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - : Institute of Electrical and Electronics Engineers (IEEE). - 0196-2892 .- 1558-0644. ; 52:10, s. 6674-6687
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a factorized geometrical autofocus (FGA) algorithm, specifically suitable for ultrawideband synthetic aperture radar. The strategy is integrated in a fast factorized back-projection chain and relies on varying track parameters step by step to obtain a sharp image; focus measures are provided by an object function (intensity correlation). The FGA algorithm has been successfully applied on synthetic and real (Coherent All RAdio BAnd System II) data sets, i.e., with false track parameters introduced prior to processing, to set up constrained problems involving one geometrical quantity. Resolution (3 dB in azimuth and slant range) and peak-to-sidelobe ratio measurements in FGA images are comparable with reference results (within a few percent and tenths of a decibel), demonstrating the capacity to compensate for residual space variant range cell migration. The FGA algorithm is finally also benchmarked (visually) against the phase gradient algorithm to emphasize the advantage of a geometrical autofocus approach.
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| 23. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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Factorized Geometrical Autofocus for UWB UHF-band SAR with a GPS-supported Linear Track Model
- 2022
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Ingår i: IEEE Transactions on Aerospace and Electronic Systems. - Piscataway : IEEE. - 1557-9603 .- 0018-9251. ; 58:4, s. 3147-3161
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes how to form a SAR image without proper motion quantities. That is within the scope of factorized geometrical autofocus (FGA). The FGA algorithm is a fast-factorized back-projection formulation with adjustable geometry parameters. Sub-apertures are tuned and merged pair by-pair (base-2) and step-by-step. With this technique, we can correct an erroneous geometry and form a focused image. The FGA algorithm has been applied on two data sets, acquired by the UWB CARABAS 3 system at UHF-band. The tracks are measured accurately by means of a DGPS. We however adopt and modify a geometry model. Equidistant linear tracks at fixed altitudes are assumed. These tracks are then regulated via a two-stage search strategy and a reverse processing procedure. As this is a first experiment at UHF-band, we provide GPS-based length values for the sub-apertures, to simplify the search. Multiple geometry solutions are tested for each sub-aperture pair, i.e. at each resolution level. Resulting FGA images are compared to reference images and verified to be focused. This indicates that it is feasible to form a focused image with wavelength resolution at UHF-band, i.e. with minimum support from a motion measurement system.
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| 24. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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Factorized Geometrical Autofocus: On the geometry search
- 2016
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Ingår i: IEEE Radar Conference (RadarConf), Philadelphia, PA, USA, May 02-06, 2016. - 1097-5764. - 9781509008636 ; , s. 293-297
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Konferensbidrag (refereegranskat)abstract
- This paper deals with local geometry optimization within the scope of Factorized Geometrical Autofocus (FGA). The FGA algorithm is a Fast Factorized Back-Projection (FFBP) formulation with six free geometry parameters. These are tuned until a sharp image is obtained, i.e. with respect to an object function. To optimize the geometry (from a focus perspective) for a small image area, we propose an efficient routine based on correlation, sensitivity analysis and Broyden-Fletcher-Goldfarb-Shanno (BFGS) minimization. The new routine is evaluated using simulated Ultra-WideBand (UWB) data. By applying the FGA algorithm step-by-step, an erroneous geometry is compensated. This gives a focused image. In regard to run time, the new routine is approximately 100 times faster than a brute-force approach, i.e. for this FGA problem. For a general problem, the run time reduction will be far greater. To be more specific: with x parameters and N values to assess for each parameter; it is anticipated that the computational effort will decrease exponentially by a factor close to N-x.
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| 25. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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SAR processing UWB VHF data without a motion measurement system
- 2018
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 10647
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Konferensbidrag (refereegranskat)abstract
- SAR processing usually requires very accurate navigation data, i.e. to form a focused image. The track must be measured within fractions of the centre wavelength. For high frequencies (e.g. X-band) this condition is too strict. Even with a cutting-edge motion measurement system, autofocus is a necessity. For low frequencies (e.g. VHF-band) a differential GPS (DGPS) is often an adequate solution (alone). However, for this case, it is actually conceivable to rely on autofocus capability over the motion measurement system. This paper describes how to form a SAR image without support from navigation data. That is within the scope of factorized geometrical autofocus (FGA). The FGA algorithm is a base-2 fast factorized back-projection realization with six free geometry parameters (per sub-aperture pair). These are tuned step-by-step until a sharp image is obtained. This procedure can compensate for an erroneous geometry (from a focus perspective). The FGA algorithm has been applied successfully on an ultra-wideband (UWB) data set, acquired at VHF-band by the CARABAS 3 system. The track is measured accurately by means of a DGPS. We however adopt and modify a basic geometry model. A linear equidistant flight path at fixed altitude is assumed and adjusted at several resolution levels. With this approach, we emulate a stand-alone processing chain without support from navigation data. The resulting FGA image is compared to a reference image and verified to be focused. This indicates that it is feasible to form a VHF-band SAR image without a motion measurement system.
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| 26. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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SAR processing without a motion measurement system
- 2017
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Ingår i: Proceedings of EURAD. - 9782874870491 ; Volume 2018-January, s. 1-4
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Konferensbidrag (refereegranskat)abstract
- This paper leads a discussion on how to form a SAR image without knowing the track. That is within the scope of Factorized Geometrical Autofocus (FGA). The FGA algorithm is a base-2 fast factorized back-projection formulation with six free geometry parameters (per sub-aperture pair). These are tuned step-by-step until a sharp image is obtained. This innovative autofocus technique can compensate completely for an erroneous geometry. The FGA algorithm has been applied successfully on two UWB data sets, acquired by the CARABAS II system at VHF-band. The tracks are known (measured accurately). We however adopt and modify a basic geometry model. A linear equidistant track at fixed altitude is initially assumed. Apart from deviations due to linearization, a ~2.5 m/s along-track velocity error is also introduced. Resulting FGA images are compared to reference images and verified to be focused. This indicates that it is feasible to form a SAR image without support from a motion measurement system.
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| 27. |
- Torgrimsson, Jan, 1982, et al.
(författare)
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SAR Processing Without a Motion Measurement System
- 2019
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 57:2, s. 1025-1039
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Tidskriftsartikel (refereegranskat)abstract
- This paper leads a discussion on how to form a Synthetic Aperture Radar (SAR) image without knowing the relative track. That is, within the scope of factorized geometrical autofocus (FGA). The FGA algorithm is a base-2 fast factorized backprojection (FFBP) formulation with six free geometry parameters (per subaperture pair). These are tuned step by step until a sharp image is obtained. This innovative autofocus concept can compensate completely for an erroneous geometry. The FGA algorithm has been applied successfully on two ultrawideband (UWB) data sets, acquired by the CARABAS II system at very high frequency (VHF)-band. The relative tracks are known (measured accurately). We, however, adopt and modify a basic geometry model. A linear equidistant track at fixed altitude is initially assumed. Apart from deviations due to linearization, a similar to 2.5-m/s along-track velocity error is also introduced. Resulting FGA images are compared to reference images and verified to be focused. This indicates that it is feasible to form a wavelength-resolution SAR image at VHF-band without support from a motion measurement system. The execution time for the examples in this paper is about five times longer with autofocus than without. Hence, the FGA algorithm is now fit for use on a regular basis.
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| 28. |
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| 29. |
- Åström, Daniel Oudin, et al.
(författare)
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High risk of developing dementia in Parkinson's disease : a Swedish registry-based study
- 2022
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Dementia have substantial negative impact on the affected individual, their care partners and society. Persons living with Parkinson's disease (PwP) are also to a large extent living with dementia. The aim of this study is to estimate time to dementia in PD using data from a large quality register with access to baseline clinical and patient reported data merged with Swedish national health registries. Persons with Parkinson's disease in the Swedish Neuro Registries/Parkinson's Disease Swedish PD Registry (PARKreg) in Sweden were included and linked to national health registries and matched by sex and age to controls without PD. Time to dementia was analysed with Cox regression models assuming proportional hazards, with time since diagnosis as the underlying time variable. In this large prospective cohort study, PwP had approximately four times higher risk of developing dementia as compared to age and sex-matched controls, a finding which remained after adjusting for potential confounders. The present resultsunderline the high risk of dementia in PD and further emphasize the importance of developing symptomatic and ultimately disease modifying strategies to counteract this part of the non-motor symptomatology in PD.
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