Swedish Defence Research Agency

About: Swedish Defence Research Agency is a government organization based out in Stockholm, Sweden. It is known for research contribution in the topics: Radar & Synthetic aperture radar. The organization has 1413 authors who have published 2731 publications receiving 56083 citations. The organization is also known as: Totalförsvarets forskningsinstitut.

Atmospheric impact on beryllium isotopes as solar activity proxy

Abstract: Reconstructing solar activity variability beyond the time scale of actual measurements provides invaluable data for modeling of past and future climate change. The 10 Be isotope has been a primary ...

Array interpolation and DOA MSE reduction

Abstract: Interpolation or mapping of data from a given real array to data from a virtual array of more suitable geometry is well known in array signal processing. This operation allows arrays of any geometry to be used with fast direction-of-arrival (DOA) estimators designed for linear arrays. In an earlier companion paper , a first-order condition for zero DOA bias under such mapping was derived and was also used to construct a design algorithm for the mapping matrix that minimized the DOA estimate bias. This bias-minimizing theory is now extended to minimize not only bias, but also to consider finite sample effects due to noise and reduce the DOA mean-square error (MSE). An analytical first-order expression for mapped DOA MSE is derived, and a design algorithm for the transformation matrix that minimizes this MSE is proposed. Generally, DOA MSE is not reduced by minimizing the size of the mapping errors but instead by rotating these errors and the associated noise subspace into optimal directions relative to a certain gradient of the DOA estimator criterion function. The analytical MSE expression and the design algorithm are supported by simulations that show not only conspicuous MSE improvements in relevant scenarios, but also a more robust preprocessing for low signal-to-noise ratios (SNRs) as compared with the pure bias-minimizing design developed in the previous paper.

Nonuniformity correction of infrared focal plane arrays

Abstract: The non-linearity of the responsivity of infrared (IR) focal plane arrays (FPA) is a major concern when IR images are evaluated. This will affect both the detection and classification ability of the IR sensor. The nonuniformity correction (NUC) method proposed here is based on polynomial fitting to pixel responsivities. It is applied to infrared data obtained from two cooled multi-band sensors in the MWIR region. The method is shown to give both suitable data restoration and a simple and efficient computer implementation. Bad pixels were identified by evaluating the coefficents obtained from the multi degree polynomial. Bad pixels were also replaced by using a nearest neighbour-hood algorithm. The NUC method, including the bad pixel identification and replacement, was implemented in a user friendly GUI (graphic user interface) for calibration of registered IR data.

Numerical investigation of the flow over an axisymmetric hill using LES, DES, and RANS

Abstract: The flow around an axisymmetric hill, mounted in a channel with a fully developed approach flow, is investigated. The flow contains complex structures such as a turbulent boundary layer with several unsteady separations and reattachments. It is highly three-dimensional due to both streamwise and spanwise pressure gradients on the leeside of the hill. The shallowness of the separation region makes the flow a very demanding test case for any computational fluid dynamics model. Three different strategies are used in this study: Reynolds-averaged Navier–Stokes (RANS), large eddy simulation (LES), and detached eddy simulation (DES). The computed flow, in terms of velocity and pressure profiles, compared with measurement data and the results show that LES and DES are indeed capable of handling this complicated flow in a correct way whereas RANS clearly fails to predict several important flow features. Furthermore, the influence of the size of the computational domain, the grid resolution and the inflow boundary...

Sustained diffusive alternating current gliding arc discharge in atmospheric pressure air

Abstract: Rapid transition from glow discharge to thermal arc has been a common problem in generating stable high-power non-thermal plasmas especially at ambient conditions. A sustained diffusive gliding arc discharge was generated in a large volume in atmospheric pressure air, driven by an alternating current (AC) power source. The plasma column extended beyond the water-cooled stainless steel electrodes and was stabilized by matching the flow speed of the turbulent air jet with the rated output power. Comprehensive investigations were performed using high-speed movies measured over the plasma column, synchronized with simultaneously recorded current and voltage waveforms. Dynamic details of the novel non-equilibrium discharge are revealed, which is characterized by a sinusoidal current waveform with amplitude stabilized at around 200 mA intermediate between thermal arc and glow discharge, shedding light to the governing mechanism of the sustained spark-suppressed AC gliding arc discharge.