Defence Research Agency

About: Defence Research Agency is a based out in . It is known for research contribution in the topics: Synthetic aperture radar & Radar. The organization has 1211 authors who have published 1109 publications receiving 31542 citations.

Segmentation and simulated annealing

Abstract: In this paper we present a new algorithm for segmenting SAR images. A common problem with segmentation algorithms for SAR imagery is the poor placement of the edges of regions and hence of the regions themselves. This usually arises because the algorithm considers only a limited number of placements for regions. The new algorithm circumvents this shortcoming, and produces an optimal segmentation into a prescribed number of regions. An objective function is derived from a statistical model of SAR imagery. This objective function is then minimized by the method of simulated annealing which is, assuming some weak constraints, guaranteed to give the global minimum. Starting with an initial segmentation, the algorithm proceeds by randomly changing the current state. The annealing then decides whether or not to accept the new configuration by calculating the difference between the likelihoods of the data fitting these segmentations. In practice there are many possible implementations of the algorithm. We describe an implementation which uses a free topological model and alters the segmentation on a pixel by pixel basis. This makes it possible to get results of high resolution, as shown in results obtained by applying the new algorithm to both airborne X-band and ERS1 imagery.

Many body shakeup in quantum well luminescence spectra.

Abstract: The observation of many body shakeup in the photoluminescence spectra of InGaAs-InP quantum wells, in the quantum Hall regime, is reported. The occurrence of this many body effect is demonstrated from the observation of low energy satellites, corresponding to inter-Landau-level excitations of the degenerate electron gas. Comparison of the zero and finite magnetic field spectra shows that the low energy tail at B=0 also arises from Fermi sea shakeup. The strength of the shakeup is controlled by the localization of the recombining hole, in agreement with recent theoretical predictions.

Computer simulation of atomic displacements in Si, GaAs, and AlAs

Abstract: Low energy displacement events produce simple point defects such as Frenkel pairs. Computer simulations of these events are useful as they provide information concerning the nature of damage in semiconductors, and can generate input parameters required for a theoretical study of cascades. Using molecular dynamics simulations we have investigated low energy displacement in Si, GaAs and AlAs for ejections below 32 eV. These materials are modelled using the Si Tersoff potential, which has been modified for the compound semiconductors. For each atomic species we have determined the displacement threshold energy, and its angular dependence together with an estimate of recombination distances. Several ejection processes have been identified. Their directional and energy dependencies will be discussed. The results for Si, GaAs and AlAs are compared and physical reasons are given for the similarities and differences.

The surface properties of silicone elastomers Exposed to seawater

Abstract: The performances of some silicone elastomers as compliant coatings which are resistant to marine fouling have been assessed from a sea‐water exposure trial covering three fouling seasons. Measurements of contact angles (polar and non‐polar liquids, recently‐advanced and recently‐receded liquid drops and air bubbles) have been used to investigate the surface properties of materials and of coatings resistant to fouling after two years’ exposure. The unmodified poly (dimethyIsiloxane) elastomer General Electric (GE) 21 was still resistant to marine settlement after three seasons and the poly(dimethyldiphenylsiloxane) GE655 only became fouled during the third season. No other unmodified material showed resistance to fouling beyond two seasons. The addition of a low‐viscosity poly(dimethylsiloxane) oil to GE655 in a sufficient quantity (20 mass %) to cause blooming resulted in a material that remained free of fouling. Time‐dependent behaviour by drops of all liquids on freshly prepared samples was observed in ...