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.

An adaptive Bayesian network for low-level image processing

Abstract: Probability calculus, based on the axioms of inference, is the only consistent scheme for performing inference; this is also known as Bayesian inference. The objects which this approach manipulates, namely probability density functions (PDFs), may be created in a variety of ways, but the focus of this paper is on the use of adaptive PDF networks. Adaptive mixture distribution (MD) networks are already widely used. In this paper, an extension of the standard MD approach is presented, it is called a partitioned mixture distribution (PMD). PMD networks are designed specifically to scale sensibly to high-dimensional problems, such as image processing. Several numerical simulations are performed, which demonstrate that the emergent properties of PMD networks are similar to those of biological low-level vision processing systems. The use of PDFs as a vehicle for solving inference problems is discussed, and the standard theory of MDs is summarised. The new theory of PMDs is then presented.

Effects of curved mirrors in waveguide resonators

Abstract: Waveguide gas lasers (CO2 ones especially) continue to be widely used. We have previously studied simple resonator designs with plane mirrors close to each end of the waveguide. Here we examine theoretical predictions concerning hybrid waveguide/free-space resonators with square-bore guides and curved mirrors. We show how resonator mode losses vary as functions of guide length and width, guide-to-mirror distance, mirror radius of curvature, and mirror tilt. We have tested a 7-W cw rf-excited CO2 laser with unusually good near-TEM00 transverse-mode quality; it is one of many promising resonator geometries not covered by earlier published research. The common case 3 reflector, sometimes viewed as guaranteeing near-TEM00 mode performance, is shown to be alarmingly sensitive to small misalignments for certain guide geometries.

Application of retrodirective beams to achieve low sidelobe levels in small phased arrays

Abstract: Synthesis of beams with low sidelobe levels is a difficult problem for the case of small phased arrays with few elements Mutual coupling between elements means that conventional weighting algorithms are not applicable A technique is presented that calculates a complex weight vector for a five element linear array, giving a reduced sidelobe beam pattern Sidelobes are reduced by the addition of retrodirective beams to the quiescent beam pattern No knowledge of the coupling coefficients or element radiation patterns is required