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.

Zero estimation for blind deconvolution from noisy sampled data

Abstract: Two- and higher-dimensional bandlimited functions are almost always non-factorizable, meaning that their zeros form a single analytic curve. In principle one can use this property to separate the product of two bandlimited functions into its respective factors; this is important in Fourier phase retrieval and deconvolution problems. The intersection of this zero structure with the real plane is at points, closed curves or lines stretching to infinity. The location of zero points, curves or lines can only be estimated, in practice, from available noisy data. The estimated locations can be used to write a factorizable approximation to the original function and we explore the consequences of doing this. Of importance is the fact that point zeros can be used to represent a 2D bandlimited function. Hence from intensity data, point zero locations in the intensity can be used directly to estimate the complex spectrum and provide an approximate solution to the phase retrieval problem. Examples will be given and their importance for the general blind deconvolution problem discussed.

Comparison of Polarimetric Algorithm With Other Algorithms in Detecting Radio Frequency Interference

Abstract: Anthropogenic radio frequency interference (RFI) is an increasing problem in microwave remote sensing radiometry. Therefore, there is growing interest in RFI counteraction methods, including detection and blanking. In this study, the performance of polarimetric algorithm has been studied and compared with several other detection algorithms with respect to their capability to detect different potential types of RFI, such as continuous wave (CW), pulse modulated, and quadrature phase shift keying (QPSK) modulated. The mission scenario and parameters applied correspond to a typical space-borne, polar-orbiting, conically scanning microwave radiometer. Therefore, the results obtained help to estimate the achievable RFI detection limits (in terms of the brightness temperature) for similar future satellite instruments if polarimetry or other algorithms analyzed here were applied. The results obtained suggest that the RFI detection limit in the 1-K range can be achieved in the scenario under study. Incorporating multiple and complementary RFI detection algorithms in the overall system, e.g., in a digital RFI processor, enables a wide range of different RFI types to be detected. The results suggest that polarimetry and spectral density estimation are sensitive in detecting constant envelope signals (such as CW or QPSK), while kurtosis and spectral kurtosis are effective in detecting pulsed RFI. A detection limit as low as 0.1 K can be achieved for pulsed RFI with low duty cycles. These findings confirm and extend earlier results. To the best knowledge of the authors, this is the first quantitative study on the performance of the RFI detection algorithms to include polarimetry.

Cross-section estimation by simulated annealing

Abstract: Presents an algorithm which performs radar cross-section estimation by using techniques based on simulated annealing. Standard simulated annealing approaches to image restoration attempt to categorise each image element as belonging to one of a small number of predefined image states or values. This is restrictive for tasks such as radar cross-section estimation and the authors present an algorithm which is capable of producing a real-valued output. This is achieved by introducing an edge detection stage into the simulated annealing process. The action of the annealing algorithm may be viewed as a filter which adapts to local image structure. The authors present results which demonstrate this behaviour and in so doing allow them to estimate the residual noise levels that might be expected. >

Experimental investigation by stimulated Brillouin scattering of incomplete phase conjugation

Abstract: We use stimulated Brillouin scattering to investigate phase conjugation of a laser beam aberrated by one or more phase screens when the aberrated beam is incompletely sampled by the phase-conjugate mirror. Good agreement is found between our experimental results and previous theoretical work. We find that the fraction of the reflected beam that is phase conjugate after the double pass is proportional to the fraction of the original beam sampled by the phase-conjugate mirror. We also show that it is possible to increase the resolution of a phase-conjugate mirror by inserting an aberrator.

Changes in the perceived head transversal plane and the subjective visual horizontal induced by Coriolis stimulation during gondola centrifugation.

Abstract: For studying the influence of the vertical semicircular canals on spatial orientation in roll, the subjective visual horizontal (SVH) and the subjective transversal plane of the head (STP) were measured in a situation where the vertical canals sense a roll-velocity stimulus while the otolith organs persistently signal that the head is upright in roll. During gondola centrifugation (resultant gravitoinertial force vector 2.5 G, gondola inclination 66 degrees) subjects were exposed to controlled rotational head movements (angular speed 27 degrees/s, magnitude 40 degrees) about the yaw (body z-) axis, produced by means of a motor-driven helmet. This causes a roll-plane Coriolis stimulus to the canals, while the otoliths persistently sense upright head position in roll. The subjects reported intense sensations of rotation and tilt in the roll plane. This was reflected in tilts of both the SVH and STP. The initial tilt of the SVH was 13.0 +/- 9.7 degrees (mean +/- S.D., n=10). The STP was changed in the opposite direction. The initial tilt was 23.8 +/- 12.2 degrees (mean +/- S.D., n=5). The changes in the SVH and STP were not of equal magnitude. A few subjects who had almost no deviations in the SVH showed pronounced tilts of the STP. The time constant for exponential decay of the tilts of the SVH and STP was on average approximately 1 minute. These findings indicate that a difference in activity of the vertical canals in the right versus left ear may cause substantial tilts of the SVH even if there is no asymmetry in the activity of the otolith system. Further, the canal stimulus may induce a tilt of the fundamental egocentric frame of reference.