Karl F. Warnick

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—Interferometric image synthesis in radio astronomy is plagued by signal corruption from man-made sources. The very weak signals of interest can be overwhelmed by such interference. Recent work has proposed using array signal processing techniques of adaptive beamforming, adaptive filtering, and subspace projection to remove interference prior to image(More)
8m3 Spatial filtering and subspace projection methods have been pro-imaging arrays [1][2]. Given the fact that RA signal levels are often difficult to form the accurate interference signal parameter an imaging may, it is possible to overcome this problem. Using rejection by tens of decibels. New extensions to subspace projec-posed for removing interference(More)
— The calculus of differential forms has significant advantages over traditional methods as a tool for teaching electromagnetic (EM) field theory: First, forms clarify the relationship between field intensity and flux density, by providing distinct mathematical and graphical representations for the two types of fields. Second, Ampere's and Faraday's laws(More)
  • Chad K Hansen, Karl F Warnick, Brian D Jeffs, J Richard Fisher, Richard Bradley
  • 2008
We consider the use of spatial filtering algorithms for radio frequency interference (RFI) mitigation in conjunction with a focal plane feed array of electrically small elements. Numerical simulations are used to study the performance of 7 and 19 element hexagonal dipole arrays with a 25 meter reflector at an operating frequency of 1612 MHz. Using the(More)
— Relative to traditional waveguide feeds, phased array feeds (PAFs) for radio telescopes can increase the instrument field of view and sky survey speed. Unique challenges associated with PAF observations, including extremely low signal levels, long-term system gain stability requirements, spatially correlated noise due to mutual coupling, and tight(More)
We present a numerical comparison of the so-called 'linear sampling' inverse scattering methods developed by Colton and Kirsch, published in this journal, and linearized tomographic inverse scattering algorithms based on either holographic filtered backpropagation principles or a plain matrix inversion scheme. Although we restrict ourselves to(More)
Adaptive interference canceling beamformers are known to suffer from beampattern distortion, particularly when inter-ferers approach the nominal mainlobe. In some applications such as radio astronomy, even small pattern variations can be problematic. We address the issue of computing power spectral density (PSD) estimates at the output of a beamform-ing(More)
—A figure of merit for array antennas can be defined using the signal processing concept of signal-to-noise ratio (SNR) gain. The resulting array efficiency metric reduces to aperture efficiency for an isolated array and to a combination of aperture and spillover efficiency for an array feed. These results provide a practical way to measure aperture(More)
—We address the issue of computing power spectral density (PSD) estimates at the output of a beamforming sensor array in the presence of strong moving interference. It is shown that the time-varying spatial response of an adaptive beamformer (" pattern rumble ") causes estimation bias in the PSD of both the signal of interest (SOI) and noise. In(More)