Showing papers on "Beamforming published in 1971"

Errors in Time Delay Measurements

Abstract: Simple delay and sum of sensors in a seismic array is an effective method for noise suppression. However, unless we have precise steering delays, much of the signal energy is lost during the beam forming process too. We have investigated possible error sources in time delay measurements, using a computerized cross-correlation procedure. Parameters perturbed are correlation window length and positioning, signal frequency content and signal to noise ratio (SNR). Our results indicate that relative low frequency waves and using the very first part of theP-signals give the most reliable and stable time delay values. High frequency bandpass filtering improves SNR, but signal correlation and the precision in beam steering corrections decrease. Significant loss of high frequency energy during beamforming seems to be unavoidable.

Adaptive beamformer with time constant control

Abstract: An adaptive beamformer and signal processor for sonar and other signal receptor arrays, in which beamforming is accomplished by correlation feedback loops providing matched weighting across the array. For improved performance of the adaptive beamformer thus comprised in the presence of large input transients, while preserving its steady state performance essentially unchanged, the time constant of the correlation feedback loops is placed under automatic control and adjusted thereby to prevent saturation of the loops even with large interference transients.

Seismic array design

Abstract: : We present a method for predicting the noise reduction by simple beamforming of an array. The method gives an accurate estimate of the known capabilities of LASA, and is applied to the design of possible new arrays. The method requires field measurement of noise and signal correlations as a function of distance.

Space-diversity reception for v.h.f. mobile radio

Abstract: Work on aerial diversity reception to combat multipath fading is described. A 3-channel adaptive array has been built around a conventional v.h.f.?a.m. receiver, and been modified to measure signal amplitudes and relative phases. Experimental results show the effects of typical multipath environments, and the improvement obtained with an adaptive array.

A signal detectability approach to optimization of the geometry of distributed aperture (array) receivers

Abstract: A technique which allows the analytical specification of optimal array geometry that enhances array reception is developed. The technique is illustrated for small antenna arrays of up to four elements in two classes of interference fields, with and without spatial constraints. The analytical treatment is predicated upon signal detectability and employs error-free arrays of noninteracting nondirectional elements. The interference fields considered are those due to isotropic sources (case I) and sources uniformly distributed on an unbounded plane (case II). The optimality criterion is minimization of the signal detection error probability. For a phased receiving array, this is equivalent to maximizing the array detectability gain function, which has the effect of maximizing the predetection signal-to-noise ratio (SNR). The optimal array configuration is found to be two-dimensional in both cases (circular configuration in case I and elliptical configuration in case II, with the antennas equally spaced in angle along the periphery of the configuration). Numerical results are given which indicate that practical increases in predetection SNR of as much as 10 to 15 dB can be achieved by employing the array configuration most appropriate for the space-time correlation structure of the interference fields.

Electric field bounds over a phased array aperture

Abstract: In the following, a bound on the electric field magnitude existing over a phased array antenna is presented. Use of this bound in predicting the power handling capability of the array is discussed. It is shown that, although the overall array is large with respect to wavelength, the narrow beamwidth often required significantly affects the near field over the aperture and just outside the antenna. The result is compared to a previously existing bound obtained for a single rectangular aperture.