James M. Alsup

Linear Signal Processing and Ultrasonic Transversal Filters

The role of linear transversal filters in signal processing is discussed in Section I. Linear filters for signal processing must often have complicated impulse responses, with large bandwidth and large time bandwidth product. The linear transversal falter, a delay line with weighted and summed taps, is ideally suited for the implementation of such filters because of its simplicity of synthesis. The filters impulse response is derived by the application of some concepts from the theories of vector spaces and sampling, and is shown to be equal to the tap weighting function. Thus, the synthesis procedure consists merely of sampling the specified impulse response at appropriate intervals and using the sample values as the tap weights. The utility of the transversal filter in signal processing is illustrated by an example from scatterer distribution mapping. The illustration is applied to two hypothetical systems--a SONAR and an astronomical RADAR. In both these cases, it is not possible for a single filter to process the signal in real time. Signal processing in compressed time is discussed as an alternative to the use of a large number of filters in parallel. If the processing filter has a bandwidth capability in excess of the signals bandwidth, the signal can be time compressed and processed serially in time. A generalized receiver, employing time compression, frequency translation, and multiple-output-port transversal filtering, is developed from these ideas. In Section II, a generalized transversal filter is described and analyzed. A delay line with multiple arrays of taps, each array with a multiplicity of weighting functions, has as the impulse response between any pair of ports the cross-correlation function of the weighting functions for the two ports. A number of implementations of transversal filters employing a variety of delay line types are described and some aspects of transduction and wave propagation in bounded media are presented in relation to these implementations.

Hermite functions and regularized deconvolution in sonar waveform design and processing

A special type of comb waveform based on the Hermite function space is described. This family of waveforms is well-suited for providing simultaneous range and Doppler resolution, is relatively efficient in its use of sonar projector power, and is especially amenable to the use of constrained regularized deconvolution methods to robustly reduce sidelobes and retain intrinsic range and Doppler resolution.

Deconvolutional beamforming for air and underwater acoustic sensor arrays

Autonomous acoustic sensor arrays are often plagued with poor spatial resolution capabilities, especially at the lowest frequencies. Beamforming techniques that are data‐adaptive can improve upon the resolution capability of conventional beamformers under certain conditions. An alternate nonadaptive technique that is simply implemented and that achieves sidelobe reduction while retaining or narrowing main lobe beamwidth is presented. The method for weight matrix construction is based on regularized, constrained deconvolution. These deconvolutional beamformer (DBF) beamspace weights, which are precomputed off‐line using this nonlinear process, are applied to the data using a linear process. The physical array may be smaller when using the DBF since this beamformer simultaneously reduces the width of the main lobe at low frequencies while reducing the integrated sidelobe level. This is something even ‘‘optimal’’ beamformers have difficulty doing since the main lobe generally increases as the sidelobes are r...

High-resolution techniques for two-dimensional estimation of angle-of-arrival for planar arrays

Two-dimensional beamforming can be a valuable technique for obtaining pure-mode signals in a multipath environment, if adequate resolution is available in both elevation and azimuth dimensions. Early simulations have shown that, for adequate signal/noise ratios and for selected signal and noise types, two sources separated by as little as .01 conventional beamwidth can be resolved. Thus, current arrays may still have adequate aperture for resolving such multipaths. The MUSIC algorithm and related eigenvalue/eigenvector analysis techniques appear to be the key to allowing the kind of superior performance which would eventually lead to an order-of-magnitude improvement in source-location accuracy.