Krishna M. Pasala

Mutual coupling effects and their reduction in wideband direction of arrival estimation

The performance of multiple signal classification (MUSIC) algorithm using three different inputs over a wideband of frequencies is considered. These inputs are: 1) ideal voltages, 2) actual voltages which include coupling effects and are obtained with the method or moments, and 3) corrected voltages which are obtained from the actual voltages so that the mutual coupling effects are removed. Linear arrays of dipoles, sleeve dipoles, and spiral antennas are considered over 200 MHz to 400 MHz band. >

Effects of mutual coupling on the performance of STAP antenna arrays

The effect of mutual coupling on the performance of space-time adaptive processing (STAP) antenna arrays is investigated. A signal model that includes the effects of mutual coupling is derived and used to compute the optimum solution for the fully adaptive and a variety of partially adaptive algorithms. The simulations indicate that if the mutual coupling is not properly accounted for there is significant degradation of the signal-to-interference-plus-noise ratio (SINR). In addition, the clutter notch is widened resulting in a larger minimum detectable velocity (MDV) of the target. When the mutual coupling is properly accounted for, the performance can be restored to the ideal level. However, STAP algorithms, in general, are very sensitive to errors in the mutual coupling matrix, requiring a very complete knowledge of this matrix for good performance. Of all the algorithms considered here, beam space algorithms appear to be the most robust with respect to uncertainties in the mutual coupling matrix.

Theory of angle estimation using a multiarm spiral antenna

An analysis of angle estimation using a single aperture, multimode spiral is presented. It is demonstrated that in addition to the classical Comparison method, modern parameter estimation techniques such as Multiple Signal Classification (MUSIC) and MLM (Maximum Likelihood Method) can be applied to multimode antenna technology for angle estimation. These techniques, coupled with the advent of digital receivers make possible the elimination of the mode-forming network. Results of computer simulations show that these new approaches give lower bias and variance, and also extend coverage toward boresight compared with the traditional Comparison method.

Scattering from a perfectly conducting cube

The cube epitomizes the complex, three-dimensional scatterer with its multiple interactions and vertex diffraction playing a critical part in the far-field patterns of some bistatic planes. the results presented are for a cube on the order of 1.5-3 wavelengths on edge which is illuminated by a plane wave at broadside incidence. The method employed is the hybrid iterative method (HIM) which utilizes an initial approximation of the surface currents on the cube faces. These currents are inserted into the magnetic-field integral equation (MFIE) to produce improved or updated approximations to these surface currents. This process is repeated to convergence by the method of successive approximations. These currents are then used to find the bistatic radar cross section (RCS) for an arbitrary plane of measurement (emphasis has been placed upon the H-plane and the E-plane). Of particular interest is the development of cross-polarized currents, which are initially approximated by zero. As the iteration process progresses, it is seen that all physical scattering processes present in this body are introduced by enforcing the MFIE. >

Performance analysis of conformal conical arrays for airborne vehicles

Conformal array apertures have great potential for providing high performance, low weight systems with little or no impact to the aerodynamic design of the air vehicle. A performance analysis of conformal conical arrays for a national airborne radar application is presented. The conical array geometry is chosen for its similarity to an aircraft or missile nosecone. Performance capabilities are analyzed for a number of antenna performance parameters including scan volume, sidelobe levels, grating lobes, beamwidth, directivity, element count, and cross-polarization

Diagnosis of subdural and intraparenchymal intracranial hemorrhage using a microwave based detector

A novel method for identifying and localizing brain hemorrhage is presented. The method uses electromagnetic waves in the microwave and RF region and a modified algorithm previously used for the estimation of the angle of arrival of radar signals. Results are presented applying this device for detecting subdural and intraparenchymal hemorrhages in anesthetized pig.

A novel multi-mode interferometer system

In this paper we consider a new interferometer comprising wideband multi-mode antenna elements rather than conventional antenna elements. As a result, the phase ambiguities associated with the long baselines of the interferometer are resolved using the angle estimates provided by the multi-mode elements. This results in the elimination of the short baseline interferometers of the conventional five-element interferometer; the hybrid multimode interferometer needs only three elements. In addition, since the multi-mode elements are wide band, the hybrid multi-mode interferometer is capable of giving good performance over a wide bandwidth.

Broadband Microstrip Leaky Wave Antenna With Inhomogeneous Materials

Analytical solutions for the fields and propagation constant in an open microstrip line above an inhomogeneous substrate are derived and the finite element method (FEM) is used as a tool to validate the theory. Excellent agreement between theoretical and simulated results is obtained and the theory is used to determine the driving point impedances. The complex propagation constant corresponding to the leaky mode is derived from the fields computed using the FEM technique and compared to the values obtained using the transverse resonance method (TRM). It is demonstrated that by periodically loading the microstrip line with air gaps and keeping the period much less than a wavelength, it is possible to realize a leaky wave antenna with lower dielectric constant whose bandwidth is increased by a factor of approximately 50% or so. The proposed inhomogeneous LWA has been fabricated, and measured with results corroborating the theory shown here.

Automatic target classification of slow moving ground targets in clutter

A new approach is proposed which will allow air-to-ground target classification of slow moving vehicles in clutter. A wideband space-time adaptive (STAP) filter architecture, based on subbanding, is developed and coupled with a one dimensional template-based minimum mean squared error (MMSE) classifier. The performance of this STAP/ATC (automatic target classification) algorithm is quantified using an extensive simulation. The level of residual clutter afforded by various filter configurations and the associated incremental improvement in ATC performance is quantified, revealing the potential for realizable hardware and software implementations to achieve acceptable ATC performance.

Hybrid antenna and scattering matrix based beamformer for wide-angle scanning

This paper presents a new hybrid antenna design and beamformer, which is capable of electronic scanning over hundreds of beamwidths with little degradation in the antenna pattern. The new compound hybrid antenna designs are discussed, with emphasis on one design that is composed of a planar array surrounded by polynomially-defined reflector surfaces. The ratio between the reflector aperture and array areas is compared to the theoretical limit. A new approach to beamforming in hybrid antennas is also discussed. Using a scattering matrix model for the hybrid antenna system., a bi-directional transformation is developed that relates the signals at the hybrid system feed to the signals that would be present in a planar array at the location of the reflector aperture. This allows standard planar array analysis and synthesis techniques to be used with the hybrid system. Examples are provided for transmit and receive weight synthesis.