Saba Mudaliar

Scattering of Electromagnetic Waves in the Presence of Wave Turbulence Excited by a Flow With Velocity Shear

It is well known that an incompressible sheared flow with an inflection point in the velocity profile will result in the formation of turbulent vortices. In the case of compressible plasma flow with velocity shear, ion-acoustic fluctuations in addition to vortices will be generated. We present detailed analysis of the excitation of such low-frequency oscillations in a compressible plasma flow with velocity shear. To examine the process of the excitation and nonlinear saturation of low-frequency oscillations in the presence of a flow shear, a nonlinear system of equations was derived. We employ a predictor-corrector method to solve this system numerically. Spectral analysis of obtained numerical solutions allows to calculate the turbulent density spectra for different velocity profiles. We find that the impact of this turbulence associated with ion-acoustic wave fluctuations is considerably more significant and dominant than that due to the turbulent vortices. Another goal for our research is to understand the influence of this turbulent flow on electromagnetic (EM) signals. Since the thickness of the flow can be fairly small in the case of hypersonic vehicles, we employed a single-scattering perturbation theory to study the scattering of the EM signals from the plasma sheath. We observe that the EM scattering from the turbulent density fluctuations of the flow results in shifted signal spectra above and below that of the source. Such shifts can have rather adverse effects on the sensor performance. For instance, shifts in temporal spectra will result in channel interference and crosstalk in communication systems. Because of the nature of the dispersion characteristics of the ion-acoustic waves, the shifts in spatial spectra of scattered EM waves can be very large. This will lead to large fluctuations in integrated phase shifts and hence, results in significant signal distortion. We carried out detailed theoretical analyses and numerical calculations to understand the nature of the influence of such hypersonic turbulent flow on EM signals. The complete loss of the EM signal because of the overdense condition of the plasma sheath is well known. We find that even in underdense conditions, GPS-based navigation can be significantly impaired because of the GPS signal distortion by the turbulence caused by such ion-acoustic wave fluctuations.

Some Issues With Using Radiative Transfer Approach to Scattering From Layered Random Media With Rough Interfaces

The radiative transfer (RT) approach is widely used in applications involving scattering from layered random media with rough interfaces. Although it has been successful in several disciplines it is well known that this approach involves certain approximations. In this paper these assumptions and approximations are reexamined. To enable this a statistical wave approach is employed to this problem and the governing equations for the first and second moments of the wave functions are derived. A transition is hence made to arrive at a system of equations corresponding to that of the RT approach. It is hence found that more conditions are implicitly involved in the RT approach than generally believed to be sufficient.

Bistatic radar clutter simulations using scattering phenomenology

Most models used by the radar community for clutter simulations are empirical and intended for monostatic radars. For bistatic applications, the monostatic-bistatic equivalence theorem of Crispin, Goodrich, and Siegel (1959) is often used to estimate the bistatic clutter. We found that this theorem has serious limitations when applied to simulating scattering from clutter scenes. To address this issue we have developed a phenomenology-based model for efficient simulation of scattering from clutter scenes. The underlying algorithm is founded on a multiple scattering wave theory for a random medium layer over a two-scale rough surface. It works equally well for monostatic and bistatic applications and is computationally efficient and suitable for radar simulation studies.

Radiation Characteristics of Antennas Embedded in a Medium With a Two-Temperature Electron Population

It is well-known that low-frequency electromagnetic (EM) signals are heavily attenuated in a medium with dense electron population. All signals below the plasma frequency of the medium get cut off. If we create in this medium, a small population of relatively hot electrons the composite medium then supports low-frequency electrostatic oscillations known as electron acoustic waves (EAW) [e.g., Gary and Tokar, Phys. Fluids 28, 2439]. The dispersion relation of this composite medium shows that it supports EAW in the frequency band where EM signals are cut off. Thus, it is possible, in principle, to employ EAW to transmit signals across an overdense plasma medium. Our primary interest in this paper is to study the radiation characteristics of a source current distribution embedded in a half-space of our composite medium. To enable this, we derive the Greens functions for our problem and, hence, study the radiation characteristics of antennas. When the source signal frequency is below the plasma frequency, only EAW exist in the composite medium, while only EM waves can exist in the free space above. We find that the far-zone radiation fields of any current distribution consist only of θ-polarized waves. Explicit expressions for the radiated fields are obtained for horizontally- and vertically-polarized Hertzian dipoles embedded in our composite medium. We, hence, find that in both cases the radiation patterns are skewed towards the horizon. In particular, we find that the radiation pattern of a horizontal dipole has two lobes as opposed to one in the underdense case.

Radar communications via random sequence encoding

In this paper we propose a novel approach to joint radar-communication waveform design with low probability of intercept characteristics. The data are encoded onto a certain parameter of a particular random distribution; using this distribution, random sequences are generated, which become amplitude weights for the orthogonal frequency division multiplexing (OFDM) sub-carriers. We assume and describe a worst-case scenario, in which an interceptor possesses a certain amount of knowledge about our approach and attempts to reconstruct the data from intercepted radar/communication signals - we then show, via simulations, that with more than 32 OFDM sub-carriers used in signal design, the interceptor cannot improve upon its bit error rate (BER) even with increasing number of intercepts.

Error analysis and comparison of Riemann and average fluxes for a spacetime discontinuous Galerkin electromagnetic formulation

We present a time domain discontinuous Galerkin (TDDG) method for electromagnetics problem that directly discretizes space and time by unstructured grids satisfying a specific causality constraint. This enables a local and asynchronous solution procedure. We show that the numerical method is dissipative, thus ensuring its stability. Numerical results show the convergence rate of 2p + 1 for energy dissipation. We also investigate the choice of Riemann versus average numerical fluxes for noncausal faces and demonstrate that while the more dissipative nature of Riemann fluxes may render it unsuitable for low order elements, it provides a cleaner solution for high order elements.

A spacetime adaptive approach to characterize complex dispersive media

We present a time domain approach that can obtain reflection and transmission coefficients of a material for a wide range of frequencies. The advanced method of spacetime discontinuous Galerkin method is used to obtain the time domain response of a unit cell to an incident wave. Adaptive operations in space and time permits very efficient and accurate tracking of wave fronts. By Fourier analysis and inversion of the obtained transmission and reflection coefficients in the frequency domain, we obtain equivalent impedance, wave speed, permittivity, and permeability of the unit cell for the given frequencies. The linear solution cost of the SDG method, its powerful adaptive operations, and derivation of the entire spectrum with one time domain simulation are attractive attributes of the proposed method.

Radiative transfer in turbulent flow using spacetime discontinuous Galerkin finite element method

The radiative transfer equation for a problem that involves scattering, absorption and radiation is solved using spacetime discontinuous Galerkin (SDG) method. The strength of finite element method to handle scattering problems in heterogeneous media with complex geometries is well known. Adaptive operations in spacetime facilitates very accurate and efficient solution algorithm. We investigated the accuracy of the SDG method by using the method of manufactured solutions. For the case of harmonic phase functions we illustrate how the L2 norm error decreases with the choice of high order polynomial and more refined element size. Key merits of the use of SDG for our problem enamates from its linear solution cost, and the ability to obtain the solution for a wide frequency spectrum in one time domain simulation.

An h-adaptive time domain discontinuous galerkin method for electromagnetics

We present an h-adaptive time domain discontinuous Galerkin (TDDG) method for electromagnetics problem in which space and time are directly discretized by unstructured grids that satisfy a specific causality constraint. This enables a local and asynchronous solution procedure with arbitrary high and per element spacetime orders of elements. Our numerical results demonstrate that by using energy dissipation as an error indicator and local adaptive operations in spacetime we can significantly improve the efficiency of the method relative to nonadaptive solutions.

Experimental radar-enabled navigation with UWB system in indoor environments

In this paper, we describe an original multi-functional UWB software-defined radar systems application to dead-reckoning navigation in indoor scenarios. The radar is the only sensor available and no other navigational aids are present, except simple targets of opportunity (TOP), which are then used as references. This work provides the experimental primer on using software-defined sensors for navigational means in, e.g. GPS-denied, uncooperative environments. It can also serve as an educational instrument for undergraduate and early graduate students exploring radar applications.