Radio Science

Caorsi, Salvatore; Gragnani, Gian Luigi

doi: 10.1029/1998RS900009pmid: N/A

Usual inverse‐scattering methods for the reconstruction of dielectric objects are limited by the information that can be collected at the measurement locations. In this paper, an inverse‐scattering method is proposed that allows one to reconstruct the unmeasured components of the equivalent current density and then to improve the dielectric reconstruction. The mathematical formulation is provided, and the theory is supported by the results of numerical simulations.

Tariq, Salim; Mahmoud, Samir F.; Laghari, Mohammad S.

doi: 10.1029/1998RS900011pmid: N/A

In this paper we present analysis and design of a chiral lens that is used to split a linearly polarized Gaussian beam into right‐circularly and left‐circularly polarized beams traveling in different directions. It will be shown that split beams remain Gaussian if the curvature of the lens is large in comparison with the beam width. Moreover, the split angle can be varied by changing the point of beam incidence on the curved surface of the lens. Parameters like the beam width and cross‐polarization ratio for the split beams are derived and computed.

Zhang, Geng Ji; Wang, Gong Li; Wang, Han Ming

doi: 10.1029/98RS02767pmid: N/A

The hybrid method is employed in the numerical simulation of electromagnetic waves propagating in an axisymmetrical and longitudinally stratified medium. Two categories of basis functions are introduced, one of which sustains the amplitude and the other of which sustains the slope, of the function in question at nodal points. An interval extending to infinity is devised, whereby infinity is treated more reasonably than in previous works. The application of the above two contrivances makes possible the reduction of nodal points from 45, as necessitated by triangular basis functions, to 4 and also a two‐orders‐of‐magnitude improvement of the overall computational efficiency. Such a substantial improvement will facilitate the inversion of induction logging.

Nedlin, G. M.; Chubb, S. R.; Cooper, A. L.

doi: 10.1029/1998RS900008pmid: N/A

A version of the integral equation method is developed, which applies to scattering from a surface of a substance with a large dielectric constant ε. An “impedance” boundary condition for the tangential components of electric and magnetic fields on the scattering surface is used, and the integral equation for the tangential components of the total magnetic field on a surface is formulated. This equation is applied to the problem of electromagnetic scattering from a slightly rough surface. It is demonstrated that the commonly used perfect conductor approximation (PCA) (ε = ∞) can adequately describe scattering in the case of extremely large ε only. The greatest sensitivity in scattering occurs when the incident and/or scattered waves are vertically polarized. For such cases the PCA does not work until ε > 104. In the particular case of scattering from the ocean (∣ε∣ ∼ 65), the PCA fails to provide an adequate description of the phenomena for either horizontally or vertically polarized waves, for practically all incidence and scattering angles.

Weile, D. S.; Michielssen, E.

doi: 10.1029/1998RS900006pmid: N/A

A hybrid genetic algorithm/local optimization technique for designing cascaded planar periodic structures such as frequency selective surfaces is developed. The genetic algorithm optimizes the shape and size of the periodic metallization in each of the screens as well as the thickness and dielectric constant of intervening dielectric layers in order to achieve a desired frequency response characteristic. Careful local optimization to fine‐tune the dielectric layer thicknesses is shown to accelerate both the spectral analysis of the cascades and the convergence of the stochastic search. Results are presented for the design of a satellite subreflector with a 3:1 ratio of stop band center to passband center, a circular polarizer with an axial ratio of less than 1 dB between 47 and 64 GHz, and a twist polarizer with a better than −1‐dB reflection into the cross‐polarized field component for 47–65 GHz.

Hirose, Masanobu; Miyake, Masayasu; Takada, Jun‐ichi; Arai, Ikuo

doi: 10.1029/1998RS900010pmid: N/A

We have derived a new form of the integral equation formulation of the measured equation of invariance (IE‐MEI). The new formulation clarifies the existence of a relationship between scattered electric and magnetic fields at consecutive nodes in the IE‐MEI and indicates that the relationship in a problem for a perfect electric conductor (PEC) holds for a problem with arbitrary materials. In a scattering problem of a two‐dimensional cylinder with an impedance boundary condition (IBC), every matrix in the IE‐MEI is a band‐like sparse matrix. That is, the solution process in the IE‐MEI with an IBC is the same as that for a PEC. Therefore the IE‐MEI with an IBC has the same merits of the IE‐MEI for a PEC: The more efficient computation can be achieved with the smaller memory than those of the method of moments (MOM). The IE‐MEI with an IBC is validated by numerical examples for a circular cylinder and a square cylinder by comparison with a combined field MOM that satisfies exact boundary conditions. Numerical examples show that the IE‐MEI with an IBC is applicable to the case where the generalized skin depth is less than half the width of a scatterer.

Zamora, R. J.; Kropfli, R. A.

doi: 10.1029/98RS02429pmid: N/A

During a month‐long experiment near San Clemente Island, California, the San Clemente Ocean Probing Experiment (SCOPE), unusual radar echoes were observed from an X‐band Doppler radar operated by the National Oceanic and Atmospheric Administration Environmental Technology Laboratory from a cliff 573 m above mean sea level. These radar images were often obtained whenever a strong marine inversion was observed below the level of the radar. The unusual characteristics of the specific event analyzed here were the observed wavelength of the periodic pattern, about 2 km, and its lack of movement during the 15‐min period during which specific features of the pattern, for example, the “wave” crest, could be identified. Several explanations were considered but rejected because of their conflict with the observations and known characteristics of ocean waves. The simplest explanation consistent with all of the observations was that of microwave propagation through a marine inversion which was supporting a low‐amplitude standing wave in the lee of the island. The plausibility of this hypothesis was supported by a simple ray‐trace calculation that simulated propagation through an analytical two‐dimensional field of modified refractive index that represented the essential characteristics of the observed refractive index profile.

Ruiz, Diego P.; Gallego, Antolino; Carrión, María C.

doi: 10.1029/98RS02599pmid: N/A

A comparison of the resonance annihilation filter (RAF) method and the extinction pulse (E‐pulse) technique for radar target discrimination is presented in this paper. It is theoretically demonstrated that the RAF synthesized using finite impulse‐response digital filters is a particular forced E‐pulse waveform expanded using impulse‐basis functions. This fact reveals the RAF method to be a very poor technique for discrimination in a noisy environment. Improvement is possible with suitable data windowing equivalent to using E‐pulse waveforms synthesized with continuous and smooth basis functions. Numerical simulations are employed to compare the two techniques when different levels of additive white Gaussian noise are present in the target response. This comparison shows that the use of the Gaussian window originally proposed in the RAF method produces results comparable to rectangular E‐pulses. However, these results can be improved by the introduction of alternative smoothing windows or subsectional‐polynomial E‐pulses.

Myers, Theodore J.; Scales, Wayne A.; Marshall, Robert E.

doi: 10.1029/98RS02776pmid: N/A

Remote detection and tracking of wing‐tip‐generated wake vortices are important for hazard avoidance, especially near airports. Aircraft that fly through these hazardous vortices experience sudden induced roll. Experiments have demonstrated that there is sufficient radar cross section for remote detection at frequencies ranging from VHF to C band (100 MHz to 5 GHz). The mechanism that yields this radar cross section is Bragg scattering from the index of refraction variations due to the atmospheric water vapor being mixed by the wake vortex system. Refractive index variations of the size that correspond to half the operating radar wavelength produce the observed radar return. Previous analysis has postulated turbulence within the wake vortex to be the generator of the index of refraction variations. In this work, a new mechanism is identified that does not assume turbulence within the wake vortex system. This “laminar flow mechanism” mixes the stratified atmosphere as the wake vortex system swirls and descends, which causes refractive index structure that stretches into successively smaller spirals over time. The results are quantitatively consistent with experimental data.

Liénard, Martine; Degauque, Pierre

doi: 10.1029/1998RS900007pmid: N/A

This paper describes the main electromagnetic characteristics of a radiating cable installed in a gallery. The emphasis is on the effect of the cable environment on the coherence bandwidth and on the delay spread of the radio channel. The direction of arrival of the waves inside the tunnel has also been deduced from the complex impulse response in order to optimize a method of localization of a mobile moving in the gallery.