摘要:

The solution is determined for the problem of a plane wave at skew incidence on the junction of two coplanar resistive half planes. Using duality and the fact that coplanar electric and magnetic current sheets do not interact, it is then shown that for any pair of half planes subject to first‐order transition or boundary conditions, the diffracted field can be expressed in terms of the diffracted field for the resistive half plane

ISSN:0048-6604    

DOI:10.1029/90RS02684

年代:1991

数据来源: WILEY

摘要:

The diffraction problem associated with a multilayer material slab recessed in a perfectly conducting ground plane is formulated and solved via the generalized scattering matrix formulation (GSMF) in conjunction with the dual integral equation approach. The multilayer slab is replaced by a surface obeying a generalized impedance boundary condition (GIBC) to facilitate the computation of the pertinent Wiener Hopf split functions and their zeros. BothEzandHzpolarizations are considered and a number of scattering patterns are presented, some of which are compared to exact results available for a homogeneous recessed slab.

ISSN:0048-6604    

DOI:10.1029/90RS02654

年代:1991

数据来源: WILEY

摘要:

Millimeter‐wave (MMW) remote sensing of ground snow has attracted considerable interest in recent years. Because the size of the snow ice particle is comparable to the wavelength in the millimeter‐wave region, we can no longer use a simple Rayleigh phase function or the small particle approximation usually used at microwave frequencies for calculating the extinction coefficient. In this paper we present a model for MMW scattering from snow using the vector radiative transfer theory and a Mie phase function. Assuming snow to consist of randomly distributed spherical particles embedded in a mixture of air and water, the vector radiative transfer theory is solved using the discrete ordinate method. The values of the extinction coefficient used in the calculations are based on a combination of experimental data and calculations using the quasi‐crystalline approximation. The backscattering coefficient is calculated for different liquid water contents at 35, 95, and 140 GHz. We show that the backscattering coefficient is sensitive to liquid water content at all three frequencies, with 35 GHz being the most sensitive. Except for normal incidence, the effect of snow surface roughness is negligibly small for dry snow, and it is somewhat significant for wet snow at 35 GHz, but not at the higher freque

ISSN:0048-6604    

DOI:10.1029/90RS02560

年代:1991

数据来源: WILEY

摘要:

Using a truck‐mounted platform, backscatter measurements were made at 35, 95, and 140 GHz for a variety of snow conditions to evaluate the radar response to incidence angle, surface roughness, and liquid water content. Good agreement was obtained between the experimental observations and theoretical calculations based on the numerical solution of the radiative transfer equation presented in the preceding paper. A notable exception is when the snowpack is in the refreezing phase of the diurnal cycle, during which the snowpack is characterized by a dry surface boundary with wet layers underneath. To accommodate this type of condition, a hybrid first‐order numerical solution is proposed. The hybrid approach provides excellent agreement between theory and experim

ISSN:0048-6604    

DOI:10.1029/90RS02559

年代:1991

数据来源: WILEY

摘要:

Measurements have been made of the radiation pattern of a symmetrical, prime‐focus paraboloidal antenna which is used as a radio telescope at 1420 MHz. A transmitter was placed on a nearby hilltop, and the test antenna, used as a receiver, was driven through the range of directions permitted by its mounting; about 55% of the radiation pattern was accessible to measurement. The main beam, near sidelobes, and spillover lobes have been measured, and the conical sidelobes generated by scattering from the feed support struts are clearly seen. The effective temperature of the ground at 1420 MHz has been determined by radiometry. By convolving the radiation pattern with the ground temperature distribution the antenna temperature has been calculated and the contribution from various parts of the radiation pattern estimated. Of a measured total antenna temperature of 26.8 K with the antenna pointing at the zenith, the following contributions can be accounted for: cosmic microwave background (2.7 K), galactic emission (1.0 K), atmospheric emission (2.0 K), direct spillover from the ground into the feed (8.0 K), leakage through the reflector mesh (5.9 K), and diffraction around the reflector rim (0.6 K). It is concluded that ground radiation scattered from the feed support struts into the aperture is a significant contributor to antenna noise, and 5.8 ± 2.5 K has been attributed to this cause. Of the total noise from the ground, 1.1 K is contributed by hills surrounding the site; most of this enters the antenna through the sidelobes generated by the feed support struts. On a completely flat site, strut scattering would contribute about 4.7

ISSN:0048-6604    

DOI:10.1029/90RS02556

年代:1991

数据来源: WILEY

摘要:

The struts supporting the feed or subreflector of a symmetrical paraboloidal antenna generate sidelobes around cones of wide opening angle by scattering energy from the plane wave leaving the reflector. On the basis of simple assumptions about these “scatter cone” sidelobes, an approximate formula is derived to predict their level relative to the main beam; it is tested against published measurements. The noise added to the antenna when the scatter cone sidelobes receive radiation from the ground is calculated, and this mechanism is seen as a significant contributor to antenna noise. The use of struts of triangular cross section in place of circular ones redirects sidelobe energy away from the back hemisphere of the radiation pattern to the front hemisphere. For most antenna pointings these sidelobes will not strike the ground, and antenna noise temperature is likely to be reduced. Radiometric measurements at 1420 MHz have been made using a 9‐m antenna equipped with struts of various cross sections and sizes. These measurements have been used to isolate the strut contribution to antenna noise, and have verified that this contribution can be reduced by using triangular struts. The strut contribution to antenna noise is calculated as a function of zenith angle of the main beam of the antenna. Triangular struts are superior to circular ones at all main‐beam zenith angles. When three struts are used, the upright Y configuration of the tripod is better than the inv

ISSN:0048-6604    

DOI:10.1029/90RS02557

年代:1991

数据来源: WILEY

摘要:

A discussion of statistical properties of wave fields in randomly layered media is presented. It is shown that such media, because of quasi‐stationary states existing therein, represents an efficient fluctuation waveguid

ISSN:0048-6604    

DOI:10.1029/90RS01251

年代:1991

数据来源: WILEY

摘要:

A theoretical foundation for the use of the parabolic wave equation/Fourier split‐step method for modeling electromagnetic tropospheric propagation is presented. New procedures are used to derive a scalar Helmholtz equation and to subsequently transform to a rectangular coordinate system without requiring approximations. The assumptions associated with reducing the resulting exact Helmholtz equation to the parabolic wave equation that is used for computations are then described. A similar discussion of the error sources associated with the Fourier split‐step solution technique is provided as well. These discussions provide an important indication of the applicability of the parabolic equation/split‐step method to electromagnetic tropospheric propagation problems. A rigorous method of incorporating an impedance boundary at the Earth's surface in the split‐step algorithm is also presented for the first time. Finally, a few example calculations which demonstrate agreement with other propagation models are p

ISSN:0048-6604    

DOI:10.1029/91RS00109

年代:1991

数据来源: WILEY

摘要:

An extensive international campaign to observe Polar Mesospheric Summer Echoes was conducted with the European Incoherent Scatter radars in July and August 1988. During this period, two short experiments used the flexibility of the VHF system to transmit two simultaneous beams which could be used to determine the horizontal variation of the scattering irregularities. On the first occasion the echoes appear to originate from noncoplanar points near 84 km altitude. On the second occasion, only the temporal behaviour could be studied for a short interval.

ISSN:0048-6604    

DOI:10.1029/90RS01404

年代:1991

数据来源: WILEY

摘要:

The problem of delay‐Doppler mapping of overspread targets, i.e., targets where the product of delay extent θtand Doppler widthBtis greater than unity is discussed. For a transmitter modulation waveform which is believed to be representative of continuous wave modulation, namely, a random Gaussian noise modulation, the effect of the “self‐noise” and the additive noise on the output signal‐to‐noise ratio is evaluated, and expressions are derived which allows the signal‐to‐noise ratio to be predicted and the quality of the mapping assessed for various types of targets and modulation waveform properties. The problem of the self‐noise is shown to be a serious problem, particularly for targets where the power returned varies greatly with range. A scheme involving a combination of random noise modulation and polarization modulation is developed which will greatly improve the capability to map this type of overspread targets. The particular case of the mapping of Mars with an existing facility is dealt with in detail, and it is shown that a limited amount of mapping with a resolution corresponding to 100 by 100 points on the planet is possible

ISSN:0048-6604    

DOI:10.1029/91RS00108

年代:1991

数据来源: WILEY