摘要:

Certain diagnostic methods, based on electromagnetic backscatter phenomena, seek to provide information on selected scattering volumes that are part of a larger randomly fluctuating medium. Under these circumstances, the effects of the fluctuating medium on the signal passing to and from the scattering cell cannot be ignored. Inhomogeneities in the background refractive index may also significantly affect the characteristics of the backscattered signal. Assuming large scale, small amplitude, refractive index fluctuations, the present study explores the influence of the propagation path on the scattering cross section, as well as on the first and second moments of the temporal spectrum of the backscattered signal. In a homogeneous background, fluctuations are found to provide the same mean frequency shift as the Born approximation, but to enhance the second Doppler moment. In an inhomogeneous background, the latter effect may be overshadowed by deterministic refraction, especially for scattering volumes located far from the source. These aspects are illustrated by application to a radially stratified plasma column.

ISSN:0048-6604    

DOI:10.1029/RS012i001p00001

年代:1977

数据来源: WILEY

摘要:

The effects of phase‐front distributions on the radiation patterns of an echosonde (acoustic echo‐sounding) antenna are studied. A radiation integral is used to compute an equivalent excitation at the transmitting antenna aperture at a heightHabove the illuminating aperture of smaller radius. A two‐dimensional Fourier transform utilizing the Kirchhoff‐Fresnel diffraction formula is used to obtain the far‐field expressions. The results reported for linear, square‐law, cubic, and periodic phase‐front distributions, indicate that (while details of the antenna pattern depend upon the specific phase distribution used) all distributions tended to raise the sidelobes and broaden the mainlobe for stronger phase modulations. In some cases, the stronger phase modulations actually reduced the central portion of the mainlobe, producing a hollow conical beam. Some computed beam patterns at a carrier frequency of 2 kHz and with different phase modulation coefficients were compared with two recent sets of experimentally measured patterns. The agreement overall is good with any variations probably being due to the fact that the actual phase distributions on the antennas were not exactly known and also due to the lateral drifts that might be experienced by the tethered weather balloon supporting the microphone probe (used for some of the pattern measurements) as a result of

ISSN:0048-6604    

DOI:10.1029/RS012i001p00011

年代:1977

数据来源: WILEY

摘要:

Transmission line theory for the impedances of wire antennas immersed in a cold magnetoplasma is proposed. The theory is applicable to antennas which are not necessarily small in terms of a wavelength measured along the antenna wire. The transmission line theory for the impedance of a very thin cylindrical dipole antenna leads to the Balmain quasi‐static theory when the antenna length reduces to zero. New impedance resonances which would be observed for antennas of finite length in a magnetoplasma in the frequency region below and near the electron cyclotron frequency are predicted by the present theor

ISSN:0048-6604    

DOI:10.1029/RS012i001p00023

年代:1977

数据来源: WILEY

摘要:

As an aid in the design of an electromagnetic system for the detection, monitoring, and mapping of the burn front of an underground (in situ) coal gasification process, an analytical model has been developed that will predict the electromagnetic field reflected from the gasified portion of the coal seam. The coal and gasified section have been modeled electrically as lossy media, surrounded above and below by relatively high conducting overburden and underburden. Vertical and horizontal electric and magnetic dipoles have been investigated, utilizing a geometrical optics formulation which also takes into account the physical roughness of the coal‐overburden (underburden) and coal‐gasified section boundaries. Indications are that a swept frequency monitoring system is capable of locating the burn front of thein situgasification process, with the vertical electric dipole being the most attractive elem

ISSN:0048-6604    

DOI:10.1029/RS012i001p00033

年代:1977

数据来源: WILEY

摘要:

Forward‐scattering amplitudes of raindrops are computed at 11, 13, 19.3, and 34.8 GHz for a real raindrop shape given by Pruppacher and Pitter, assuming plane waves at normal incidence to the drop axis. Horizontal and vertical polarizations are considered in the incident field. These forward‐scattering amplitudes are then used to estimate cross polarization by rain. Cross‐polarization factors are calculated for two kinds of raindrop canting angle distributions: gaussian distributions and a photographically measured distribution. The results show that the scatter of the drop canting angles reduces the cross polarization considerably as compared with that for equioriented rain

ISSN:0048-6604    

DOI:10.1029/RS012i001p00041

年代:1977

数据来源: WILEY

摘要:

A hypothetical array of terrestrial microwave links has been designed at radar ranges 36 to 110 km and all azimuths, such that useful attenuation statistics can be derived from the weather‐radar record on digital tape of the few hours' passage of one storm system across the array. These statistics cannot be used as the basis for design in the way that the data for a whole season might be used, but one can compare one storm with another, and hope to relate attenuation to meteorological aspects of the storm. From one case of a line of intense storms it has been found that: (i) Total duration of attenuation in excess ofAdecreased approximately exponentially with increasingAfor a given link length, and for a given value ofAincreased approximately linearly with link length, at any rate up to 48 km. (ii) Durations varied with link direction, but less than ±20%. (iii) Individual fades of attenuation in excess ofAwere more numerous but more brief for links perpendicular to the direction of motion of the precipitation pattern than for those parallel. (iv) “Synthetic links” based on raingauge data have statistics like links parallel to the direction of pattern motion. Thus they are representative of links generally for “total duration” statistics (see i) where direction is unimportant (see ii) but not for individual‐fade duration and frequency (see iii). These findings for one storm passage were confirmed by two others: a less intense line and a case of widely scattered l

ISSN:0048-6604    

DOI:10.1029/RS012i001p00053

年代:1977

数据来源: WILEY

摘要:

Long distance radio systems at 11 GHz are considered with respect to rain outage. Conventional tandem systems and simple route diversity systems are analyzed and compared on the basis of rain attenuation statistics obtained from radar observations. The comparisons appear to be very favorable to diversity systems in terms of the number of repeaters. Unfortunately, the available descriptions of joint probabilities of attenuation for pairs of radio paths allow only conservative analysis of very simple diversity systems, so that effective designs are not yet possible. Therefore, the need for further investigations on intense precipitation patterns still exists.

ISSN:0048-6604    

DOI:10.1029/RS012i001p00063

年代:1977

数据来源: WILEY

摘要:

Several case studies of propagation in offshore San Diego tropospheric ducting environments are examined via waveguide formalism. Both ground‐based and earth‐detached ducts produced by elevated layers at about 600 and 2000 ft respectively are considered. The model assumes a trilinear refractivity profile and the layer to be stationary and horizontally homogeneous. The ground‐based duct study includes cases ranging from single‐mode propagation (65 MHz) to cases where more than a hundred modes have been evaluated (3300 MHz). The earth‐detached duct studies are for the single frequency of 3087.7 MHz with the transmitter at 68 ft. Approximate agreement has been found between measurements and calculations in the case of the ground‐based duct. Calculations for earth‐detached ducts dramatically overestimate the observed path losses for beyond the horizon fields. This discrepancy is probably due to layer fluctuations and/or inhomogeneity, along with the possible existence of an unaccounted for, persistent, ground‐based duct produced by a low level layer. The evaporation duct for which duct heights are generally less than 100 ft would be suc

ISSN:0048-6604    

DOI:10.1029/RS012i001p00075

年代:1977

数据来源: WILEY

摘要:

With the advent of radars capable of detecting backscatter from turbulent inhomogeneities in the clear air, there is renewed interest in the refractive character of the atmosphere. The radio refractive index structure constant and its spatial and temporal distribution are of particular importance. The general refractive properties of air masses are discussed in this paper. Based on certain assumptions, the distribution of optical and radio refractive index structure constants are then calculated and the results are compared with the limited observational data available.

ISSN:0048-6604    

DOI:10.1029/RS012i001p00089

年代:1977

数据来源: WILEY

摘要:

A recently proposed means of combining models of ionosphericF‐layer peak electron density and irregularity incremental electron density (ΔN) so as to simulate the global occurrence probability of the frequency‐spreading component of spread‐Fis discussed. This procedure is then used to model experimental spread‐Foccurrence results. It is found possible to readily simulate the sunspot‐maximum results, independently of season, with only small adjustments to the amplitudes of the empirical expressions used to model ΔNin the several latitude regimes. However, at sunspot minimum and for each season, the ΔNmodel requires modification in the equatorial and midlatitude regions of high irregularity incidence, before successful simulations of the spread‐Fdata can be obtained. These modifications, which include a broadening of the equatorial region and a poleward shift of the midlatitude region with decreasing sunspot number, are discussed in detail. It is concluded that the scintillation data base, from which the original ΔNmodel derives, is not sufficiently representative with regard to sunspot number and magnetic index. The use of the spread‐Fadaptation of the ΔNmodel, as well as its original scintillation version, to rectify these failings of the ΔNmodel

ISSN:0048-6604    

DOI:10.1029/RS012i001p00107

年代:1977

数据来源: WILEY