摘要:

The conventional wisdom that the incoherent portion of the forward‐scattered signal off a rough surface approaches a constant asymptote as the roughness parameter (RP) grows large is reexamined and shown to be incorrect. Essential use is made of the fact that the incoherent‐scattered component has been observed to come mainly from the specular direction. With this approximation, a remarkably close fit to the data is achieved when one further assumes that the surface is illuminated by a very narrow beam. Asymptotic expressions show that the rms of the incoherent portion of the signal first rises linearly with RP, and then eventually falls off as the inverse square root of the RP. This general shape is independent of the form of the surface autocorrelation funct

ISSN:0048-6604    

DOI:10.1029/RS010i006p00585

年代:1975

数据来源: WILEY

摘要:

Increasing accuracy in the conversion ofh′(f) toN(h) profiles is obtained by increasing the order of the polynomial used to interpolate between measured points. Linear and parabolic lamination techniques correspond to first‐ and second‐order interpolation. Fourth‐order interpolation (as in the 5‐term overlapping polynomial method) is about optimum. In comparing different methods, it is essential that fixed boundary conditions be employed; when this is done an adjacent polynomial technique is much less accurate than overlapping polynomials. All methods (including least‐squares procedures) are equally sensitive to errors in the virtual height data. Possible procedures for reducing the errors caused by underlying and valley ionization are critically reviewed. It is concluded that, in general, only a single parameter is necessary or desirable to describe these regions. This parameter can be determined from ordinary and extraordinary ray measurements, at a

ISSN:0048-6604    

DOI:10.1029/RS010i006p00589

年代:1975

数据来源: WILEY

摘要:

Comparisons of the variation of atmospheric radio noise intensities for 20–24 hr to sunspot numbers have been completed. Statistical dependence between the noise intensities and sunspot numbers was found for different seasons at a number of frequencies for many locations in the global network of ARN‐2 noise recorders. The noise intensities generally tended to decrease with sunspot number in the range from 50 kHz to 5 MHz, which is presumed to be due to increases in residual ionospheric absorption during nighttime. At frequencies greater than 5 MHz, noise intensities increased with sunspot number in many cases, which would be expected from our present knowledge of ionospheric behavior in the HF range. By convention, CCIR treats year‐to‐year variation in the noise intensities as random and includes them in the prediction uncertainty σFam(for which one value is given at a frequency for a seasonal time block for all locations) in system performance evaluation. An error analysis on a global basis shows that a large portion of the year‐to‐year variability is due to sunspot variation. This suggests the possibility of improved noi

ISSN:0048-6604    

DOI:10.1029/RS010i006p00601

年代:1975

数据来源: WILEY

摘要:

An improved matrix method of calculating the solutions of wave equations in a horizontally stratified anisotropic medium is described. An inhomogeneous medium is divided into a number of thin horizontal and homogeneous slabs. The solutions of a differential equation with constant coefficients in each slab are connected by applying the boundary conditions, and the wave fields are given as the power series expansion of wave fields. In calculating the wave fields, a matrix is divided into two matrices for two independent solutions and Gram‐Schmidt orthogonalizing process is applied to prevent numerical swamping. Comparisons are made between this and other full wave methods regarding step size and computer time, and this method is found to be more efficient than the full wave method especially at high frequency wave fields. The magnetoionic modes separated from the resultant wave are shown for a model ionospher

ISSN:0048-6604    

DOI:10.1029/RS010i006p00611

年代:1975

数据来源: WILEY

摘要:

A novel formulation of the complex power radiated from planar antenna apertures is presented in terms of the cross‐correlation functions of the aperture fields. The analysis makes use of the Fourier transform relationship between the cross‐correlation function and the cross spectral power density. We start with the well‐known Poynting vector integration which involves plane wave intrinsic impedances and which can be given in five different forms according to the choice of the field representation. Through simple mathematical manipulations we arrive at the cross‐correlation form which is applicable not only to simple apertures but also to nonperiodic as well as periodic arrays either with or without dielectric sheets. This formulation is specially useful in analyzing aperture problems having a statistical nature where the conventional Poynting vector approach is not directly app

ISSN:0048-6604    

DOI:10.1029/RS010i006p00619

年代:1975

数据来源: WILEY

摘要:

Using an idealized coaxial model, we consider the transmission, in the frequency range from 20 to 1000 kHz, down a mine hoist shaft from a symmetric source at the surface. The shaft is circular in cross section and the metal hoist cable is represented by a concentric conductor. To facilitate the analysis, the source is idealized as a voltage‐excited annular slot in a circular ground plane that is located in the air‐earth interface. The earth itself is taken to be a homogeneous conducting half space. Using a modal type analysis, the total power supplied to the annular slot is calculated as the sum of the power delivered to the lower half space, the power dissipated at the air‐earth interface and the power radiated into the atmosphere. Using these results, the relative power transmitted down the shaft to a specified depth is estimated. The resulting transmission efficiency is found to be almost completely dependent on the attenuation characteristics of the dominant TEM mode in the

ISSN:0048-6604    

DOI:10.1029/RS010i006p00625

年代:1975

数据来源: WILEY

摘要:

Through application of the basic equation of geometrical optics, the eikonal equation, we have derived an expression for the refractive defocusing of a critical ray in a medium which has circular symmetry. The equation of the ray itself is simply a circle, since by definition its radius of curvature is exactly the same as its distance from the center of symmetry. This mathematical development has a practical significance in that it permits for the first time the calculation of the strength of a signal which travels in the atmosphere of a planet at a fixed critical height. Our specific interest in this matter was due to the probable existence of such a critical ray about 33 km high in the atmosphere of Venus; at this altitude the pressure and temperature are reasonably Earth‐like and, as a result, some advanced space exploration studies have considered telecommunication by means of the critical ray (J. E. Blamont, personal communication, 1974). Through the application of the result given here, the strength of such a propagation mode can be determined. The result also has potential applications in geocentric studies and in other media which possess either spherical or cylindrical symmetr

ISSN:0048-6604    

DOI:10.1029/RS010i006p00633

年代:1975

数据来源: WILEY

摘要:

In this paper, integral and differential equations are derived for the two‐frequency mutual coherence function in a random distribution of stationary and moving particles. The differential equation is then solved for a plane wave case, and the coherent bandwidth is obtained for both the weak and strong fluctuation cases. Using the two‐frequency correlation function, the output pulse shape is calculated for different particle densities. When the optical distance is small compared with unity, the pulse shape is substantially unchanged, but a long tail develops. When the optical distance is large, the coherent bandwidth is reduced and considerable pulse spread and delay occur. Numerical calculations are given for a nanosecond optical pulse propagating through cloud. It is shown that in dense cloud (density 0.5×109m−3), the pulse delay and spread are 3 and 1.28 μsec respectively over a distance of 5 km. This agrees with available experiment

ISSN:0048-6604    

DOI:10.1029/RS010i006p00637

年代:1975

数据来源: WILEY

摘要:

The concept of nonmetallic, resistive, and “conductive” sheets is discussed, and for a plane electromagnetic wave incident on a half plane composed of any one of these materials, the exact solution of the boundary value problem is obtained for incidence in a plane perpendicular to the edge. In each case the edge diffraction coefficient is proportional to a product of split functions arising from the Wiener‐Hopf method of solution. Since each function depends on only one angular variable, the coefficient for arbitrary directions of incidence and observation is expressible in terms of the backscattering coefficient for edge‐on incidence on a half plane with an equivalent impedance. This last is rather easily computed, and data are presented to illustrate the scattering b

ISSN:0048-6604    

DOI:10.1029/RS010i006p00645

年代:1975

数据来源: WILEY

ISSN:0048-6604    

DOI:10.1029/RS010i006p00651

年代:1975

数据来源: WILEY