摘要:

The ionospheric reflection channel is known to be dispersive, time varying, and random. The dispersive nature of the ionosphere and the propagation geometry make the transfer function frequency selective, i.e., frequency dependent; while the ionospheric motions cause the communication channel to be time dependent or fading. Frequently, the background ionosphere is permeated by random irregularities. A radio wave propagating through such an irregular ionosphere will have its phase randomized, followed by phase mixing through diffraction. As a result, the transfer function also acquires a random character. The transfer function for a random, dispersive, and time‐varying ionospheric reflection channel can be simulated numerically using the phase‐screen‐diffraction‐layer method. The detailed behavior of the calculated transfer function is examined in several different ways. First, the spatial behavior of the transfer function, both magnitude and phase, is depicted in three‐dimensional plots. This spatial behavior can also be interpreted as a temporal behavior for a moving ionosphere. A curious phase jump of 180° accompanied by a deep fade at some frequency within the passband of the signal is found to occur. To facilitate the examination of this phase jump, locus plots are made. Using real and imaginary parts of the transfer function as coordinates, the transfer function traces out a locus with increasing frequency on this complex plane. In the presence of severe selective fading the locus is shown to pass through the origin occasionally. When this happens, the magnitude of the transfer function dips down to zero with an accompanied phase jump of 180°. Such an event is caused by destructive interference among multipath components. Second, the channel behavior is examined by taking temporal moments of the received pulse. This is done for several sample pulses, each up to the moments of the fourth order, giving the arrival time, pulsewidth, skewness, and kurtosis. Each of these four quantities is then treated as a time series in the computation of the mean, standard deviation, histogram, power spectrum, and autocorrelation function. The stochastic behavior of these four quantities is thus thoroug

ISSN:0048-6604    

DOI:10.1029/90RS01844

年代:1991

数据来源: WILEY

摘要:

A technique for measuring ionospheric echo arrival times and strengths is described. The technique uses ordinary HF‐SSB transceivers and an easy to generate probe signal. It is based on the cepstral signal processing tool for deconvolution. Identification of ionospheric echoes in a received sky wave signal was viewed as a deconvolution problem, and the cepstral transformation was used to obtain information about the multipath profile of the channel. Field tests were carried out where multipath characteristics of the ionospheric channel was measured using this techniqu

ISSN:0048-6604    

DOI:10.1029/90RS01611

年代:1991

数据来源: WILEY

摘要:

A new class of proton cyclotron echoes observed on topside sounder ionograms is presented; these echoes occur at frequencies above the electron plasma frequency and at the third and fourth harmonics of the electron cyclotron frequency, 3ƒHand 4ƒH. Some features are noted such as the dependence on low dip angle values. It is suggested that the 3ƒHand 4ƒHproton cyclotron echoes may be generated by wave‐particle interaction between the electrostatic waves and the protons that are both energized by the dipole antenna on Alouette II, ISIS I, or IS

ISSN:0048-6604    

DOI:10.1029/90RS01543

年代:1991

数据来源: WILEY

摘要:

As the Galileo probe Relay Radio Link (RRL) signal propagates outward through the Jovian atmosphere, the atmosphere will manifest itself in two ways. First, the geometric path length of the signal is increased, resulting in a small change of the RRL signal departure angle from the probe (transmitter). Secondly, the velocity of the signal is decreased. For a spherical, static atmosphere with a known profile of refractivity versus altitude the effects of refraction on the RRL frequency can be found using a variation of standard ray‐tracing techniques, whereby the ray departure angle is found by an iterative process. From the dispersive characteristics of a mixture of hydrogen and helium with trace amounts of methane and ammonia a simple model of the Jovian atmosphere is constructed assuming spherical symmetry and uniform mixing. The contribution to the RRL Doppler frequency arising from refraction is calculated, and its effect on the Doppler wind measurements is discusse

ISSN:0048-6604    

DOI:10.1029/90RS02529

年代:1991

数据来源: WILEY

摘要:

In the present work, radio ray tracing equations are derived for linear refractive index profiles. The solution is obtained in closed form without approximations. Wave front reconstruction is proposed to infer wave parameters for a large number of points from few ray traces. A computer program to evaluate radar coverage diagrams was developed utilizing a closed form solution in a complex troposphere consisting of several linear refractive index profile layers in conjunction with complex topography. The computer program is expected to be highly superior in speed compared to similar programs due to the use of wave front reconstruction and closed form ray tracing.

ISSN:0048-6604    

DOI:10.1029/89RS00280

年代:1991

数据来源: WILEY

摘要:

In this paper, numerical simulations of the scattering from time‐dependent realizations of one‐dimensional ocean surface waves are described. A new technique is used that allows efficient generation of ocean surface realizations that preserve the dominant nonlinear hydrodynamic characteristics. Thus unique scattering effects of real ocean surface waves can be explored. Until very recently, numerical simulations of rough‐surface scattering were used mainly to test and/or improve theoretical models that predict the average bistatic scatter cross section. We carry the simulations further by generating Doppler spectra from dynamically evolving surface realizations. Doppler spectra of signals scattered from the ocean surface are affected by both hydrodynamic nonlinearities and higher‐order scatter terms. The simulated Doppler spectra from nonlinear surface realizations reproduce the measured characteristics of ocean and wave‐tank data for low and high wind conditions. We also show that the results are essentially reproduced by the second‐order Kirchhoff ap

ISSN:0048-6604    

DOI:10.1029/90RS01687

年代:1991

数据来源: WILEY

摘要:

An analytic model has been developed for predicting the electromagnetic interference from multiconductor power lines over the frequency range from 100 kHz to 20 MHz. Results are provided for typical transmission lines. These include lateral and vertical profiles of the interference as well as radiation patterns.

ISSN:0048-6604    

DOI:10.1029/90RS02328

年代:1991

数据来源: WILEY

摘要:

The Canadian geophysical long baseline interferometer, a new very long baseline interferometer system, has been developed and put into operation. The system tracks source delay and fringe rotation during observation using a wave front clock. After the low‐noise receiver the signals from radio sources are channelized and down converted to baseband, each baseband channel is one bit digitized at a rate of 12 mbar/s and then recorded on a single video cassette recorder for processing. The total bandwidth of the system can be easily increased by replication of baseband channels. During processing the tapes are played back in synchronism and the recovered astronomy data are processed in a simple correlator to obtain the source visibility. The use of the wave front clock allows bursts of wideband astronomy data to be sampled at a high rate at each station and then recorded at a lower rate for processing. Because bursts recorded at different stations at the same wave front clock epoch are emitted by the source at the same time, they are correlated. The fringe visibility obtained from processing the bursts would provide high‐delay resolution geodetic measurements commensurate with the wide bandwidth of the bur

ISSN:0048-6604    

DOI:10.1029/90RS00241

年代:1991

数据来源: WILEY

摘要:

We have obtained a general, numerical solution of an ideal biconical transmitting antenna, with arbitrary arm length and conic angle. The solution evaluates the field coefficients associated with the electric multipolar expansion terms both within the antenna region and external to it; the set must necessarily be truncated, and we truncate our set with 16 terms. From the field coefficients we obtain surface current and charge densities resulting from a normalized input current, for several antennas. We evaluate resonant frequencies versus cone angle for the first three resonances. Using a normalized input voltage, we obtain the complex power at all radii; conservation of energy, in turn, permits us to solve for the reactive energy of the field per unit radius.

ISSN:0048-6604    

DOI:10.1029/90RS02469

年代:1991

数据来源: WILEY

摘要:

The scattering properties of a homogeneous elliptic cylinder with a confocal dielectric or magnetic coating are investigated analytically. The method of separation of variables is used to determine, exactly, the field distributions in each region for both transverse electric and transverse magnetic excitations. The technique can be easily extended to handle any number of layers. The behavior of the scattered field in the far zone is illustrated with numerical results for different core and coating material types, axial ratios, and electrical sizes.

ISSN:0048-6604    

DOI:10.1029/90RS01843

年代:1991

数据来源: WILEY