摘要:

A uniform geometrical theory of diffraction solution is developed for the two‐dimensional problem of high‐frequency plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes. Each material half plane in this two‐part configuration is assumed to be electrically thin so that it can be replaced in the analysis by a generalized resistive boundary condition of 0(t), wheretis the corresponding material slab thickness. The solution obtained is based on the Wiener‐Hopf technique, and it is shown that the present boundary value problem can be completely solved by imposing a junction condition at the junction of the two dielectric half planes in addition to the boundary and radiation conditions as well as the usual edge condition. This junction condition can be obtained if the field near the junction is modeled by a quasi‐static solution. The solution developed here can be further specialized to limiting cases for which either of the material half planes can become free space, perfectly conducting, or purely resistive, respectively. Several numerical examples are presented, and it is shown that the present solution reduces to known results and agrees very well with a moment method

ISSN:0048-6604    

DOI:10.1029/91RS00247

年代:1991

数据来源: WILEY

摘要:

In common with all wideband ionospheric sounding systems the backscatter technique is susceptible to contamination by radio frequency interference. The eradication of such interference is imperative in systems required to automatically interrogate the backscatter sounder data base. This paper presents the results of the development of an algorithm designed for that purpose. The philosophy guiding the design is presented, together with the results of a computer simulation designed to simultaneously validate the proposed approach and derive an appropriate threshold. Details of the implementation are provided together with data demonstrating the effectiveness. The algorithm is capable of being implemented in any backscatter sounder system featuring an adequate digital signal processing capability.

ISSN:0048-6604    

DOI:10.1029/90RS02758

年代:1991

数据来源: WILEY

摘要:

Fade, radar, and propagation modeling results are described for two nearby over‐the‐water, line‐of‐sight, propagation links operating at 4.7 GHz in the Mid‐Atlantic coast of the United States. During a 5 day “intense experimental period” in August 1989 a focused examination of the propagation mechanisms relating to time series fading was conducted. Cumulative fade distributions associated with both propagation paths are presented for the individual days and the overall 5‐day period. During this period, helicopter measurements of the refractivity structures along the propagation paths, boat measurements of the refractivity structure near the ocean surface at the center of the paths, and radar measurements of the clear atmosphere and ocean backscatter were executed. The refractivity measurements were interfaced with a complex parabolic equation propagation model which incorporated the two‐dimensional structure of the refractive index to yield normalized power density structures as a function of range and height for the geometry of the propagation links. These results were compared with the time series fade measurements and provided clear indications of the mechanisms for fading and nonfading events. Azimuthal and vertical radar scans employing a high‐resolution radar at the NASA Wallops Flight Facility showed excellent correlation between strong backscatter returns from the ocean surface well beyond the standard horizon and ducting events as evidenced by signal fading along th

ISSN:0048-6604    

DOI:10.1029/91RS00101

年代:1991

数据来源: WILEY

摘要:

Quasi‐periodic scintillations at a mid‐latitude station, Wakkanai, Japan, are examined using 136‐MHz geostationary satellite transmissions. Observations are compared with the ionospheric parameter obtained at the same station and random scintillation records. The results indicate that the quasi‐periodic scintillations are most likely produced by plasma blobs within the sporadicElayers. Discussion focuses on characteristics of the ringing pattern which precedes and follows the primary deep fade‐out, in field strength. In the majority of events the ringing pattern tends to develop after the distinct deep fade‐out, i.e., the pattern is asymmetric. Quasi‐periodic scintillation patterns are produced by the movement of plasma blobs in the case of geostationary satellite experiments. Thus the shape of the blob must be deformed so that a steep density gradient is attained on the backside. When the blob is highly deformed by the plasma instability which grows at the steep density gradient, burstlike random scintillations may be produce

ISSN:0048-6604    

DOI:10.1029/91RS00357

年代:1991

数据来源: WILEY

摘要:

A general spectral domain formulation to the problem of radiation of arbitrary distribution of sources embedded in a horizontally stratified arbitrarily magnetized linear plasma is presented. The fields are obtained in terms of electric and magnetic type dyadic Green's functions. The formulation is considerably simplified by using the kDB system of coordinates in conjunction with the Fourier transform. The distributional singular behavior of the various dyadic Green's functions in the source region is investigated and taken into account by extracting the delta function singularities. Finally, the fields in any arbitrary layer are obtained in terms of appropriately defined global upward and downward reflection and transmission matrices.

ISSN:0048-6604    

DOI:10.1029/91RS00654

年代:1991

数据来源: WILEY

摘要:

In this paper the results of precipitation observed by Chung‐Li VHF radar during the passage of typhoon Susan through the Taiwan area on June 2, 1988, are presented. We find that VHF radar can not only observe the falling speed of the rain drops but can also detect that of the ice particles. The observational results show that the former is about 6–8 m/s, while the latter is about 1 m/s, consistent with those observed by the conventional Doppler meteorological radar. The echo power due to precipitation is generally far smaller than that due to atmospheric refractivity fluctuations. However, it is enhanced strikingly at the height of melting layer (about 5 km in Taiwan area), and its value can be greater than that due to refractivity fluctuations by about 5 dB. Moreover, it also shows that the feature of bright band only occurs in the echo power profile of precipitation, and it cannot be observed in that of atmospheric refractivity. The echo power profiles of the vertical and oblique beams (with zenith angle 17°) are also examined. They indicate that there is no aspect sensitivity for the echoes scattered from the atmospheric refractivity fluctuations during the precipitating environment. However, for the echo power of precipitation the feature of aspect sensitivity does exist above the melting layer and disappears below this altitude. The correlation between the horizontal wind speed and the Doppler spectral width of precipitation is found to be fairly high, with a correlation coefficient of 0.86. This suggests that the beam‐broadening effect due to the advection of horizontal wind may be one of the important factors contributed to the precipitation spectral width and has to be taken into consideration when the precipitation spectra are an

ISSN:0048-6604    

DOI:10.1029/91RS00830

年代:1991

数据来源: WILEY

摘要:

The sensitivity of five evaporation duct height algorithms to errors in shipboard meteorological data is examined, and the algorithms are intercompared. The data set used was generated parametrically so a large variation of environmental conditions could be considered. The errors in the meteorological data consisted of two types: uncorrelated random errors associated with sensor inaccuracies and both random and systematic errors due to the influence of ship‐induced distortions. Before considering any errors, however, algorithm‐to‐algorithm differences of 10%–70% in the computed duct height are demonstrated and related to the assumptions and simplifications used during algorithm development. The sensitivity study shows that although the evaporation duct height algorithms have different genealogies, they have similar sensitivities. The sensitivities to errors caused by sensor inaccuracies range from 10% to 50% relative uncertainty in the calculated duct height, except for extremely low duct heights where the uncertainties are greater. During the daytime the relative uncertainties due to errors caused by ship‐induced distortions are approximately 10%–20% higher than those due to the sensor errors, and they are about 50%–75% smaller at night because of a lack of solar heating of the ship. These conclusions represent a best‐case scenario for Navy operational applications because of the omission of some sources of error, the optimistic ship error characteristics used, and the assumption of horizontal homogeneity in the near‐surface r

ISSN:0048-6604    

DOI:10.1029/91RS00835

年代:1991

数据来源: WILEY

摘要:

Coherent backscattering of scalar waves from a semi‐infinite discrete nonabsorbing random medium of nonisotropic scatterers is considered. The dependence of the line shape of the backscattering peak of the scattering nonisotropy is studied in the frameworks of the diffusion approximation. Various versions of this approximation are applied in which, in particular, the gradiental terms are taken into account, and various forms of boundary conditions are used. It is shown that the peak line shape is almost independent of the form of boundary conditions only in the vicinity of the backscattering direction. The validity of the diffusion approximation is discusse

ISSN:0048-6604    

DOI:10.1029/90RS01875

年代:1991

数据来源: WILEY

摘要:

Microwave field strength measurements are conducted on a round the clock basis on an operational communication link in northern India. In winter and premonsoon months, nighttime signal exhibits deep fades with fall in median signal level. The investigations revealed that nighttime fading is due to the refraction of signal from ground‐based layers. To minimise the amount of energy refracted by the ground‐based layers, the antennae were tilted upward. The tilting of antennae has decreased the fade depths, and an improvement in the performance of the link is obtai

ISSN:0048-6604    

DOI:10.1029/90RS02690

年代:1991

数据来源: WILEY

摘要:

Postsunset ionospheric scintillation measurements at 4 GHz from the INSAT 1B (74°E) satellite taken during the increasing half of the current solar cycle 22 at Sikandarabad (22.0°N magnetic latitude sub ionospheric) along 149°E geomagnetic meridian in the Indian zone have been analyzed and presented here. Results show that during the low solar activity period, only weak scintillations (peak‐to‐peak fluctuations<2 dB) were observed mainly during the summer months. As the solar activity increased, occurrence of summer scintillations more or less remained the same, but the intensity and occurrence probability increased substantially in the equinoxes and to some extent in the December solstice also. During October 1989, severe scintillations, with peak‐to‐peak fluctuations exceeding 20 dB were observed at this latitude, which is normally beyond the daytime crest of the equatorial anomaly. The characteristics of scintillations during equinoctial periods of high sunspot year were such that their onsets were mostly abrupt and they developed fully within a few seconds. Simultaneous measurements taken from an equatorial station Chenglepet (5.2°N magnetic latitude) along the same geomagnetic meridian plane during September–October 1989 show that scintillations at Sikandarabad were conditional to their prior occurrence at Chenglepet. As compared to the equatorial location, the onset of scintillations at 22.0°N magnetic latitude was always delayed and also died out first at the higher latitude. Simultaneous observations showed that the intensity of scintillations at the equatorial location never exceeded 5 dB peak‐to‐peak, whereas the intensity of scintillations at 22.0°N magnetic latitude frequently exceeded 10 dB peak‐to‐peak. This is explained on the basis of background ionization and the geometry of the ray paths relevent for these locations. Scintillation activity is in general found to be suppressed during geomagnetic disturbances, but it is observed to be enhanced in the post midnight hours of the same night for those magnetic storms whose recovery phase starts between the midnight and dawn local time sector. Results are also compared with the observations reported from oth

ISSN:0048-6604    

DOI:10.1029/91RS00386

年代:1991

数据来源: WILEY