摘要:

One aim of the Propagation en Air Clair et Météorologie 2 (PACEM2) experiment was to obtain a model for clear air propagation applicable to space diversity reception. The statistical results derived from nearly 4 years of continuous recording show an asymmetry between both diversity channels, the higher antenna being significantly more affected by multipath effects than the lower one. A possible effect of the antenna heights on multipath propagation occurrence is included in the latest version of the International Radio Consultative Committee (CCIR) prediction formula, which takes the inclination angle of the path into consideration, but the difference experimentally observed between both channels is much greater than the prediction derived from this CCIR formula. The object of this paper is to give a physical interpretation of the effect of antenna height on multipath occurrence leading to a quantitative agreement with the data. The physical model we use is built on available statistics of radio meteorological data. For each particular meteorological situation the shape and size of the region of interference between multiple propagation paths are then obtained by means of ray tracing. The results of these computations lead to the determination of the relative probability of occurrence of multipath propagation as a function of path length and antenna heights. A rather good agreement with the data is obtained when these parameters are given the values of the experimental lin

ISSN:0048-6604    

DOI:10.1029/94RS02227

年代:1995

数据来源: WILEY

摘要:

The quarter‐wave monopole base driven over a circular ground plane with a finite radius has applications in over‐the‐horizon radar and on surveillance aircraft. A new use, for which the analysis is given in this paper, is as an over‐the‐head‐mounted antenna for cellular telephones. With this design, the electromagnetic field in the head and the associated specific absorption rate of electromagnetic energy are greatly reduced when compared with the conventional hand‐held transceiver. A complete analysis is carried out of the electromagnetic field on the surface of the head and throughout its interior when the head is modeled as a cylinder with the electrical properties of the brain enclosed in a wall with the thickness and electrical properties of the skull. Graphs and tables are provided that give the field in the air on the surface of the head and in the skull and brain. The far field is also determined. The results are compared with those obtained with the hand‐held radiotelepho

ISSN:0048-6604    

DOI:10.1029/94RS02226

年代:1995

数据来源: WILEY

摘要:

The recursiveT‐matrix algorithm is applied to Monte Carlo simulations of multiple scattering by random distribution of dielectric spheres. The method is a fast algorithm for calculating the exact solution of Maxwell's equations for a large number of scattering objects. The extinction rate is calculated by averaging over many realizations. The computed results are compared with those from analytic approximations, namely, the quasi‐crystalline approximat

ISSN:0048-6604    

DOI:10.1029/94RS01567

年代:1995

数据来源: WILEY

摘要:

The fundamental relationship between self‐similar, that is, fractal, arrays and their ability to generate radiation patterns which possess fractal features is examined in this paper. The theoretical foundation and design procedures are developed for using fractal arrays to synthesize fractal radiation patterns having certain desired characteristics. A family of functions, known as generalized Weierstrass functions, are shown to play a pivotal role in the theory of fractal radiation pattern synthesis. These functions are everywhere continuous but nowhere differentiable and exhibit fractal behavior at all scales. It will be demonstrated that the array factor for a nonuniformly but symmetrically spaced linear array can be expressed in terms of a Weierstrass partial sum (band‐limited Weierstrass function) for an appropriate choice of array element spacings and excitations. The resulting fractal radiation patterns from these arrays possess structure over a finite range of scales. This range of scales can be controlled by the number of elements in the array. For a fixed array geometry, the fractal dimension of the radiation pattern may be varied by changing the array current distribution. A general and highly flexible synthesis technique is introduced which is based on the theory of Fourier‐Weierstrass expansions. One of the appealing attributes of this synthesis technique is that it provides the freedom to select an appropriate generating function, in addition to the dimension, for a desired fractal radiation pattern. It is shown that this synthesis procedure results in fractal arrays which are composed of a sequence of self‐similar uniformly spaced linear subarrays. Finally, a synthesis technique for application to continuous line sources is presented which also makes use of Fourier‐Weierstrass e

ISSN:0048-6604    

DOI:10.1029/94RS02315

年代:1995

数据来源: WILEY

摘要:

Powerful ground‐based VLF transmitters resonantly scatter ∼100‐keV electrons from the inner radiation belt onto trajectories from which they precipitate into the atmosphere as they drift eastward. To obtain a better understanding of the underlying wave‐particle mechanisms, we have studied energetic electron data from the Burst and Transient Source Experiment (BATSE) on NASA's Compton Gamma Ray Observatory (GRO) satellite. Wave‐particle interactions are detected when the satellite traverses a cloud of energetic electrons which have been scattered into quasi‐trapped trajectories by VLF waves. The previously reported seasonal variation in the frequency of occurrence of cyclotron resonance interaction is confirmed with the continuous coverage provided by GRO. These data also provide the first detailed measurements of the frequency of occurrence of the quasi‐trapped electron fluxes as a function of local time at the transmitter. The variations are more complex than just a day‐night effect, and the precipitation is most often seen in the predawn hours. As with the seasonal changes, the local time variations are governed by the transmission of VLF waves through

ISSN:0048-6604    

DOI:10.1029/94RS02592

年代:1995

数据来源: WILEY

摘要:

An important novel feature of the tethered sounding rocket experiment OEDIPUS A (Observations of Electric‐field Distributions in the Ionospheric Plasma—A Unique Strategy) was its direct excitation and detection of electromagnetic waves on conductors in space plasmas. We present quantitative evidence about sheath waves excited in the ionosphere by a high‐frequency transmitter on one end of the 1‐km tether and detected by a synchronized receiver on the other end. An important characteristic of sheath waves is their sequence of sharply defined passbands and stop bands in the frequency range 0.1–5 MHz. The lowest passband is between 0.1 MHz and the plasma frequency near 2 MHz, the bandwidth where existing theory predicts sheath waves. Resonance fringes in this band have been scaled to determine the phase and group refractive indices of sheath waves. These agree reasonably well with the theory, considering the approximations therein. Passbands and stop bands observed in the range between 2 and 5 MHz are not expected on the basis of the current theory. In this range, band limits have clear signatures of the interaction of the tether fields with electrostatic cyclotron waves. Finite wire moment method modeling of the payload shows that in the low‐frequency passband, RF coupling along the tether is increased by 20 dB over vacuum conditions. Similarly, isolation is greater than vacuum isolation in the stop bands. Because sheath waves at frequencies up to 2 MHz are guided efficiently along conductors in plasma, they are a significant design issue in the electromagnetic compatibility of avionics at frequencies up to HF on large metal space

ISSN:0048-6604    

DOI:10.1029/94RS02412

年代:1995

数据来源: WILEY

摘要:

This paper describes a new generation frequency‐modulated continuous wave (FMCW) radar using state‐of‐the‐art electronics and computerized data processing that greatly enhance the use of the radar as a practical tool for atmospheric research. The system senses at high resolution (∼2‐m range and 12 s for obtaining each profile), has ultrasensitivity (<−165 dBm), and is accurately calibrated for the refractive index structure parameter (C2n). The authors present salient features, discuss the calibration procedure, and present and discuss examples of various types of fine detail wave and frontal activity, boundary layer convection, and a light winter storm sensed by the radar over the last 2 years. The authors also show associated data from radiosonde and tower‐mounted sensors that are relevant to the episodal events shown from the FMCW radar observations. Since the FMCW radar technique can resolve and sense individual insects, these point targets are shown to act as tracers and enhance flo

ISSN:0048-6604    

DOI:10.1029/94RS01937

年代:1995

数据来源: WILEY

摘要:

Data collection for the first ground‐based ionospheric tomography campaign in North America was conducted over a 48‐hour period in mid‐November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U. S. Navy Navigation Satellite System spacecraft over a chain of ground‐based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual‐frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two‐dimensional (2‐D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2‐D contour “maps” of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the expe

ISSN:0048-6604    

DOI:10.1029/94RS02225

年代:1995

数据来源: WILEY

摘要:

A computerized ionospheric tomography (CIT) experiment utilizing an array of nine Navy Navigation Satellite System (NNSS) receivers deployed along a north‐south line from South Dakota to south Texas (spanning over 2000 km) is currently underway. The Mid‐America Computerized Ionospheric Tomography Experiment (MACE '93) began collecting data from three receivers deployed in Texas on June 1, 1993. This “short communiqué” presents preliminary results from the ex

ISSN:0048-6604    

DOI:10.1029/94RS01538

年代:1995

数据来源: WILEY

摘要:

A volume scatter situation is assumed, and results expected from radar interferometric techniques are derived. It is shown that fictitious scattering points will appear, with the property that the points for different Doppler frequencies lie along a straight line. This line is not generally in the direction of the wind; only if the scatter is isotropic in azimuth will the line coincide with the wind direction. In the presence of turbulence the effective scattering points still lie along a straight line. However, the scattering point for a particular Doppler frequency will be closer to the zenith than it would be in the absence of turbulence. The results apply if smoothed cross spectra are used. If the cross spectra are not smoothed, the expected effects are harder to predict.

ISSN:0048-6604    

DOI:10.1029/94RS02415

年代:1995

数据来源: WILEY