摘要:

During August 1989, extended twilight and nighttime measurements with the European incoherent scatter (EISCAT) UHF radar were performed under PCA conditions. This provided an excellent data quality in the altitude region of 70 to 90 km throughout the three nights of August 12 to 15, 1989. A sophisticated control program allowed the measurement of the spectral width in the altitude region mentioned. In general, the measured spectral width deviates significantly from model values based on temperatures measured simultaneously by Na lidar combined with CIRA 88 temperature and density values. The observed spectra are up to 2 or 3 times narrower. In our observations the deviation tends to increase with increasing altitude. We also find that earlier spectral width measurements published by other workers are often narrower than currentDregion theory predicts. The possible reasons for this phenomenon are discussed.

ISSN:0048-6604    

DOI:10.1029/91RS01430

年代:1991

数据来源: WILEY

摘要:

The scattering problem of obliquely incident waves on a multilayered elliptical cylinder is considered. The cylinder is assumed homogeneous and isotropic but lossy. Both polarization of the incident wave, theEwave, and theHwave are considered. In the theoretical analysis the number of layers are unlimited, but in the numerical analysis we limit the number of layers so we can avoid excessively long computer running time. The numerical results include the bistatic radar cross sections, the specular differential cross sections, and the efficiency factors for absorption, scattering, and extinction. The computed results show the dependence of the scattering on the size, shape, material, and the angle of the incident.

ISSN:0048-6604    

DOI:10.1029/91RS01706

年代:1991

数据来源: WILEY

摘要:

This paper gives a description of the transionospheric radiosonde (TIR) designed for simultaneous measurements of the main parameters of the ETS 2 radio signal at 136 MHz (polarization, angles of arrival, frequency Doppler shift, and amplitude, with a time resolution of 3O s; amplitude scintillations using spaced antennas with a resolution of 0.2 s). These data, referenced to exact time, are stored on magnetic tape and diskettes for the purpose of making a subsequent digital processing on a PDP‐11 computer and personal computers. Data from the TIR radio interferometer can be useful when studying the dynamics of a geostationary satellite in orbit; when making a complex study of the transionospheric propagation of radio waves; and when studying the structure, dynamics, interaction, and transformation of ionospheric irregularities simultaneously in a wide range of scales (from 100 m to 1000 km). The objective of this paper is also to attract the researcher's attention to the data obtained by these authors, for possible cooperation on the level of problem formulation or joint secondary treatment and interpretation of dat

ISSN:0048-6604    

DOI:10.1029/91RS01109

年代:1991

数据来源: WILEY

摘要:

A procedure for the mapping of radar targets which are spread both in delay and frequency offset is described. The procedure is based on transmitted pulses with linearly chirped frequency variation. The reconstruction of the properties of the target scattering function is achieved by combining observations made with a large number of different chirp rates. The procedure is closely related to tomography methods employed in other observational contexts. The relationship between tomographic reconstruction and multiple chirp reconstruction is discussed. Possible applications of this reconstruction method exist in HF ionospheric modification experiments, in mesosphere‐stratosphere‐troposphere radar applications as well as in the radar astronomy of planetary bod

ISSN:0048-6604    

DOI:10.1029/91RS01414

年代:1991

数据来源: WILEY

摘要:

An improved asymptotic series for an integral representing transient current on an infinite cylindrical antenna caused by a step voltage source, applied at the gap, is given. A generating function for obtaining the general coefficient is derived. The method is applicable to types of integrals whose integrands consist of a product of a slowly varying function and a rapidly varying function. A simple accurate formula obtained before, by the averaging method, is shown to be the leading term of this infinite series. The averaging used in evaluating integrals is shown to improve the accuracy of the leading term.

ISSN:0048-6604    

DOI:10.1029/91RS01415

年代:1991

数据来源: WILEY

摘要:

Generalizing radar interferometry to three‐dimensional space, we present a model which predicts the variation with respect to frequency of the phase of the cross‐spectrum obtained between two separate antennas. The slope of this variation is proportional to the cosine of the angle between the horizontal wind and the interferometer baseline, and inversely proportional to the horizontal wind amplitude. The zero‐frequency phase depends on the horizontal wind speed and direction, on the vertical wind velocity, and on the anisotropy of the backscattered power with respect to the zenith. The model also allows us to predict qualitatively the effects of specularity, beam shape, and random turbulence on the phase versus frequency variation. Assuming a spaced receiver setup with three baselines rotated by 120°, we present an implementation scheme of the radar interferometry technique which yields a three‐dimensional wind estimate and a parameter characterizing the anisotropy of the echo power distribution. Finally, we argue that the radar interferometry horizontal wind measurements will suffer from limitations similar to those of the so‐called apparent velocity as both techniques are related by Fourier transform, and that the vertical wind component will be unbiaised by anisotropy in the backscatt

ISSN:0048-6604    

DOI:10.1029/91RS01229

年代:1991

数据来源: WILEY

摘要:

The salient features of the sidebands generated on the HF waves in an ionospheric plasma modified by two pump frequencies are presented and compared to theories. Particular attention is focused upon the properties of the nonlinear modulation process by comparing the modulation index theory and the observations. The observations show that the amplitudes of the sidebands are related to the HF power and the difference frequency of the two pump waves. The relationships of the amplitude of the sideband with HF frequency, the scale height, and the temperature of the electrons in the ionosphere are discussed. The modulation index theory shows agreement with the observations over a range of conditions.

ISSN:0048-6604    

DOI:10.1029/91RS01218

年代:1991

数据来源: WILEY

摘要:

The ground wave propagation of VHF radio waves over an inhomogeneous sea surface of variable conductivity and under various sea roughness conditions is studied both theoretically and experimentally. Theoretical simulations predict a high sensitivity of the ground wave attenuation factor upon large variations of the surface conductivity, as can be found in an estuary. This sensitivity can be influenced appreciably by the sea state under high winds in fetch‐limited conditions and under a broader range of wind speeds for a fully developed sea. Results from a VHF radar experiment over a river plume with appropriate in situ documentation are reported here. They validate the theoretical expectations on the influence of the conductivity profile on radio wave propagation. Because of the low winds encountered, the wind/wave influence is evident only on the variations of the radar cross‐section coefficient (linearly related to Bragg wave spectral energy) in fetch‐limited conditions. Also, these variations can be attributed in some cases to the influence of slicks in the river plume. The study suggests that the VHF radar technique may be used for remote sensing of sea surface conduct

ISSN:0048-6604    

DOI:10.1029/91RS01413

年代:1991

数据来源: WILEY

摘要:

By making use of some new results in optical coherence theory we show that spectral analysis, rather than fringe visibility measurements, can be used to determine correlation properties of the field. A principle is also formulated, according to which, in many interferometric measurements a trade‐off can be made between the length of the baseline and the frequency at which measurements are made. This principle is a rigorous generalization of the so‐called space‐frequency equivalence theorem for two antenna detection systems, and it applies to radiation from a broader class of sources than previously consi

ISSN:0048-6604    

DOI:10.1029/91RS01032

年代:1991

数据来源: WILEY

摘要:

The wind and VHF radar reflectivity fields around frontal zones are examined. While even weak upper level baroclinic zones can be seen in both the reflectivity and wind fields, the reflectivity signature of low level fronts may be obscured. Humidity terms dominate the refractive index gradient in the lower troposphere and can mask the enhancement of the gradient as a result of the stable frontal layers.

ISSN:0048-6604    

DOI:10.1029/91RS00964

年代:1991

数据来源: WILEY