摘要:

This paper derives useful expressions for the synthesized beam statistics using poststatistic steering (PSS), when three antennas are used in a spatial interferometer configuration for two‐dimensional steering. These antennas may have arbitrary position and can have nonorthogonal baselines. The use of three (noncollinear) antennas allows altitude dependent two‐dimensional steering. This technique is verified using spatial interferometer data obtained in October 1989 using the MU radar, including PSS in directions parallel and perpendicular to the wind direction, which is a very convenient advantage of the techni

ISSN:0048-6604    

DOI:10.1029/RS025i006p01105

年代:1990

数据来源: WILEY

摘要:

The Doppler beam‐swinging (DBS) and the spaced antenna (SA) techniques for radar wind measurement are compared on theoretical and practical grounds. It is shown that the information content is similar for both techniques and that errors introduced by horizontal variations of the vertical wind field affect both equivalently. A radar equation for a bistatic radar with close transmitting and receiving antennas is derived, and it is shown that the backscattered power is proportional to an equivalent area. The equivalent area is equal to the antennas' physical area for a monostatic radar and is twice the area of the smallest antenna in the limit of one antenna being much larger than the other. Other factors affecting the techniques' sensitivity and accuracy are discussed. VHF radars operating at about 50 MHz should give similar sensitivity using both techniques, but for UHF wind profilers the DBS method is preferabl

ISSN:0048-6604    

DOI:10.1029/RS025i006p01111

年代:1990

数据来源: WILEY

摘要:

A new method for the investigation of anisotropic electromagnetic wave propagation in the atmosphere is developed using parabolic approximations. Model equations for the electric field components are formulated which include the effects of both the inhomogeneous atmosphere and the static magnetic field of the Earth. Application of parabolic‐type approximations produces different systems of coupled parabolic equations. Each is valid for different relative magnitudes of components of the electric field. All admissible cases are then synthesized into one system which can be numerically examined, yielding solutions without a priori knowledge of electric field ratios. A specific example is presented and examined to understand static magnetic field effects on electromagnetic wave propagation. The influences of the Earth's magnetic field are discussed and displayed in terms of electric components and the Poynting vector. Results demonstrate that the geomagnetic field can significantly influence HF atmospheric propagatio

ISSN:0048-6604    

DOI:10.1029/RS025i006p01121

年代:1990

数据来源: WILEY

摘要:

A convenient polarimetric radar calibration technique is presented and used to calibrate anXband polarimetric radar. The technique uses a distortion matrix model for the errors introduced by the transmitter and receiver, and it is applicable to any polarimetric radar system where the concept of an invariant distortion matrix is valid. A sphere and two arbitrary passive targets are used to calibrate the radar with respect to the unknown polarization transmitted when the v‐polarized channel is energized. The unknown transmitted polarization is then recovered by measuring any nondepolarizing target. Because knowledge of the scattering matrices for the two arbitrary targets is not required, the technique is insensitive to errors in the orientation of calibration targets. Experimental results are presented, and they indicate magnitude and relative phase measurement errors of less than 0.2 dB and 2°, respective

ISSN:0048-6604    

DOI:10.1029/RS025i006p01137

年代:1990

数据来源: WILEY

摘要:

A new technique is discussed and developed for correcting atmospherically induced errors in phase data collected by radio astronomy interferometers and synthetic aperture antenna arrays. The main features of this technique are modeling and filtering the information content of the complex visibility data in time sequences. Because the atmospheric phase variations are highly correlated in time, they can be described by stochastic time series models. In conjunction with other radio astronomy data processing algorithms, a time series modeling and parameter estimation technique is developed to obtain noise models and source models from observed phase data. These models can be in the form of stochastic difference equations, autocorrelation, power spectral density, and a state space format ready for further data processing. Once the models are in a state space format, the Kalman filter is used for optimally extracting source information from the noisy data. The synthetic image is then improved by reducing the phase error. The quality of the corrected phase data is quantitatively described by the error covariance matrix of the Kalman filter. One useful application of this technique is for reducing atmospherically induced phase errors of small synthesis arrays that have too few antennas to apply self‐calibration. Another application of this technique is for improving the performance of large synthesis arrays when the standard calibration methods are insufficient for correcting very noisy phase data. This technique has been tested using the very large array (VLA) (operated by the National Radio Astronomy Observatory) and the Hat Creek millimeter interferomete

ISSN:0048-6604    

DOI:10.1029/RS025i006p01145

年代:1990

数据来源: WILEY

摘要:

Riometer recordings of cosmic radio noise in polar regions show a variability in sidereal variation during subsequent quiet days. The fitting of a low‐order Fourier series to maximum density points in sidereal time intervals has the advantage that as few as 7 days may be used to determine a quiet day curve. Quiet day curves determined for periods of 7 days in a sliding sequence show a short‐term variability of about 0.13 dB in signal strength. For periods of 30 days this variability is 10 times less. Absorption events should be obtained from the quiet day curve appropriate for the period of the event. Absorption obtained from quiet day curves determined for maximum density points will be about 0.05 dB less for 30 MHz than using the inflection point met

ISSN:0048-6604    

DOI:10.1029/RS025i006p01159

年代:1990

数据来源: WILEY

摘要:

When pulsed electromagnetic signals propagate through a stratified, inhomogeneous, isotropic ionosphere, their shapes are usually distorted because of dispersion. To the first‐order approximation, this distortion is determined by the dispersion coefficient ф″, the second derivative of the phase with respect to the angular frequency. In order to derive analytic expressions of the group path and the dispersion coefficient in model ionospheres for the spherical Earth‐ionosphere geometry the quasi‐linear and quasi‐parabolic electron density models already in existence in the literature have been used. When generalized to an ionosphere with known virtual height, the secant law applicable to the plane geometry must be modified for application to the spherical geometry. In this regard, a correction factor can be introduced. This correction factor is computed, and a dispersion coefficient in terms of a correction factor is derived using a modified secant law and the geometry of a spherical Earth‐ionosphere. All calculations are made under conditions which neglect the Earth's magnetic field, absorption, and horizontal gradients. The results show the dependence of the group path and the dispersion coefficient on propagation geometry and ionospheri

ISSN:0048-6604    

DOI:10.1029/RS025i006p01167

年代:1990

数据来源: WILEY

摘要:

Digital ionosondes such as the Dynasonde disclose that the ionosphere is seldom horizontal even when it is plane stratified to a good approximation. The local magnetic dip does not then determine correctly the radiowave propagation angle for inversion of the ionogram to a plasma density profile. The measured echo direction of arrival can be used together with the known dip for an improved propagation angle. The effects are small for simple one‐parameter laminae but become important when differential (ordinary, extraordinary) retardations are used to aid correction for “valley” and “starting” ambiguities. The resulting profile describes the plasma distribution along the direction of observation, rather than the vertical; it thus conveys information about horizontal gradients. Observations suggest that advances in inversion methods may be practicable for application to modern ionosonde recordings, by which local lateral structure can be described in great

ISSN:0048-6604    

DOI:10.1029/RS025i006p01175

年代:1990

数据来源: WILEY

摘要:

Electromagnetic scattering from a raindrop splash on a water surface is examined in terms of the splash structures that have been disclosed by high‐speed photography. Of the three basic scattering features, the “crown,” the “stalk,” and the “ring wave,” the first two are modeled as dielectric cylinders, while the third is treated by a perturbation approximation. Cross‐section predictions based on heuristic application of these models are found to be in good agreement with laboratory measurements of individual splash‐scattering events. A statistical model in which surface slope and stalk height distributions are taken into account is proposed for extending these results to natural rain on a real water surface. Unfortunately, there is at present insufficient experimental data to allow a reasonable test of the theory. Nevertheless, a few qualitative conclusions can be drawn from the formalism: the major scattering feature is the stalk, and while the vertically polarized returns will have only a weak dependence on rain rate, the horizontally polarized returns will depend strongly on both the rain rate and the shape of the stalk (or drop) size di

ISSN:0048-6604    

DOI:10.1029/RS025i006p01183

年代:1990

数据来源: WILEY

摘要:

This paper is a sequel to our earlier paper (Lee and Mudaliar, 1988) where the backscattering coefficients of a half‐space anisotropic random medium were obtained. Here we calculate bistatic scattering coefficients by solving the modified radiative transfer equations under a first‐order approximation. The effects of multiple scattering on the results are observed. We also calculate emissivities and compare them with those obtained using the Born approximation (single scattering). Several interesting properties of our model are brought to notice using numerical examples. Finally, as an application, we use our theory to interpret the passive remote sensing data of multiyear sea ice in the microwave frequency range. We observe a quite close agreement between our theoretical prediction and the measured d

ISSN:0048-6604    

DOI:10.1029/RS025i006p01199

年代:1990

数据来源: WILEY