Showing papers in "Radio Science in 1978"

Vegetation modeled as a water cloud

Abstract: Because the microwave dielectric constant of dry vegetative matter is much smaller (by an order of magnitude or more) than the dielectric constant of water, and because a vegetation canopy is usually composed of more than 99% air by volume, it is proposed that the canopy can be modeled as a water cloud whose droplets are held in place by the vegetative matter. Such a model was developed assuming that the canopy “cloud” contains identical water droplets randomly distributed within the canopy. By integrating the scattering and attenuation cross-section contributions of N droplets per unit volume over the signal pathlength through the canopy, an expression was derived for the backscattering coefficient as a function of three target parameters: volumetric moisture content of the soil, volumetric water content of the vegetation, and plant height. Regression analysis of the model predictions against scattering data acquired over a period of four months at several angles of incidence (0°–70°) and frequencies (8–18 GHz) for HH and VV polarizations yields correlation coefficients that range from .7 to .99 depending on frequency, polarization, and crop type. The corresponding standard errors of estimate range from 1.1 to 2.6 dB.

STARE: A new radar auroral backscatter experiment in northern Scandinavia

Abstract: As part of the ground-based program for the International Magnetospheric Study (IMS), a new two-station VHF radar auroral experiment, STARE, has been constructed in northern Scandinavia. Each of these stations can provide good spatial and temporal resolution measurements of the intensity and Doppler velocity of radar auroral irregularities within a 300,000 km2 scattering region. Approximately 230,000 km2 of these scattering regions are common to both radars and within this area it is possible to compare the backscattered signals observed by the two radars and to combine the Doppler data to derive the mean irregularity drift velocity. The drift velocity can ultimately be related to the ionospheric electric field. In this paper we describe the operation of these radars and the method by which the data are processed. We also describe the assumptions used in the drift velocity analysis and present some initial measurements supporting their validity. Finally, we present several examples of the STARE data during two periods of counterstreaming currents in the late evening auroral oval. The irregularity drift patterns are consistent with the expected electron drift patterns for these periods and, from these patterns, one can determine the two-dimensional structure of the ionospheric electric field.

Early results from the DNA Wideband satellite experiment—Complex‐signal scintillation

Abstract: A multifrequency (ten spectral lines between VHF and S band) coherent radio beacon is presently transmitting continuously from a 1000-km, high-inclination orbit for the purpose of characterizing the transionospheric communication channel. Its high phase-reference frequency (2891 MHz) permits direct observation of complex-signal scintillation, and its very stable, sun-synchronous orbit allows repeated pre-midnight observations at low latitudes and near-midnight observations at auroral latitudes. We present here early results of the observations; salient points include the following. First, most of the data are consistent with phase-screen modeling of the production of ionospheric scintillation, including an ƒ−2 frequency dependence for phase variance. Second, propagation theories invoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompanied by phase fluctuations comparable to or greater than a radian. Third, under conditions producing GHz scintillation (near the geomagnetic equator), lower frequencies show marked diffraction effects, including breakdown of the simple ƒ−2 behavior of phase variance and loss of signal coherenceacross a band as narrow as 11.5 MHz at UHF.

Vertical profiles of refractivity turbulence structure constant: Comparison of observations by the Sunset Radar with a new theoretical model

Abstract: Vertical profiles of refractivity turbulence structure constant C2n (which is proportional to the radar volume reflectivity) from about 5 to 15 km are measured by the Sunset Radar every 50 seconds. The method of determining such profiles from the radar Doppler spectra is described. The C2n profiles for about an hour are averaged to form (C2n(radar)). The profiles of (C2n(radar)) are quite variable: on the average they decrease by about two orders of magnitude from about 5 to 15 km, and they often change by one order of magnitude from day to day at a given height. A theoretical model is developed that enables the calculation of C2n from routine rawinsonde profiles of wind, temperature, and humidity. This model is based on the assumption that the fluctuations of refractivity that scatter the radio waves are in equilibrium with homogeneous, isotropic, steady-state turbulence in the inertial subrange. An essential and new feature of this model is an estimate of the mean fraction of the radar-observed volume that is turbulent. The resulting profiles of (C2n(model)) agree well with the measured profiles of (C2n(radar)) in general shape, in changes from day to day, and in many details from kilometer to kilometer. This agreement implies that: (1) The vertical profile of C2n can be measured by Doppler radar. (2) The vertical profile of C2n can also be estimated by calculation from routine rawinsonde profiles, using our theoretical model.

The accuracy of water vapor and cloud liquid determination by dual‐frequency ground‐based microwave radiometry

Abstract: A dual frequency ground-based radiometer operating in the 1 to 1.4 cm wavelength range can provide continuous measurements of integrated water vapor and cloud liquid water. Using climatological data, the accuracy of the vapor and liquid determinations is estimated as a function of cloud amount. Limiting factors in the water determination are uncertainties in water vapor absorption coefficients and, during cloudy conditions, uncertainties in cloud temperature. For integrated water vapor content greater than 10 mm, the accuracy of the vapor determination is better than 15% for a wide range of cloudy conditions.

Evidence for specular reflection from monostatic VHF radar observations of the stratosphere

Abstract: The received signal obtained on the vertical antenna of the 40 MHz Sunset radar often reveals an order-of-magnitude enhancement over the received signal obtained nearly simultaneously on an antenna pointed 30° from the zenith. The enhancement is most pronounced in regions of the atmosphere which are hydrostatically very stable. The enhanced vertical echoes are consistent with specular or partial reflection from thin stable laminae of radio index of refraction. An order-of-magnitude calculation is used to show that the expected reflectivity from a thin stable layer would easily be detected by the radar.

Approximate method for determining ELF eigenvalues in the earth‐ionosphere waveguide

Abstract: A simple approximate expression is derived for the TEM eigenvalue for ELF propagation in the earth-ionosphere wave-guide under conditions where anisotropy due to the earth's magnetic field may be neglected. Eigenvalues obtained in this manner are in excellent agreement with full-wave test calculations. It is shown that the eigenvalue is determined primarily by the location of two altitudes at which the conductivity attains certain values, and on the local conductivity scale heights at those altitudes.

The universal digital ionosonde

Abstract: The digital ionospheric sounding system Digisonde 128PS explores ionospheric structure and dynamics by exploiting all observables of the reflected electromagnetic wave: range, amplitude, phase, Doppler, incidence angle, and wave polarization. The Digisonde operates in two complementary modes of observation: the Ionogram Mode with full range and frequency display but limited resolution in Doppler and incidence angle, and the Doppler-Drift Mode operating with a limited number of frequency-range bins but full resolution in Doppler and incidence angle. This technique reduces the volume of magnetically recorded output data to a manageable size. Special digital-analog hybrid techniques are used for the presentation of the multi-parameter ionograms and the sky maps.

Differential reflectivity and differential phase shift: Applications in radar meteorology

Abstract: The differential scattering properties of classes of hydrometeors at linear orthogonal polarizations provide potentially important differences which may be exploited for radar measurements of precipitation. The ratio of the reflectivity at horizontal (ZH) and vertical (Zv) polarizations may be combined with other radar measurements such as absolute reflectivity and differential phase shift to determine drop size distributions. Previous model calculations of differential reflectivity ZDR = 10 log (ZH/Zy) were made using Gans' theory of scattering by spheroids. New calculations, using Waterman's T-matrix method, are in excellent agreement with Gans' theory for raindrops at S-band wavelengths. The ZDR-differential phase shift combination, which need not depend on any absolute measurements, is shown to be a reasonable radar method for measuring two-parameter raindrop size distributions. Finally, calculations of ZDR for oblate hailstones indicate the possibility of uniquely detecting large, dry hail when the hail is falling with preferred orientation.

Excitation of the Zenneck surface wave by a vertical aperture

Abstract: Excitation of the Zenneck wave is examined for both finite and infinit vertical apertures over a homogeneous conducting flat ground. The infinit vertical aperture with a Zenneck wave variation is found to excite only the Zenneck wave with no radiation field. The finite vertical aperture excites a field which is similar to the Zenneck wave near the aperture but resembles the usual ground wave at large distances. Numerical results for various aperture heights are given for frequencies of 1 and 10 MHz. Both homogeneous ground and sea water paths are considered.

Spectra of fluctuations in refractivity, temperature, humidity, and the temperature-humidity cospectrum in the inertial and dissipation ranges

Abstract: Recent theoretical and experimental advances have produced a model of the power spectrum of quantities mixed by turbulence. This model is applicable for arbitrary diffusivity and is thereby capable of predicting the high wavenumber shape of the spectra of refractivity, temperature, humidity, and the temperature-humidity cospectrum. Equations for the mean refractivity and the refractivity variance are formed from temperature, pressure, and humidity fluctuations; the accuracy of neglecting the higher-order terms in the fluctuations is determined. In order to apply the model power spectrum for temperature and humidity fluctuations in the atmosphere, it is first determined under what conditions temperature and humidity satisfy a simple continuity equation that balances temporal changes, advection, and diffusion. Experiments have shown that the temperature spectrum in air has a “bump” at high wave numbers. This feature is understood in terms of the theoretical model, and the bump is predicted to appear in the refractivity and humidity spectra and in the temperature-humidity cospectrum as well as in the temperature spectrum. The bump in the humidity spectrum is smaller than that in the temperature-humidity cospectrum, which is in turn smaller than the bump in the temperature spectrum; this effect is caused by the fact that the diffusivity of water vapor in air is slightly greater than that of heat in air. The shape of the refractivity spectrum depends on the relative contributions of the temperature spectrum, humidity spectrum, and temperature-humidity cospectrum. The refractivity spectrum can have an enhanced bump if the contribution of the temperature-humidity cospectrum is negative and nearly cancels the contributions of the temperature and humidity spectra. The impact of these developments on radio wave propagation is discussed.

Simulation of radiation from lightning return strokes - The effects of tortuosity

Abstract: A Monte Carlo simulation has been developed for the electromagnetic fields radiated from a tortuous lightning channel This was done using a piecewise linear model for the channel and employing for each element the field radiated by a traveling wave on an arbitrarily oriented filament over a conducting plane The simulation reproduces experimental data reasonably well and has been used to study the effects of tortuosity on the fields radiated by return strokes Tortuosity can significantly modify the radiated waveform, tending to render it less representative of the current pulse and more nearly unipolar than one would expect based on the theory for a long straight channel In the frequency domain the effect of tortuosity is an increase in high frequency energy as compared with an equivalent straight channel The extent of this increase depends on the mean length of the elements comprising the channel and can be significant

Radiative transfer theory for active remote sensing of half-space random media

Abstract: In active remote sensing of low-loss and scattering-dominant areas, the effect of volume scattering can be modeled as a half-space random medium with lateral and vertical fluctuations. Correlation functions are assumed to be Gaussian laterally and exponential vertically. A radiative transfer theory is developed in this paper to calculate backscattering cross sections. An iteration process is used. When calculated to first order in albedo, the result gives the single-scattering effects and confirms previous work. Calculated to second order, it is found that the backscattered power exhibits depolarization effects. This important consequence is studied numerically by illustrating the backscattering cross sections as a function of incidence angles and frequencies.

Solar wind velocity estimation from multi-station IPS

Abstract: Three-station and four-station observations at 74 MHz are used to study the spatial structure and temporal rearrangement of the intensity pattern of interplanetary scintillations (IPS) in weak scattering. Velocity estimators from three models, two of which include rearrangement, are applied to the data and evaluated. It is shown that for certain radio sources the spatial anisotropy of the pattern is determined by elongated source structure. An upper bound on the average pattern axial ratio for an isotropic ecliptic source is 1.3 for data of mid-1972. An upper bound on the average random velocity for the same period is 40% of the bulk velocity. We discuss a simple velocity estimator that is independent of the spatial anisotropy and has a low sensitivity to the random velocity.

Spectral analysis of high‐frequency diffraction of an arbitrary incident field by a half plane—Comparison with four asymptotic techniques

Abstract: The knowledge of high-frequency diffraction of an arbitrary wave incident on an edge is important in many applications, such as antennas mounted on aircraft and reflector antennas illuminated by complex feeds. In this paper the problem of a half plane illuminated by a nonplanar wave is investigated using the concept of the plane wave spectral representation. For large wave number k, a new higher-order asymptotic solution for the total field up to and including terms of order k−5/2 relative to the incident field is derived. The behavior of the solution for the observation points which coincide with shadow boundary directions of a multipole line source is discussed in detail. Furthermore, numerical solution of the field integral representation is constructed for the observation angles in the transition regions. The results are compared with those of the Geometrical Theory of Diffraction (GTD), the Uniform Asymptotic Theory (UAT), the Uniform Theory of Diffraction (UTD) and the Modified Slope Diffraction (MSD).

Remote sensing of directional gravity wave spectra and surface currents using a microwave dual‐frequency radar

Abstract: The modulation of small-scale centimeter (and later decimeter) water waves induced by larger-scale 2 to 18 m gravity waves has been studied by using a coherent, dual-frequency radar technique. Experiments using a prototype CW X-band radar operating at 9.3 GHz have been performed. Experience with the X-band system led to the subsequent development of a pulsed dual-frequency L-band radar operating at 1.2 GHz. The gravity wave modulation manifests itself as a narrow, Doppler-shifted, resonance peak in the product power spectrum of the backscattered returns. The dispersion relation (for both deep and shallow water) of the modulation pattern matches that of gravity waves. Modulation amplitude spectra have been experimentally obtained which, after sufficient averaging, closely resemble directional gravity wave spectra simultaneously obtained from capacitance wave probe and Sea Photo Analysis measurements. Temporal stationarity of the large-scale gravity wave structure may only be assumed for finite data acquisition times of, perhaps, the order of one hour, or less. Experiments with the X-band system, however, have shown that high resolution, sufficiently averaged, densely sampled spectra require measurement periods much longer than one hour. To reduce the overall data acquisition time, multiplexing of the individual spectral samples has been employed. The amount of spectral averaging required was substantially reduced by developing and using a dual-frequency L-band radar (Bragg resonant with 12 cm short-gravity waves) which proved to be much less sensitive to wind-induced fluctuations in the small-scale wave return. A method is also presented for enhancing the amplitude of the resonance peak through selective Doppler filtering. This technique was subsequently used to facilitate detection of small tidal currents flowing in the Chesapeake Bay.

Rain-rate estimates for an attenuating radar

Abstract: An iterative method has been used to produce rain-rate estimates for an attenuating frequency radar. An infinite number of higher-order estimates are shown to converge in the limit to the Hitschfeld-Bordan solution under certain conditions. An error analysis was performed by a model accounting for the randomness of the radar return power, the k-Z, Z-R relations, and offsets in the radar calibration constant. Since the behavior of the estimates strongly depends on system errors, the choice of the best estimate requires a knowledge of the variance and range of offsets in meteorological data and the calibration constant. As the errors increase, the use of the lower orders avoids significant overpredictions. In order to obtain reliable rain-rate predictions in the presence of realistic errors, an antenna pointing angles away from the horizontal at frequencies in the lower end of the X-band may be used. Such antenna configurations insure low attenuation.

Numerical approximations of the Sommerfeld integral for fast convergence

Abstract: Several approximation techniques for the Hertz potential of an infinitesimal dipole in the presence of a conducting half space are presented. The formal solutions obtained by rigorous mathematical procedures are transformed, with the aid of some approximation, to fast convergent forms. The approximation techniques are different depending on the media where the observer and source are located. All results calculated are compared to those of the brute-force integrations of the formal solutions. Restrictions of the validity of these approximations are discussed.

Modal theory of long horizontal wire structures above the earth: 2, Properties of discrete modes

Abstract: The characteristics of the discrete propagation modes on horizontal thin-wire structures located above a dissipative earth are investigated. In addition to the well-known transmission line mode, a new root of the characteristic equation is found which is identified as a “surface-attached” mode because of its close connection with the Sommerfeld pole (Zenneck wave) in some parameter ranges. Under many conditions the surface-attached mode suffers substantially less attenuation along the propagation direction than does the transmission line mode. Numerical investigation of the propagation constants of the two modes is made, and field plots for the modes for a variety of wire parameters are presented.

A numerical study of the Kirchhoff approximation in horizontally polarized backscattering from a random surface

Abstract: The backscattering characteristics of a computer-generated known one-dimensional random surface are studied by computing first the exact surface current distribution over the illuminated area (using four or more points per wavelength) by the moment methods and then calculating the backscattered field by the gaussian quadrature technique. These computations are repeated 40 to 65 times (depending on the incident frequency) over different surface segments to obtain enough scattered field samples to estimate the average scattered power. If the surface current distribution over the surface segment is estimated by the Kirchhoff approximation, the above calculations may be repeated to obtain the average scattered power under the Kirchhoff approximation. By comparing these two average scattered powers at different frequencies, it was found that (1) the two averaged powers can agree to within 2 dB over a range of frequencies in the incident angular range 0° ≤ θ ≤ 40°; (2) within the range of agreement specified in (1), the average backscattered power need not be proportional to the slope distribution of the random surface; and (3) further study is needed to establish the range of validity of the Kirchhoff approximation. Comparisons were also made between existing approximations to the Kirchhoff integral representing the average backscattered power and the numerically computed backscattered power under the Kirchhoff approximation. It was found that the choice of approximations depends upon the incident frequency and the shape of the surface correlation function.

Theory of short-scale field-aligned density striations due to ionospheric heating

Abstract: The theoretical saturation spectrum of parametrically excited Langmuir waves in a locally uniform ionosphere is shown by the present calculations to produce, by ohmic dissipation, short-scale field-aligned density striations. The spectrum of the calculated striations is not inconsistent with observations of field-aligned scatter of VHF and UHF waves in ionospheric modification experiments if local increases of the pump field due to focusing are invoked.

Measurements at 13.9 GHz of the radar backscattering cross section of the North Sea covered with an artificial surface film

Abstract: The reduction of the Ku-band (13.9 GHz) normalized radar cross section (NRCS) by an artificial monomolecular surface film (oleyl alcohol) on the sea surface was measured in the North Sea during the 1975 Joint North Sea Wave Project, JONSWAP 75 experiment. The aim of the surface film experiment was to simulate natural surface films which often occur on the ocean surface and are produced by plankton or fish. NRCS measurements were obtained from an aircraft at incidence angles of 41 deg and 47 deg at vertical and horizontal polarizations. For winds between 3.5 and 4.4 m/sec the maximum measured reduction was 7.3 plus or minus 3.5 dB relative to the mean. In-situ measurements showed that the oleyl alcohol film reduced the surface tension from 74 to 43 dyne/cm.

Numerical analysis of three-dimensional arbitrarily-shaped conducting scatterers by trilateral surface cell modelling

Abstract: A new technique has been developed to calculate the scattering characteristics of a three-dimensional arbitrarily-shaped conducting body. The analysis employs a moment method which divides the surface of the scatterer into trilateral surface cells. The physical-optics type of current on each cell has been utilized to form the set of basis functions. A generalized computer program has been written and has been tested on the case of a sphere to demonstrate the numerical advantages of the present technique as compared with other existing surface-type integral approaches and wire-grid modelling techniques.

Signatures of snow in the 5 to 94 GHz range

Abstract: A long-term observational study of the microwave emission and scattering behavior of snow under quasi-controlled conditions was started recently on a high-altitude Alpine test site. Results of the first set of measurements, carried out with five radiometers at frequencies of 4.9, 10.5, 21, 36, and 94 GHz, are reported and preliminary interpretations are given. The spectral reversal of the brightness temperature and its dependence on look angle and polarization provides a means to distinguish between different snow states and to estimate the wetness factors of the surface layer of the snow pack. From an electromagnetic scattering standpoint, snow cover is an excellent medium to use for studying volume scattering.

Depolarization of 19 and 28 GHz earth‐space signals by ice particles

Abstract: Strong depolarization (sometimes > −20 dB) at 19 and 12 GHz occurring with low signal attenuation (often <1 dB) and associated with summer thunderstorms was described earlier. This paper describes winter observations at 19 and 28 GHz of what appears to be the same phenomenon. The depolarization is a few dB stronger at 28 GHz but is highly correlated between the two frequencies. The winter depolarization events appear when ground temperatures are near or below freezing and clouds are along the path. These events are caused by differential phase shift through ice crystals in the clouds.

Techniques for the study of TID's with multi‐station rapid‐run ionosondes

Abstract: Individual daytime traveling ionospheric disturbances (TID's) in the F-layer are studied using three ionosondes mutually approximately 150 km apart in northern New Hampshire and Vermont. The ionosondes were operated continuously for long periods of time, making ionograms every two minutes. Methods of scanning the continuous data for TID's, and of analyzing them, are described. Iso-height contours are emphasized as superior to iso-ionic contours for analysis. It is found that the direction toward which a TID travels is essentially independent of height within the disturbance and that the directions of travel cluster about the south-southeast. Other parameters are found to be height-dependent and will be the subject of future papers.

Large-amplitude ULF electromagnetic fields from BART

Abstract: Starting in 1972, persistent large-amplitude magnetic signals were observed in recordings of ultra-low-frequency (ULF, frequencies less than 5 Hz) geomagnetic field fluctuations made on the Stanford campus. The energy of these ULF signals is concentrated predominantly at frequencies below about 0.3 Hz and their amplitude, which has increased greatly since 1972, is at least ten times greater than the normal natural background level of geomagnetic activity, i.e., it is comparable to the level observed during great geomagnetic storms. ULF magnetic field measurements at additional locations away from the Stanford campus have enabled us to identify the San Francisco Bay Area Rapid Transit (BART) system as the source of these large-amplitude magnetic fields, which appear to be produced throughout the entire Bay area and vicinity (area ≈100 km2). Primary power for the BART trains is provided by a 1000 V dc third rail (total length ≈270 km) and a single loaded 10-car train can draw as much as 7 Mw during acceleration. Large fluctuating currents (5 to 10 kA) therefore flow in the third rail and associated conductors, with the result that the entire BART system constitutes a large ULF “antenna.” Similar systems planned or already in operation in other parts of the US (e.g., the Washington, DC, Metro) will probably also produce similar large-amplitude ULF electromagnetic fields.

Incoherent backscatter from rough surfaces: The two‐scale model reexamined

Abstract: An expression for the incoherent backscatter cross section per unit area of a two-scale rough surface is derived that includes the effects of local polarization tilt angles, thus providing a first-order cross-polarized scattering component. A rigorous numerical method of evaluating the derived expression is developed and two approximate solutions are also derived. The approximate solutions clarify the two-scale interaction for the cases of diffuse and quasi-specular small-scale scattering. Calculated scattering distributions are compared with experimental data at radar and optical frequencies using both perturbation theory and physical optics polarization coefficients. Good agreement with copolarized radar data is obtained using perturbation theory polarization coefficients while physical optics polarization predictions are observed at optical frequencies. Ambiguities in the comparisons between experimental data and model predictions are discussed.

Numerical calculation of the scattered field from a periodic deformed cylinder using the smoothing process on the mode-matching method

Abstract: The smoothing process on the mode-matching method is applied to scattering problems for a periodic deformed cylinder with perfect conductivity. If the body is unaltered on rotation through an angle 2π/ν(ν = 2, 3, 4, …) about some axis, the polyphase wavefunction allows us to deal with the problems only in a sector region with the angle 2π/ν. Consideration of the symmetry of the scatterer reduces the dimension of the linear equations to one part of ν in comparison with the formulation without consideration. The smoothing process together with the improved point-matching method can be regarded as the computer-aided algorithm; the algorithm is simple and the accuracy of the solutions can be evaluated explicitly. The numerical results show that the speed of convergence of the solutions by the smoothing process is faster than that of the conventional mode-matching method, and that for a moderately large truncation size the smoothing process highly improves the accuracy of the solutions. The radar cross section for a non-convex body is calculated.