Showing papers in "Radio Science in 2000"

Vertical structure of vegetated land surfaces from interferometric and polarimetric radar

Abstract: This paper describes the estimation of vertical-structure parameters from combined interferometric and polarimetric radar data.

Dense media radiative transfer theory based on quasicrystalline approximation with applications to passive microwave remote sensing of snow

Abstract: Dense media radiative transfer (DMRT) equations based on quasicrystalline approximation (QCA) for densely distributed moderate size particles are developed. We first compute the effective propagation constant and coherent transmission into a dense medium on the basis of the generalized Lorentz-Lorenz law and the generalized Ewald-Oseen extinction theorem. The absorption coefficient of the dense media is then calculated. The distorted Born approximation is next applied to a thin layer to determine the bistatic scattering coefficients and the scattering coefficient. The phase matrix in DMRT is then obtained as bistatic scattering coefficient per unit volume. The model is applied to multiple sizes and for sticky particles. Numerical results are illustrated for extinction and brightness temperatures in passive remote sensing using typical parameters in snow terrain. The QCA-based DMRT is also used to compare with satellite Special Sensor Microwave Imager (SSM/I) brightness temperatures for four channels at 19 and 37 GHz with vertical and horizontal polarizations and for two snow seasons. It shows reasonable agreement to snow depth of 1 m.

The application of GPS observations to equatorial aeronomy

Abstract: Routine observations of the ionospherically imposed propagation effects upon GPS satellite signals are available online from the International GPS Service for Geodynamics. With over 200 ground-based stations now reporting data, ionospheric studies ranging from the analysis of single-site observations to the full global network have demonstrated the geophysical science yield possible from this remarkable resource. In this paper we expand on the use of GPS data for comprehensive “regional studies” at low latitudes. Using the GPS observing sites in South America, we show how routine observations are processed to form reliable total electron content (TEC) values capable of describing the latitudinal, longitudinal, local time, and storm time behavior over the full span of the continent. To study the dominant F region structure at low latitudes, the Appleton anomaly, two indices are developed to assess its strength (Is) and asymmetry (Ia). TEC data at 30 s intervals are used to form phase fluctuation indices that capture plasma irregularity patterns at 15-min (fp) and hourly (Fp) time resolution. Tests of Fp at Atlantic and Pacific sector locations show them to reproduce accurately all known occurrence patterns for equatorial spread F (ESF). The use of the three indices (Is,Ia, and Fp) to formulate predictive capabilities for ESF on the basis of the enhancement or suppression of growth rate indicators was not particularly successful.

Estimating evaporation duct heights from radar sea echo

Abstract: The evaporation duct is a downward refracting layer that results from the rapid decrease in humidity with respect to altitude occurring in the atmospheric surface layer above bodies of water. The evaporation duct affects radar detection ranges at frequencies of approximately 1 GHz and above. Models based on Monin-Obukhov similarity theory are usually used to calculate evaporation duct refractivity profiles from bulk measurements of air temperature, humidity, wind speed, and the sea surface temperature. Modeling results by Pappert et al. [1992] indicated that the falloff of radar sea echo as a function of range was an increasing function of the evaporation duct height. On the basis of those results, the authors proposed inferring the evaporation duct height by a slope fit to modeled clutter power, a nonlinear least squares inversion procedure. Data for testing the inversion procedure were obtained using the S band Space Range Radar at Wallops Island, Virginia. Evaporation duct heights were inferred from the radar data on the basis of the assumption of a range-independent evaporation duct height and sea clutter radar cross section (σ°). Validation data consist of buoy and boat in situ bulk measurements. The result of comparing the radar-inferred evaporation duct heights and those calculated from bulk measurements indicates that the radar-inferred duct heights are strongly correlated with those from the in situ measurements, but there is some uncertainty as to whether they are biased or unbiased. That uncertainty arises from the assumed dependence of σ° on the grazing angle ψ. That dependence is currently a matter of debate in the open literature, with the lower and upper ends of modeling results being σ° ∝ ψ0; and σ° ∝ ψ4, respectively. We show results for both dependencies and note that the σ° ∝ ψ0; provides the best agreement with our measurements. It should be noted that inferring the evaporation duct height from radar sea echo is a problem that stresses the modeling of low-grazing-angle backscatter.

GPS global detection of the ionospheric response to solar flares

Abstract: This author suggests the concept of a new technology for global detection (GLOBDET) of atmospheric disturbances of natural and technogenic origin, on the basis of phase measurements of the total electron content (TEC) in the ionosphere using an international GPS network. Temporal dependencies of TEC are obtained for a set of spaced receivers of the GPS network simultaneously for the entire set of “visible” (over a given time interval) GPS satellites (up to 5–10 satellites). These series are subjected to filtering in the selected range of oscillation periods using algorithms for spatiotemporal analysis of signals of nonequidistant GPS phased antenna arrays which are adequate to the detected disturbance. An analysis is made of the possibilities of using the GLOBDET when detecting the ionospheric response of solar flares. In this case it is best to make the coherent summation of the filtered series of TEC. Powerful impulsive flares of July 29, 1999, and December 28, 1999, were chosen to illustrate the practical implementation of the proposed method.

On the ionosphere calibration in GPS radio occultation measurements

Abstract: Elimination of the ionospheric contribution to atmospheric delays in GPS radio occultation measurements is a key issue in the retrieval of accurate pressure and temperature profiles above the tropopause. The traditional so-called “ionosphere-free” combination of the L1 and L2 phase paths, eliminating the first-order ionospheric effects, is not sufficient because of the nonnegligible bending of the two ray paths. Because of the dispersive nature of the ionosphere the L1 and L2 signals will follow slightly different paths, giving rise to an ionospheric residual, in this paper referred to as the “dispersion” residual. Other higher-order ionospheric effects contribute to the total residual, but the dispersion residual is the most dominant. A linear combination of the L1 and L2 bending angles gives better results in most cases and has in recent years become the method of choice. In this paper a new phase path correction method is presented, dealing with the problem of L1 and L2 ray path separation. Formulas are derived showing how the dispersion residual can be evaluated using measurements of the satellite-to-satellite total electron content. The residuals for various conditions in the ionosphere are estimated numerically and compared to analytic estimates. A mathematical formulation of the difference between this improved phase path correction method and the bending angle correction method is obtained, showing that the two methods are nearly equivalent.

Modeling ELF radio atmospheric propagation and extracting lightning currents from ELF observations

Abstract: Observations of extremely low frequency (ELF) radio atmospherics (sferics), the transient electromagnetic fields radiated by lightning discharges, are used to determine the current moment waveforms of vertical lightning discharges. In order to extract this information the propagation of radio atmospherics from source to receiver must be modeled accurately, especially in view of the important role played by the D and E regions of the ionosphere at these long (> 200 km) wavelengths. We model broadband ELF sferic waveforms by adapting a single-frequency ELF propagation code to calculate an ELF propagation impulse response under the assumption of horizontal ionospheric homogeneity, with which we extract the source lightning current waveform from an observed ELF sferic waveform using a deconvolution method based on linear regularization. Tests on modeled sferics indicate that the method is accurate and relatively insensitive to noise, and we demonstrate the application of the technique with a sprite-associated sferic. Since ELF sferics can often be observed many thousands of kilometers from the source discharge, the technique developed here represents a powerful new method of remotely sensing lightning current waveforms.

Nighttime equatorial ionosphere: GPS scintillations and differential carrier phase fluctuations

Abstract: The presence of scintillation-producing irregularities in the nighttime equatorial ionosphere, in the path of Global Positioning System (GPS) signals received at an equatorial station, causes dual-frequency measurements of the differential carrier phase of GPS L1 and L2 signals to have a contribution from phase scintillations on the two signals. Dual-frequency data for fluctuations in the total electron content (TEC) along the path of GPS signals to the equatorial station Ancon (1.5° dip), sampled at a rate of 1 Hz, are used to separate this contribution from the slower TEC variations. Rapid fluctuations in the differential carrier phase, usually on timescales < 100 s, which result from diffraction, are seen to follow the pattern of intensity scintillations on the L1 signal. Intensity scintillations are also related to the variations in TEC which arise from density fluctuations associated with ionospheric irregularities. An approximate version of the transport-of-intensity equation, based on a phase screen description of the irregularities, suggests that a quantitative measure of intensity scintillations may be provided by the derivative of rate of change of TEC index (DROTI), obtained from the second derivative of TEC. This equation also yields the dependence of the scaling factor between DROTI and S4 on the Fresnel frequency. Comparison of DROTI computed from relative TEC data to corresponding S4 indices indicates that there may be lesser uncertainity in a quantitative relation between the two than between the index ROTI, introduced in recent years, and S4. Power spectral analysis of TEC fluctuations and simultaneous intensity scintillations on L1 signal, recorded at Ancon, does not indicate any simple dependence of the scaling factor between DROTI and S4 on the spectral characteristics.

An excitation theory for bound modes, leaky modes, and residual-wave currents on stripline structures

Abstract: The nature of the current on a general multilayered printed-circuit stripline structure excited by a delta-gap source is investigated. The current is obtained through the construction of a semianalytical three-dimensional (3-D) Green's function, which accounts for the presence of the infinite conducting strip and the layered background structure. The 3-D Green's function is obtained by Fourier transforming the delta-gap source in the longitudinal (z) direction, which effectively resolves the 3-D problem of a delta-gap source into a superposition of 2-D problems, each of which is infinite in the z direction. The analysis allows for a convenient decomposition of the strip current into a sum of constituent parts. In particular, the strip current is first resolved into a set of bound-mode current waves and a continuous-spectrum current. The continuous-spectrum current is then represented as a set of physical leaky-mode currents in addition to a set of “residual-wave” currents, which arise from the steepest-descent integration paths. An asymptotic analysis reveals that the residual-wave currents decay algebraically as z−3/2. Far away from the source, the residual-wave currents dominate the continuous-spectrum strip current. Results are shown for a specific type of stripline structure, but the analysis and conclusions are valid for arbitrary multilayer stripline structures.

An analysis of the scattering of high‐frequency electromagnetic radiation from rough surfaces with application to pulse radar operating in backscatter mode

Abstract: The scattering of high-frequency (HF) electromagnetic radiation from slightly rough, good conducting surfaces is presented. The analysis is based on a decomposition of the relevant space using generalized functions. The fundamental analysis incorporates a general source and involves all scattering orders for the normal component of the field. Subsequently, derivation of the scattered electric field (to third order in scatter) using a pulsed dipole source is effected. The first 2 orders are used to deduce an estimate of radar cross sections of bounded regions or targets when operation is carried out in the backscatter mode. Conditions of small height and small slope are imposed. Application is made to the determination of the first-order cross section of a perfectly conducting sphere (within the limits of the imposed contraints) and of an exponential boss. The results are shown to be consistent with Rayleigh scattering theory.

A hybrid uniform geometrical theory of diffraction–moment method for efficient analysis of electromagnetic radiation/scattering from large finite planar arrays

Abstract: A hybrid uniform geometrical theory of diffraction (UTD)-moment method (MOM) approach is introduced to provide an efficient analysis of the electromagnetic radiation/scattering from electrically large, finite, planar periodic arrays. This study is motivated by the fact that conventional numerical methods become rapidly inefficient and even intractable for the analysis of electrically large arrays containing many antenna or frequency-selective surface (FSS) elements. In the present hybrid UTD-MOM approach, the number of unknowns to be solved is drastically reduced as compared to that which is required in the conventional MOM approach. This substantial reduction in the MOM unknowns is essentially made possible by introducing relatively few, special ray-type (or UTD) basis functions to efficiently describe the unknown array currents. The utility of the present hybrid approach is demonstrated here for the simple case of a large rectangular phased array of short and thin metallic dipoles in air, which are excited with a uniform amplitude and linear phase distribution. Some numerical results are presented to illustrate the efficiency and accuracy of this hybrid method.

Method of moments analysis of electromagnetic scattering from a general three‐dimensional dielectric target embedded in a multilayered medium

Abstract: The method of moments (MOM) is applied to the problem of electromagnetic scattering from general three-dimensional dielectric targets in an arbitrary multilayered environment. The dyadic multilayered Green's function is computed via the method of complex images, and the Galerkin MOM solution is effected by employing triangular patch basis functions. Several example frequency and time domain results are presented, with application to radar-based sensing of plastic land mines.

On the predictability of f0F2 using neural networks

Abstract: Recently, there has been much interest in the use of neural networks (NNs) in ionospheric prediction models [Williscroft and Poole, 1996; Altinay et al., 1997]. This paper is divided into two parts. The first part presents an extension of the work of Williscroft and Poole [1996], in which a NN is trained to use nonionospheric geophysical parameters representing time, season, solar cycle, and magnetic activity to estimate f0F2. The NN is trained with data from two sunspot cycles at the midlatitude station of Grahamstown (26.5°E, 33.3°S, geographic) and predicts f0F2 for various combinations of the input parameters. It is further shown how the squared errors between f0F2 estimated from the NN and the measured values are themselves functions of the input parameters, and it is demonstrated how a second NN can be trained to predict the squared error, and hence the rms error, thus providing a measure of the uncertainty in the estimation. These uncertainties lie in the range 0.4–0.9 MHz, a considerable improvement on current non-NN-based models. In the second part, the input data to the net are expanded to include recent measured values of f0F2, which leads to a further improvement in the estimation of future values. We conclude that the inclusion of this ionospheric information in the input data is only justified for prediction times up to 4–5 hours ahead, whereafter a knowledge of the most recent values of f0F2 does not improve the prediction significantly, and the nonionospheric parameters described in the first part are adequate.

An optimization approach to two‐dimensional time domain electromagnetic inverse problems

Abstract: An optimization approach to a two-dimensional electromagnetic inverse problem in the time domain is considered. Wave-splitting is integrated in the optimization algorithm. The permittivity, permeability, and conductivity are reconstructed by minimizing an objective functional. To speed up the computation time, an explicit expression for the gradient of the objective functional is derived by introducing dual functions and using the Gauss surface divergence theorem. The parameters are then reconstructed by an iterative conjugate gradient algorithm. Numerical results for a simultaneous reconstruction of the permittivity, permeability, and conductivity are presented.

Analysis of Perseid meteor head echo data collected using the Advanced Research Projects Agency Long-Range Tracking and Instrumentation Radar (ALTAIR)

Abstract: The 1998 Perseid meteor shower was observed using the Advanced Research Projects Agency (ARPA) Long-Range Tracking and Instrumentation Radar (ALTAIR) in order to study potential meteoroid impact on orbiting spacecraft. ALTAIR is a dual-frequency radar that operates at VHF and UHF, and its high sensitivity and precise calibration make it uniquely suited for detecting meteor head echoes. ALTAIR is dually polarized and records left-circularly and right-circularly polarized signal returns, which allow the determination of polarization ratios. ALTAIR uses a multihorn feed and interferometry to measure target angle of arrival. This paper contains analysis on Perseid data that were collected at VHF (158 MHz). Meteor head echo statistics are presented, including mean altitude, radial velocity, radar cross section (RCS), and polarization ratio. ALTAIR's VHF detection rate was approximately 1 head echo per second. An in-depth analysis on select head echoes to estimate meteor decelerations and densities has also been included.

Fast inhomogeneous plane wave algorithm for electromagnetic solutions in layered medium structures: Two‐dimensional case

Abstract: An accurate and efficient algorithm is proposed for solving two-dimensional electromagnetic scattering problems in a layered medium. As a natural extension of the previously developed fast inhomogeneous plane wave algorithm (FIPWA), this approach has several inherent merits, such as being simple, versatile, and error controllable. The basic idea is first to express the spatial Green's function, encountered in layered medium studies, as Sommerfeld-type integrals. By observing the similarity of these integrals with the integral representation of the free space Green's function, FIPWA can be applied with ease to accelerate the matrix-vector multiplication. The multilevel scheme has been implemented, and the computational complexity of O(N log N) is achieved. It is noted that the proposed approach is more accurate and more efficient than the existing fast multipole method coupled with the discrete complex image method.

Inversions of radio occultation amplitude data

Abstract: Radio occultation remote sensing of the Earth's atmosphere consists of satellite-to-satellite observations of phase and amplitude of radio waves that propagate through the atmosphere. The observed excess phase along with the positions and velocities of the satellites are inverted into bending angle as a function of impact parameter and then into vertical profiles of refractivity, pressure, and temperature in the neutral atmosphere, or into electron density in the ionosphere. The retrieved profiles are assigned to the perigee points of the sounding rays. Amplitude data are normally not used, except when solving diffraction back propagation problems. In this paper a simple method to utilize amplitude radio occultation data is discussed. Equations based on geometric optics are considered for the inversion of an amplitude into bending angle. These inversions do not require high coherence of radio waves or precise orbit determination, as with phase inversions, but they do require precise calibration of the amplitude. Even though amplitude inversions are not so precise as phase inversions, they may still be useful for a number of applications. When compared to phase inversions they allow the optimization of the filter bandwidth for phase inversions, the detection of multipath propagation, and the localization of electron density irregularities in the ionosphere. These applications are demonstrated by processing of the Global Positioning System/Meteorology (GPS/MET) radio-occultation data collected onboard the satellite Microlab-1.

Comparative analysis of radioholographic methods of processing radio occultation data

Abstract: Two methods for the determination of refraction angle profiles from radio occultation measurements in multipath areas are analyzed and compared: (1) the radio-optic method based on the analysis of the local spatial spectra of the measured wave field and (2) the back propagation of the received wave field to a single-ray region. The basic limitations of the radio-optic method are (1) the restriction of the resolution of refraction angle profiles due to the uncertainty relation of refraction angle and impact parameter and (2) diffractive effects in subcaustic areas, where the spatial spectra cannot be interpreted in terms of geometric optical rays. The basic limitation of the back propagation method is related to ray and caustic structures, which may not contain single-ray areas. It is shown that strong refraction reduces the uncertainties of refraction angle and impact parameter. On the other hand, strong refraction or superrefraction is responsible for complicated caustic structures that cannot be resolved by the back propagation technique. The two methods are thus complementary to each other and can be combined for processing lower tropospheric occultation data. This analysis is corroborated by numerical simulations based on global fields of atmospheric variables from analyses of the European Centre for Medium Range Weather Forecast.

Asymptotic high‐frequency Green's function for a planar phased sectoral array of dipoles

Abstract: This paper deals with the derivation and physical interpretation of a uniform high-frequency Green's function for a planar right-angle sectoral phased array of dipoles. This high-frequency Green's function represents the basic constituent for the full-wave description of electromagnetic radiation from rectangular periodic arrays and scattering from rectangular periodic structures. The field obtained by direct summation over the contributions from the individual radiators is restructured into a double spectral integral whose high-frequency asymptotic reduction yields a series of propagating and evanescent Floquet waves (FWs) together with corresponding FW-modulated diffracted fields, which arise from FW scattering at the array edges and vertex. Emphasis is given to the analysis and physical interpretation of the vertex diffracted rays. The locally uniform asymptotics governing this phenomenology is physically appealing, numerically accurate, and efficient, owing to the rapid convergence of both the FW series and the series of corresponding FW-modulated diffracted fields away from the array plane. A sample calculation is included to demonstrate the accuracy of the asymptotic algorithm.

A possible mechanism for echo striation generation of radar backscatter from midlatitude sporadic E

Abstract: The morphological resemblance is described between the quasi-periodic scintillations of VHF radio wave transmissions from a satellite and the 46.5-MHz radar backscatters of the middle and upper atmosphere (MU) radar radio waves, which are both associated with ionospheric irregularities in the sporadic E layer. Because these observation techniques are sensitive to different wavelengths of irregularities, the resemblance may be caused by coexisting irregularities. That is, radio wave scintillations reveal medium-scale density-enhanced regions with a specific scale length of several hundred meters in which small-scale irregularities are embedded. The MU radar echoes may reveal backscatter signals from small-scale irregularities. A periodic structure on a range-time-intensity (RTI) map reflected the spatial structure of the medium-scale density-enhanced regions. A model is proposed to explain the striations on the RTI map, which extend up to heights of several tens of kilometers even though sporadic E layers are usually thin.

Comparison of atmospheric parameters derived from meteor observations with CIRA

Abstract: In this paper we compare monthly averages of the atmospheric parameter T/ p (where T is temperature and p is pressure) derived from the decay of underdense meteor echoes with the CIRA (1986) atmospheric model. The meteor data were collected with the Buckland Park VHF radar situated ∼40km north of Adelaide, Australia. We examine the overall agreement between the meteor observations and CIRA as well as seasonal differences between the two. Comparison is made with the results of Hocking et al. [1997]. Our results are complimentary to those of Hocking et al.; our data were obtained in the Southern Hemisphere as opposed to the Northern Hemisphere. A discussion on the effect of the geomagnetic field on the diffusion of meteor trails and its effect on the measurement of atmospheric parameters is also included. We note that the geomagnetic field is a very important consideration when using meteors for the derivation of atmospheric temperatures and pressures above heights of around 92–93 km. This effect is required to be taken into account above these heights as failure to do so leads to errors in the interpretation of the data. Recent researchers have avoided this problem by restricting their data to below 90 km.

Ionogram analysis using fuzzy segmentation and connectedness techniques

Abstract: We present a new procedure for the analysis of ionograms that evolves from methods developed for image analysis and utilizes techniques based on the concepts of fuzzy segmentation and connectedness. Ionogram traces are often not “crisply” defined, and we demonstrate that it is possible to approximate them as fuzzy subsets within the two-dimensional space defined by the time-of-flight and the radio frequency. A real number between 0 and 1 is assigned to each pixel in an ionogram, thereby defining the membership of that pixel to each of the fuzzy subsets, effectively creating a “gray scale” ionogram. In this context, ionogram analysis becomes a problem in fuzzy geometry, and various geometrical properties, including the topological concepts of connectedness, adjacency, height, width, and major axis, can be defined. It is shown that not only does the fuzzy segmentation process separate signals from the chaotic noise background that often characterizes ionograms, but that it can also be applied to classify ionospheric echoes according to standard nomenclature, e.g., normal E, F, or Es layers. Furthermore, in reference to the skeleton or thinning extraction procedures employed in imaging processing, the fuzzy connectedness between echoes in selected segments can be used to determine the primary layers that are characteristic of vertical incidence ionospheric reflection. This information can be provided as input to automatic scaling or true-height inversion routines, which can then be used to derive either the standard URSI set of ionospheric parameters or the electron density distribution in the overhead ionosphere, or both. This fuzzy algorithm approach has been successfully applied to midlatitude ionogram data from advanced digital ionospheric sounders operated by the National Central University and Utah State University.

Representation of a wave field in a randomly inhomogeneous medium in the form of the double-weighted Fourier transform

Abstract: New integral representation of a wave field dn a continuously inhomogeneous random medium is suggested in the form of double-weighted Fourier transformation, performed simultaneously with respect to coordinates of the source and the observer. The integral representation under consideration takes into account both the diffraction effects and the multiray effects. It incorporates many results of known techniques of wave propagation description in continuously inhomogeneous media: the methods of geometrical optics, smooth perturbations, phase screen, and two-scale expansions. The method delivers new opportunities to retrieve small-scale inhomogeneous structure of ionosphere plasma from radio-sounding data and can serve as the basis for diffraction tomography of the ionosphere.

Twenty‐four hour predictions of f0F2 using time delay neural networks

Abstract: The use of time delay feed-forward neural networks to predict the hourly values of the ionospheric F2 layer critical frequency, f0F2, 24 hours ahead, have been examined. The 24 measurements of f0F2 per day are reduced to five coefficients with principal component analysis. A time delay line of these coefficients is then used as input to a feed-forward neural network. Also included in the input are the 10.7 cm solar flux and the geomagnetic index Ap. The network is trained to predict measured f0F2 data from 1965 to 1985 at Slough ionospheric station and validated on an independent validation set from the same station for the periods 1987–1990 and 1992–1994. The results are compared with two different autocorrelation methods for the years 1986 and 1991, which correspond to low and high solar activity, respectively.

Azimuthal variations of the microwave radiation from a slightly non‐Gaussian sea surface

Abstract: Microwave radiation from the ocean rough surface is considered using a small slope expansion. The emphasis is on azimuthal variations of the brightness temperature, related to the anisotropy of wind-generated sea waves. The first and second harmonics, describing the upwind-downwind and upwind-crosswind differences, respectively, are considered using existing models of the sea spectrum. For modeling of the first harmonic the small deviation from Gaussian statistics of the sea surface is introduced to estimate the effect of longwave asymmetry and the distribution of ripples over the long waves. Numerical analysis shows that the longwave asymmetry cannot explain the observed values of the first harmonic in azimuthal variations of the brightness temperature. The ripple modulation by long waves is a possible mechanism explaining the first harmonic in moderate winds, although foam and wave breaks have to be included in electromagnetic modeling to achieve a quantitative agreement with experiments.

A simulation study of coherent radar imaging

Abstract: Coherent radar imaging (CRI) is used in an attempt to overcome the angular resolution limitation of conventional single-station radars and is used to image the horizontal structure inside the resolution volume. This recently developed technique has been successfully applied to radar observations of the ionosphere as well as the lower atmosphere. However, no statistical analysis of the robustness of the various techniques has been presented to date. In this work, three CRI techniques are reviewed: Fourier-based, Capon's, and maximum entropy (MaxEnt) methods. The Fourier-based method is the simplest of the three algorithms but has inherent resolution limitations. Although quite different in nature and performance, both Capon's and MaxEnt methods can be posed as constrained optimization problems. A statistical comparison of performance of the three CRI techniques, using various receiver configurations and two distinct cases of scattering structure, is made using simulated data. The results show that the MaxEnt method exhibits the best performance in the case of aspect-sensitive scattering with a broad characteristic. In the localized scattering case, however, Capon's method shows superior performance for signals with high signal-to-noise ratio (SNR), but MaxEnt method outperforms all methods for low SNR. In general, both Capon's and MaxEnt methods are able to reproduce the gross characteristics of the scattering media under observation.

Terrain topography measurement using multipass polarimetric synthetic aperture radar data

Abstract: A method has been investigated for the measurement of topography using airborne fully polarimetric synthetic aperture radar (SAR) data. Terrain slopes in both the range and azimuthal directions have been estimated using multipass flight geometries. Using these slope values, the Poisson equation was then solved to create a Digital Elevation Model (DEM) of the terrain topography. The method measures polarimetric orientation angles which are then converted into terrain slopes in the azimuthal direction. The conversion of these orientation angles into terrain slopes requires additional knowledge of the radar look angle and the range direction terrain slopes. The solution for slopes is, therefore, a problem coupled between the range and azimuthal directions. For specialized multipass flight geometries these orthogonal terrain slopes are solved for, and maps of terrain slopes are produced. In particular, the processing of two-pass orthogonal and two-pass antiparallel (headings Θ and Θ+ 180°) NASA - Jet Propulsion Laboratory airborne SAR data sets has been carried out for an area in central California. When orthogonal slopes are derived using either of these data sets, a digital elevation model may be generated. The L band, polarimetric SAR (POLSAR) DEM created by this solution is compared to a coregistered C band, interferometric SAR (IFSAR) DEM. Similar comparisons are made for terrain slopes in the azimuthal - range directions which are generated by the POLSAR and IFSAR elevation data. The polarimetric SAR, operating from an aircraft, or satellite, in a strip-mapping mode, is capable of measuring terrain topography for large areas provided that phase-preserving fully polarimetric data are taken. Polarimetric SAR data are also widely used for studies of crop classification, surface roughness, biomass density, and soil moisture content. All of these studies are adversely affected by scattering changes attributable to topography. The technique investigated here potentially offers a means to correct for these effects by making simultaneous, coregistered estimates of local orthogonal terrain slopes.

Direct numerical simulation of VHF radar measurements of turbulence in the mesosphere

Abstract: We simulate very high frequency (VHF) radar backscatter from turbulent irregularities in the mesosphere and examine the process of calculating key turbulence parameters from radar Doppler spectral moments. Mesospheric turbulence is represented by a three-dimensional (3-D) direct numerical simulation (DNS) of a turbulent layer generated by Kelvin–Helmholtz instability, which exhibits inertial range turbulence characteristics. A subgrid-scale turbulence advection model is used to parameterize the unresolved small-scale structure of mesospheric temperature, ion species, and electron mixing ratios and to derive the radar backscatter cross sections. Energy and thermal dissipation rates are calculated from the 3-D velocity and temperature derivatives of the DNS. Doppler spectral widths relevant to various mesospheric radar observations are then determined from explicit calculation of spectral moments. We then present a comparison between the turbulence parameters calculated from the DNS and those calculated from the Doppler spectral moments.

Inversion algorithm and measurement system for microwave tomography of buried object

Abstract: The detection and identification of buried inhomogeneities using electromagnetic waves are of crucial importance for many applications. The paper deals with reconstructed images from measurements using an inversion qualitative algorithm for microwave tomography. The algorithm is based on diffraction tomography for detecting and locating buried objects. The backscattered field is measured at different frequencies over a probing line above or in contact with the soil at different receiver locations and for different positions of a transmitting antenna (multistatic configuration). The reconstruction algorithm processes the backscattered field generated by the buried inhomogeneities and incorporates the incident near-field distribution transmitted in the soil by the broadband antenna. Tomographie reconstructions of buried objects are presented for a situation of practical interest using bow tie antennas in the frequency band [0.3–1.3] GHz.

Bistatic form of the electric field equations for the scattering of vertically polarized high‐frequency ground wave radiation from slightly rough, good conducting surfaces

Abstract: The scattering of high-frequency (HF) electromagnetic radiation from slightly rough, good conducting surfaces for the case of bistatic reception is examined. In this work, the scattering surface is considered to be time invariant and to be representable by a two-dimensional Fourier series. The source is taken to be a continuously excited elementary vertical dipole whose current distribution is arbitrary. Thus the basis is provided for the introduction of any desired source waveform. The convolution integrals resulting from earlier analyses are treated asymtotically, primarily via stationary phase techniques. The physical relevance of the stationary points derived from the first 2 orders of scatter is discussed. The various field components presented for the time-invariant surface may be easily extended to time-varying surfaces and subsequently to deducing the high-frequency cross sections of highly conducting surfaces such as that of the ocean.