Showing papers on "Amplitude published in 1997"

Measurements and global models of surface wave propagation

Abstract: A new technique for making single-station phase velocity measurements is developed and applied to a large number of globally recorded Rayleigh and Love waves in the period range 35–150 s. The method is based on phase-matched filter theory and iteratively suppresses the effect of interfering overtones by minimizing residual dispersion. The model surface wave signal is described by its amplitude and apparent phase velocity, both of which are parameterized in terms of smooth B-spline functions of frequency. A misfit function is constructed which represents the difference between the model and observed waveforms, and the optimal spline coefficients are estimated in an iterative misfit minimization algorithm. In order to eliminate cycle skips in the measurements of phase at short periods, the waveforms are first matched at long periods, and the frequency range is gradually extended to include higher frequencies. The application of the algorithm to records from the Global Seismographic Network, using earthquakes in the Harvard centroid-moment tensor catalog, results in the determination of more than 50,000 high-quality dispersion curves. The observed variations in measured dispersion for pairwise similar paths are used to estimate realistic uncertainties in the data. Phase delays at discrete periods are inverted for global maps of variations in phase velocity expanded in spherical harmonics up to degree 40. A realistic resolution test indicates that structures are well recovered up to at least degree 20. The new phase velocity maps explain 70–96% of the observed variance in phase residuals, reflecting the high internal consistency of the dispersion measurements.

On instantaneous amplitude and phase of signals

Abstract: In many questions of signal processing, it is important to use the concepts of instantaneous amplitude or phase of signals. This is especially the case in communication systems with amplitude or frequency modulation. These concepts are often introduced empirically. However, it is well known that the correct approach for this purpose is to use the concept of analytic signal. Starting from this point, we show some examples of contradictions appearing when using other definitions of instantaneous amplitude or frequency that are commonly admitted. This introduces the problem of characterizing pure amplitude-modulated or pure phase-modulated signals. It is especially shown that whereas amplitude modulated signals can be characterized by spectral considerations, this is no longer the case for phase-modulated signals. Furthermore, signals with constant amplitude have very specific properties, which are analyzed in detail. Some consequences and extensions to random signals are finally discussed.

The electric field and wave experiment for the cluster mission

Abstract: The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s-1. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes up to 700 mV m-1 with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.

Cosmological Model Predictions for Weak Lensing: Linear and Nonlinear Regimes

Abstract: Weak lensing by large-scale structure induces correlated ellipticities in the images of distant galaxies. The two-point correlation is determined by the matter power spectrum along the line of sight. We use the fully nonlinear evolution of the power spectrum to compute the predicted ellipticity correlation. We present results for different measures of the second moment for angular scales θ 1'-3° and for alternative normalizations of the power spectrum, in order to explore the best strategy for constraining the cosmological parameters. Normalizing to observed cluster abundance, the rms amplitude of ellipticity within a 15' radius is 0.01z s0.6, almost independent of the cosmological model, with zs being the median redshift of background galaxies. Nonlinear effects in the evolution of the power spectrum significantly enhance the ellipticity for θ < 10'—for θ 1' the rms ellipticity is 0.05, which is nearly twice as large as the linear prediction. This enhancement means that the signal-to-noise ratio for the ellipticity is only weakly increasing with angle for 2' < θ < 2°, unlike the expectation from linear theory that the signal-to-noise ratio is strongly peaked on degree scales. The scaling with cosmological parameters also changes because of nonlinear effects. By measuring the correlations on small (nonlinear) and large (linear) angular scales, different cosmological parameters can be independently constrained to obtain a model-independent estimate of both power spectrum amplitude and matter density Ωm. Nonlinear effects also modify the probability distribution of the ellipticity. Using second-order perturbation theory, we find that over most of the range of interest there are significant deviations from a normal distribution.

Spontaneously ordered motion of self-propelled particles

Abstract: We study a biologically inspired, inherently non-equilibrium model consisting of self-propelled particles. In the model, particles move on a plane with a velocity of constant magnitude; they locally interact with their neighbours by choosing at each timestep a velocity direction equal to the average direction of their neighbours. Thus, in the limit of vanishing velocities the model becomes analogous to a Monte Carlo realization of the classical XY ferromagnet. We show by large-scale numerical simulations that, unlike in the equilibrium XY model, a long-range ordered phase characterized by non-vanishing net flow, , emerges in this system in a phase-space domain bordered by a critical line along which the fluctuations of the order parameter diverge. The corresponding phase diagram as a function of two parameters, the amplitude of noise and the average density of the particles is calculated and is found to have the form . We also find that scales as a function of the external bias h (field or `wind') according to a power law . In the ordered phase the system shows long-range correlated fluctuations and 1/f noise.

Transverse-Wake Wave Breaking

Abstract: A finite-width laser pulse of high intensity propagating in an underdense plasma excites a transversely inhomogeneous, finite amplitude wakefield. This wake wave undergoes a transverse wave breaking due to the increase of the wake front curvature, followed by the self-intersection of electron trajectories. Transverse break occurs at much lower wave amplitudes than the conventional one-dimensional wave break. The resulting structures have generic forms that can be described by modified curves parallel to a parabola. Simulations with the particle-in-cell electromagnetic relativistic code VLPL2D show such structures appearing. {copyright} {ital 1997} {ital The American Physical Society}

A Nonlinear Model of Internal Tide Transformation on the Australian North West Shelf

Abstract: A numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide. The model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf. The model is run using observed hydrographic conditions from the Australian North West Shelf and results are compared to current meter and thermistor observations from the shelf-break region. It is found from observations that the coefficient of nonlinearity in the K-dV equation changes sign from negative in deep water to positive in shallow water, and this plays a major role in determining the form of the internal tide transformation. On the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide, which distorts the density profile over a wave period. Both the model and observations show the formation of an initial shock on the leading face of the internal tide. In shallow water, the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves. After further propagation a second shock forms on the back face of the wave, followed by a packet of solitons. The inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion. Friction is found to be important in limiting the amplitudes of the evolving waves.

Measuring non-axisymmetry in spiral galaxies

Abstract: We present a method for measuring small deviations from axisymmetry of the potential of a filled gas disc. The method is based on a higher order harmonic expansion of the full velocity field of the disc. This expansion is made by first fitting a tilted-ring model to the velocity field of the gas disc and subsequently expanding the velocity field along each ring into its harmonic terms, We use epicycle theory to derive equations for the harmonic terms in a distorted potential. The phase of each component of the distortion can vary with radius. We show that if the potential has a distortion of harmonic number m, the velocity field as seen on the sky exhibits an m - 1 and m + 1 distortion. As is to be expected, the effects of a global elongation of the halo are similar to an m = 2 spiral arm. The main difference is that the phase of the spiral arm can vary with radius, Our method allows a measurement of epsilon(pot) sin 2 phi(2), where epsilon(pot) is the elongation of the potential and phi(2) is one of the viewing angles. The advantage of this method over previous approaches to measure the elongations of disc galaxies is that, by using H I data, one can probe the potential at radii beyond the stellar disc, into the regime where dark matter is thought to be the dominant dynamical component. The method is applied to the spiral galaxies NGC 2403 and 3198 and the harmonic terms are measured up to ninth order. The residual velocity field of NGC 2403 shows some spiral-like structures. The harmonic analysis indicates that the m = 3 term is dominant, with an average value of similar to 0.02v(c). This is consistent with an average ellipticity of the potential of epsilon(pot) sin 2 phi(2) = 0.064 +/- 0.003, but spiral arms may couple significantly to this result. In the harmonic analysis of the kinematics of NGC 3198 the m = 2 and m = 3 terms are strongest (similar to 0.01v(c)), The inferred average elongation of the potential is 0.019 +/- 0.003. Since the amplitude of the elongation is coupled to the viewing angles and may be influenced by spiral arms, more galaxies should be examined to separate these effects from true elongation in a statistical way.

New Features of Time Domain Electric-Field Structures in the Auroral Acceleration Region

Abstract: The Polar Satellite carries the first three-axis electric field detector flown in the magnetosphere. Its direct measurement of electric field components perpendicular and parallel to the local magnetic field has revealed new classes and features of electric field structures associated with the plasma acceleration that produces discrete auroras and that populates the magnetosphere with plasma of ionospheric origin. These structures, associated with the hydrogen ion cyclotron mode, include very large solitary waves, spiky field structures, wave envelopes of parallel electric fields, and very large amplitude, nonlinear, coherent ion cyclotron waves. {copyright} {ital 1997} {ital The American Physical Society}

Infall regions of galaxy clusters

Abstract: In hierarchical clustering, galaxy clusters accrete mass through the aggregation of smaller systems. Thus, the velocity field of the infall regions of clusters contains significant random motion superposed on radial infall. Because the purely spherical infall model does not predict the amplitude of the velocity field correctly, methods estimating the cosmological density parameter Ω0 based on this model yield unreliable biased results. In fact, the amplitude of the velocity field depends on local dynamics and only very weakly on the global properties of the universe. We use N-body simulations of flat and open universes to show that the amplitude of the velocity field of the infall regions of dark matter halos is a direct measure of the escape velocity within these regions. We can use this amplitude to estimate the mass of dark matter halos within a few megaparsecs from the halo center. In this region dynamical equilibrium assumptions do not hold. The method yields a mass estimate with better than 30% accuracy. If galaxies trace the velocity field of the infall regions of clusters reliably, this method provides a straightforward way to estimate the amount of mass surrounding rich galaxy clusters from redshift data alone.

Active Region Magnetic Fields. I. Plage Fields

Abstract: We present observations taken with the Advanced Stokes Polarimeter (ASP) in active-region plages and study the frequency distribution of the magnetic field strength (B), inclination with respect to vertical (γ), azimuthal orientation (χ), and filling factor (f). The most common values at disk center are B = 1400 G, γ < 10°, no preferred east-west orientation, and f = 15%. At disk center, there is a component of weak (<1000 G), more horizontal fields that corresponds to arching field lines connecting footpoints of different polarities. The center-to-limb variation (CLV) of the field strength shows that, close to the limb (μ = 0.3), the field strength is reduced to 800 G from its disk-center value. This can be interpreted as a gradient of B with height in solar plages of around -3 G km-1. From this CLV study, we also deduce that magnetic field lines remain vertical for the entire range of heights involved. A similar analysis is performed for structures found in active regions that show a continuous distribution of azimuths (resembling sunspots) but that do not have a darkening in continuum. These "azimuth centers" show slightly larger values of B than normal plages, in particular at their magnetic center. Filling factors are also larger on average for these structures. The velocities in the magnetic component of active regions have been studied for both averaged Stokes profiles over the entire active region and for the spatially resolved data. The averaged profiles (more representative of high filling factor regions) do not show any significant mean velocities. However, the spatial average of Doppler velocities derived from the spatially resolved profiles (i.e., unweighted by filling factor) show a net redshift at disk center of 200 m s-1. The spatially resolved velocities show a strong dependence on filling factor. Both mean velocities and standard deviations are reduced when the filling factor increases. This is interpreted as a reduction of the p-mode amplitude within the magnetic component. Strong evidence for velocities transverse to the magnetic field lines has been found. Typical rms values are between 200 and 300 m s-1, depending on the filling factor. The possible importance of these transverse motions for the dynamics of the upper atmospheric layers is discussed. The asymmetries of the Stokes profiles and their CLV have been studied. The averaged Stokes V profiles show amplitude and area asymmetries that are positive at disk center and become negative at the limb. Both asymmetries, and for the two Fe I lines, are maximized away from disk center. The spatially resolved amplitude asymmetries show a clear dependence on filling factor: the larger the filling factor, the smaller the amplitude asymmetry. On the other hand, the area asymmetry is almost independent of the filling factor. The only observed dependence is the existence of negative area-asymmetry profiles at disk center for filling factors smaller than 0.2. Around 20% of the observed points in a given plage have negative area asymmetry. The amplitude asymmetry of Stokes V is, on the other hand, always positive. The amplitude asymmetries of the linear polarization profiles are observed to have the same sign as the Stokes V profiles. Similarly, the same CLV variation of the linear polarization amplitude asymmetries as for Stokes V has been found. The scenarios in which this similarity can exist are studied in some detail.

Infall Regions of Galaxy Clusters

Abstract: In hierarchical clustering, galaxy clusters accrete mass through the aggregation of smaller systems. Thus, the velocity field of the infall regions of clusters contains significant random motion superimposed on radial infall. Because the purely spherical infall model does not predict the amplitude of the velocity field correctly, methods estimating the cosmological density parameter Omega_0 based on this model yield unreliable biased results. In fact, the amplitude of the velocity field depends on local dynamics and only very weakly on the global properties of the universe. We use N-body simulations of flat and open universes to show that the amplitude of the velocity field of the infall regions of dark matter halos is a direct measure of the escape velocity within these regions. We can use this amplitude to estimate the mass of dark matter halos within a few megaparsecs from the halo center. In this region dynamical equilibrium assumptions do not hold. The method yields a mass estimate with better than 30% accuracy. If galaxies trace the velocity field of the infall regions of clusters reliably, this method provides a straightforward way to estimate the amount of mass surrounding rich galaxy clusters from redshift data alone.

ECG coding by wavelet-based linear prediction

Abstract: Presents a novel coding scheme for the electrocardiogram (ECG). Following beat delineation, the periods of the beats are normalized by multirate processing. After amplitude normalization, a discrete wavelet transform is applied to each beat. Due to the period and amplitude normalization, the wavelet transform coefficients bear a high correlation across beats at identical locations. To increase the compression ratio, the residual sequence obtained after linear prediction of the significant wavelet coefficients is transmitted to the decoder. The difference between the actual period and the mean beat period, and that between the actual scale factor and the average amplitude scale factor are also transmitted for each beat. At the decoder, the inverse wavelet transform is computed from the reconstructed wavelet transform coefficients. The original amplitude and period of each beat are then recovered. The approximation achieved, at an average rate of 180 b/s, is of high quality. The authors have evaluated the normalized maximum amplitude error and its position in each cycle, in addition to the normalized root mean square error. The significant feature of the proposed technique is that, while the error is nearly uniform throughout the cycle, the diagnostically crucial QRS region is kept free of maximal reconstruction error.

Flow analysis system and method

Abstract: A non-invasive flow analysis system and method wherein a sensor, such as an acoustic sensor, is coupled to a conduit for transmitting a signal which varies depending on the characteristics of the flow in the conduit. The signal is amplified and there is a filter, responsive to the sensor signal, and tuned to pass a narrow band of frequencies proximate the resonant frequency of the sensor. A demodulator generates an amplitude envelope of the filtered signal and a number of flow indicator quantities are calculated based on variations in amplitude of the amplitude envelope. A neural network, or its equivalent, is then used to determine the flow rate of the flow in the conduit based on the flow indicator quantities.

Direct Measurement of the Spatial Displacement of Bloch-Oscillating Electrons in Semiconductor Superlattices

Abstract: The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.

Saturated-cascade similitude theory of gravity wave spectra

Abstract: A theory is presented, which is based mainly on dimensional analysis (but also on gravity wave theory), that attempts to explain all the types of gravity wave power spectral densities (PSDs) now being measured. This theory is based on two concepts, namely, wave saturation and wave cascade. The immediate result of the simultaneous presence of these two processes is that there should exist a unique relation between the vertical (or horizontal) wavelength of a gravity wave and its period (provided the Brunt Period and dissipation rate are given and Doppler effects are omitted). This relation provides a way to derive all of the intrinsic spectra from the fundamental one which is the vertical wavenumber PSD of the horizontal winds. The most important suggestion to emerge from this theory is that e, the dissipation rate, is the main controlling independent variable for the amplitude of all but 3 of the 12 spectra predicted. It would also control the wavelength-period relations. Comparisons are made between observations and theory, and important experimental tests are proposed. This model presently appears to be useful in the analysis of gravity wave data obtained by means of lidars, radars, interferometers, and imagers. In addition, it raises a number of new scientific issues for future research.

Permittivity measurements of multilayered media with monostatic pulse radar

Abstract: Electromagnetic (EM) inversion is a useful tool for quantitative analysis in short-range applications of pulse radars. To estimate multilayered media properties using monostatic radar, two inverse scattering approaches are discussed: (a) layer-stripping algorithm by exploiting amplitude and time delay of radar echoes after their detection, and (b) EM inverse problem of parameter optimization by minimizing the mean square error between measured and modeled data. Redundancy in the estimation of media properties is given by spatial continuous measurements of the investigated media. This property is exploited in both the approaches investigated. In the layer-stripping approach the medium within each layer is homogeneous and the interfaces are assumed laterally continuous. In the inverse problem permittivity is assumed to be laterally smooth, implicit smoothing being given in the model parameterization. It is implicit in both methods that the inversion accuracy is strictly related to the amplitude stability of the radar and plane wave approximation. Therefore, the system calibration and the compensation of some propagation effects (e.g., near field, losses due to conductivity and to scattering from particles distributed between layers and on interfaces, pulse distortion) become crucial aspects for each specific application.

Cerebrospinal fluid pulsation amplitude and its quantitative relationship to cerebral blood flow pulsations: a phase-contrast MR flow imaging study.

Abstract: Our purpose in this investigation was to explain the heterogeneity in the cerebrospinal fluid (CSF) flow pulsation amplitudes. To this end, we determined the contributions of the cerebral arterial and jugular venous flow pulsations to the amplitude of the CSF pulsation. We examined 21 healthy subjects by cine phase-contrast MRI at the C2-3 disc level to demonstrate the CSF and vascular flows as waveforms. Multiple regression analysis was performed to calculate the contributions of (a) the arterial and venous waveform amplitudes and (b) the delay between the maximum systolic slopes of the arterial and venous waveforms (AV delay), in order to predict the amplitude of the CSF waveform. The contribution of the arterial waveform amplitude was positive (r = 0.61; p = 0.003) to the CSF waveform amplitude and that of the venous waveform amplitude was negative (r = -0.50; p = 0.006). Both in combination accounted for 56% of the variance in predicting the CSF waveform amplitude (p < 0.0006). The contribution of AV delay was not significant. The results show that the variance in the CSF flow pulsation amplitudes can be explained by concurrent evaluation of the CSF and vascular flows. Improvement in the techniques, and controlled experiments, may allow use of CSF flow pulsation amplitudes for clinical applications in the non-invasive assessment of intracranial dynamics by MRI.

The modified Korteweg - de Vries equation in the theory of large - amplitude internal waves

Abstract: The propagation of large- amplitude internal waves in the ocean is studied here for the case when the nonlinear effects are of cubic order, leading to the modified Korteweg - de Vries equation. The coefficients of this equation are calculated analytically for several models of the density stratification. It is shown that the coefficient of the cubic nonlinear term may have either sign (previously only cases of a negative cubic nonlinearity were known). Cubic nonlinear effects are more important for the high modes of the internal waves. The nonlinear evolution of long periodic (sine) waves is simulated for a three-layer model of the density stratification. The sign of the cubic nonlinear term influences the character of the solitary wave generation. It is shown that the solitary waves of both polarities can appear for either sign of the cubic nonlinear term; if it is positive the solitary waves have a zero pedestal, and if it is negative the solitary waves are generated on the crest and the trough of the long wave. The case of a localised impulsive initial disturbance is also simulated. Here, if the cubic nonlinear term is negative, there is no solitary wave generation at large times, but if it is positive solitary waves appear as the asymptotic solution of the nonlinear wave evolution.

Nonlinear distortion of short pulses radiated by plane and focused circular pistons.

Abstract: Detailed measurements of finite-amplitude pulses radiated by plane and focused circular pistons in water are presented. Comparisons of time waveforms and frequency spectra, both on and off axis, are made with numerical calculations based on the nonlinear parabolic wave equation. Emphasis is on nonlinear distortion of amplitude- and frequency-modulated tone bursts. Use of short pulses enabled resolution of the direct and diffracted waves prior to their coalescence and subsequent shock formation along the axis of the source. Because of its relevance to investigations of cavitation inception, attention is devoted to variation of the peak positive (p+) and negative (p−) pressures along the axis of a focused source. It is shown that with increasing source amplitude, the maximum of each shifts away from the focal plane, toward the source. This effect is more pronounced for p− than for p+.

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. III. Probing the Inner Jet through Multiwavelength Correlations

Abstract: In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ~10 d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with ground- based radio, IR, and optical telescopes. The light curves show a well-defined X-ray flare (x2 increase in 1/2 d, with similar decay) followed by a broader, lower amplitude EUV flare ~1 day later, and a broad, low-amplitude UV flare ~2 d later (amplitude ~35%, duration >2 d). In the preceding week there was at least one X-ray flare of comparable amplitude or perhaps ongoing stochastic X-ray variations. An extremely rapid change in UV flux was seen at the beginning of the IUE observation. Assuming the central X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous relativistic jet. We can rule out a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t<

Toward a Unified Theory of Gravity Wave Stability.

Abstract: This paper reveals relationships among linear instabilities of internal gravity waves often supposed to be independent. Using a Floquet analysis of a monochromatic wave propagating in a uniformly stratified background without shear, which accounts for finite wave amplitude, spatial and temporal periodicity, tilted phase planes, and 3D disturbances, it is demonstrated that the dominant instabilities of overturning waves are the large-wave-amplitude manifestations of resonant and slantwise instabilities of small amplitude waves, and that they possess no threshold amplitudes. An energy budget analysis examines the relation of parametric instabilities at large wave amplitude to vertical dynamic and static instabilities; however, the instability characteristics for propagating waves are very different from those inferred by analogy to Kelvin–Helmholtz instability and Benard convection in simpler backgrounds. At small amplitudes, resonant instabilities rely on horizontal or slantwise gradients of wave ...

A model for Brewster angle damping and multipath effects on the microwave radar sea echo at low grazing angles

Abstract: Brewster angle damping and local multipath effects are considered as sources of polarization differences in low grazing angle sea scatter characteristics. The authors show that at least five observed polarization differences can be explained by local multipath interference effects that occur due to the illumination of discrete nonlinear ocean surface features, such as bores and small scale breaking waves. The illumination gain factor (IGF) is defined at a point in space, as the total power at that point relative to the power in the incident plane wave. The IGF resulting from local multipath from the sea surface forward of a discrete scatterer produces strong interference patterns that can vary both with grazing angle and scatterer height. As a result, IGF values up to a factor of 16 (12 dB) can occur for horizontal polarization (HH) when the interference is constructive; a corresponding strong cancellation occurs for destructive interference. These extreme variations can cause strong HH NRCS amplitude modulations due either to a change of local wave slope or a change of scatterer shape with time. However, Brewster angle damping of the forward scatter path for grazing angles below 20/spl deg/ occurs for vertical (VV) polarization, and reduces the VV IGF in magnitude and dynamic range, eliminating such strong modulations. This effect scales with radar wavelength, and higher wave features are required to produce equivalent effects for radar frequencies far below 10 GHz. As an illustration, six radar bands are compared: L (1.4 GHz), S (3.5 GHz), C (5 GHz), X (10 GHz), K/sub u/ (15 GHz), and K/sub a/ (35 GHz), for a sea water dielectric. X-band results indicate that 12-dB IGFs can occur for water surface features just a centimeter above the mean surface. As an application of these results, the influence of these HH and VV IGF patterns is modeled for discrete scatterers distributed uniformly along an ocean gravity wave. The dynamic range of the HH IGF for a distribution of bore scatterers up to 5 cm high is found to he significantly larger than for VV at all locations on the long wave. Moreover, the IGF HHVV polarization ratio forward of the crest, where the largest number of small scale breaking wave scatterers occurs, is larger than at all other regions of the long wave, of the order of 20 dB. These results suggest that HH polarization may be sensitive to small scale breaking features on the ocean surface at low grazing angles, and thus may be a sensitive measure of air-sea fluxes.

Dynamic output feedback compensation for linear systems with independent amplitude and rate saturations

Abstract: The positive real lemma provides the basis for constructing linear output feedback dynamic compensators for multi-input plants with independent amplitude saturations. Fixed-structure techniques are used to obtain full- and reduced-order feedback compensators along with a guaranteed domain of attraction. These results are then applied to the problem of rate saturation. By using a feedback-type model, rate saturation is modelled as an amplitude saturation. The closed-loop system with amplitude and rate saturation is then treated as a system with independent 'amplitude' saturations.