Spl Nu Research Articles

A physical model to determine snowfall over land by microwave radiometry

Falling snow is an important component of global precipitation in extratropical regions. This paper describes the methodology and results of physically based retrievals of snow falling over land surfaces. Because microwave brightness temperatures emitted by snow-covered surfaces are highly variable, precipitating snow above such surfaces is difficult to observe using window channels that occur at low frequencies (/spl nu/<100 GHz). Furthermore, at frequencies /spl nu//spl les/37 GHz, sensitivity to liquid hydrometeors is dominant. These problems are mitigated at high frequencies (/spl nu/>100 GHz) where water vapor screens the surface emission, and sensitivity to frozen hydrometeors is significant. However, the scattering effect of snowfall in the atmosphere at those higher frequencies is also impacted by water vapor in the upper atmosphere. The theory of scattering by randomly oriented dry snow particles at high microwave frequencies appears to be better described by regarding snow as a concatenation of "equivalent" ice spheres rather than as a sphere with the effective dielectric constant of an air-ice mixture. An equivalent sphere snow scattering model was validated against high-frequency attenuation measurements. Satellite-based high-frequency observations from an Advanced Microwave Sounding Unit (AMSU-B) instrument during the March 5-6, 2001 New England blizzard were used to retrieve snowfall over land. Vertical distributions of snow, temperature, and relative humidity profiles were derived from the Mesoscale Model (MM5) cloud model. Those data were applied and modified in a radiative transfer model that derived brightness temperatures consistent with the AMSU-B observations. The retrieved snowfall distribution was validated with radar reflectivity measurements obtained from a ground-based radar network.

Conjugate regenerative dividers

We discuss a novel design of a self-starting regenerative divider that permits division by 3, 4, 5, 6... instead of the usual 2. This is accomplished by having the loop oscillate simultaneously at two harmonically related conjugate frequencies, e.g., at /spl nu//4 and 3/spl nu//4. A prototype of the divide-by-four circuit has been constructed for an input frequency of 400 MHz. This divider exhibits very low phase noise, l (1 kHz) = -162 dBc/Hz and l (100 kHz) = -170 dBc/Hz, which is approximately 9 dB lower than that of its constituent parts. Simple modifications of the feedback loop of this circuit enabled it to divide by 3, 5, and 8. Operation at higher division ratios appears feasible under certain conditions.

Limiting Sensitivity of a Differential Absorption Spectrometer With Direct Detection in the&amp;gt;tex&amp;lt;$2nu_3$&amp;gt;/tex&amp;lt;and&amp;gt;tex&amp;lt;$nu_2+2nu_3$&amp;gt;/tex&amp;lt;Vibration Bands

A differential absorption spectrometer, for methane detection, using tunable laser diodes in the 1.33-/spl mu/m and 1.66-/spl mu/m bands, has been studied. The spectral scanning is carried out by current modulation of the laser diode. We analyzed the performance with a multimode Fabry-Perot laser diode in the /spl nu//sub 2/+2/spl nu//sub 3/ band of methane and a monomode laser diode in the 2/spl nu//sub 3/ band. The theoretical results are validated by several experiments. To determine the sensitivity limit of the sensor, we have examined the influence of the noise sources. A sensitivity of 10 ppm/spl middot/m was obtained in the 2/spl nu//sub 3/ band. The main limiting factors are the relative intensity noise of the laser, optical interferences, and quantization noise. We also analyzed the influence of the temperature on the laser diode emission spectra and the methane absorption spectra.

Fast Fourier Transform for Discontinuous Functions

In computational electromagnetics and other areas of computational science and engineering, Fourier transforms of discontinuous functions are often required. We present a fast algorithm for the evaluation of the Fourier transform of piecewise smooth functions with uniformly or nonuniformly sampled data by using a double interpolation procedure combined with the fast Fourier transform (FFT) algorithm. We call this the discontinuous FFT algorithm. For N sample points, the complexity of the algorithm is O(/spl nu/Np+/spl nu/Nlog(N)) where p is the interpolation order and /spl nu/ is the oversampling factor. The method also provides a new nonuniform FFT algorithm for continuous functions. Numerical experiments demonstrate the high efficiency and accuracy of this discontinuous FFT algorithm.

Efficient calculation of the electromagnetic dyadic green~s function in spherical layered media

A numerical methodology is proposed for the evaluation of the electromagnetic fields of an electric or magnetic dipole of arbitrary orientation in spherical stratified media. The proposed methodology is based on the numerical solution of the differential equation in the radial coordinate that is satisfied by each coefficient in the spherical harmonics expansion of the governing Helmholtz equation. More specifically, the finite-difference solution of this equation may be cast in a pole-residue form that allows the analytic evaluation of the spherical harmonics series by means of the Watson transformation. Thus, a closed-form expression is obtained for the electromagnetic fields in terms of a short series of associated Legendre functions P/sup m//sub /spl nu//(cos(/spl theta/)) of integer order m and complex degree /spl nu/. The number of terms in the series is strongly dependent on the angle /spl theta/, and decreases very fast when the point of observation moves away from the source. The method allows for arbitrary variation in the permittivity and permeability profiles in the radial directions.

On the optimal parameter choice for v-support vector machines

We determine the asymptotically optimal choice of the parameter /spl nu/ for classifiers of /spl nu/-support vector machine (/spl nu/-SVM) type which has been introduced by Scholkopf et al. (2000). It turns out that /spl nu/ should be a close upper estimate of twice the optimal Bayes risk provided that the classifier uses a so-called universal kernel such as the Gaussian RBF kernel. Moreover, several experiments show that this result can be used to implement some modified cross validation procedures which improve standard cross validation for /spl nu/-SVMs.

Algebraic properties of space-time block codes in intersymbol interference multiple-access channels

In this paper, we study the multiple-access channel where users employ space-time block codes (STBC). The problem is formulated in the context of an intersymbol interference (ISI) multiple-access channel which occurs for transmission over frequency-selective channels. The algebraic structure of the STBC is utilized to design joint interference suppression, equalization, and decoding schemes. Each of the K users transmits using M/sub t/=2 transmit antennas and a time-reversed STBC suitable for frequency-selective channels. We first show that a diversity order of 2M/sub r/(/spl nu/+1) is achievable at full transmission rate for each user, when we have M/sub r/ receive antennas, channel memory of /spl nu/, and an optimal multiuser maximum-likelihood (ML) decoder is used. Due to the decoding complexity of the ML detector we study the algebraic structure of linear multiuser detectors which utilize the properties of the STBC. We do this both in the transform (D-domain) formulation and when we impose finite block-length constraints (matrix formulation). The receiver is designed to utilize the algebraic structure of the codes in order to preserve the block quaternionic structure of the equivalent channel for each user. We also explore some algebraic properties of D-domain quaternionic matrices and of quaternionic circulant block matrices that arise in this study.

Rate-diversity tradeoff of space-time codes with fixed alphabet and optimal constructions for psk modulation

We show that for any (Q/spl times/M) space-time code S having a fixed, finite signal constellation, there is a tradeoff between the transmission rate R and the transmit diversity gain /spl nu/ achieved by the code. The tradeoff is characterized by R/spl les/Q-/spl nu/+1, where Q is the number of transmit antennas. When either binary phase-shift keying (BPSK) or quaternary phase-shift keying (QPSK) is used as the signal constellation, a systematic construction is presented to achieve the maximum possible rate for every possible value of transmit diversity gain.

The inequalities of quantum information theory

Let /spl rho/ denote the density matrix of a quantum state having n parts 1, ..., n. For I/spl sube/N={1, ..., n}, let /spl rho//sub I/=Tr/sub N/spl bsol/I/(/spl rho/) denote the density matrix of the state comprising those parts i such that i/spl isin/I, and let S(/spl rho//sub I/) denote the von Neumann (1927) entropy of the state /spl rho//sub I/. The collection of /spl nu/=2/sup n/ numbers {S(/spl rho//sub I/)}/sub I/spl sube/N/ may be regarded as a point, called the allocation of entropy for /spl rho/, in the vector space R/sup /spl nu//. Let A/sub n/ denote the set of points in R/sup /spl nu// that are allocations of entropy for n-part quantum states. We show that A~/sub n/~ (the topological closure of A/sub n/) is a closed convex cone in R/sup /spl nu//. This implies that the approximate achievability of a point as an allocation of entropy is determined by the linear inequalities that it satisfies. Lieb and Ruskai (1973) have established a number of inequalities for multipartite quantum states (strong subadditivity and weak monotonicity). We give a finite set of instances of these inequalities that is complete (in the sense that any valid linear inequality for allocations of entropy can be deduced from them by taking positive linear combinations) and independent (in the sense that none of them can be deduced from the others by taking positive linear combinations). Let B/sub n/ denote the polyhedral cone in R/sup /spl nu// determined by these inequalities. We show that A~/sub n/~=B/sub n/ for n/spl les/3. The status of this equality is open for n/spl ges/4. We also consider a symmetric version of this situation, in which S(/spl rho//sub I/) depends on I only through the number i=/spl ne/I of indexes in I and can thus be denoted S(/spl rho//sub i/). In this case, we give for each n a finite complete and independent set of inequalities governing the symmetric allocations of entropy {S(/spl rho//sub i/)}/sub 0/spl les/i/spl les/n/ in R/sup n+1/.

In-flight spectral calibration of the atmospheric infrared sounder

Preflight testing of AIRS determined that the shapes of the detector spectral response functions (SRFs) do not vary under different instrument conditions. This reduces in-flight spectral calibration to the determination of detector spectral centroids. A spectrometer grating model has been developed to calculate detector centroids. Only two parameters of this model need to be determined in orbit. An algorithm is presented for determining these two parameters in orbit by correlating observed upwelling radiance spectra with modeled spectra. The method of selecting spectral regions against which to correlate is detailed. The in-orbit spectral calibration algorithm was tested on one day of simulated global AIRS radiances, showing that the uncertainty in the frequencies of the SRF centroids is between 0.006 /spl Delta//spl nu/ and 0.01 /spl Delta//spl nu/, compared to the spectral calibration requirement of 0.01 /spl Delta//spl nu/, where /spl Delta//spl nu/ is the SRF full width at half maximum. The simulation also indicates that the stability of the spectral calibration can be monitored at the 0.001-/spl Delta//spl nu/ level on a daily basis.

Prelaunch spectral calibration of the atmospheric infrared sounder (AIRS)

The Atmospheric Infrared Sounder (AIRS) is a high-resolution infrared sounder launched aboard the National Aeronautics and Space Administration's Aqua satellite on May 4, 2002. AIRS is a grating spectrometer with 2378 channels located between 15 and 3.8 /spl mu/m, with nominal resolving powers of /spl nu///spl Delta//spl nu/=1200. As the first of a new generation of upcoming infrared instruments with similar spectral coverage and resolution, there will be much interest in the performance of AIRS. The ability to retrieve good atmospheric profiles from AIRS observations will depend in part upon our knowledge of the spectral response of AIRS to the upwelling radiance. This paper discusses the spectral calibration of AIRS based upon an extensive set of laboratory test data generated by the instruments prime contractor, BAE. In particular, we describe the calibration of the AIRS spectral response functions, showing that our requirement for accuracies of "1% of a width" have been achieved.

Image-charge effects on the envelope dynamics of an unbunched intense charged-particle beam

The root-mean-squared (rms) envelope equations are derived and analyzed for an unbunched intense charged-particle beam in an alternating-gradient focusing field and a cylindrical conducting pipe. All higher-order image-charge effects from the cylindrical pipe are expressed in terms of so-called multiple moment factors in the rms beam envelope equations, and the multiple moment factors are evaluated. Numerical results show that for vacuum phase advance /spl sigma//sub /spl nu// < 90/spl deg/, the image-charge effects on the matched and slightly mismatched beam envelopes are negligibly small, at all orders, for all beams with arbitrary beam density profiles (including hollow density profiles) as well as for arbitrary small apertures (including beams with large aspect ratios). However, the main unstable region for the envelope evolution with image-charge effects, which occurs for 90/spl deg/ < /spl sigma//sub /spl nu// < 270/spl deg/, depending on the value of the normalized beam intensity SK / /spl epsiv/, is found to be narrower than its counterpart without image-charge effects.

Cycle-time properties of the timed token medium access control protocol

We investigate timing properties of the timed token protocol that are necessary to guarantee synchronous message deadlines. A tighter upper bound on the elapse time between the token's lth arrival at any node i and its (l + /spl nu/)th arrival at any node k is found. A formal proof of this generalized bound is presented.

Analysis of switching in uniformly magnetized bodies

Summary form only given. When defining standard problems for verifying micromagnetic calculations, one important limiting case is uniform magnetization. Exchange energy vanishes and the total energy can be simplified to the expression, E=(/spl mu//sub 0//2)M/sup T/DM-/spl mu//sub 0/H/sup T/M, where H=Hh is the applied field and by choice of coordinates the diagonal matrix D=diag(D/sub x/,D/sub y/,D/sub z/) accounts for self-magnetostatic and uniaxial anisotropy energy. This is the 3D analog to the familiar 2D Stoner-Wohlfarth model. Lagrange multiplier analysis defines M/sub /spl nu//= h/sub /spl nu///(D/sub /spl nu//-/spl lambda/) for /spl nu/= x, y, z and the constraint |M|= M/sub s/ leads to an equation to be solved for the n stationary points of total energy. Depending on the value of H, n may vary between 2 and 6. With the solutions /spl lambda//sub 1/ to /spl lambda//sub n/, in increasing order, our complete analysis shows that /spl lambda//sub 1/ corresponds to the global energy minimum and /spl lambda//sub n/ the global energy maximum. Examination of the solutions leads to an iterative calculation for the critical switching fields and a closed form calculation for the coercive field when it differs from the switching field.

Extended dominating-set-based routing in ad hoc wireless networks with unidirectional links

We extend dominating-set-based routing to networks with unidirectional links. Specifically, an efficient localized algorithm for determining a dominating and absorbant set of vertices (mobile hosts) is given and this set can be easily updated when the network topology changes dynamically. A host /spl nu/ is called a dominating neighbor (absorbant neighbor) of another host u if there is a directed edge from /spl nu/ to u (from u to /spl nu/). A subset of vertices is dominating and absorbant if every vertex not in the subset has one dominating neighbor and one absorbant neighbor in the subset. The derived dominating and absorbant set exhibits good locality properties; that is, the change of a node status (dominating/dominated) affects only the status of nodes in the neighborhood. The notion of dominating and absorbant set can also be applied iteratively on the dominating and absorbant set itself, forming a hierarchy of dominating and absorbant sets. The effectiveness of our approach is confirmed and the locality of node status update is verified through simulation.

A formal analysis of stopping criteria of decomposition methods for support vector machines

In a previous paper, the author (2001) proved the convergence of a commonly used decomposition method for support vector machines (SVMs). However, there is no theoretical justification about its stopping criterion, which is based on the gap of the violation of the optimality condition. It is essential to have the gap asymptotically approach zero, so we are sure that existing implementations stop in a finite number of iterations after reaching a specified tolerance. Here, we prove this result and illustrate it by two extensions: /spl nu/-SVM and a multiclass SVM by Crammer and Singer (2001). A further result shows that, in final iterations of the decomposition method, only a particular set of variables are still being modified. This supports the use of the shrinking and caching techniques in some existing implementations. Finally, we prove the asymptotic convergence of a decomposition method for this multiclass SVM. Discussions on the difference between this convergence proof and the one in another paper by Lin are also included.

Mechanical properties of soft, electrodeposited Fe-Co-Ni films for magnetic recording heads

Electrodeposited Fe-Co-Ni alloys are promising candidates for the writer in ultrahigh-density magnetic recording systems. In this paper, the mechanical properties of these materials have been studied by stress measurements as a function of temperature and nanoindentation. The alloy films (/spl sim/1-/spl mu/m-thick) were electrodeposited onto Cu/Ti/Si substrates. Three distinct structural regions were observed: FCC, mixed FCC+BCC, and BCC phases as Fe content in the alloy increased. The internal stress of Fe-Co-Ni films increases with decreasing current density, as reported for Ni-Fe films. The hardness of the as-deposited films was around 4.8 GPa; after annealing to 400/spl deg/C hardness was reduced to 3.6 GPa. The coefficient of thermal expansion CTE was /spl sim/10-11 /spl times/ 10-6/spl deg/C/sup -1/, comparable to typical values of bulk Ni/sub 78/Fe/sub 22/ (CTE -12 /spl times/ 10/sup -1//spl deg/C/sup -1/). Reduced Young's modulus E/(1 - /spl nu//sup 2/) changes from 150 to 180 Gpa, corresponding to the structural change from BCC to FCC. The ratio H/E is smaller than other materials investigated for write heads, indicating a lower resistance to deformation. H/E further decreases after annealing.

Optimal duct bias for transport of cathodic-arc plasmas

Carbon and titanium plasmas are used to investigate the effects of duct bias on the plasma transport through the magnetic duct of a cathodic-arc plasma source as a function of the magnetic-field strength and arc current so as to determine the optimal duct bias, at which the magnetic duct produces the maximum efficiency for plasma transport. The influence of the guiding magnetic field and arc current on the optimal duct bias is investigated. The optimal duct bias increases with the plasma density for carbon plasma, while the relationship is the opposite for the titanium plasma. The carbon-plasma behavior can be explained by a plasma-diffusion model presented in this paper, since the electron-ion collision frequency /spl nu//sub ei/ is less than the electron-cyclotron frequency /spl nu//sub c,e/. On the other hand, in a titanium plasma, /spl nu//sub ei/ is larger than /spl nu//sub c,e/, so this model is inaccurate. Our result shows that different kinds of plasmas have different transport behavior through the magnetic duct and thus, the duct parameters must be carefully chosen in order to achieve the optimal transport efficiency.

Scale-invariant nonlinear digital filters

Many signal processing applications involve procedures with simple, known dependences on positive rescalings of the input data; examples include correlation and spectral analysis, quadratic time-frequency distributions, and coherence analysis. Often, system performance can be improved with pre- and/or post-processing procedures, and one of the advantages of linear procedures (e.g., smoothing and sharpening filters) is their scale-invariance (x/sub k//spl rarr/y/sub k/ implies /spl lambda/x/sub k//spl rarr//spl lambda/y/sub k/). There are, however, important cases where linear processing is inadequate, motivating interest in nonlinear digital filters. This paper considers the general problem of designing nonlinear filters that exhibit the following scaling behavior: x/sub k//spl rarr/y/sub k/ implies /spl lambda/x/sub k//spl rarr//spl lambda//sup /spl nu//y/sub k/ for some /spl nu/>0, with particular emphasis on the case v=1. Results are presented for two general design approaches. The first is the top-down design of these filters, in which a relatively weak structural constraint is imposed (e.g., membership in the nonlinear FIR class), and a complete characterization is sought for all filters satisfying the scaling criterion for some fixed /spl nu/. The second approach is the bottom-up design of filters satisfying specified scaling behavior by interconnecting simpler filter structures with known scaling behavior. Examples are presented to illustrate both the simplicity and the utility of these design approaches.

Modal characteristic equation and dispersion curves for an elliptical step-index fiber with a conducting helical winding on the core-cladding boundary - an analytical study

A special type of elliptical step-index fiber with a conducting helical winding on the core-cladding boundary is proposed and investigated theoretically. Using elliptic cylindrical coordinates, boundary conditions are derived and longitudinal field components for the even and odd modes are obtained. The characteristic equation for the waveguide to be studied is determined by solving the fourth-order determinantal equation, which also contains complex quantities. In order to study the fundamental mode, the modal index /spl nu/ is put as /spl nu/=1. The equations obtained involve modified Mathieu functions of the first order and their derivatives. Applying approximations, cutoff conditions are obtained. Numerical computations are made. Dispersion curves are obtained for different pitch angles of the helical winding.