Focused synthetic aperture radar processing of ice-sounder datacollected over the Greenland ice sheet

The authors developed a synthetic aperture radar (SAR) processing algorithm for airborne/spaceborne ice-sounding radar systems and applied it to data collected in Greenland. By using focused SAR (phase-corrected coherent averaging), they improved along-track resolution by a factor of four and provided a 6-dB processing gain over unfocused SAR (coherent averaging without phase correction) based on a point-target analysis for a Greenland ice-sounding data set. Also, They demonstrated that the focused-SAR processing reduced clutter and enabled them to identify bedrock-interface returns buried in clutter. Using focused-SAR technique, they processed data collected over a key 360-km-long portion of the 2000-m contour line of southwest Greenland. To the best of their knowledge, these are the first high-quality radar ice thickness measurements over this key location. Moreover, these ice-thickness measurements have been used for improving mass-balance estimates of the Greenland ice sheet     

Growth of the Greenland ice sheet: a performance assessment ofaltimeter retracking algorithms

The authors compare the performance of different altimeter retracking algorithms for measuring ice sheet elevations and growth rates. The results show that the threshold, ESA, and S/V retracking algorithms produce growth rates that are 30-55% smaller than those produced by the NASA algorithm. Based upon a comparison of crossover-point standard deviations, the analysis indicates that the surface elevation estimates produced by these algorithms are more repeatable than the NASA surface elevations. An analysis of the NASA algorithm shows that a mixing of its 5 and 9 parameter functional fits on the crossover-point altimeter waveforms occurs in over 70% of the crossover data. The mixing of the functional fits is shown to reduce the repeatability of the NASA elevations and this may be responsible for the larger estimates of ice sheet growth produced by the NASA retracking algorithm. The extremely close agreement between the standard deviations and the growth-rate estimates from the threshold, ESA, and S/V retracking algorithms: lead the authors to conclude that 0.10 m/yr is a more accurate estimate of the growth of the Greenland ice sheet from 1978-1987 (south of 72°N)     

Comparison of retracking algorithms using airborne radar and laseraltimeter measurements of the Greenland ice sheet

Compares four continental ice sheet radar altimeter retracking algorithms using airborne radar and laser altimeter data taken over the Greenland ice sheet in 1991. The refurbished Advanced Application Flight Experiment (AAFE) airborne radar altimeter has a large range window and stores the entire return waveform during flight. Once the return waveforms are retracked, or post-processed to obtain the most accurate altitude measurement possible, they are compared with the high-precision Airborne Oceanographic Lidar (AOL) altimeter measurements. The AAFE waveforms show evidence of varying degrees of both surface and volume scattering from different regions of the Greenland ice sheet. The AOL laser altimeter, however, obtains a return only from the surface of the ice sheet. Retracking altimeter waveforms with a surface scattering model results in a good correlation with the laser measurements in the wet and dry-snow zones, but in the percolation region of the ice sheet, the deviation between the two data sets is large due to the effects of subsurface and volume scattering. The Martin et al. model results in a lower bias than the surface scattering model, but still shows an increase in the noise level in the percolation zone. Using an offset center of gravity algorithm to retrack altimeter waveforms results in measurements that are only slightly affected by subsurface and volume scattering and, despite a higher bias, this algorithm works well in all regions of the ice sheet. A cubic spline provides retracked altitudes that agree with AOL measurements over all regions of Greenland. This method is not sensitive to changes in the scattering mechanisms of the ice sheet and it has the lowest noise level and bias of all the retracking methods presented     

Temporal change in the extinction coefficient of snow on theGreenland ice sheet from an analysis of Seasat and Geosat altimeter data

The extinction coefficient of snow k_e along the central portion of the Greenland ice sheet is mapped using data from the Seasat (1978) and Geosat (1985-1989) altimeters. The extinction coefficient is obtained by fitting altimeter waveforms with a surface/volume scattering model. The authors find that in the lower latitudes the Seasat and Geosat extinction coefficients are very nearly the same, while in a specific higher latitude region of the ice sheet the Seasat k_e values exceed the Geosat values by over 100%. By analyzing 18 months of the Geosat data, the author quantified the variability inherent in the extinction coefficient measurements. The results show that the observed temporal variation in the extinction coefficient from 1978 to 1985 is three times larger than the measured variability. This indicates that the average grain size of the near surface snow in this region may have decreased during the time span between the two altimeter datasets. The temporal change in extinction coefficient found in this study demonstrates the important contributions that time-series analysis of satellite datasets can make to the study of the polar ice sheets. In addition, these results have important implications for the study of long-term elevation change over the ice sheets using altimeter data. The author's study demonstrates that significant biases could be introduced into ice-sheet elevation change estimates because of temporal variations in the surface conditions of the ice sheet. Future investigations of ice-sheet mass balance using altimetry data should be aware of this possibility     

Elevation change of the Antarctic ice sheet, 1995-2000, from ERS-2 satellite radar altimetry

We analyzed Antarctic ice-sheet elevation change (dH/dt) from 1995 to 2000 using 123 million elevation change measurements from European Remote Sensing 2 ice-mode satellite radar altimeter data covering an area of about 7.2 million km/sup 2/. Almost all drainage basins in east Antarctica had average dH/dt values within /spl plusmn/3.0 cm/year, whereas drainage basins in west Antarctica had substantial spatial variability with average dH/dt values ranging between -11 to +12 cm/year. The east Antarctic ice sheet had a five-year trend of 1/spl plusmn/0.6 cm/year, where 13 out of the 14 basins had either a positive trend or a trend that was not significantly different than zero. The west Antarctic ice sheet had a five-year trend of -3.6/spl plusmn/1.0 cm/year due largely to strong negative trends of around 10 cm/year for basins in Marie Byrd Land along the Pacific sector of the Antarctic coast. The continent as a whole had a five-year dH/dt trend of 0.4/spl plusmn/0.4 cm/year. Finally, time series constructed for the Pine Island, Thwaites, De Vicq, and Land glaciers in west Antarctic showed five-year dH/dt trends from -26 to -135 cm/year that were significantly more negative than the average dH/dt trends in their respective basins. The strongly negative dH/dt values for these coastal glacier outlets are consistent with recently reported results indicating increased basal melting at these glaciers' grounding lines caused by ocean thermal forcing.     

Adaptation of the sheet resistance of an YBaCuO layer to the substrate impedance,applications

We have studied the Far Infrared transmission spectra of an MgO plate covered with an YBaCuO film of thickness t=200 Å, at different temperatures. By cooling from room temperature to temperatures Θ < 140 K, the Far Infrared interference fringes are displaced by half a period. Explanation in terms of different adaptations of the YBaCuO sheet resistance to the MgO substrate impedance during cooling.     

Frequency dependence of scattering and extinction of dense media based on three-dimensional simulations of Maxwell's equations with applications to snow

The frequency dependence of scattering by geophysical media at microwave frequencies is an important topic because multifrequency measurements are used in remote sensing applications. In this paper, we study rigorously the frequency dependence of scattering by dense media using Monte Carlo simulations of the three-dimensional solutions of Maxwell's equations. The particle positions are generated by deposition and bonding techniques. The extinction, scattering, and absorption properties of dense media are calculated for dense media of sticky and nonsticky particles. Numerical solutions of Maxwell's equations indicate that the frequency dependence of densely packed sticky small particles are much weaker than that of independent scattering. Numerical results are illustrated using parameters of snow in microwave remote sensing. Comparisons are made with extinction measurements as a function of frequency.     

运用测量仪器完成无线电信号自动监测的一种尝试

矢量信号分析仪89610是一个测量分析仪器,它具有多种调制信号的测量功能.但是,它只能被动地测量已知调制信号的参数,而对于一个未知调制方式的信号,它无法测量分析该信号的相关参数.     

Beaufort-Chukchi seas summer and fall ice margin data from Seasat:conditions with similarities to the Labrador Sea

The margin of the sea-ice pack of the Beaufort and Chukchi seas is examined using the microwave data from Seasat taken during the summer to early fall, July 4 through October 8, 1978, and the observations are compared to the analogous observations taken in LIMEX'87. The sensors used are synthetic-aperture radar (SAR), the Seasat-A scatterometer system, and the Scanning Multichannel Microwave Radiometer     

国家战略性新兴产业引领IPV6部署应用

2011全球IPv6高峰会议近日在北京隆重开幕,以"国家战略性新兴产业引领IPv6部署应用:世界与中国"为主题,集结了下一代互联网领域全球一流专家和重要人士,呈现全球范围内最专业、全面的IPv6交流平台。IPv4地址告罄2011年2月3日,ICANN官方宣布:全球最后一批IPv4地址分配完     

Convergence of filters with applications to the Kalman-Bucy case

For each N, and each fixed time T, a signal X^N and a `noisy' observation Y^N are defined by a pair of stochastic difference equations. Under certain conditions (X^N, Y^N) converges in distribution to (X, Y, where dX(t)= f(t, X(t))dt+dV( t), dY(t)=g(t, X( t))dt+dW(t). Conditions are found under which convergence in distribution of the conditional expectations E{F(X^N)|Y^N} to E{F(X)|Y} follows, for every bounded continuous function F. The case in which the conditional expectations still converge but the limit is not E{ F(X)|Y} is also studied. In the situation where f and g are linear functions of X, an examination of this limit leads to a Kalman-Bucy-type estimate of X ^N which is asymptotically optimal and is an improvement on the usual Kalman-Bucy estimate     

On the partial MAP detection with applications to MIMO channels

We investigate a multidimensional detection problem with a partial information of the a posteriori probability, which is referred to as the partial maximum a posteriori probability (MAP) detection problem. We show that the maximum likelihood (ML) detection of a higher dimension can be reduced to the ML detection of a lower dimension with cancellation under a certain condition through the formulation of the partial MAP detection problem. Using this, we can propose a computationally efficient algorithm to apply to the detection problem for multiple input multiple output (MIMO) channels including multiple transmit and multiple receive antenna (MTMR) channels and intersymbol interference (ISI) channels. It is shown that the proposed method has less error propagation effect, and its performance is close to that of the full ML detection with a lower computational complexity.     

New applications of a sinusoidally driven InGaAsP electroabsorptionmodulator to in-line optical gates with ASE noise reduction effect

The authors propose new applications of a sinusoidally driven InGaAsP electroabsorption modulator to an inline optical gate for a 2R (reshape and retiming) repeater in optical amplifier systems, an n :1 optical demultiplexer in time division multiplexing systems, and an optical switch. The small polarization dependence of the modulator is essential for inline use. By utilizing the monotonic increase of the extinction ratio with increasing applied voltage, the electroabsorption modulator driven by a large-signal sinusoidal voltage can produce a time domain square-shaped gate function with variable gate width. Furthermore, amplified spontaneous emission noise of optical amplifier systems can be reduced in both time and wavelength domains at the off-state of the modulator, due to noninterferometric wide wavelength operation of the modulator. Experimental results for a 2R repeater, an n:1 (n=4, 8) optical demultiplexer, and optical gates for switching are also demonstrated at over 10-Gb/s repetition rate     

An investigation into the use of iteration methods for the analysis of axially symmetric and sheet beam electrode shapes with an emitting surface

The techniques for analyzing an axially symmetric or two-dimensional electrode system with an emitting surface have been studied with a computer program. The program is very easy to use and performs in one pass the steps which have usually previously been followed on analog and digital devices separately. The program alternately solves Poisson's equation by a relaxation technique, traces trajectories, recomputes the space-charge distribution and again performs a relaxation process. This procedure continues until satisfactory convergence has been achieved. The ease of use of the computer program has allowed the investigation of different methods of speeding up the convergence. The results are illustrated with two specific examples.     

Convergence and accuracy criteria of iteration methods for the analysis of axially symmetric and sheet beam electrode shapes with an emitting surface

Convergence and accuracy criteria have been determined for computer techniques analyzing an axially symmetric or two-dimensional electrode system with an emitting surface. The convergence criterion has been drawn from analytical studies of the one-dimensional model and verified numerically by the computer program for an analytically designed, axially symmetric Pierce-type electrode system. The accuracy of the computer program is illustrated with two electron gun configurations of known analytical solutions, and comparison of cathode current emission, beam minimum radius, and its location is made with experimental data of a high perveance Pierce-type gun.     

Duality bounds on the cutoff rate with applications to Ricean fading

We propose a technique to derive upper bounds on Gallager's cost-constrained random coding exponent function. Applying this technique to the noncoherent peak-power or average-power limited discrete time memoryless Ricean fading channel, we obtain the high signal-to-noise ratio (SNR) expansion of this channel's cutoff rate. At high SNR, the gap between channel capacity and the cutoff rate approaches a finite limit. This limit is approximately 0.26 nats per channel-use for zero specular component (Rayleigh) fading and approaches 0.39 nats per channel-use for very large values of the specular component. We also compute the asymptotic cutoff rate of a Rayleigh-fading channel when the receiver has access to some partial side information concerning the fading. It is demonstrated that the cutoff rate does not utilize the side information as efficiently as capacity, and that the high SNR gap between the two increases to infinity as the imperfect side information becomes more and more precise.     

Syntheses,characterizations and electrochromic applications of polymers derived from carbazole containing thiophene rings in side chain with electrochemical and FeCl3 methods

1,8-Bis(3,6-di(thiophen-3-yl)-9H-carbazol-9-yl)octane (BDTCO) was synthesized via Ullmann and Suzuki couplings. Additionally, its homopolymer and copolymers with 3,4-ethylenedioxythiophene (EDOT), pyrrole (Py) and thiophene (Th) were synthesized and coated onto an ITO–glass surface via electrochemical oxidative polymerization. The spectroelectrochemical and electrochromic properties of the polymers were also investigated. The switching ability of these polymers was measured as the percent transmittance (T%) at their point of maximum contrast. Moreover, the polymer of BDTCO (P-BDTCO) was successfully synthesized through FeCl₃ oxidative polymerization. The compounds were characterized by FT-IR, NMR and X-ray Photoelectron Spectroscopy (XPS), and their thermal stabilities were determined via TG measurements. The solid state conductivities of the polymeric films coated onto the ITO–glass surface were measured via the four point probe technique using an electrometer. In addition, the surface morphological properties were investigated by Scanning Electron Microscopy (SEM) for different magnifications.     

Simplified Lyapunov matrix equation with applications to the control of distributed-parameter systems

The Lyapunov matrix equation is shown to be of importance in generating the optimal gains for distributed-parameter feedback control systems with controllers based on the values of the state variables at discrete points. It is shown that the Lyapunov matrix equation can be considerably reduced for such systems. In a typical example, the number of unknowns is reduced from 1225 to 49.