Time Filtering Research Articles

The Average Behavior of Large-Scale Westward Traveling Disturbances Evident in the Northern Hemisphere Geopotential Heights

Abstract The average structure of westward traveling disturbances that contribute to relative maxima found in space-time spectra from 13–32 days at northern latitudes is determined for each season. A compositing method used employs a minimum of space and time filtering in order to avoid biasing the results. The average latitudinal structure is “global” in that it is discernible in the Southern Hemisphere during December–February and September–November. It is primarily confined to northern latitudes during March–August. In all seasons the disturbance is out of phase between northern high latitudes and subtropical and tropical latitudes. The longitudinal structure is primarily zonal wavenumber 1 in all seasons. Further work is suggested to confirm the structures determined here and to learn if they reflect the superposition of a number of occasionally excited Rossby normal modes.

The seasonal evolution of planetary waves in the southern hemisphere stratosphere and troposphere

AbstractSeasonality in planetary wave structure in the southern hemisphere stratosphere and troposphere (1000‐1 mb) is studied based on eight years (1979–1986) of daily geopotential data. Emphasis is placed on the distinctive seasonality in the stratosphere, along with tracing the stratospheric fluctuations into the troposphere. Time filtering is used to separate stationary and transient components. Maximum geopotential height variance is observed in the middle to upper stratosphere in late winter‐spring, with a distinct secondary, smaller maximum in late fall‐early winter; these maxima sandwich the strongest mid‐winter zonal winds. This sequence occurs approximately one month earlier in the upper stratosphere. Two corresponding peaks are observed in stratospheric zonal mean wind and temperature fluctuations. The two maxima in stratospheric wave variance are not in agreement with observed seasonal changes in quasi‐geostrophic refractive index of the zonal mean flow.The stratospheric geopotential variance maxima result from nearly equal amounts of stationary and transient zonal wave‐1 fluctuations, with transient wave 2 also contributing during late winter. Stationary wave 1 shows intriguing seasonal evolution: the early winter maximum is predominantly equivalent barotropic, whereas that of late winter‐spring is highly baroclinic in the stratosphere. Stratospheric poleward fluxes of heat and momentum both exhibit pronounced maxima during September‐October; these result primarily from the stationary wave‐1 contributions, and the transients assume a secondary role. Although not as pronounced as the stationary wave seasonality, seasonal differences in transient wave behaviour are also documented, including three‐dimensional propagation characteristics and wave‐mean flow interaction diagnostics.

An Array Study of Ground Tremors and its Application to Exploration

Micro ground tremors were observed by an array of multiple 3-D geophones of 1 Hz with a short spacing along a traverse in a volcanotectonic zone of northeastern Japan. Instead of line spectral methods by single observation point for earthquake engineering purposes, a continuous spectrum was processed by cross correlation and 2-D filtering in time and then in the frequency domain. In addition to the power spectral profiling, 2-D representation of ground motion vectors assists in understanding a realistic structure. Frequency to depth conversion was made by using the law of a quarter wavelength in assumption of velocity distribution of S- or P-waves. Advanced discussion is proposed on this point considering dispersion of surface waves dominant in the tremors, especially of the longer period. As an example of interpretation of the results, geomagnetic intensity was correlated in detail, and the volcanotectonism was seen as double caldera walls (post Miocene).

The distribution of the amplitude and continuous phase of a sinusoid in noise

A derivation of the joint distribution of the amplitude and angle of a sinusoid in Gaussian noise is given. No assumptions about the structure of the noise at the output of the post-detection filter are required. However, it is assumed that the Gaussian noise at the input is generated by passing white noise through the bandpass equivalent of a single-pole low-pass filter. Hence, the noise is a two-dimensional Ornstein-Uhlenbeck or Gauss-Markov process. In practice, a higher-order bandpass filter would be encountered in FM reception. However, the approach is intended as a first step towards the goal of an understanding of the phase process. The method used is to derive and solve the Fokker-Planck partial differential equation that governs the joint distribution. An explicit integral formula for the solution is obtained. The integral is, in general, rather difficult to evaluate. A further integration over amplitude is required if the angle distribution rather than the joint distribution is required. For some special-cases, where the filtering time is large and the signal-to-noise power ratio is very large or very small, explicit approximate expressions are given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of 3 dimensional space and time filtering on ECG gated 201Tl myocardial images.

Stress 201Tl myocardial images were obtained using a standard ECG gated acquisition protocol for 10 min per view. These images were filtered using a three dimensional space and time filter. The effects of filtering were evaluated by sectorial analysis. Comparing raw and filtered dynamic images, quantification demonstrated that filtering did not produce artefacts. A conventional static image was also obtained as the sum of the series of unfiltered dynamic images. Comparison of raw static and filtered diastolic images indicated that the latter demonstrated myocardial abnormalities more clearly (P less than 0.01) in 33 patients. Improvement in the signal to noise ratio produced by filtering made cinematic display and visual analysis possible which was not feasible with raw gated images. Clinical evaluation is in progress.

Optimal nearly singular estimation of continuous linear stationary uniform rank systems

The problem of optimal estimation of linear continuous systems is considered for the case where all the noise signals are stationary white and the intensity of the measurement noise tends uniformly to zero. Explicit expressions for the estimation procedures and the corresponding minimum error covariance matrices of the estimate are obtained in terms of the measurement noise intensity and the zero properties of the system. These expressions are derived for the case where the first non-zero Markov parameter of the system is of full rank, for the infinite and the finite time filtering problems. They provide a simple geometric interpretation to the structure of the error covariance matrices and they distinctly show the difference between the estimation of minimum-and non-minimum-phase systems. The theory is developed both for square and non-square systems.

Signal shaping and tail cancellation for gas proportional detectors at high counting rates

A low noise, wide bandwidth preamplifier and signal processing filter were developed for high counting rate proportional counters. The filter consists of a seven pole Gaussian integrator with symmetrical weighting function and continuously variable shaping time, τ s , of 8–50 ns (fwhm) preceded by a second order pole zero circuit which cancels the long (1/ t) tails of the chamber signals. The preamplifier is an optimized common base input design with 2 ns rise time and an equivalent noise input charge /lt 2000 r.m.s. electrons, when connected to a chamber with 10 pF capacitance and at a filtering time, τ s , of 10 ns.

Numerical procedure for determining the dynamic conductance of an electronic device

A numerical procedure for differentiating current/voltage data of electronic devices is discussed within the framework of an example that uses a Pb–PbO–Pb superconducting tunnel junction. The derivative is obtained from measured values of current and voltage by a combination of numerical integration and digital filtering. Advantages are that the data can be acquired rapidly and the filtering time constant can be adjusted during the analysis, which is not performed in real time.

Signal Shaping and Tail Cancellation for Gas Proportional Detectors at High Counting Rates

A low noise, wide bandwidth preamplifier and signal processing filter were developed for high counting rate proportional counters. The filter consists of an eight pole Gaussian integrator with symmetrical weighting function and continuously variable shaping time, ?s, of 8 ns to 50 ns (FWHM) preceded by a second order pole/zero circuit which cancels the long (1/t) tails of the chamber signals. The preamplifier is an optimized common base input design with 2 nsec rise time and an equivalent noise input charge of < 2000 rms electrons, when connected to a chamber with 10 pF capacitance and at a filtering time, ?s, of 10 ns.

Adaptive Modal Control of Large Flexible Spacecraft

A recently developed strategy for adaptive sampled-data control of distributed parameter systems based on a plant modal expansion description and modal simultaneous identification and regulation algorithms is presented with frequent reference to the annular momentum control device (AMCD) test example. The requirements of observation spillover reduction and modal eigenvector shape prespecification, which are especially crucial to the proposed adaptive control strategy, are addressed. Individual low pass time filtering of sensed AMCD particle displacements is proposed for observation spillover reduction. A layered scheme incorporating 'eigenvector' shape improvement is outlined to combat the expansion basis prespecification requirement.

Solution of the Kalman filtering problem for stationary noise and finite data records

A solution is obtained in the s-domain to the finite-time optimal filtering problem where the system is constant and the noise is stationary. This enables the time-varying Kalman filter gain matrix to be calculated using transform techniques. In the solution of this problem, to find the optimal time-varying linear filter, a new fixed point filtering problem is posed and solved. That is, the optimal time-invariant filter is found which will give the best state estimate after some fixed time interval T. This state estimate is the same as would be obtained from the equivalent Kalman filter at this time. It follows that this time-invariant filter can give a better state estimate than the Wiener filter, when the filtering time is finite Sub-optimal versions of both the time-invariant filter and the Kalman filter are defined from a simple approximation which can be made in the s-domain solution. This considerably simplifies the calculation of the gain matrix for the continuous-time Kalman filter. There are important advantages over the Kalman filter in the implementation of both the time-invariant filter and the sub-optimal filters.

高周波ノイズを効果的にダンプさせる時間積分スキーム

By combining the ideas of the iterative time integration scheme and the time filtering, a time integration scheme for a primitive equations model is formulated. This scheme turns out to be identical with the generalized two time level iterative time integration scheme. However, it is shown that a weight parameter used at the corrector step of time integration is permissible to exceed unity. It is also shown that a weight parameter larger than unity has the character to damp high-frequency noises more efficiently than that less than unity.Along the Kurihara and Tripoli's idea, terms in the primitive equations are divided into two parts, that is, terms which contribute to the relatively slow temporal variation and those which yield the high-frequency noises. By adapting a weight parameter less than unity to the former terms and one larger than unity to the latter ones, it is shown that the amplitude of low-frequency meteorological waves are preserved fairly well while the high-frequency noises are damped effectively.Numerical examples corresponding to the differences of a weight parameter are presented and the results are compared with each other. The results of application of this scheme to the actual numerical prediction model are also presented.

Operations on ultrashort pulses using “flat” lenses

A scheme for passive discrimination of picosecond pulses from longer ones by means of a “flat” lens is described. The system will preferentially transmit long pulses for cw light. The discrimination factors available are calculated for various apertures and for single or multiple lenses. The procedure can be used in studying the time behavior of mode-locked lasers, rapid fluorescence, etc. The system can also be used for time filtering against short or long pulses, where the associated time smearing can be removed if necessary. Less immediate applications involve the time shaping of pulses and the design of a wholly passive “streak camera” for the ir region.

Factors affecting DTPA‐extractable Zn, Fe, Mn, and Cu from soils

Abstract Tests were made to determine the effects of grinding, type of extraction vessel, type of shaker, speed of shaking, time of shaking, time of filtering, soil to solution ratio and other variables on DTPA‐extractable Zn, Fe, Mn, and Cu from soils. Time of grinding, force of grinding, and the quantity of soil being ground greatly affected the amount of extractable Fe. At the lower grinding force, the quantity of soil being ground only slightly affected extractable Fe, but at the higher grinding force, more Fe was extracted from the smaller sized samples especially at the longer grinding period. Extractable Zn was also increased by longer grinding time and greater grinding force, but increases were much less than increases for Fe. Increasing grinding time tended to increase extractable Mn. The effects of grinding on Cu was inconclusive. Increasing the ratio of extractant to soil increased the amount of extractable Fe from soils and tended to increase Zn, Mn, and Cu but to a lesser extent. Both shaker ...

High-speed Bode- and Nichols-diagram plotter

A Bode- or Nichols-diagram plotter with a new type of rectifying and filtering system is proposed. From the performance point of view, this system can be compared with a standard-type plotter operating with a ‘self-adapting’ time constant. The advantage of the design is that a high-speed detection is no longer dependent on the filtering time constant. A quadratic sweeping waveform optimises the plotter display time.

On the design and use of digital filters in certain experimental situations

The difficulty of defining a digital filter whose frequency characteristics match those of a desired continuous filter is discussed theoretically. The 'impulse invariant' filter design is used to derive a formula for calculating the digital filter parameters. Important aspects of the actual use of digital filters are analysed with special reference to the single section RC, three pole Butterworth and three pole Bessel filters. It is shown that reverse time filtering of signals which start discontinuously but decay slowly to zero enables output signal discontinuities to be avoided. Such filtering enhances the slow decay components of multicomponent exponential decays but leaves the exponential nature of the data unchanged. The optimum reverse time filter for exponential signals is an RC filter whose time constant equals that of the decay. The use of digital filters whose bandwidth echoes changes in the signal information bandwidth are examined with special reference to multicomponent exponential decays.

A Quantitative Comparison of Pigment Extraction by Membrane and Glass‐Fiber Filters

Absorbances of acetone extracts from lake phytoplankton collected on glass‐fiber filters, ground and extracted 2 hr, were 6–18% greater than from phytoplankton collected on membrane filters and processed identically. Use of glass‐fiber filters cut filtering time by a factor of 10 and material cost by a factor of 4 or more.

Digital filters with equiripple or minimax responses

Techniques for determining the coefficients of digital filters which have equiripple or minimax errors are reviewed and occasionally extended. These techniques include: 1) mapping to provide equiripple errors in recursive filters; 2) windows for making Fourier spectrum measurements with minimax leakage; 3) the simplex method of linear programming to provide minimax errors in a nonrecursive filter's time response to a known pulse or Fourier transform of its coefficients; 4) nonlinear programming to provide minimax errors for nominally any response and filter; and 5) an integer programming technique to provide minimax error despite quantizing the coefficients of a nonrecursive filter. Some sources of computer programs embodying these techniques are indicated.

Investigations of Charge States inCo57-Doped Oxides Using the Mössbauer-Effect and Delayed-Coincidence Techniques

The time dependence of ${\mathrm{Fe}}^{57}$ chemical charge states in sources of ${\mathrm{Cu}}_{2}$O, MgO, and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ doped with ${\mathrm{Co}}^{57}$ has been studied by means of the M\"ossbauer effect. Charge states of ${\mathrm{Fe}}^{1+}$, ${\mathrm{Fe}}^{2+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Fe}}^{4+}$ resulting from the $K$-capture decay of the ${\mathrm{Co}}^{57}$ have been detected. Delayed-coincidence measurements showed no time dependence of the relative peak intensities of these charge states. A time-dependent narrowing of the resonance lines is observed which is not ascribable to "time filtering" alone. The isomer shifts of the different valence states are discussed in terms of $3d$-electron shielding.

Coincidence-Mössbauer Spectroscopy

Coincidence Mossbauer spectroscopy (CMS) is a technique which merges the methods of the Mossbauer effect and γ-γ angular correlation. The ultimate goal is to obtain solid-state information which would otherwise be unavailable. The usual procedure is to observe two γ-rays in delayed coincidence. The first γ-ray signals the formation of the Mossbauer state, the second γ-ray signals its decay. The Mossbauer absorption spectrum using a particular delay time T is, in the simplest cases, characteristic of T. This is called “time filtering.” Experimental and computational techniques used in CMS are presented. Experimental data are shown that can be explained in terms of time filtering. Additional data indicate the improved resolution and increased percentage effect obtained using CMS. Numerical results are given so that one can estimate the necessary T needed to obtain a particular linewidth. Difficulties associated with interpreting multiline spectra are pointed out. A brief discussion of possible uses of CMS is also given.