Transfer function

About: Transfer function is a research topic. Over the lifetime, 14362 publications have been published within this topic receiving 214983 citations. The topic is also known as: system function & network function.

Scatterometer data interpretation: Estimation and validation of the transfer function CMOD4

Abstract: In this paper we estimate the 18 coefficients of the CMOD4 σ0-to-wind transfer function using a maximum likelihood estimation (MLE) method in order to improve the prelaunch function. We show that a MLE method has to be used with caution when dealing with a nonlinear relationship or with measurement errors that depend on the measured values. In the transfer function estimation it is crucial to use the components of the wind, rather than wind speed and direction, to use σ0 in logarithmic units rather than physical ones, and to use well-sampled input data. In Stoffelen and Anderson [1997a] we showed that the triplets of measured backscatter are very coherent and, when plotted in a three-dimensional measurement space, they lie on a well-defined conical surface. Here we propose a strategy for validation of a transfer function, the first step of which is to test the ability of a transfer function to fit this conical surface. We derive an objective measure to compute the average fit of the transfer function surface to the distribution of measured σ0 triplets. The transfer function CMOD4, derived in the first part of this paper, is shown to fit the cone surface to within the observed scatter normal to the cone, i.e., within roughly 0.2 dB, equivalent to a root-mean-square wind vector error of ∼0.5 m s−1 The second step in the validation strategy is the verification of retrieved scatterometer winds at each position on the cone surface. Scatterometer winds computed from CMOD4 compare better to the European Centre for Medium-Range Weather Forecasts model winds than real-time conventional surface wind data (ship, buoy, or island reports) with the root-mean-square wind vector difference typically 3.0 m s−1. This surprising result can be explained by the so-called representativeness error. We further show that no significant spatial wind error correlation is present in scatterometer data and therefore conclude that the ERS 1 scatterometer provides winds useful for weather forecasting and climate studies.

Structured uncertainty in control system design

Abstract: This paper reviews control system analysis and synthesis techniques for robust performance with structured uncertainty in the form of multiple unstructured perturbations and parameter variations. The structured singular value, µ, plays a central role. The case where parameter variations are known to be real is considered.

Control oriented system identification: a worst-case/deterministic approach in H/sub infinity /

Abstract: The authors formulate and solve two related control-oriented system identification problems for stable linear shift-invariant distributed parameter plants In each of these problems the assumed a priori information is minimal, consisting only of a lower bound on the relative stability of the plant, an upper bound on a certain gain associated with the plant, and an upper bound on the noise level The first of these problems involves identification of a point sample of the plant frequency response from a noisy, finite, output time series obtained in response to an applied sinusoidal input with frequency corresponding to the frequency point of interest This problem leads naturally to the second problem, which involves identification of the plant transfer function in H/sub infinity / from a finite number of noisy point samples of the plant frequency response Concrete plans for identification algorithms are provided for each of these two problems >

Fast B-spline transforms for continuous image representation and interpolation

Abstract: Efficient algorithms for the continuous representation of a discrete signal in terms of B-splines (direct B-spline transform) and for interpolative signal reconstruction (indirect B-spline transform) with an expansion factor m are described. Expressions for the z-transforms of the sampled B-spline functions are determined and a convolution property of these kernels is established. It is shown that both the direct and indirect spline transforms involve linear operators that are space invariant and are implemented efficiently by linear filtering. Fast computational algorithms based on the recursive implementations of these filters are proposed. A B-spline interpolator can also be characterized in terms of its transfer function and its global impulse response (cardinal spline of order n). The case of the cubic spline is treated in greater detail. The present approach is compared with previous methods that are reexamined from a critical point of view. It is concluded that B-spline interpolation correctly applied does not result in a loss of image resolution and that this type of interpolation can be performed in a very efficient manner. >

Modulation Transfer Function Associated with Image Transmission through Turbulent Media

Abstract: The aim of this paper is to determine exactly the optical transfer function corresponding to the time-averaged image-degrading effects of atmospheric turbulence. First the average transfer function is shown to be related to the spatial coherence function for the light entering the imaging system. Next an exact closed solution is found for the coherence propagation equation. This yields the desired coherence function in terms of the statistics of the random fluctuations of the atmospheric index of refraction. Published meteorological data are analyzed to determine empirical values for the required index statistics. Of particular interest is the variation of turbulent index fluctuations with altitude. Finally, quantitative predictions of image degradation are made and shown to agree with observed data.