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271.
During encounters with comet Halley, the experiment PICCA onboard GIOTTO measured the gas phase organic ion composition of the coma and the experiment PUMA onboard VEGA-1 measured the dust composition. Joining both results, we obtain a consistent picture of the parent organic matter from which dust and gas is produced. One recognizes a complex unsaturated polycondensate, which splits during coma-formation into the more refractory C=C,C-N-containing dust part, and the more volatile C=C,C-O-containing gas part. The responsible exothermal chemical reactions, which are triggered by the sunlight, may play a major role in the dynamics of coma formation.This paper is a shortened and upgraded version of Krueger, F.R., Korth, A., and Kissel, J.: 1989, in S. Chang (ed.) Proc. of the ROSETTA Conf., Milpitas CA, January 1989, submitted. 相似文献
272.
John F. Kerridge 《Space Science Reviews》1991,56(1-2):177-184
Carbon isotope ratios have been measured for CN in the coma of comet Halley and for several CHON particles emitted by Halley. Of these, only the CHON-particle data may be reasonably related to organic matter in the cometary nucleus, but the true range of 13C/12C values in those particles is quite uncertain. The D/H ratio in H2O in the Halley coma resembles that in Titan/Uranus. The next decade should substantially improve our understanding of the distribution of C, H, N, and O isotopes in cometary organics. The isotopic composition of meteoritic organic matter is better understood and can serve as a useful analog for the cometary case. 相似文献
273.
Drumheller D.M. Titlebaum E.L. 《IEEE transactions on aerospace and electronic systems》1991,27(1):2-10
The problem of determining the cross-correlation properties of signals based on algebraically constructed Costas arrays is addressed by examining the discrete cross-correlation of the algebraically constructed Costas arrays for a given construction and dimension. Finding two arrays that minimally correlate implies that the signals based on these arrays also minimally correlate. The properties of finite fields are reviewed, and the major algebraic constructions for Costas arrays are presented, i.e. the Welch construction and the Golomb construction. The discrete cross-correlation properties of the Costas arrays are derived for arrays of the same dimension derived from the same construction. The use of Costas arrays in the signal design problem is discussed, and examples are given to show the cross-correlation of the signals based on the algebraically constructed arrays 相似文献
274.
The proposed KAAD (knowledge-based automated air defense) system demonstrates a man-machine environment for airspace defense systems. When the unknown aircraft is hostile, a threat rating and response methods are generated by the system. It serves as a double-check decision-making system for a war control center. In addition to this application, the KAAD system can also be a useful tool as a training program for the war controller. The capabilities of the system are limited due to the shortage of knowledge resources. It requires communications among war controllers and air fighter pilots to organize a practical knowledge base. It is shown that the KAAD system can be combined with an automated ATC (air traffic control) system to become a practical system for air defense applications 相似文献
275.
Calculation of optimum gain for minimum distortion due to A/D (analog-to-digital) conversion requires the estimation of the input signal strength. To use a common AGC (automatic gain control) for both the I/Q (in-phase and quadrature) signals, it is efficient to estimate the input signal strength using the quantized A/D output from both channels. Assuming a Gaussian input, the relationship between σ of the input of the A/D converter and E (|x |+|y |) and E (max(|x |,|y |)+1/2 min (|x |,|y |)) for t quantized I/Q output x and y is derived. Numerical results obtained using the derived expression and the statistical data obtained through simulation show excellent agreement. It is concluded that, because of its simplicity, the cubic equation obtained by fitting the numerical results should be useful 相似文献
276.
A method for estimating parameters of K-distributed clutter 总被引:1,自引:0,他引:1
A method for estimating the parameters of K -distributed clutter when the available sample size of the data is limited is proposed. In this method, the arithmetic mean and geometric mean of the given data are used to estimate the model parameters. Expressions characterizing the performance of the proposed estimator are presented, along with some simulation results. For spiky clutter, simulations show that parameter estimates obtained from the arithmetic and geometric mean are approximately equal to the numerically evaluated maximum-likelihood (ML) estimates. The method is also used to estimate the parameter of the Weibull density 相似文献
277.
Beamspace ML bearing estimation incorporating low-angle geometry 总被引:1,自引:0,他引:1
A problem in low-angle radar tracking, namely, bearing estimation in the presence of a strong specular multipath component that arrives within the beamwidth of the direct path signal, is studied. Three-dimensional beamspace domain maximum likelihood (3D-BDML) is a computationally simple ML bearing estimation algorithm applicable in this scenario which operates in a 3-D beamspace. A variation of 3D-BDML incorporating the multipath geometry as a priori information is presented. In symmetric 3D-BDML the pointing angle of the center beam is equal to the bisector angle between the direct path ray and the image ray, which may be estimated a priori given only the radar height and the target range. The effect of the inclusion of a priori information on the performance of 3D-BDML is analyzed in terms of the dependence on the relative phase difference between the direct and specular path signals, the sensitivity to error in the bisector angle estimate, and the results of operation when no specular multipath component is present in the data. In addition, computationally simple schemes for coherently incorporating multifrequency data into 3D-BDML are investigated 相似文献
278.
Varghese M. Fuchs A. Mukundan R. 《IEEE transactions on aerospace and electronic systems》1991,27(5):784-796
The authors attempt to integrate two areas in robotics that are maturing rapidly: the control of kinematically redundant rigid robots and feedback linearization. Kinematically redundant robots have several advantages over nonredundant robot arms. The extra degrees of freedom are used to reach around or between obstacles. Unlike nonredundant rigid link robots that do not exhibit zero dynamics, the self motions of kinematically redundant robots can be used for torque optimization or avoiding obstacles. It is shown that induced self motions can be chaotic. The main contribution is to demonstrate chaos in cases of feedback linearization when the decoupling matrix is nonsquare and there are less outputs than inputs 相似文献
279.
An optimum filter for direct A/D conversion 总被引:1,自引:0,他引:1
The author describes a design procedure to find the optimum finite-impulse-response (FIR) filter coefficients when the characteristics of the bandpass filter preceding the analog-to-digital (A/D) converter are known. A simulation of direct A/D conversion performance shows the improvement over the filter coefficients designed to minimize the maximum error over a specified band of frequencies. DC cancellation may be added to the process with very little change in the coefficients of the optimum FIR filter or its performance.<> 相似文献
280.
The author describes a simplified derivation of the representation of the circular error probability (CEP) integral, which is the integral over a disk centered at the origin of a zero mean two-dimensional Gaussian random variable, as a one-dimensional integral. In addition, two series are presented which can be used to compute efficiently the CEP integral. The domain of applicability of the series and methods for acceleration of the convergence of these series are discussed. The integral occurs in the evaluation of communication and radar signals, and in other statistical applications.<> 相似文献