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891.
Wahl D.E. Eichel P.H. Ghiglia D.C. Jakowatz C.V. Jr. 《IEEE transactions on aerospace and electronic systems》1994,30(3):827-835
The phase gradient autofocus (PGA) technique for phase error correction of spotlight mode synthetic aperture radar (SAR) imagery is examined carefully in the context of four fundamental signal processing steps that constitute the algorithm. We demonstrate that excellent results over a wide variety of scene content, and phase error function structure are obtained if and only if all of these steps are included in the processing. Finally, we show that the computational demands of the fun PGA algorithm do not represent a large fraction of the total image formation problem, when mid to large size images are involved 相似文献
892.
A computational algorithm to approximate an n-dimensional function of the form F(x,y,z, ...) by an algebraic polynomial in the least square (LSQ) sense is described. The basis of this algorithm is that the n-dimensional function can be approximated by a set of polynomials in each one of the independent variables x,y,z, ... . This technique is illustrated by deriving the algebraic polynomial approximation in the LSQ sense for three-dimensional functions of the form F(x,y,z). 相似文献
893.
C. Potard P. Dusserre 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):105-108
Contactless positioning, manipulation and shaping of liquids using the gas bearing principle is demonstrated by ground experiments. Extensions to microgravity applications are presented also associated with solidification of molten materials. 相似文献
894.
The detection search problem, one of distributing limited resources so as to maximize the detection probability in the single-try search for a concealed target with known probability density, is analyzed. Under fairly general assumptions, the optimal search density uniquely exists when the detection index is governed by the law of diminishing returns and another simple regularity condition. Numerical procedure based on the bisection method, which is guaranteed to converge if the solution uniquely exists, may be used to solve for the optimal search density and the associated Lagrange multiplier. When it is not possible to confidently estimate the target a priori probability density, the minimax solution guarantees a positive detection probability at the expense of degradation in performance 相似文献
895.
P.N. Bhat S.K. Gupta P.V.Ramana Murthy B.V. Sreekantan S.C. Tonwar P.R. Vishwanath 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,3(10-12)
In a further study of sporadic emission from pulsars we find evidence for short lived intense emission from the Crab pulsar. 相似文献
896.
H. Wiedemeier R.C. Whiteside 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):1
Previous chemical vapor transport experiments of the GeSe-GeI4 system performed under reduced gravity conditions /1/ yielded crystals of considerably improved surface and bulk morphology. In addition, the mass transport rates observed in microgravity environment were significantly greater than predicted. A quantitative thermodynamic analysis of the solid-gas phase reactions of the GeSe-GeI4 system revealed the multi-component, multi-reaction nature of the vapor phase /2/. Continued transport studies on ground of the GeSe-GeI4 system in the presence of inert gases provided experimental evidence for the existence of a boundary layer /2/ and its thickness dependence on GeI4 pressure in closed tube systems. Systematic transport rate measurements for different orientations of the density gradient relative to the gravity vector demonstrated the effects of ampoule inclination on mass flux /3/. Based on a computational model for simultaneous chemical vapor transport, sublimation, and Stefan flow /3/, the excellent agreement of predicted with ground-based experimental mass transport rates over wide pressure ranges /3/ confirmed the validity of the model and the discrepancy between observed and expected transport rates of the GeSe-GeI4 system in microgravity. 相似文献
897.
E. C. Pavlis 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,16(12):55-58
The last two Global Positioning System (GPS) spacecraft launched on August 30, 1993 and March 10, 1994 carried identical laser retroreflector arrays that allow laser ranging from ground stations. Tracking of these targets is a low priority for most of the ground stations due to the present mandatory support of missions with no alternative tracking. The available data is sparse and concentrated primarily on the first of the two spacecraft, GPS-35 (PRN 5). Despite this, analysis of this data set indicates that there is great potential for engineering and scientific experiments in a synergistic application of Satellite Laser Ranging (SLR) and radiometric GPS techniques. 相似文献
898.
A major factor that will drive the definition and design of future avionics systems is affordability. Affordability is being addressed on numerous fronts such as hardware re-use, software re-use, COTS leveraging, and reduced cycle times. Each of these thrusts provide potential cost saving along with unique challenges. What is needed is a process that integrates these initiatives while ensuring the overall system objectives are achieved. An Open Systems-based process is key to integrating these initiatives and balancing affordability and system performance goals. Although Open Systems are being widely recognized as a key to affordability, most interpret Open Systems as a set of system attributes that need to be achieved. There are numerous claims of vendors saying they have an Open Systems architecture; or how their system will evolve to an Open Systems architecture. This emphasis is not on increasing affordability, but on attaching a politically-correct label to their product. In this paper, we focus on the Open Systems process as the key to affordability. An Open Systems process is based on Open Systems principles. This paper discusses the Open Systems principles and process in detail and shows how this process integrates numerous affordability initiatives 相似文献
899.
This paper reports on EXOSAT observations of four bright QSO's and active galaxies. The QSO PHL 1657 has a simple spectrum that can be fit with power law plus some low energy absorption. A soft excess is observed for both MR 2251 and E 1352.24+1830. Finally, PKS 2126-158 was detected and the results are consistent with a 0.7 energy index. 相似文献
900.
This paper analyzes a buck/boost regulator employing current-injected control (CIC). It reveals the complex interactions between the dc loop and the current-injected loop and underlines the fundamental principle that governs the loop gain determination. Three commonly used compensation techniques are compared. The integral and lead/lag compensation are shown to be most desirable for performance optimization and stability. 相似文献