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901.
Wennstrom M. Oberg T. Rydberg A. 《IEEE transactions on aerospace and electronic systems》2001,37(2):549-562
Adaptive antennas are now used to increase the spectral efficiency in mobile telecommunication systems. A model of the received carrier-to-interference plus noise ratio (CINR) in the adaptive antenna beamformer output is derived, assuming that the weighting units are implemented in hardware, The finite resolution of weights and calibration is shown to reduce the CINR. When hardware weights are used, the phase or amplitude step size in the weights can be so large that it affects the maximum achievable CINR. It is shown how these errors makes the interfering signals “leak” through the beamformer and we show how the output CINR is dependent on power of the input signals. The derived model is extended to include the limited dynamic range of the receivers, by using a simulation model. The theoretical and simulated results are compared with measurements on an adaptive array antenna testbed receiver, designed for the GSM-1800 system. The theoretical model was used to find the performance limiting part in the testbed as the 1 dB resolution in the weight magnitude. Furthermore, the derived models are used in illustrative examples and can be used for system designers to balance the phase and magnitude resolution and the calibration requirements of future adaptive array antennas 相似文献
902.
H.A. Taylor J.M. Grebowsky H.G. Mayr H.B. Niemann L.H. Brace P.A. Cloutier R.E. Daniell J.T. Coulson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):291-295
The Bennett rf ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter was particularly designed to provide variable temporal resolution for measurements of thermal ion composition and density. An Explore-Adapt mode is used to obtain priority for measurement of most prominent ion species, and in a 2/16 configuration, the two dominant ions within the available range of 16 species are selectively sampled at the highest rate of 0.2 sec/sample. The high resolution measurements are combined with independent observations from the magnetic field (OMAG), neutral mass spectrometer (ONMS), and electron temperature (OETP) experiments to investigate sharply structured troughs in the low altitude nightside ion concentrations. The results indicate a close correlation between the structure in the ion distributions and the structured configuration of the magnetic field which is draped about the planet. In the regions of the ion depletions, sharp fluctuations in electron temperature and anomalous increases in the density of neutral gases indicate that the ion depletion may be associated either with dynamic perturbation in the ion and neutral flows, and/or local joule heating. The configuration of the ion flow/magnetic field draping and consequent electric fields for these events must be analyzed in detail to understand the relationships. 相似文献
903.
R.C. Canfield T.A. Gunkler H.S. Hudson L.W. Acton J.W. Leibacher A.L. Kiplinger 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(11):145-149
We have observed the flare of 1980 May 7 1456 UT with several Solar Maximum Mission instruments, in coordination with the Sacramento Peak Observatory Vacuum Tower Telescope. From the X-ray data we determine the total amount of plasma at T > 2 × 106 K, commonly attributed to chromospheric evaporation. From Hα we have determined the amount of plasma that has been evaporated from the chromosphere. We find that enough material has been evaporated from the chromosphere to account for the X-ray plasma. Taken together, the Hα, soft and hard X-ray images suggest that chromospheric evaporation is driven both by flare accelerated electrons, during the impulsive phase, and conduction, during the thermal phase. 相似文献
904.
Among the major objectives of NASA's program of space exploration is a better understanding of the origin and evolution of the solar system. Crucial to this objective is the study of comets, which are thought to be the most primitive, pristine bodies remaining in the solar system. The importance of the study of comets has led NASA to plan a mission to rendezvous with comet Tempel 2 in 1997. Critical to the understanding of comets will be measurements of the nucleus material to determine its elemental and isotopic composition, its mechanical properties, and its thermal state and properties. This paper describes a proposal for a Comet Nucleus Penetrator to accomplish these measurement goals. The Comet Nucleus Penetrator will implant instruments into the comet's nucleus beneath a probable volatile-depleted surface mantle into material more representative of the bulk composition of the nucleus. 相似文献
905.
D. Rees M. Carlson T.J. Fuller-Rowell P. Charleton C.R. Philbrick 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):129-132
This paper summarizes the dynamical information obtained in the lower thermosphere during the Energy Budget Campaign, by three experimental techniques: rocket-borne falling spheres instrumented with accelerometers and Tri-Methyl-Aluminium (TMA) trails, and from a ground-based Fabry-Perot interferometer. Winds of 200–400 m/sec, accelerated by the momentum and energy inputs from the magnetosphere, were observed during the ‘B’ and ‘A2’ salvos (15/16 Nov 1980 and 30 Nov/1 Dec resp.), with perturbations as low as 100 km altitude during the ‘B’ salvo. A global model has been used to simulate the wide-scale consequences of these disturbances, and to aid estimation of the integrated energy and momentum inputs. 相似文献
906.
Adaptive arrays for use in communication systems require the generation of a so-called reference signal, which is usually derived from the array output. A particular problem associated with this technique, the problem of reference loop phase shift, is discussed. It is shown that phase shift in the reference loop causes the array weights to cycle, and also causes the array to frequency-modulate the signal. In spite of this frequency change, the array maintains a maximum SNR at the output. 相似文献
907.
Analytical and experimental considerations of the common-mode noise in a dc-to-dc connverterare discussed. Analyzing the mechanism of the common-mode noise generation by means of a high-frequency equivalent circuit, we estimate the output noise voltage generated during the transistor's turn-on time. This commonmode noise consists mainly of two frequency components distributed in several megahertz. The effects of parasitic parameters on the noise voltage and frequencies are clarified. In addition, the effect of a common-mode choke on the noise suppression is discussed experimentally. 相似文献
908.
A discrete time model for simulation of the dynamics of samarium cobalt-type permanent magnet brushless dc machines is presented. The similation model includes modeling of the interaction between these machines and their attached power conditioners. These are transistorized conditioner units. This model is part of an overall discrete-time analysis of the dynamic performance of electromechanical actuators, which was conducted as part of prototype development of such actuators studied and built for NASA-Johnson Space Center as a prospective alternative to hydraulic actuators presently used in shuttle orbiter applications. The resulting numerical simulations of the various machine and power conditioner current and voltage waveforms gave excellent correlation to the actual waveforms collected from actual hardware experimental testing. These results, numerical and experimental, are presented here for machine motoring, regeneration and dynamic braking modes. Application of the resulting model to the determination of machine current and torque profiles during closed-loop actuator operation were also analyzed and the results are given here. These results are given in light of an overall view of the actuator system components. The applicability of this method of analysis to design optimization and trouble-shooting in such prototype development is also discussed in light of the results at hand. 相似文献
909.
F Bonde-Petersen Y Suzuki T Sadamoto 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(9):205-208
Isometric exercise induces profound cardiovascular adaptations increasing mean arterial pressure and heart rate. We investigated effects of simulated +Gz and -Gz respectively on the central and peripheral cardiovascular system. Sustained handgrip exercise was performed at 40% of maximum for 2 minutes in five subjects. This maneuver increased mean arterial pressure by 40-45 mm Hg both during head out water immersion which simulates weightlessness, as well as bedrest during -25, 0, and +25 degrees tilt from the horizontal. Lower body negative pressure (-60 mm Hg for 10 min) attenuated the response to handgrip exercise to 30 mm Hg. It also increased the heart rate minimally by about 20 beats per minute while the water immersion, as well as head up, head down and horizontal bedrest showed increments of about 50 beats per min. It was concluded that the response to isometric contraction is mediated through the high pressure baroreceptors, because similar responses were seen during stresses producing a wide variation in central venous pressure. During lower body negative pressure the increased sympathetic nervous activity itself increased resting heart rate and mean arterial pressure. The responses to static exercise were, therefore, weaker. 相似文献
910.
T.L. Cline 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):175-183
The first interplanetary gamma-ray burst spacecraft network is making possible the precise determination of gamma-ray burst source locations. This network is an international cooperation involving the Helios-2, Pioneer Venus, Venera-11, Venera-12, ISEE-3 and Prognoz-7 spacecraft. The celestial regions that have been defined, with one exception, have no correlations either to known x-ray emitters or to steady optical counterparts, to ~ 22nd mag. The event of 1979 March 5 has a very small source field located within the contour of the supernova remnant N49 in the LMC; the possibility of this measurement as a source identification, the 55 kpc distance of N49 as opposed to the nearby source distances assumed for typical bursts, and the very different characteristics of this event, however, are three arguments for its separate classification. The recent identification of an archived, 50-year old, optical transient within the high-precision source field of a typical burst [1] suggests both that events may repeat and that sources may be localized with even greater accuracy optically.This review outlines the precise source location data being produced by the first and the second spacecraft networks, the possibilities of additional networks and of related studies in other disciplines, and the prospects both for real-time optical transient observations and for the definition of gamma-ray burst sources by optical transient astronomy. 相似文献