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201.
H. Borg L. -A. Holmgren B. Hultovist F. Cambou H. Rème A. Bahnsen G. Kremser 《Space Science Reviews》1978,22(5):511-535
Some preliminary new results are presented of the keV plasma experiment on GEOS-1. Electrons and ions have been observed to stream along the magnetic field lines in the dayside magnetosphere from the ionospheric side of the satellite towards the equatorial plane during magnetic storms, with streaming velocities corresponding to a kinetic energy of the order of a keV. The opposite streaming velocity has also been seen, but primarily in the ions only and with a smaller flux ratio for the two opposite directions along the field lines. The transition between the two opposite streaming directions, as seen by the satellite, has been found to occur even in a fraction of a second.In magnetic storm conditions azimuthal asymmetries in ion fluxes corresponding to electric field intensities of several tens of millivolts per meter have been observed. 相似文献
202.
J. L. Birck 《Space Science Reviews》1991,56(1-2):141-146
According to their chemical composition, rich in volatile compounds, comets are thought to be primitive materials. They may provide prime samples for the study of nucleosynthetic components of the solar system and of the processes occurring during the formation of the outer planets. Their origin is largely a matter of conjecture. Chromium isotopic measurements in carbonaceous chondrites illustrate how the non-volatile part of cometary material can be investigated both for isotopic heterogeneity and for the extinct nuclide 53Mn. Questions like the possible presence of 26Al as a heat source can also be addressed by these measurements. 相似文献
203.
Simplified analysis of PWM converters using model of PWM switch.II. Discontinuous conduction mode 总被引:1,自引:0,他引:1
For pt.I see ibid., vol.26, no.3, p.490-6 (1990). According to the method of state-space averaging, when a pulsewidth modulation (PWM) converter enters discontinuous conduction mode (DCM), the inductor current state is lost from the average model of the converter. It is shown that there is neither theoretical nor experimental justification for the disappearance of the inductor state as claimed by the method of state-space averaging. For example, when the model of the PWM switch in DCM is substituted in the buck, boost, or buck-boost converter while the inductor is left intact, the average model has two poles: the first pole f p1 agrees with the single pole of state-space averaging, while the second pole f p2 occurs in the range f p2⩾F s/π. It is shown that the right-half plane zeros present in the control-to-output transfer functions of the boost, buck-boost, and Cuk converters in continuous conduction mode are also present in discontinuous conduction mode 相似文献
204.
Trunk G.V. Cantrell B.H. Gordon W.B. 《IEEE transactions on aerospace and electronic systems》1979,(6):883-887
The Cramer-Rao bound for an unbiased estimate of the elevation angle of a target in the presence of multipath is calculated for the symmetric (target and image symmetric about the elevation symmetry plane of antenna) and nonsymmetric cases for an antenna consisting of 21 elements. These bounds are compared to the maximum likelihood estimates and it is found that the rms error of the maximum likelihood estimate (which has a bias) is below the Cramer-Rao bound for unbiased estimates. 相似文献
205.
206.
Trunk G.V. Gordon W.B. Cantrell B.H. 《IEEE transactions on aerospace and electronic systems》1990,26(1):146-153
A technique which uses maximum-likelihood estimates (MLEs) of target Doppler and target amplitude is developed for rejecting clutter residues. Multiple estimates are made and consistency checks are applied to the estimates. Simulation results indicate that for large clutter-to-noise ratios (C /N ⩾55 dB) the probability of false alarm from clutter residues is reduced from 1.0 to below 0.01 相似文献
207.
J. D. Anderson J. W. Armstrong J. K. Campbell F. B. Estabrook T. P. Krisher E. L. Lau 《Space Science Reviews》1992,60(1-4):591-610
The gravitation and celestial mechanics investigations during the cruise phase and Orbiter phase of the Galileo mission depend on Doppler and ranging measurements generated by the Deep Space Network (DSN) at its three spacecraft tracking sites in California, Australia, and Spain. Other investigations which also rely on DSN data, and which like ours fall under the general discipline of spacecraft radio science, are described in a companion paper by Howard et al. (1992). We group our investigations into four broad categories as follows: (1) the determination of the gravity fields of Jupiter and its four major satellites during the orbital tour, (2) a search for gravitational radiation as evidenced by perturbations to the coherent Doppler link between the spacecraft and Earth, (3) the mathematical modeling, and by implication tests, of general relativistic effects on the Doppler and ranging data during both cruise and orbiter phases, and (4) an improvement in the ephemeris of Jupiter by means of spacecraft ranging during the Orbiter phase. The gravity fields are accessible because of their effects on the spacecraft motion, determined primarily from the Doppler data. For the Galilean satellites we will determine second degree and order gravity harmonics that will yield new information on the central condensation and likely composition of material within these giant satellites (Hubbard and Anderson, 1978). The search for gravitational radiation is being conducted in cruise for periods of 40 days centered around solar opposition. During these times the radio link is least affected by scintillations introduced by solar plasma. Our sensitivity to the amplitude of sinusoidal signals approaches 10-15 in a band of gravitational frequencies between 10-4 and 10-3 Hz, by far the best sensitivity obtained in this band to date. In addition to the primary objectives of our investigations, we discuss two secondary objectives: the determination of a range fix on Venus during the flyby on 10 February, 1990, and the determination of the Earth's mass (GM) from the two Earth gravity assists, EGA1 in December 1990 and EGA2 in December 1992. 相似文献
208.
Halpin S.M. Grigsby L.L. 《IEEE transactions on aerospace and electronic systems》1992,28(4):1033-1041
Alternative methods for power flow and fault analysis of single-phase distribution systems are presented. The algorithms for both power flow and fault analysis utilize a generalized approach to network modeling. The generalized admittance matrix, formed using elements of linear graph theory, is an accurate network model for all possible single-phase network configurations. Unlike the standard nodal admittance matrix formulation algorithms, the generalized approach uses generalized component models for the transmission line and transformer. The standard assumption of a common node voltage reference point is not required to construct the generalized admittance matrix. Therefore, truly accurate simulation results can be obtained for networks that cannot be modeled using traditional techniques 相似文献
209.
Philippe L. Masson 《Space Science Reviews》1991,56(1-2):9-12
The main aspects of the Martian stratigraphy have been determined from the detailed study of Mariner 9 and Viking Orbiter images. Three major stratigraphic systems, the Noachian System, the Hesperian System, and the Amazonian System, are inferred from these studies. The global geological evolution of Mars is essentially derived from its stratigraphy. It reveals that tectonism and volcanism were widespread during two major periods (Noachian and Lower Hesperian) and became more localized during the Upper Hesperian and Amazonian periods. The transition between these two major periods occurred about 2 Ga ago, and significant geologic activity could still be present. However, a number of geologic features and processes remain little understood. Future investigations, including complete high resolution imaging and detailed mapping, geochemical mapping, in situ chemical analyses, etc., will be necessary in order to improve our knowledge of the Martian stratigraphy and geologic evolution and are essential to prepare any future Mars Sample Return mission and the Human Exploration of this planet. 相似文献
210.
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