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511.
G. J. Peters 《Space Science Reviews》1989,49(3-4):9-22
The circumstellar plasma that produces H emission in Algol binaries has been investigated using phase-resolved, high dispersion data acquired from CCD and image tube detectors. Results are summarized in this paper, including discussions of the disk geometry and size, asymmetry in the distribution of material, long-term or non-phase dependent variability, mass outflow, the mean electron density, and how the latter properties vary with the system's period or location in the r-q diagram. Five systems which display permanent emission with periods ranging from 4.5 to 261 days (SW Cyg, UX Mon, TT Hya, AD Her, and RZ Oph) are intercompared. If P < 4.5 days, no permanent disks are observed, while if P > 6 days, stable disks with only slight long-term variations in their H brightness are seen. The most variable systems appear to be those in the 5 – 6 day range, but the star's position in the r-q diagram has the largest influence on its behavior. The trailing side of the accretion disk, where the gas stream impacts the inner disk, is usually brighter, and the leading side is often times more extended. The disk extends out to at least 95% of the Roche surface of the primary and is highly flattened (RP). Mass outflow near phase 0.5 is commonplace. 相似文献
512.
I. Hubeny 《Space Science Reviews》1989,49(3-4):117-126
A brief review of various theoretical approaches to model accretion disks is presented. Emphasis is given to models that determine self-consistently the structure of a disk together with the radiation field. It is argued that a proper treatment of the vertical structure is essential for calculating theoretical spectra to be compared with observations. In particular, it is shown that hot layers above an accretion disk (sometimes called disk chromospheres or coronae), whose presence is indicated by recent UV observations of strong emission lines of highly ionized species, may be explained using simple energy balance arguments.1987–88 JILA Visiting Fellow.This work was in part supported by a NASA grant ADP U-003-88 (Plavec and Hubeny). I also wish to thank the organizers of the IAU Colloquium 107 for the travel grant which enabled me to attend the meeting. 相似文献
513.
Performance prediction for a detection system employing noncoherent integration is carried out for a chi-square family of fluctuating targets in K -distributed clutter plus noise. The detection performance for Swerling 11 targets in the K -distributed clutter plus noise is compared with that in exponentially correlated Rayleigh clutter. The results show that the performance prediction based on N pulses integrated in clutter plus noise using the K -distributed clutter model may be approximately equivalent to that using the exponentially correlated Rayleigh-distributed clutter model 相似文献
514.
Application of digital cross-correlation spectroscopy to the spectra of the W Serpentis binaries SX Cas and RX Cas has allowed an accurate determination of the orbits and rotations of the (mass-losing) K-subgiant secondary components. The distortion of the primary radial-velocity curves due to the influence of the prominent accretion disks in these systems has been modelled to first order. This enables us to estimate k 1, and thereby the mass ratio q ≈ 0.30, to within ~ ± 20%. The absolute radii of the secondaries are derived independently from the observed rotations and periods, assuming synchronous rotation. They show that the stars fill their Roche lobes, or at least very nearly so. Rough fits to the available photometry shows both primaries to be unevolved mid-B stars; that in RX Cas appears completely obscured by the disk. Preliminary spectroscopic data for W Ser and W Cru show some promise for similar analyses of these systems. 相似文献
515.
The ordered-statistics (OS) constant false-alarm rate (CFAR) is relatively immune to the presence of interfering targets among the reference cells used to determine the average background. OS CFAR performance in a multitarget environment was previously studied by simulation. The author obtains analytic expressions for the added detection loss, assuming strong interfering targets. The real target is assumed to be a Rayleigh fluctuating target. Numerical examples are included 相似文献
516.
Dabbous T.E. Ahmed N.U. McMillan J.C. Liang D.F. 《IEEE transactions on aerospace and electronic systems》1988,24(1):85-102
A refined stochastic model for the errors of the Loran-C radio navigation aid is described, and it is shown how this model can be used to improve the performance of integrated navigation systems. In addition to the usual propagation errors, Loran-C time of arrival measurements are occasionally plagued with sudden intermittent errors of a particular magnitude and caused by receiver cycle selection errors. These result in sudden large jumps in the calculated position solution. The Loran-C error has been modeled as the sum of a diffusion process, representing the normal propagating errors, and a pure jump process of Poisson type, representing the cycle selection errors. A simple integrated navigation system is then described, based on the Loran-C model and the standard dead reckoning (heading and speed) system model. Assuming that the observed process is governed by a linear stochastic difference equation, a recursive linear unbiased minimum variance filter is developed, from which the Loran-C and dead reckoning errors, and hence position and velocity, can be estimated 相似文献
517.
The Galileo spacecraft was launched by the Space Shuttle Atlantis on October 18, 1989. A two-stage Inertial Upper Stage propelled Galileo out of Earth parking orbit to begin its 6-year interplanetary transfer to Jupiter. Galileo has already received two gravity assists: from Venus on February 10, 1990 and from Earth on December 8, 1990. After a second gravity-assist flyby of Earth on December 8, 1992, Galileo will have achieved the energy necessary to reach Jupiter. Galileo's interplanetary trajectory includes a close flyby of asteroid 951-Gaspra on October 29, 1991, and, depending on propellant availability and other factors, there may be a second asteroid flyby of 243-Ida on August 28, 1993. Upon arrival at Jupiter on December 7, 1995, the Galileo Orbiter will relay data back to Earth from an atmospheric Probe which is released five months earlier. For about 75 min, data is transmitted to the Orbiter from the Probe as it descends on a parachute to a pressure depth of 20–30 bars in the Jovian atmosphere. Shortly after the end of Probe relay, the Orbiter ignites its rocket motor to insert into orbit about Jupiter. The orbital phase of the mission, referred to as the satellite tour, lasts nearly two years, during which time Galileo will complete 10 orbits about Jupiter. On each of these orbits, there will be a close encounter with one of the three outermost Galilean satellites (Europa, Ganymede, and Callisto). The gravity assist from each satellite is designed to target the spacecraft to the next encounter with minimal expenditure of propellant. The nominal mission is scheduled to end in October 1997 when the Orbiter enters Jupiter's magnetotail.List of Acronyms ASI
Atmospheric Structure Instrument
- EPI
Energetic Particles Instrument
- HGA
High Gain Antenna
- IUS
Inertial Upper Stage
- JOI
Jupiter Orbit Insertion
- JPL
Jet Propulsion Laboratory
- LRD
Lightning and Radio Emissions Detector
- NASA
National Aeronautics and Space Administration
- NEP
Nephelometer
- NIMS
Near-Infrared Mapping Spectrometer
- ODM
Orbit Deflection Maneuver
- OTM
Orbit Trim Maneuver
- PJR
Perijove Raise Maneuver
- PM
Propellant Margin
- PDT
Pacific Daylight Time
- PST
Pacific Standard Time
- RPM
Retropropulsion Module
- RRA
Radio Relay Antenna
- SSI
Solid State Imaging
- TCM
Trajectory Correction Maneuver
- UTC
Universal Time Coordinated
- UVS
Ultraviolet Spectrometer
- VEEGA
Venus-Earth-Earth Gravity Assist 相似文献
518.
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
519.
Efron A.J. Swaszek P.E. Tufts D.W. 《IEEE transactions on aerospace and electronic systems》1992,28(4):932-943
A detector which is designed to operate in a correlated Gaussian-plus-impulsive-noise environment is presented. The detector whitens the data robustly and then uses a two-sided threshold test to determine the presence of impulsive samples. The impulsive samples are discarded, and the remaining samples are used to detect the presence or absence of a signal using a matched filter. An approximate analysis is presented, and simulations are used to demonstrate the effectiveness of this approach 相似文献
520.
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