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271.
272.
We discuss mass loss relations for OB-type stars as a function of luminosity, effective temperature, and mass. We conclude that a simple first order linear regression relation is as good as any other more sophisticated relation, with the advantage that the simple form consumes much less computer time when used in evolutionary codes. 相似文献
273.
A novel VHF localizer system has been designed, built and successfully tested to provide increased reliability and safety of commercial and general aviation air transportation. Additional benefits are more precise tracks for aircraft executing a missed approach, reduced volume of the airspace needed for missed approaches, and reduced sizes of areas affected by noise. The design uses contemporary instrument landing system (ILS) hardware to provide dual independent front and back course directional localizer operation with two carriers in the receiver passband offset 4 kHz from the nominal carrier frequency. An example is given of an application and solution to an ILS problem at Reno, NV. Relevant data are presented 相似文献
274.
The continued development of the symmetric measurement equation (SME) filter for track maintenance in multiple target tracking (MTT) is considered, focusing on the case in which the SMEs are generated by forming sums of products of the original position measurements. The SME filter is developed for the case of N targets whose motions consist of random perturbations about constant-velocity trajectories. It is assumed that measurements of x -coordinate positions are available, and that the number of measurements is equal to the number of targets. Various analytical properties of the SME filter are studied. It is shown that under a very weak condition, the estimation error equation is locally exponentially stable. The performance of the SME filter is investigated by comparing it with an optimal (minimum-variance) estimator and by generating a computer simulation in the six-target case 相似文献
275.
Light scattering in planetary atmospheres 总被引:45,自引:0,他引:45
This paper reviews scattering theory required for analysis of light reflected by planetary atmospheres. Section 1 defines the radiative quantities which are observed. Section 2 demonstrates the dependence of single-scattered radiation on the physical properties of the scatterers. Section 3 describes several methods to compute the effects of multiple scattering on the reflected light. 相似文献
276.
D. Lal 《Space Science Reviews》1972,14(1):3-102
Recent examinations of extraterrestrial materials exposed to cosmic rays for different intervals of time during the geological history of the solar system have generated a wealth of new information on the history of cosmic radiation. This information relates to the temporal variations in
- the flux and energy spectrum of low energy (solar) protons of ? 10 MeV kinetic energy;
- the flux and energy spectrum of (solar) heavy nuclei of Z > 20 of kinetic energy, 0.5–10 MeV/n;
- the integrated flux of protons and heavier nuclei of ? 0.5 GeV kinetic energy, and
- the flux and energy spectrum of nuclei of Z > 20 of medium energy — 100–2000 MeV/n kinetic energy.
277.
The modulation of galactic cosmic rays in the heliosphere seems to be dominated by four major mechanisms: convection, diffusion,
drifts (gradient, curvature and current sheet), and adiabatic energy losses. In this regard the global structure of the solar
wind, the heliospheric magnetic field (HMF), the current sheet (HCS), and that of the heliosphere itself play major roles.
Individually, the four mechanisms are well understood, but in combination, the complexity increases significantly especially
their evolvement with time - as a function of solar activity. The Ulysses observations contributed significantly during the past solar minimum modulation period to establish the relative importance
of these major mechanisms, leading to renewed interest in developing more sophisticated numerical models, and in the underlying
physics, e.g., what determines the diffusion tensor. With increased solar activity, the relative contributions of the mentioned
mechanisms change, but how they change and what causes these changes over an 11-year solar cycle is not well understood. It
can therefore be expected that present and forthcoming observations during solar maximum activity will again produce very
important insights into the causes of long-term modulation. In this paper the basic theory of solar modulation is reviewed
for galactic cosmic rays. The influence of the Ulysses observations on the development of the basic theory and numerical models are discussed, especially those that have challenged
the theory and models. Model-based predictions are shown for what might be encountered during the next solar minimum. Lastly,
modulation theory and modelling are discussed for periods of maximum solar activity when a global reorganization of the HMF,
and the HCS, occurs.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
278.
An Overview of the Fast Auroral SnapshoT (FAST) Satellite 总被引:3,自引:0,他引:3
The FAST satellite is a highly sophisticated scientific satellite designed to carry out in situ measurements of acceleration physics and related plasma processes associated with the Earth's aurora. Initiated and conceptualized by scientists at the University of California at Berkeley, this satellite is the second of NASA's Small Explorer Satellite program designed to carry out small, highly focused, scientific investigations. FAST was launched on August 21, 1996 into a high inclination (83°) elliptical orbit with apogee and perigee altitudes of 4175 km and 350 km, respectively. The spacecraft design was tailored to take high-resolution data samples (or `snapshots') only while it crosses the auroral zones, which are latitudinally narrow sectors that encircle the polar regions of the Earth. The scientific instruments include energetic electron and ion electrostatic analyzers, an energetic ion instrument that distinguishes ion mass, and vector DC and wave electric and magnetic field instruments. A state-of-the-art flight computer (or instrument data processing unit) includes programmable processors that trigger the burst data collection when interesting physical phenomena are encountered and stores these data in a 1 Gbit solid-state memory for telemetry to the Earth at later times. The spacecraft incorporates a light, efficient, and highly innovative design, which blends proven sub-system concepts with the overall scientific instrument and mission requirements. The result is a new breed of space physics mission that gathers unprecedented fields and particles observations that are continuous and uninterrupted by spin effects. In this and other ways, the FAST mission represents a dramatic advance over previous auroral satellites. This paper describes the overall FAST mission, including a discussion of the spacecraft design parameters and philosophy, the FAST orbit, instrument and data acquisition systems, and mission operations. 相似文献
279.
The problem with aviation COTS 总被引:1,自引:0,他引:1
Commercial Off the Shelf (COTS) has become a byword for acquisition reform, but there are significant risks associated with the use of COTS products in military systems. These risks are especially acute for aviation systems. This paper explains how COTS can negatively affect military acquisitions and gives ideas on how to plan and resolve COTS caused problems 相似文献
280.
Fabrizio G.A. Gershman A.B. Turley M.D. 《IEEE transactions on aerospace and electronic systems》2004,40(2):510-525
Adaptive beamforming is used to enhance the detection of target echoes received by high frequency (HF) surface wave (HFSW) over-the-horizon (OTH) radars in the presence of spatially structured interference. External interference from natural and man-made sources typically masks the entire range-Doppler search space and is characterized by a spatial covariance matrix that is time-varying or nonstationary over the coherent processing interval (CPI). Adaptive beamformers that update the spatial filtering weight vector within the CPI are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal decorrelation of the clutter which severely degrades subclutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses spatially nonstationary interference without degrading subclutter visibility is proposed here. The proposed algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HFSW OTH radar, located near Darwin in far north Australia. 相似文献