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441.
Velocities of fluid flows and solid objects can be measured by performing photon count correlations on laser light scattered from the measuring region, the most common optical configuration being the laser Doppler system. A statistical analysis for the counting process is presented for both the constant velocity case and for turbulent fluid flows with formulations for specific laser Doppler configurations. For turbulent flow, relationships are derived for the shape of the count correlation in terms of the mean velocity and rms turbulent intensity, taking into account the shape of the turbulence correlation curve. Experimental results recorded for both a constant velocity solid object and for an unseeded turbulent air flow in a wind tunnel are presented, and, in the latter case, comparisons are made with hot-wire measurements. 相似文献
442.
Barnard T.W. Wasserman J. Schroeder J.B. 《IEEE transactions on aerospace and electronic systems》1972,(2):156-160
A system concept for remote measurement of vehicle orientation with only passive devices mounted on a vehicle is presented. Three channels of polarization-modulated light are used to determine all three degrees of freedom of vehicle orientation. Two important system components are described, a polarization rotator and a retroreflector which leaves the polarization state invariant. Also, the method of data reduction is outlined and the system accuracy is discussed. 相似文献
443.
Optical guidance for autonomous landing of spacecraft 总被引:7,自引:0,他引:7
Miso T. Hashimoto T. Ninomiya K. 《IEEE transactions on aerospace and electronic systems》1999,35(2):459-473
An autonomous rendezvous guidance scheme for spacecraft to descend to small celestial bodies by using optical information is presented. First, a new guidance, navigation, and control (GNC) method based on fixation-point (FP) inheritance is proposed. A spacecraft can safely descend toward the target point on the celestial body by tracking and autonomously renewing the FPs on the surface. Next, we deal with the method of extracting the FPs. A spatial band-pass filter (BPF) is applied to pictures taken to enhance features having comparable size with the tracking window. Local variance of the filtered image is used as a criterion of the extraction. Then, the relative information between the spacecraft and the celestial body (position, velocity, attitude, etc.) is calculated from the image coordinates and the range measurements of the FPs from the spacecraft. To suppress observation noise and improve navigation accuracy, an application of the extended Kalman filter is also presented. Finally, simulations are conducted to verify the guidance precision and the fuel consumption of the proposed guidance scheme 相似文献
444.
Interplanetary origin of geomagnetic storms 总被引:8,自引:0,他引:8
Gonzalez Walter D. Tsurutani Bruce T. Clúa de Gonzalez Alicia L. 《Space Science Reviews》1999,88(3-4):529-562
Around solar maximum, the dominant interplanetary phenomena causing intense magnetic storms (Dst<−100 nT) are the interplanetary
manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of storms,
involving intense southward IMFs: the sheath region just behind the forward shock, and the CME ejecta itself. Whereas the
initial phase of a storm is caused by the increase in plasma ram pressure associated with the increase in density and speed
at and behind the shock (accompanied by a sudden impulse [SI] at Earth), the storm main phase is due to southward IMFs. If
the fields are southward in both of the sheath and solar ejecta, two-step main phase storms can result and the storm intensity
can be higher. The storm recovery phase begins when the IMF turns less southward, with delays of ≈1–2 hours, and has typically
a decay time of 10 hours. For CMEs involving clouds the intensity of the core magnetic field and the amplitude of the speed
of the cloud seems to be related, with a tendency that clouds which move at higher speeds also posses higher core magnetic
field strengths, thus both contributing to the development of intense storms since those two parameters are important factors
in genering the solar wind-magnetosphere coupling via the reconnection process.
During solar minimum, high speed streams from coronal holes dominate the interplanetary medium activity. The high-density,
low-speed streams associated with the heliospheric current sheet (HCS) plasma impinging upon the Earth's magnetosphere cause
positive Dst values (storm initial phases if followed by main phases). In the absence of shocks, SIs are infrequent during
this phase of the solar cycle. High-field regions called Corotating Interaction Regions (CIRs) are mainly created by the fast
stream (emanating from a coronal hole) interaction with the HCS plasma sheet. However, because the Bz component is typically highly fluctuating within the CIRs, the main phases of the resultant magnetic storms typically have
highly irregular profiles and are weaker. Storm recovery phases during this phase of the solar cycle are also quite different
in that they can last from many days to weeks. The southward magnetic field (Bs) component of Alfvén waves in the high speed stream proper cause intermittent reconnection, intermittent substorm activity,
and sporadic injections of plasma sheet energy into the outer portion of the ring current, prolonging its final decay to quiet
day values. This continuous auroral activity is called High Intensity Long Duration Continuous AE Activity (HILDCAAs).
Possible interplanetary mechanisms for the creation of very intense magnetic storms are discussed. We examine the effects
of a combination of a long-duration southward sheath magnetic field, followed by a magnetic cloud Bs event. We also consider the effects of interplanetary shock events on the sheath plasma. Examination of profiles of very
intense storms from 1957 to the present indicate that double, and sometimes triple, IMF Bs events are important causes of such events. We also discuss evidence that magnetic clouds with very intense core magnetic
fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense
storms. Finally, we argue that a combination of complex interplanetary structures, involving in rare occasions the interplanetary
manifestations of subsequent CMEs, can lead to extremely intense storms.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
445.
From magnetic fields and coronal heating observed in flares, active regions, quiet regions, and coronal holes, we propose
that exploding sheared core magnetic fields are the drivers of most of the dynamics and heating of the solar atmosphere, ranging
from the largest and most powerful coronal mass ejections and flares, to the vigorous microflaring and coronal heating in
active regions, to a multitude of fine-scale explosive events in the magnetic network, driving microflares, spicules, global
coronal heating, and, consequently, the solar wind.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
446.
K. Miyamoto M. Oka R. Yamamoto Y. Masuda T. Hoson S. Kamisaka J. Ueda 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(12):2033-2036
Activity of auxin polar transport in inflorescence axes of Arabidopsis thaliana grown under simulated microgravity conditions was studied in relation to the growth and development. Seeds were germinated and allowed to grow on an agar medium in test tubes on a horizontal clinostat. Horizontal clinostat rotation substantially reduced the growth of inflorescence axes and the productivity of seeds of Arabidopsis thaliana (ecotypes Landsberg erecta and Columbia), although it little affected seed germination, development of rosette leaves and flowering. The activity of auxin polar transport in inflorescence axes decreased when Arabidopsis plants were grown on a horizontal clinostat from germination stage, being ca. 60% of 1 g control. On the other hand, the auxin polar transport in inflorescence axes of Arabidopsis grown in 1 g conditions was not affected when the segments were exposed to various gravistimuli, including 3-dimensional clinorotation, during transport experiments. Pin-formed mutant of Arabidopsis, having a unique structure of the inflorescence axis with no flower and extremely low levels of the activity of auxin polar transport in inflorescence axes and endogenous auxin, did not continue its vegetative growth under clinostat rotation. These facts suggest that the development of the system of auxin polar transport in Arabidopsis is affected by microgravity, resulting in the inhibition of growth and development, especially during reproductive growth. 相似文献
447.
A Sato T Hamazaki T Oomura H Osada M Kakeya M Watanabe T Nakamura Y Nakamura N Koshikawa I Yoshizaki S Aizawa S Yoda A Ogiso M Takaoki Y Kohno H Tanaka 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(6):807-813
The paper summarizes the data on proliferation and gravity-related gene expression of osteoblasts that were obtained from an experiment conducted under simulated and real microgravity conditions. Simulated microgravity conditions obtained in a clinostat depress proliferation of both osteoblast-like MC3T3-E1 and HeLa carcinoma cells. This depression of proliferation occurs in a collagen gel culture in which the flow of culture medium by rotation may be reduced. Interestingly, MC3T3-E1 cells which are probably one of target cells to microgravity are more sensitive than the HeLa cells. Simulated microgravity inhibited the epidermal growth factor (EGF)-induced c-fos gene expression in the MC3T3-El cells. To examine in detail the effect of real microgravity on the EGF signal transduction cascade in osteoblasts, MC3T3-E1 cells were cultured in the Cell Culture Experiment Module of the sounding rocket TR-1A6. The EGF-induced c-fos expression in cells was depressed under short-term microgravity conditions in the sounding rocket, while the phosphorylation of mitogen-activated protein kinase (MAPK) was not affected compared with the controls grown on the ground. These results suggest that an action site of microgravity in the signal transduction pathway may be downstream of MAPK. 相似文献
448.
M R Sims C T Pillinger I P Wright J Dowson S Whitehead A Wells J E Spragg G Fraser L Richter H Hamacher A Johnstone N P Meredith C de la Nougerede B Hancock R Turner S Peskett A Brack J Hobbs M Newns A Senior M Humphries H U Keller N Thomas J S Lingard T C Ng 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(11):1925-1928
The aim of the proposed Beagle 2 small lander for ESA's 2003 Mars Express mission is to search for organic material on and below the surface of Mars and to study the inorganic chemistry and mineralogy of the landing site. The lander will have a total mass of 60kg including entry, descent, and landing system. Experiments will be deployed on the surface using a robotic arm. It will use a mechanical mole and grinder to obtain samples from below the surface, under rocks, and inside rocks. Sample analysis by a mass spectrometer will include isotopic analysis. An optical microscope, an X-ray spectrometer and a Mossbauer spectrometer will conduct in-situ rock studies. 相似文献
449.
Stone E.C. Cohen C.M.S. Cook W.R. Cummings A.C. Gauld B. Kecman B. Leske R.A. Mewaldt R.A. Thayer M.R. Dougherty B.L. Grumm R.L. Milliken B.D. Radocinski R.G. Wiedenbeck M.E. Christian E.R. Shuman S. von Rosenvinge T.T. 《Space Science Reviews》1998,86(1-4):357-408
The Solar Isotope Spectrometer (SIS), one of nine instruments on the Advanced Composition Explorer (ACE), is designed to provide
high- resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z=2 to 30) over the energy range
from ∼10 to ∼100 MeV nucl−1. During large solar events SIS will measure the isotopic abundances of solar energetic particles
to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet
times SIS will measure the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic-ray component,
which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that
provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories
are measured with a pair of two-dimensional silicon-strip detectors instrumented with custom, very large-scale integrated
(VLSI) electronics to provide both position and energy-loss measurements. SIS was especially designed to achieve excellent
mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry
factor of ∼40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers. A microprocessor controls the
instrument operation, sorts events into prioritized buffers on the basis of their charge, range, angle of incidence, and quality
of trajectory determination, and formats data for readout by the spacecraft. This paper describes the design and operation
of SIS and the scientific objectives that the instrument will address.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
450.
Wong K.T. Zoltowski M.D. 《IEEE transactions on aerospace and electronic systems》1998,34(4):1320-1336
A novel sparse array geometry embedding two sizes of spatial invariances is presented for use with a new ESPRIT-based (estimation of signal parameters via rotational invariance techniques) algorithm for aperture extension. The half-wavelength invariance yields unambiguous but high-variance direction cosine estimates to disambiguate low-variance but cyclically ambiguous estimates from the larger invariance. With larger invariance at 60 half-wavelengths, resolution threshold for two closely spaced emitters is reduced by 50 dB and estimation error by 100-fold. Array design formulas are also presented 相似文献