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711.
Bird M.K. Dutta-Roy R. Heyl M. Allison M. Asmar S.W. Folkner W.M. Preston R.A. Atkinson D.H. Edenhofer P. Plettemeier D. Wohlmuth R. Iess L. Tyler G.L. 《Space Science Reviews》2002,104(1-4):613-640
A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction
and strength of Titan's zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ∼160 km down to the surface. The Probe's wind-induced horizontal motion will
be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit
and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas,
thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information
on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after
Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted
signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout,
as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests),
are described. Ongoing efforts to measure and model Titan's winds using various Earth-based techniques are briefly reviewed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
712.
从理论上研究了不同楼层,位于楼房边侧或中间不同位置的房间发生火灾时,外界吹来正向的不同风速,对房间内外稳态温度场与流场的作用。特别研究了房间内热气流流出窗外向上运动的浮力和外界风力相互作用发生的现象及其对房间内流动的影响,引入了关于风的临界速度(或流量),作为外界风速与建筑物内火灾流场相互作用有标志性的重要参数,本文还对不同外界风速情况下,房间内外速度中性面倾斜的角度,房间内流出热气流温度的差别等诸多问题进行了详细的讨论。 相似文献
713.
A grid algorithm for autonomous star identification 总被引:1,自引:0,他引:1
Padgett C. Kreutz-Delgado K. 《IEEE transactions on aerospace and electronic systems》1997,33(1):202-213
An autonomous star identification algorithm is described that is simple and requires less computer resources than other such algorithms. In simulations using an 8/spl times/8 degree field of view (FOV), the algorithm identifies the correct section of sky on 99.7% of the sensor orientations where spatial accuracy of the imaged star is 1 pixel (56.25 arc seconds) in standard deviation and the apparent brightness deviates by 0.4 units stellar magnitude. This compares very favorably with other algorithms in the literature. 相似文献
714.
Modeling and Design of Switching Regulators 总被引:1,自引:0,他引:1
Various building blocks of a switching regulator are described in detail and mathematical models are developed for all building blocks in terms of transfer functions, which enable one to design a switching regulator for stability, desirable bandwidth, line rejection, and transient response. A step-by-step procedure to design compensation is illustrated using two examples. Various networks for compensation and their transfer functions are presented which the author hopes will be very handy to use and will become the reference source. 相似文献
715.
Ion-induced nucleation has been suggested to be a potentially important mechanism for atmospheric aerosol formation. Ions
are formed in the background atmosphere by galactic cosmic rays. A possible connection between galactic cosmic rays and cloudiness
has been However, the predictions of current atmospheric nucleation models are highly uncertain because the models are usually
based on the liquid drop model that estimates cluster thermodynamics based on bulk properties (e.g., liquid drop density and
surface tension). Sulfuric acid (H2SO4) and water are assumed to be the most important nucleating agents in the free troposphere. Measurements of the molecular
thermodynamics for the growth and evaporation of cluster ions containing H2SO4 and H2O were performed using a temperature-controlled laminar flow reactor coupled to a linear quadrupole mass spectrometer as well
as a temperature-controlled ion trap mass spectrometer. The measurements were complemented by quantum chemical calculations
of the cluster ion structures. The analysis yielded a complete set of H2SO4 and H2O binding thermodynamics extending from molecular cluster ions to the bulk, based on experimental thermodynamics for the small
clusters. The data were incorporated into a kinetic aerosol model to yield quantitative predictions of the rate of ion-induced
nucleation for atmospheric conditions. The model predicts that the negative ion-H2SO4-H2O nucleation mechanism is an efficient source of new particles in the middle and upper troposphere. 相似文献
716.
An analysis is described of a kinematic state vector fusion algorithm when tracks are obtained from dissimilar sensors. For the sake of simplicity, it is assumed that two dissimilar sensors are equipped with nonidentical two-dimensional optimal linear Kalman filters. It is shown that the performance of such a track-to-track fusion algorithm can be improved if the cross-correlation matrix between candidate tracks is positive. This cross-correlation is introduced by noise associated with target maneuver that is common to the tracking filters in both sensors and is often neglected. An expression for the steady state cross-correlation matrix in closed form is derived and conditions for positivity of the cross-correlation matrix are obtained. The effect of positivity on performance of kinematic track-to-track fusion is also discussed 相似文献
717.
The fundamental problems of automatic target recognition (ATR) are discussed. A new approach to ATR is suggested that includes: a new method of scoring ATR performance, a new concept of artificial images, a new method called probing for extracting target signature knowledge from image experts, and suggestions for coping with the problem of insufficient test data and algorithm obsolescence 相似文献
718.
The Federal Aviation Administration plans to independently monitor signals in space from the Global Positioning System (GPS) for the purpose of providing immediate awareness to civil aviation users of the operational status of GPS when it is used in the National Airspace System. The operational status will be disseminated to Air Traffic Control and will possibly be broadcast from ground monitoring stations to GPS aviation users via a dedicated integrity channel. An algorithm is described that measures the coverage of a configuration of ground monitoring station locations, and is applied to several different configurations of ground monitoring stations to compare the coverage provided. Also included are the resulting ground monitoring station configurations that give the best coverage of GPS signals for several specific geographical areas, the conterminous United States (CONUS), Canada, and Alaska. 相似文献
719.
C. M. Lisse M. F. A’Hearn T. L. Farnham O. Groussin K. J. Meech U. Fink D. G. Schleicher 《Space Science Reviews》2005,117(1-2):161-192
As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules
and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface
temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 102 km), it is in collisional equilibrium, at larger distances (≥104 km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply
dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics
of the coma, as well as with its variability with time, orbital position, and heliocentric distance.
The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive
measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several
gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986.
This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current
state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards
that the coma presents for the spacecraft.
An erratum to this article is available at . 相似文献
720.
Since its discovery in 1867, periodic comet 9P/Tempel 1 has been observed at 10 returns to perihelion, including all its returns
since 1967. The observations for the seven apparitions beginning in 1967 have been fit with an orbit that includes only radial
and transverse nongravitational accelerations that model the rocket-like thrusting introduced by the outgassing of the cometary
nucleus. The successful nongravitational acceleration model did not assume any change in the comet’s ability to outgas from
one apparition to the next and the outgassing was assumed to reach a maximum at perihelion. The success of this model over
the 1967–2003 interval suggests that the comet’s spin axis is currently stable. Rough calculations suggest that the collision
of the impactor released by the Deep Impact spacecraft will not provide a noticeable perturbation on the comet’s orbit nor
will any new vent that is opened as a result of the impact provide a noticeable change in the comet’s nongravitational acceleration
history. The observing geometries prior to, and during, the impact will allow extensive Earth based observations to complement
the in situ observations from the impactor and flyby spacecraft. 相似文献