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71.
Jin Young Choi Dongkyoung Chwa 《IEEE transactions on aerospace and electronic systems》2006,42(2):659-669
An adaptive control against uncertainties in tail-controlled STT (skid-to-turn) missiles is presented. First, we derive an analytic uncertainty model from a parametric affine missile model developed by the authors. Based on this analytic model, an adaptive feedback linearizing control law accompanied by a sliding mode control law is proposed. We provide analyses of stability and output tracking performance of the overall adaptive missile system. The performance and validity of the proposed adaptive control scheme are demonstrated by simulation. 相似文献
72.
This paper reports a first application of contingent valuation method to measure the value of information generated by earth science data from an environmental geostationary satellite payload called Geostationary Environmental Monitoring Satellite. The purpose of the space project is to improve the accuracy of air pollution information by enhancing air pollution monitoring and forecasting system coupled with conventional ground level monitoring stations located throughout South Korea. 相似文献
73.
74.
The advent of far infrared arrays will change fundamentally the means of analyzing observations in this spectral region. Sources much fainter than traditional confusion limits will be extracted from images by using computer algorithms similar to CLEAN or DAOPHOT. We have conducted numerical experiments to evaluate these techniques and show that they will permit long integrations (10,000 sec at 60 m, 200 sec at 100 m) to achieve nearly photon-background-limited performance and hence very deep detection limits. The dominant noise sources—photon noise, confusion by distant galaxies, and confusion by IR cirrus — scale with nearly the same power of the telescope aperture. As a result, the integration times required to reach confusion limits are nearly aperture-independent. 相似文献
75.
Boynton WV D'Uston LC Young DT Lunine JI Waite JH Bailey SH Berthelier JJ Bertaux JL Borrel V Burke MF Cohen BA McComas DH Nordholt JE Evans LG Trombka JI 《Acta Astronautica》1997,40(9):663-674
The determination of the composition of materials that make up comets is essential in trying to understand the origin of these primitive objects. The ices especially could be made in several different astrophysical settings including the solar nebula, protosatellite nebulae of the giant planets, and giant molecular clouds that predate the formation of the solar system. Each of these environments makes different ices with different composition. In order to understand the origin of comets, one needs to determine the composition of each of the ice phases. For example, it is of interest to know that comets contain carbon monoxide, CO, but it is much more important to know how much of it is a pure solid phase, is trapped in clathrate hydrates, or is adsorbed on amorphous water ice. In addition, knowledge of the isotopic composition of the constituents will help determine the process that formed the compounds. Finally, it is important to understand the bulk elemental composition of the nucleus. When these data are compared with solar abundances, they put strong constraints on the macro-scale processes that formed the comet. A differential scanning calorimeter (DSC) and an evolved gas analyzer (EGA) will make the necessary association between molecular constituents and their host phases. This combination of instruments takes a small (tens of mg) sample of the comet and slowly heats it in a sealed oven. As the temperature is raised, the DSC precisely measures the heat required, and delivers the gases to the EGA. Changes in the heat required to raise the temperature at a controlled rate are used to identify phase transitions, e.g., crystallization of amorphous ice or melting of hexagonal ice, and the EGA correlates the gases released with the phase transition. The EGA consists of two mass spectrometers run in tandem. The first mass spectrometer is a magnetic-sector ion-momentum analyzer (MAG), and the second is an electrostatic time-of-flight analyzer (TOF). The TOF acts as a detector for the MAG and serves to resolve ambiguities between fragments of similar mass such as CO and N2. Because most of the compounds of interest for the volatile ices are simple, a gas chromatograph is not needed and thus more integration time is available to determine isotopic ratios. A gamma-ray spectrometer (GRS) will determine the elemental abundances of the bulk cometary material by determining the flux of gamma rays produced from the interaction of the cometary material with cosmic ray produced neutrons. Because the gamma rays can penetrate a distance of several tens of centimeters a large volume of material is analyzed. The measured composition is, therefore, much more likely to be representative of the bulk comet than a very small sample that might have lost some of its volatiles. Making these measurements on a lander offers substantial advantages over trying to address similar objectives from an orbiter. For example, an orbiter instrument can determine the presence and isotopic composition of CO in the cometary coma, but only a lander can determine the phase(s) in which the CO is located and separately determine the isotopic composition of each reservoir of CO. The bulk composition of the nucleus might be constrained from separate orbiter analyses of dust and gas in the coma, but the result will be very model dependent, as the ratio of gas to dust in the comet will vary and will not necessarily be equal to the bulk value. 相似文献
76.
The role of calcium in the regulation of hormone transport in gravistimulated roots. 总被引:5,自引:0,他引:5
M L Evans L M Young K H Hasenstein 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(1):211-218
Prior research has shown that gravistimulation induces preferential movement of calcium toward the lower side of the tips of maize roots and that roots depleted of calcium show impaired gravitropism. To further investigate the role of calcium in root gravitropism, we examined the effects of calcium on auxin movement in both vertical and gravistimulated roots of maize. Longitudinal movement of auxin was basipetally polar in intact roots but acropetally polar in decapped roots. Treatment of the root tip with calcium increased basipetal auxin movement in both intact and decapped roots. Gravistimulation induced asymmetric auxin movement toward the lower side of the root tip. Both asymmetric auxin movement and gravicurvature were inhibited by treatment of the root tip with auxin transport inhibitors or with EGTA. The results indicate that there is a close correlation between curvature and gravity-induced asymmetric auxin movement across the root cap. Since gravistimulation causes calcium movement toward the lower side of the root tip, our observation that calcium promotes basipetal auxin movement supports the idea that gravity-induced calcium asymmetry is a key step linking gravistimulation to the establishment of auxin asymmetry during root gravitropism. 相似文献
77.
D. C. Hyland H. A. Altwaijry S. Ge R. Margulieux J. Doyle J. Sandberg B. Young X. Bai J. Lopez N. Satak 《Cosmic Research》2010,48(5):430-436
In the later stages of a combined mission of exploration and damage mitigation of a hazardous Near Earth Asteroid (NEA), sufficiently
detailed tracking, orbit prediction, mass distribution, and physical properties data have been accumulated. It may be desirable
to implement a “slow push” damage mitigation technique that is capable of altering the NEA orbit continuously and permanently.
This possibility was suggested in a companion paper that outlined an archetypal exploration/mitigation mission to the NEA
99942 Apophis, which is achieved using a novel albedo change approach. The details and apparatus used in the albedo modification
technique are described in the paper. We describe the design details and the constraints on particle size (to prevent electrostatic
levitation and escape) and on the dispensing speed (to achieve the desired coverage zone and prevent particles from orbiting
or escaping). 相似文献
78.
Tarter JC Backus PR Mancinelli RL Aurnou JM Backman DE Basri GS Boss AP Clarke A Deming D Doyle LR Feigelson ED Freund F Grinspoon DH Haberle RM Hauck SA Heath MJ Henry TJ Hollingsworth JL Joshi MM Kilston S Liu MC Meikle E Reid IN Rothschild LJ Scalo J Segura A Tang CM Tiedje JM Turnbull MC Walkowicz LM Weber AL Young RE 《Astrobiology》2007,7(1):30-65
Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute. 相似文献
79.
The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) NASA Mission-of-Opportunity 总被引:3,自引:0,他引:3
D. J. McComas F. Allegrini J. Baldonado B. Blake P. C. Brandt J. Burch J. Clemmons W. Crain D. Delapp R. DeMajistre D. Everett H. Fahr L. Friesen H. Funsten J. Goldstein M. Gruntman R. Harbaugh R. Harper H. Henkel C. Holmlund G. Lay D. Mabry D. Mitchell U. Nass C. Pollock S. Pope M. Reno S. Ritzau E. Roelof E. Scime M. Sivjee R. Skoug T. S. Sotirelis M. Thomsen C. Urdiales P. Valek K. Viherkanto S. Weidner T. Ylikorpi M. Young J. Zoennchen 《Space Science Reviews》2009,142(1-4):157-231
Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) is a NASA Explorer Mission-of-Opportunity to stereoscopically image the Earth’s magnetosphere for the first time. TWINS extends our understanding of magnetospheric structure and processes by providing simultaneous Energetic Neutral Atom (ENA) imaging from two widely separated locations. TWINS observes ENAs from 1–100 keV with high angular (~4°×4°) and time (~1-minute) resolution. The TWINS Ly-α monitor measures the geocoronal hydrogen density to aid in ENA analysis while environmental sensors provide contemporaneous measurements of the local charged particle environments. By imaging ENAs with identical instruments from two widely spaced, high-altitude, high-inclination spacecraft, TWINS enables three-dimensional visualization of the large-scale structures and dynamics within the magnetosphere for the first time. This “instrument paper” documents the TWINS design, construction, calibration, and initial results. Finally, the appendix of this paper describes and documents the Southwest Research Institute (SwRI) instrument calibration facility; this facility was used for all TWINS instrument-level calibrations. 相似文献
80.
P. M. E. Decreau J. Etcheto K. Knott A. Pedersen G. L. Wrenn D. T. Young 《Space Science Reviews》1978,22(5):633-645
An attempt has been made to combine data from five instruments on GEOS-1 in order to determine the characteristics of the ambient cold plasma, assess the effects of spacecraft sheaths on the different techniques and establish cross calibration criteria. In addition to measuring plasma density and temperature it is necessary to consider the influence of satellite potential and motion, ionic composition, ion drifts (electric fields), electrons emitted from spacecraft surfaces and any consequent departures from isotropy or Maxwellian distribution.9 October 1977 was selected for the study, the orbit covers a range of L-values between 3.5 and 7.5 giving outbound and inbound plasmapause crossings with plasma densities spanning more than two orders of magnitude. Eclipse data from 7 February 1978 is used to determine the influence of photoelectron emission.Three techniques — active sounding of the plasma frequency resonance (S-301), a mutual impedance measurement (S-304) and DC electric field probe determination of floating potential (S-300) — utilize the long boom sensors in either AC or DC modes. Electrons and ions are measured directly by electrostatic analysers (S-302), mounted on one short radial boom. On the day chosen for study here ions are predominantly protons as determined by the body mounted ion composition experiment (S-303).The techniques using the 20 m long booms agree well in determining density whereas the short boom and body mounted ion detectors are seriously compromised when satellite potential becomes several volts positive. Measurements of satellite potential and plasma temperature agree within 20% among the several instruments making these measurements. Data obtained during the transition from eclipse to sunlight conditions show no discontinuity in the long boom instruments caused by the sudden appearance of photoelectrons. 相似文献