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731.
Approaches in the determination of plant nutrient uptake and distribution in space flight conditions. 总被引:2,自引:0,他引:2
A G Heyenga A Forsman L S Stodieck A Hoehn M Kliss 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,26(2):299-302
The effective growth and development of vascular plants rely on the adequate availability of water and nutrients. Inefficiency in either the initial absorption, transportation, or distribution of these elements are factors which impinge on plant structure and metabolic integrity. The potential effect of space flight and microgravity conditions on the efficiency of these processes is unclear. Limitations in the available quantity of space-grown plant material and the sensitivity of routine analytical techniques have made an evaluation of these processes impractical. However, the recent introduction of new plant cultivating methodologies supporting the application of radionuclide elements and subsequent autoradiography techniques provides a highly sensitive investigative approach amenable to space flight studies. Experiments involving the use of gel based 'nutrient packs' and the radionuclides calcium-45 and iron-59 were conducted on the Shuttle mission STS-94. Uptake rates of the radionuclides between ground and flight plant material appeared comparable. 相似文献
732.
The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar
system water - (720±120)×10−6 in clay minerals and (88±11)×10−6 in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in
the solar nebula between deuterium-rich interstellar water and protosolar H2. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking
720×10−6 as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical
simulations show that the water D/H ratio decreases via an isotopic exchange with H2. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and
the isotopic homogenization of the solar nebula was completed in 106 years, reaching a D/H ratio of 88×10−6. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H2. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs
and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement
with these chondritic data and their interpretation (Texeira et al., 1999).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
733.
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735.
This article presents an innovative time-domain nonlinear mapping-based identification method. The method reported is applied to identify the unknown parameters of multivariable dynamic systems which are mapped by nonlinear differential equations. A systematic identification method is introduced, and a novel algorithm is developed using nonlinear error maps. An analysis of parameter convergence is provided and the regions of convergence can be found using the second method of Lyapunov. Innovative nonquadratic Lyapunov functions are designed and used. Analytical and numerical studies are performed to illustrate and validate the identification concept. The unsteady flight of a high-alpha aircraft in the longitudinal axis is chosen as a nonlinear case study. The unknown parameters are identified. Simulation results show that the model dynamics match the experimental data. The reported example demonstrates that the time-domain nonlinear mapping-based identification method ensures robustness and reduces major shortcomings in stability, convergence, and computational efficiency compared with other algorithms available 相似文献
736.
Metallic Power has demonstrated a regenerative zinc air fuel cell for applications in industrial and specialty vehicles. The fuel cell uses zinc pellets and atmospheric oxygen to generate electric current; the reaction product is zinc oxide, which is collected in a tank. In its present stage of development the 36 V fuel cell will deliver approximately 6 kWh, with a maximum power of 4 kW. The device is refueled at a zinc recycling/refueling station where zinc pellets are pumped into each cell; ZnO is pumped from the tank and replaced with KOH electrolyte. The recycling/refueling unit uses an electrolytic process to convert zinc oxide powder into zinc pellets 相似文献
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738.
Lobanov V.S. Tarasenko N.V. Shulga D.N. Zboroshenko V.N. Fedotov V.P. 《Aerospace and Electronic Systems Magazine, IEEE》2007,22(4):23-29
In article the opportunity of use strapdown inertial navigation system (SINS) on the base of fiber-optic gyroscopes and quartz accelerometers corrected from star sensors and satellite navigation equipment (SNE) for perspective interplanetary spacecrafts motion control on phases of interplanetary trajectory insertion, trajectory correction, and braking during transition to Mars orbit is investigated. Results of onboard control complex accuracy characteristics estimation are presented at the given dynamic spacecraft scheme which is taking into account the liquid oscillations in tanks and structure elements elasticity. At modelling the errors of measuring devices installation, errors of SINS initial alignment and instrumental errors of SINS sensitive elements, variation of control engines parameters were taken into account. The structure of the developed complex of imitation modelling of interplanetary spacecraft controlled motion is resulted. Estimations of active flight legs realization accuracy were received by a method of statistical modelling of spacecraft controlled motion 相似文献
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740.
McEneaney W.M. Fitzpatrick B.G. Lauko I.G. 《IEEE transactions on aerospace and electronic systems》2004,40(4):1191-1216
A command and control (C/sup 2/) problem for military air operations is addressed. Specifically, we consider C/sup 2/ problems for air vehicles against ground-based targets and defensive systems. The problem is viewed as a stochastic game. We restrict our attention to the C/sup 2/ level where the problem may consist of a few unmanned combat air vehicles (UCAVs) or aircraft (or possibly teams of vehicles), less than say, a half-dozen enemy surface-to-air missile air defense units (SAMs), a few enemy assets (viewed as targets from our standpoint), and some enemy decoys (assumed to mimic SAM radar signatures). At this low level, some targets are mapped out and possible SAM sites that are unavoidably part of the situation are known. One may then employ a discrete stochastic game problem formulation to determine which of these SAMs should optimally be engaged (if any), and by what series of air vehicle operations. We provide analysis, numerical implementation, and simulation for full state-feedback and measurement feedback control within this C/sup 2/ context. Sensitivity to parameter uncertainty is discussed. Some insight into the structure of optimal and near-optimal strategies for C/sup 2/ is obtained. The analysis is extended to the case of observations which may be affected by adversarial inputs. A heuristic based on risk-sensitive control is applied, and it is found that this produces improved results over more standard approaches. 相似文献