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271.
Nitrous oxide as a rocket propellant 总被引:1,自引:0,他引:1
Nitrous oxide is introduced as a multi-purpose propellant for spacecraft. Potential space applications of this propellant are given. Based on comparison to conventional systems, a multi-mode nitrous oxide propulsion concept is expected to deliver higher performance. Main features of a self-pressurising, nitrous oxide storage system are described. A nitrous oxide catalytic decomposition technique is suggested for restartable spacecraft propulsion. Up-to-date experimental results are presented. A conclusion describes the long-term feasibility of novel nitrous oxide propulsion option concepts. 相似文献
272.
Road-map assisted ground moving target tracking 总被引:3,自引:0,他引:3
Martin Ulmke Wolfgang Koch 《IEEE transactions on aerospace and electronic systems》2006,42(4):1264-1274
Tracking ground targets with airborne GMTI (ground moving target indicator) sensor measurements proves to be a challenging task due to high target density, high clutter, and low visibility. The exploitation of nonstandard background information such as road maps and terrain information is therefore highly desirable for the enhancement of track quality and track continuity. The present paper presents a Bayesian approach to incorporate such information consistently. It is particularly suited to deal with winding roads and networks of roads. The target dynamics is modeled in quasi one-dimensional road coordinates and mapped onto ground coordinates using linear road segments taking road map errors into account. The case of several intersecting roads with different characteristics, such as mean curvature, slope, or visibility, is treated within an interacting multiple model (IMM) scheme. Targets can be masked both by the clutter notch of the sensor and by terrain obstacles. Both effects are modeled using a sensor-target state dependent detection probability. The iterative filter equations are formulated within a framework of Gaussian sum approximations on the one hand and a particle filter approach on the other hand. Simulation results for single targets taken from a realistic ground scenario show strongly reduced target location errors compared with the case of neglecting road-map information. By modeling the clutter notch of the GMTI sensor, early detection of stopping targets is demonstrated 相似文献
273.
Neural network development for the forecasting of upper atmosphere parameter distributions 总被引:1,自引:0,他引:1
Jeffrey D. Martin Yu T. Morton Qihou Zhou 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2480-2485
This paper presents a neural network modeling approach to forecast electron concentration distributions in the 150–600 km altitude range above Arecibo, Puerto Rico. The neural network was trained using incoherent scatter radar data collected at the Arecibo Observatory during the past two decades, as well as the Kp geomagnetic index provided by the National Space Science Data Center. The data set covered nearly two solar cycles, allowing the neural network to model daily, seasonal, and solar cycle variations of upper atmospheric parameter distributions. Two types of neural network architectures, feedforward and Elman recurrent, are used in this study. Topics discussed include the network design, training strategy, data analysis, as well as preliminary testing results of the networks on electron concentration distributions. 相似文献
274.
Martin J. Duncan 《Space Science Reviews》2008,138(1-4):109-126
It is widely believed that cometary orbits contain important clues to both the outer solar system’s current structure and its past dynamical evolution. The first part of this paper summarizes the results of numerical simulations designed to study the dynamical origins of observed comets and to link the observed populations to the reservoirs from which they are currently leaking. The second part reviews simulations which are designed to study the dynamical origin of the reservoirs themselves. The paper concludes with a brief discussion of the currently unresolved issue of where in the primordial solar nebula the different dynamical classes of observed comets originated. 相似文献
275.
276.
Vytenis M. Vasyliūnas 《Space Science Reviews》2011,158(1):91-118
Many widely used methods for describing and understanding the magnetosphere are based on balance conditions for quasi-static
equilibrium (this is particularly true of the classical theory of magnetosphere/ionosphere coupling, which in addition presupposes
the equilibrium to be stable); they may therefore be of limited applicability for dealing with time-variable phenomena as
well as for determining cause-effect relations. The large-scale variability of the magnetosphere can be produced both by changing
external (solar-wind) conditions and by non-equilibrium internal dynamics. Its developments are governed by the basic equations
of physics, especially Maxwell’s equations combined with the unique constraints of large-scale plasma; the requirement of
charge quasi-neutrality constrains the electric field to be determined by plasma dynamics (generalized Ohm’s law) and the
electric current to match the existing curl of the magnetic field. The structure and dynamics of the ionosphere/magnetosphere/solar-wind
system can then be described in terms of three interrelated processes: (1) stress equilibrium and disequilibrium, (2) magnetic
flux transport, (3) energy conversion and dissipation. This provides a framework for a unified formulation of settled as well
as of controversial issues concerning, e.g., magnetospheric substorms and magnetic storms. 相似文献
277.
William Marshall Mark Shirley Zachary Moratto Anthony Colaprete Gregory Neumann David Smith Scott Hensley Barbara Wilson Martin Slade Brian Kennedy Eric Gurrola Leif Harcke 《Space Science Reviews》2012,167(1-4):71-92
The Lunar CRater Observations and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket stage into a permanently shadowed region near the lunar south pole. The Sheperding Spacecraft (SSC) separated ~9 hours before impact and performed a small braking maneuver in order to observe the Centaur impact plume, looking for evidence of water and other volatiles, before impacting itself. This paper describes the registration of imagery of the LCROSS impact region from the mid- and near-infrared cameras onboard the SSC, as well as from the Goldstone radar. We compare the Centaur impact features, positively identified in the first two, and with a consistent feature in the third, which are interpreted as a 20 m diameter crater surrounded by a 160 m diameter ejecta region. The images are registered to Lunar Reconnaisance Orbiter (LRO) topographical data which allows determination of the impact location. This location is compared with the impact location derived from ground-based tracking and propagation of the spacecraft’s trajectory and with locations derived from two hybrid imagery/trajectory methods. The four methods give a weighted average Centaur impact location of ?84.6796°, ?48.7093°, with a 1σ uncertainty of 115 m along latitude, and 44 m along longitude, just 146 m from the target impact site. Meanwhile, the trajectory-derived SSC impact location is ?84.719°, ?49.61°, with a 1σ uncertainty of 3 m along the Earth vector and 75 m orthogonal to that, 766 m from the target location and 2.803 km south-west of the Centaur impact. We also detail the Centaur impact angle and SSC instrument pointing errors. Six high-level LCROSS mission requirements are shown to be met by wide margins. We hope that these results facilitate further analyses of the LCROSS experiment data and follow-up observations of the impact region. 相似文献
278.
279.
Although macroscale features dominate astrophysical images and energetics, the physics is controlled through microscale transport processes (conduction, diffusion) that mediate the flow of mass, momentum, energy, and charge. These microphysical processes manifest themselves in key (all) boundary layers and also operate within the body of the plasma. Crucially, most plasmas of interest are rarefied to the extent that classical particle collision length- and time-scales are long. Collective plasma kinetic phenomena then serve to scatter or otherwise modify the particle distribution functions and in so-doing govern the transport at the microscale level. Thus collisionless plasmas are capable of supporting thin shocks, current sheets which may be prone to magnetic reconnection, and the dissipation of turbulence cascades at kinetic scales. This paper lays the foundation for the accompanying collection that explores the current state of knowledge in this subject. The richness of plasma kinetic phenomena brings with it a rich diversity of microphysics that does not always, if ever, simply mimic classical collision-dominated transport. This can couple the macro- and microscale physics in profound ways, and in ways which thus depend on the astrophysical context. 相似文献
280.
Bernhard Schläppi Kathrin Altwegg Timm Riesen Martin Rubin 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
The Pioneer anomaly, an unexpected acceleration of the Pioneer 10 and 11 spacecraft of ∼8.5 × 10−10 ms−2 directed towards the inner Solar System, has been of great interest for the physics community during the past decade: considered explanations range from new physical concepts to conventional mechanism. It is shown that non-isotropic outgassing of the complete spacecraft structure is comparable in magnitude and direction to the effect and should be considered as a significant contribution to the anomalous acceleration. Although gas leaks from e.g. the propulsion system and propulsive mass loss mechanism have been discarded as possible explanations for the anomaly, the arguments used against such mechanisms do not apply to global outgassing from the spacecraft. 相似文献