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151.
K. Le Bail F.G. Lemoine D.S. Chinn 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The NASA GSFC DORIS analysis center has provided weekly DORIS solutions from November 1992 to January 2009 (839 SINEX files) of station positions and Earth Orientation Parameters for inclusion in the DORIS contribution to ITRF2008. The NASA GSFC GEODYN orbit determination software was used to process the orbits and produce the normal equations. The weekly SINEX gscwd10 submissions included DORIS data from Envisat, TOPEX/Poseidon, SPOT-2, SPOT-3, SPOT-4, SPOT-5. The orbits were mostly seven days in length (except for weeks with data gaps or maneuvers). The processing used the GRACE-derived EIGEN-GL04S1 gravity model, updated modeling for time-variable gravity, the GOT4.7 ocean tide model and tuned satellite-specific macromodels for SPOT-2, SPOT-3, SPOT-4, SPOT-5 and TOPEX/Poseidon. The University College London (UCL) radiation pressure model for Envisat improves nonconservative force modeling for this satellite, reducing the median residual empirical daily along-track accelerations from 3.75 × 10−9 m/s2 with the a priori macromodel to 0.99 × 10−9 m/s2 with the UCL model. For the SPOT and Envisat DORIS satellite orbits from 2003 to 2008, we obtain average RMS overlaps of 0.8–0.9 cm in the radial direction, 2.1–3.4 cm cross-track, and 1.7–2.3 cm along-track. The RMS orbit differences between Envisat DORIS-only and SLR & DORIS orbits are 1.1 cm radially, 6.4 cm along-track and 3.7 cm cross-track and are characterized by systematic along-track mean offsets due to the Envisat DORIS system time bias of ±5–10 μs. We obtain a good agreement between the geometrically-determined geocenter parameters and geocenter parameters determined dynamically from analysis of the degree one terms of the geopotential. The intrinsic RMS weekly position repeatability with respect to the IDS-3 combination ranges from 2.5 to 3.0 cm in 1993–1994 to 1.5 cm in 2007–2008. 相似文献
152.
V. Florinski G.P. Zank J.A. le Roux 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(2):361-367
Low-energy termination shock particle populations observed by the Voyagers upstream of the shock exhibited strong field-aligned beaming with anisotropies of the order of unity. The Parker transport equation is valid only for nearly isotropic phase space distributions and is inapplicable to these highly beamed populations. The usual approach is to revert to the more general focused transport equation retaining pitch-angle information. We developed a complimentary technique employing a three-moment expansion of the Skilling equation using Legendre polynomials. We investigate the effects of adiabatic focusing and reflection on the diffusive acceleration process at oblique shock waves. It is shown that low-energy particle intensities are discontinuous and sharply peaked at the shock, consistent with the observations. Particle spectra are not only harder than the power laws predicted from diffusive transport theory, but also exhibit spectral gaps near the low-energy acceleration threshold due to more efficient acceleration by scattering and mirroring. Our model also predicts upstream anisotropies as high as 100% for highly oblique shocks whereas downstream distributions are nearly isotropic. 相似文献
153.
配置管理是PDM的核心业务基础,本文从论述实现PDM应用中配置管理功能的重要性出发,介绍分析了国际上多种标准对配置管理的基本要求和方法. 相似文献
154.
One long-standing difficulty in estimating the large-scale ocean circulation is the inability to observe absolute current
velocities. Both conventional hydrographic measurements and altimetric measurements provide observations of currents relative
to an unknown velocity at a reference depth in the case of hydrographic data, and relative to mean currents calculated over
some averaging period in the case of altimetric data. Space gravity missions together with altimetric observations have the
potential to overcome this difficulty by providing absolute estimates of the velocity of surface oceanic currents. The absolute
surface velocity estimates will in turn provide the reference level velocities that are necessary to compute absolute velocities
at any depth level from hydrographic data.
Several studies have been carried out to quantify the improvements expected from ongoing and future space gravity missions.
The results of these studies in terms of volume flux estimates (transport of water masses) and heat flux estimates (transport
of heat by the ocean) are reviewed in this paper. The studies are based on ocean inverse modeling techniques that derive impact
estimates solely from the geoid error budgets of forthcoming space gravity missions. Despite some differences in the assumptions
made, the inverse modeling calculations all point to significant improvements in estimates of oceanic fluxes. These improvements,
measured in terms of reductions of uncertainties, are expected to be as large as a factor of 2.
New developments in autonomous ocean observing systems will complement the developments expected from space gravity missions.
The synergies of in situ and satellite observing systems are considered in the conclusion of this paper.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
155.
P Chagvardieff B Dimon A Souleimanov D Massimino S Le Bras M Pean D Louche-Teissandier 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,20(10):1971-1974
Wheat, potato, pea and tomato crops were cultivated from seeding to harvest in a controlled and confined growth chamber at elevated CO2 concentration (3700 microL L-1) to examine the effects on biomass production and edible part yields. Different responses to high CO2 were recorded, ranging from a decline in productivity for wheat, to slight stimulation for potatoes, moderate increase for tomatoes, and very large enhancement for pea. Mineral content in wheat and pea seeds was not greatly modified by the elevated CO2. Short-term experiments (17 d) were conducted on potato at high (3700 microL L-1) and very high (20,000 microL L-1) CO2 concentration and/or low O2 partial pressure (approximately 20,600 microL L-1 or 2 kPa). Low O2 was more effective than high CO2 in total biomass accumulation, but development was affected: Low O2 inhibited tuberization, while high CO2 significantly increased production of tubers. 相似文献
156.
Baqué M Le Postollec A Ravelet C Peyrin E Coussot G Desvignes I Incerti S Moretto P Dobrijevic M Vandenabeele-Trambouze O 《Astrobiology》2011,11(3):207-211
Biochips are promising instruments for the search for organic molecules in planetary environments. Nucleic acid aptamers are powerful affinity receptors known for their high affinity and specificity, and therefore are of great interest for space biochip development. A wide variety of aptamers have already been selected toward targets of astrobiological interest (from amino acids to microorganisms). We present a first study to test the resistance of these receptors to the constraints of the space environment. The emphasis is on the effect of cosmic rays on the molecular recognition properties of DNA aptamers. Experiments on beam-line facilities have been conducted with 2 MeV protons and fluences much higher than expected for a typical mission to Mars. Our results show that this irradiation process did not affect the performances of DNA aptamers as molecular recognition tools. 相似文献
157.
Gauvrit H. Le Cadre J.P. Jauffret C. 《IEEE transactions on aerospace and electronic systems》1997,33(4):1242-1257
Traditional multihypothesis tracking methods rely upon an enumeration of all the assignments of measurements to tracks. Pruning and gating are used to retain only the most likely hypotheses in order to drastically limit the set of feasible associations. The main risk is to eliminate correct measurement sequences. The probabilistic multiple hypothesis tracking (PMHT) method has been developed by Streit and Luginbuhl in order to reduce the drawbacks of "strong" assignments. The PMHT method is presented in a general mixture densities perspective. The Expectation-Maximization (EM) algorithm is the basic ingredient for estimating mixture parameters. This approach is then extended and applied to multitarget tracking for nonlinear measurement models in the passive sonar perspective. 相似文献
158.
159.
将三维MHD双温入口模型的计算结果作为入口条件,运用DSMC(Direct Simulation Monte-Carlo)/PIC(Particle in Cell)流体混合算法,模拟实验室PPT样机羽流。验证计算显示,该模型具有模拟脉冲等离子体推力器羽流的能力。对不同初始电压和电容下的羽流场进行了模拟,给出了出口平面返流质量流率的变化情况和20μs羽流中离子和CEX离子的分布情况。计算结果显示,高能量状态对应高质量流率,高质量流率对应高动量,高动量离子和中性粒子对航天器撞击会造成更强影响。 相似文献
160.