A multi-objective approach to the design of low thrust space trajectories using optimal control     

Michael Dellnitz  Sina Ober-Blöbaum  Marcus Post  Oliver Schütze  Bianca Thiere 《Celestial Mechanics and Dynamical Astronomy》2009,105(1-3):33-59

In this article, we introduce a novel three-step approach for solving optimal control problems in space mission design. We demonstrate its potential by the example task of sending a group of spacecraft to a specific Earth L ₂ halo orbit. In each of the three steps we make use of recently developed optimization methods and the result of one step serves as input data for the subsequent one. Firstly, we perform a global and multi-objective optimization on a restricted class of control functions. The solutions of this problem are (Pareto-)optimal with respect to ΔV and flight time. Based on the solution set, a compromise trajectory can be chosen suited to the mission goals. In the second step, this selected trajectory serves as initial guess for a direct local optimization. We construct a trajectory using a more flexible control law and, hence, the obtained solutions are improved with respect to control effort. Finally, we consider the improved result as a reference trajectory for a formation flight task and compute trajectories for several spacecraft such that these arrive at the halo orbit in a prescribed relative configuration. The strong points of our three-step approach are that the challenging design of good initial guesses is handled numerically by the global optimization tool and afterwards, the last two steps only have to be performed for one reference trajectory.  相似文献   

Ground-Water Exploration Based on Lineament Analysis and Reproducibility Tests   总被引：6，自引：0，他引：6  

Per Sander  Timothy B. Minor  Matthew M. Chesley 《Ground water》1997,35(5):888-894

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The Potential Response of Terrestrial Carbon Storage to Changes in Climate and Atmospheric CO2   总被引：3，自引：0，他引：3  

Anthony W. King  Wilfred M. Post  Stan D. Wullschleger 《Climatic change》1997,35(2):199-227

We use a georeferenced model of ecosystem carbon dynamics to explore the sensitivity of global terrestrial carbon storage to changes in atmospheric CO₂ and climate. We model changes in ecosystem carbon density, but we do not model shifts in vegetation type. A model of annual NPP is coupled with a model of carbon allocation in vegetation and a model of decomposition and soil carbon dynamics. NPP is a function of climate and atmospheric CO₂ concentration. The CO₂ response is derived from a biochemical model of photosynthesis. With no change in climate, a doubling of atmospheric CO₂ from 280 ppm to 560 ppm enhances equilibrium global NPP by 16.9%; equilibrium global terrestrial ecosystem carbon (TEC) increases by 14.9%. Simulations with no change in atmospheric CO₂ concentration but changes in climate from five atmospheric general circulation models yield increases in global NPP of 10.0–14.8%. The changes in NPP are very nearly balanced by changes in decomposition, and the resulting changes in TEC range from an increase of 1.1% to a decrease of 1.1%. These results are similar to those from analyses using bioclimatic biome models that simulate shifts in ecosystem distribution but do not model changes in carbon density within vegetation types. With changes in both climate and a doubling of atmospheric CO₂, our model generates increases in NPP of 30.2–36.5%. The increases in NPP and litter inputs to the soil more than compensate for any climate stimulation of decomposition and lead to increases in global TEC of 15.4–18.2%.  相似文献   

An experimental study of turbulent boundary layer with slit suction     

Sander M. Calisal   《Ocean Engineering》1978,5(5-6):325-345

Experiments are performed on a flat plate with a transverse suction slit in the Reynolds number range 5 × 10⁵ < R_e < 1.1 × 10⁶. Mean velocity profiles, RMS values are measured with hot wire anemometry. Friction velocity is numerically calculated. The experiments showed that a classical boundary layer parameter α is related to the suction coefficient S_c with an equation of the form: .The value of A seems to depend strongly on the relative location with respect to suction slit and possibly weakly on Reynolds number.  相似文献   

Species diversity of the Okavango Delta, Botswana   总被引：4，自引：0，他引：4  

Lars Ramberg  Peter Hancock  Markus Lindholm  Thoralf Meyer  Susan Ringrose  Jan Sliva  Jo Van As  Cornelis Vander Post 《Aquatic Sciences - Research Across Boundaries》2006,68(3):310-337

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Crustal residence history and garnet Sm–Nd ages of high-grade metamorphic rocks from the Windmill Islands area,East Antarctica     

Andreas Möller  Nicholas J. Post  Bas J. Hensen 《International Journal of Earth Sciences》2002,91(6):993-1004

Nd whole-rock data from the Windmill Islands area yield early Proterozoic to middle Archaean Nd model ages. These crustal residence times are consistent with regional correlations with other parts of Antarctica (Bunger Hills, Denman Glacier area) and the Albany-Fraser Orogen of south-western Australia during the Mid-Proterozoic and thus support reconstructions with a continuous Mid-Proterozoic orogen in these areas. The new Nd isotope data provide strong evidence that no age boundary exists between the higher- and lower-grade parts of the Windmill Islands area, and that the metamorphic complex represents a single terrane with a common crustal history. The data support the notion of a time-link between the occurrence of intrusive charnockites (C-type magmas) and high-grade metamorphism. The magmatic rocks and orthogneisses in the area are interpreted to have a mixed source consisting of older crustal components, i.e. older sediments (ca. 3.2-2.6 Ga) and a younger mafic component (ca. 1.9 Ga). Two garnet Sm-Nd isochrons yield ages of 1156ᆥ Ma and 1137DŽ.5 Ma and are identical to SHRIMP U-Pb results on monazite from these samples. A garnet Sm-Nd age of 1123ᆡ Ma for the Ford granite is significantly younger than the SHRIMP U-Pb zircon age for this sample. The difference relates to the different closure temperature of each isotopic system and is thus interpreted as initial cooling after granulite facies metamorphism.  相似文献   

Iodine Chemistry and its Role in Halogen Activation and Ozone Loss in the Marine Boundary Layer: A Model Study   总被引：6，自引：0，他引：6  

Rainer Vogt  Rolf Sander  Roland von Glasow  Paul J. Crutzen 《Journal of Atmospheric Chemistry》1999,32(3):375-395

A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICl, and I2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO-3. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(1D) + H2O 2OH, HO2 + O3 OH + 2O2, and OH + O3 HO2 + O2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.  相似文献   

A simplified hydrological rainfall-runoff model — Comment     

J.-Y. Parlange  G. Sander 《Journal of Hydrology》1985,80(3-4):395-397

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Holocene flooding history of the Lower Tagus Valley (Portugal)     

Geert‐Jan Vis  Sjoerd J. P. Bohncke  Heike Schneider  Cornelis Kasse  Sander Coenraads‐Nederveen  Koen Zuurbier  Jouke Rozema 《第四纪科学杂志》2010,25(8):1222-1238

The present paper aims to reconstruct the Lower Tagus Valley flooding history for the last ca. 6500 a, to explore the suitability of pollen‐based local vegetation development in supporting the reconstruction of flooding history, and to explain fluvial activity changes in terms of allogenic (climate, human impact) and autogenic (system intrinsic) processes. The flooding history has been determined by cored sedimentary records located ～18 km apart in distal, low‐energy backswamps on both sides of the Tagus channel. In these low‐energy backswamps, fine‐grained sediment layers deposited from suspended load of overbank flood water reflect periods with multiple overbank floods. By means of a multi‐proxy approach (sedimentology, magnetic susceptibility, grain size, loss‐on‐ignition, carbonate content and pollen), sedimentary and environmental changes were identified. At both sites, synchronous lithological intervals accumulated, suggesting a common origin for the changes in fluvial activity since ca. 6500 cal. a BP. Based on lithological changes, three phases of high fluvial activity (6500–5500, 4900–3500 and 1000–0 cal. a BP) and two phases of low fluvial activity (5500–4900 and 3500–1000 cal. a BP) were identified. Two periods with dominant allogenic controls on fluvial activity in the Lower Tagus Valley were identified: relative sea level (6500–5500 cal. a BP) and human impact (1000–0 cal. a BP). During the intermediate period, changes in fluvial activity may have been caused by climate (5500–1000 cal. BP), but unambiguous correlations are difficult to make. This is due to the way allogenic controls are translated through the fluvial system, the geomorphological differences between upstream and downstream studies and autogenic processes. The comparison of local vegetation development and flooding phases as reconstructed using sedimentology shows a limited added value of using local palynology as a proxy for fluvial activity. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Aeromagnetic surveying using a simulated unmanned aircraft system     

Raymond M. Caron  Claire Samson  Paul Straznicky  Stephen Ferguson  Luise Sander 《Geophysical Prospecting》2014,62(2):352-363

Carleton University and Sander Geophysics are developing an unmanned aircraft system (UAS) for aeromagnetic surveying. As an early indication of the expected performance of the unmanned aircraft system, a simulated unmanned aircraft system (sUAS) was built. The simulated unmanned aircraft system is a T‐shaped structure configured as a horizontal gradiometer with two cesium magnetometers spaced 4.67 m apart, which is the same sensor geometry as planned for the unmanned aircraft system. The simulated unmanned aircraft system is flown suspended beneath a helicopter. An 8.5 km² area in the Central Metasedimentary Belt of the Grenville Province, near Plevna, Ontario, Canada, was surveyed with the simulated unmanned aircraft system suspended 50 m above ground. The survey site was chosen on the basis of its complex geological structure. The total magnetic intensity (TMI) data recorded were compared to that obtained during a conventional fixed‐wing survey and a ground survey. Transverse magneto‐gradiometric data were also recorded by the simulated unmanned aircraft system. The simulated unmanned aircraft system total magnetic intensity data have a higher resolution than the conventional fixed‐wing data and were found to have a similar resolution to that of the ground survey data. The advantages of surveying with the simulated unmanned aircraft system were: (1) the acquisition of a detailed data set free of gaps in coverage at a low altitude above the terrain and (2) substantial saving of time and effort. In the survey site, the 4.67 m simulated unmanned aircraft system gradiometer measured the transverse magnetic gradient reliably up to an altitude of 150 m above ground.  相似文献