Minimum control for spacecraft formations in a J2 perturbed environment     

M. Sabatini  D. Izzo  G. B. Palmerini 《Celestial Mechanics and Dynamical Astronomy》2009,105(1-3):141-157

Large ΔV amounts are often required to maintain the relative geometry which is needed to implement a formation flying concept. A wise use of the orbital environment makes the orbit keeping phase easier, reducing the overall propellant consumption. A first important step in this direction is the selection of formation configurations and orbits which, while still satisfying the mission requirements, are less subject to perturbations resulting naturally in closed relative motion. Within this frame, a number of studies have been recently carried out in order to identify possible sets of invariant relative orbits under the effects of the Earth oblateness, a conservative force commonly referred to as J2 which is also the most important perturbation for Low Earth Orbit. These efforts clearly marked the difficulties connected with achieving genuine periodic relative motion under J2 effect, but they also showed the existence of a set of conditions on the orbital parameters which allow for quasi-periodic J2 trajectories. This paper presents these particular trajectories, by means of deeper theoretical explanations, showing the dependency of the shape of the relative configurations on the orbital inclination. Since the quasi-periodic trajectories cannot be written analytically, and moreover, they are very sensitive with respect to the initial conditions, difficulties arise when trying to exploit these paths as reference for the control of a formation. This paper proposes a novel approach to find, from the actual quasi periodic natural trajectories, minimal control periodic reference trajectories. Next, it evaluates quantitatively the amount of propellant which is needed to control a spacecraft formation along these paths. The choice of Hill’s classical circular projected configuration as a nominal trajectory is considered as a comparison, showing the clear advantages of the proposed guidance design, which assumes low-perturbed periodic reference orbits as nominal trajectories.  相似文献   

Ground Water Discharge of Agricultural Pesticides and Nutrients to Estuarine Surface Water   总被引：2，自引：0，他引：2  

Daniel L. Gallagher  rea M. Dietrich  William G. Reay  Mary C. Hayes  George M. Simmons Jr. 《Ground Water Monitoring & Remediation》1996,16(1):118-129

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Field Studies on the Fate and Transport of Pharmaceutical Residues in Bank Filtration   总被引：5，自引：0，他引：5  

Thomas Heberer  y Mechlinski  Britta Fanck  rea Knappe  Gudrun Massmann  Asaf Pekdeger  Birgit Fritz 《Ground Water Monitoring & Remediation》2004,24(2):70-77

Bank filtration and artificial ground water recharge are important, effective, and cheap techniques for surface water treatment and removal of microbes, as well as inorganic, and some organic, contaminants. Nevertheless, physical, chemical, and biological processes of the removal of impurities are not understood sufficiently. A research project titled Natural and Artificial Systems for Recharge and Infiltration attempts to provide more clarity in the processes affecting the removal of these contaminants. The project focuses on the fate and transport of selected emerging contaminants during bank filtration at two transects in Berlin, Germany. Several detections of pharmaceutically active compounds (PhACs) in ground water samples from bank filtration sites in Germany led to furthering research on the removal of these compounds during bank filtration. In this study, six PhACs including the analgesic drugs diclofenac and propyphenazone, the antiepileptic drugs carbamazepine and primidone, and the drug metabolites clofibric acid and 1-acetyl-1-methyl-2-dimethyl-oxamoyl-2-phenylhydrazide were found to leach from the contaminated streams and lakes into the ground water. These compounds were also detected at low concentrations in receiving public supply wells. Bank filtration either decreased the concentrations by dilution (e.g., for carbamazepine and primidone) and partial removal (e.g., for diclofenac), or totally removed PhACs (e.g., bezafibrate, indomethacine, antibiotics, and estrogens). Several PhACs, such as carbamazepine and especially primidone, were readily transported during bank filtration. They are thought to be good indicators for evaluating whether surface water is impacted by contamination from municipal sewage effluent or whether contamination associated with sewage effluent can be transported into ground water at ground water recharge sites.  相似文献   

Development of a GIS-based Methodology for Quantifying Predation Risk in a Spatial Context     

Andrew D Kliskey  rea E Byrom 《Transactions in GIS》2004,8(1):13-22

We used GIS‐based techniques and inductive‐spatial modelling to analyse spatial interactions between predator and prey populations, a unique example of an extension to an existing ecological approach. The approach identifies spatial predictors of predation and is demonstrated for braided riverbed habitat in the Mackenzie Basin, New Zealand. This habitat is highly modified and is the focus of intensive management because of the presence of introduced mammalian predators and their native prey (primarily nests of ground‐nesting native birds that are extremely vulnerable to predators). Native prey are currently in decline due to predation; GIS techniques therefore have the potential to augment conventional ecological techniques and management practices. Predation risk to nests of ground‐nesting birds was interpolated across an area using inverse distance weighting. A linear regression model for spatio‐temporal predictors of nest predation was deter‐mined using a predation risk surface and three separate surfaces representing predator abundance, prey abundance, and habitat structure. These measures can enhance the non‐spatial results from conventional ecological inquiry and help solve pest management problems affecting the natural environment.  相似文献   

Critical reflections on practice: the changing roles of three physical geographers carrying out research in a developing country     

Jayalaxshmi Mistry  rea Berardi  Matthew Simpson 《Area》2009,41(1):82-93

To date, discussions on positionality and the relationship with research collaborators have been very much in the human geography realm. In this paper, we explore issues of expertise, positionality, collaboration and participation from our perspective as physical geographers working in a developing country context. We trace our journey from identifying ourselves as top-down 'experts' to participatory 'facilitators', and the difficulties and dilemmas encountered during this journey as we coped with the contrasting challenges of academic demands and local necessities. Our experiences highlight the many assumptions we make about doing research in developing countries and the real lack of capacity in these places to undertake typical short-term research projects designed in the developed world. We conclude with a call for a longer term and deeper commitment by physical geographers to the people that we engage with in our research .  相似文献   

Fluorescent Dye and Media Properties Affecting Sorption and Tracer Selection   总被引：4，自引：0，他引：4  

Torez Kasnavia  De Vu  David A. Sabatini 《Ground water》1999,37(3):376-381

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Distribution pattern and population structure of Calanus australis Brodsky, 1959 over the southern Patagonian Shelf off Argentina in summer     

Sabatini  M. E.; Ramirez  F. C.; Martos  P. 《ICES Journal of Marine Science》2000,57(6):1856-1866

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Community structure of bathyal decapod crustaceans off South-Eastern Sardinian deep-waters (Central-Western Mediterranean)     

Maria Cristina Follesa  Cristina Porcu  Alice Gastoni  Antonello Mulas  rea Sabatini  & Angelo Cau 《Marine Ecology》2009,30(S1):188-199

Community structure and faunal composition of bathyal decapod crustaceans off South-Eastern Sardinian deep-waters (Central-Western Mediterranean) were investigated. Samples were collected during 32 hauls between 793 and 1598 m in depth over the 2003–2007 period. A total of 1900 decapod specimens belonging to 23 species were collected. Multivariate analysis revealed the occurrence of three faunistic assemblages related to depth: (i) an upper slope community at depths of 793–1002 m; (ii) a middle slope community at depths of 1007–1212 m and (iii) a lower slope community at depths greater 1420 m. In the upper and middle slopes the benthic ( Polycheles typhlops ) and epibenthic–endobenthic feeders (mainly Aristeus antennatus and Geryon longipes ), which eat infaunal prey, were dominant, followed by the macroplankton–epibenthic feeders such as Acanthephyra eximia and Plesionika acanthonotus . In the deepest stratum, the most remarkable feature was the prevalence of macroplankton–epibenthic feeders ( A. eximia and P. acanthonotus ). A small percentage of the benthic deep-sea lobster Polycheles sculptus was also present. The biomass presented higher values in the middle slope and declined strongly in the lower slope. There was no general pattern of mean individual weight/size versus depth among decapods, and the changes seemed to be species-specific with different trends.  相似文献   

Estimating copepods' abundance with paired nets: Implications of mesh size for population studies     

J.C. Antacli  D. Hernández  M.E. Sabatini 《Journal of Sea Research》2010,63(1):71-77

Three representative copepods occurring in the southern Patagonian shelf, i.e., Calanus australis, Drepanopus forcipatus and Oithona helgolandica were sampled by the first time with paired nets of 66 and 150 µm mesh size. The stage-specific estimates of abundance of their populations were statistically analyzed to assess differences in the catchability by both plankton nets. Differences between nets were significant only for smaller developmental stages and species, with higher catchabilities by the 66 µm net, while no differences were detected for most of the stages of the medium and large size species. A significant effect of the spatial distribution on the estimates of abundance was detected for the majority of the species and stages but the interaction between mesh size and spatial distribution was not significant. This means that differences between both nets were maintained across the latitudinal gradient. Our results strongly suggest the convenience of using the 66 µm plankton net to estimate the abundance of the three copepods' populations. Furthermore, we propose a correction factor to adjust past estimates of abundances from 150 µm mesh collections.  相似文献   

Effects of sewage pollution and bacterial load on growth and oxidative balance in the freshwater mussel Diplodon chilensis     

Sebastián E. Sabatini  Iara Rocchetta  Carlos M. Luquet  Mariana I. Guido  María del Carmen Ríos de Molina 《Limnologica》2011,41(4):356-362

The effect of chronic exposure to Escherichia coli on morphometrical parameters, different antioxidant defenses, lipid peroxidation and protein oxidation has been studied in digestive gland of the freshwater mussel Diplodon chilensis in the laboratory during 6 weeks. In, a parallel field study, we evaluated morphometrical and oxidative stress parameters in digestive glands of mussels collected from a sewage polluted area (SMA) and from a pristine area (control). Both sites were characterized by analyzing bacteriological and physic-chemical parameters. In the laboratory D. chilensis was able to filter bacteria at high concentrations with a clearing rate of 0.510 ± 0.036 L/h per gram of dry soft tissue mass. The digestive gland mass to shell length ratio (DGM/SL), reduced glutathione (GSH), lipid peroxidation, as concentration of thiobartituric acid reactive substances (TBARS), and glutathione-S-transferase (GST) activity of mussels fed with bacteria were significantly higher than those of control mussels after the fourth week. Fecal bacteria in lake water samples were undetectable in the control, and higher than 24,000 MPN (most probable number)/100 mL in SMA. DGM/SL was higher in SMA. No differences between sites were observed in total lipid and protein content, neither in superoxide dismutase activity. GSH content was higher in SMA, with no difference in the oxidized form. GST activity and MDA were significantly higher in SMA but protein oxidation was not affected.  相似文献