Evaluation of GPS orbit prediction strategies for the IGS Ultra-rapid products   总被引：2，自引：1，他引：1  

Kevin K. Choi  Jim Ray  Jake Griffiths  Tae-Suk Bae 《GPS Solutions》2013,17(3):403-412

The International GNSS Service (IGS) provides Ultra-rapid GPS & GLONASS orbits every 6 h. Each product is composed of 24 h of observed orbits with predicted orbits for the next 24 h. We have studied how the orbit prediction performance varies as a function of the arc length of the fitted observed orbits and the parameterization strategy used to estimate the empirical solar radiation pressure (SRP) effects. To focus on the dynamical aspects of the problem, nearly ideal conditions have been adopted by using IGS Rapid orbits and known earth rotation parameters (ERPs) as observations. Performance was gauged by comparison with Rapid orbits as truth by examining WRMS and median orbit differences over the first 6-h and the full 24-h prediction intervals, as well as the stability of the Helmert frame alignment parameters. Two versions of the extended SRP orbit model developed by the Centre for Orbit Determination in Europe (CODE) were tested. Adjusting all nine SRPs (offsets plus once-per-revolution sines and cosines in each satellite-centered frame direction) for each satellite shows smaller mean sub-daily, scale, and origin translation differences. On the other hand, eliminating the four once-per-revolution SRP parameters in the sun-ward and the solar panel axis directions yields orbit predictions that are much more rotationally stable. We found that observed arc lengths of 40–45 h produce the most stable and accurate predictions during 2010. A combined strategy of rotationally aligning the 9 SRP results to the 5 SRP frame should give optimal predictions with about 13 mm mean WRMS residuals over the first 6 h and 50 mm over 24 h. Actual Ultra-rapid performance will be degraded due to the unavoidable rotational errors from ERP predictions.  相似文献   

Integrating the LISFLOOD‐FP 2D hydrodynamic model with the CAESAR model: implications for modelling landscape evolution     

Tom J. Coulthard  Jeff C. Neal  Paul D. Bates  Jorge Ramirez  Gustavo A. M. de Almeida  Greg R. Hancock 《地球表面变化过程与地形》2013,38(15):1897-1906

Landscape evolution models (LEMs) simulate the geomorphic development of river basins over long time periods and large space scales (100s–1000s of years, 100s of km²). Due to these scales they have been developed with simple steady flow models that enable long time steps (e.g. years) to be modelled, but not shorter term hydrodynamic effects (e.g. the passage of a flood wave). Nonsteady flow models that incorporate these hydrodynamic effects typically require far shorter time steps (seconds or less) and use more expensive numerical solutions hindering their inclusion in LEMs. The recently developed LISFLOOD‐FP simplified 2D flow model addresses this issue by solving a reduced form of the shallow water equations using a very simple numerical scheme, thus generating a significant increase in computational efficiency over previous hydrodynamic methods. This leads to potential convergence of computational cost between LEMs and hydrodynamic models, and presents an opportunity to combine such schemes. This paper outlines how two such models (the LEM CAESAR and the hydrodynamic model LISFLOOD‐FP) were merged to create the new CAESAR‐Lisflood model, and through a series of preliminary tests shows that using a hydrodynamic model to route flow in an LEM affords many advantages. The new model is fast, computationally efficient and has a stronger physical basis than a previous version of the CAESAR model. For the first time it allows hydrodynamic effects (tidal flows, lake filling, alluvial fans blocking valley floor) to be represented in an LEM, as well as producing noticeably different results to steady flow models. This suggests that the simplification of using steady flow in existing LEMs may bias their findings significantly. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

The influence of large dams building on resettlement in the Upper Mekong River     

Yong Zhang  Daming He  Ying Lu  Yan Feng  Jake Reznick 《地理学报(英文版)》2013,23(5):947-957

This paper seeks to quantify the social and economic impact of resettlement based on the physiographic element changes post relocation. We focus on communities affected by the Nuozhadu hydropower project, the largest existing hydropower project on the mainstream of the Upper Mekong River. Soil and meteorological data were collected from the Soil Spatial Database and the China Terrestrial Ecological Information Spatial Meteorology Database, while social and economic data were collected via field surveys. We have three major con- clusions： （1） Communities will be relocated to a new climate and new elevation, moving from a north tropical climate zone under 700 m to a subtropical climate zone above 700 m. （2） Physiographic element changes due to relocation will reduce household economic income. After relocation, the annual family income of the Shidaimao group decreased by 62%; the annual family income of the other 5 study groups （Lasa, Hani, Nochangchangyi, Mengsa, and Dawazi） dropped by 65%. （3） Communities relocated across the study township are 61.1% less connected with their former relatives after relocation while family-to-family free labor exchange, a previous community norm, decreased by 91%. China＇s dam resettlement compensation system focuses on the loss of economic resources after relocation. However, this study finds that the physiographic elements of the relocation sites are an important driver of ensuring economic growth and stability after relocation. As a result, we recommend more attention be paid to physiographic continuity when designing relocation models.  相似文献   

The discrete dynamics of small-scale spatial events: agent-based models of mobility in carnivals and street parades     

Michael Batty  Jake Desyllas  Elspeth Duxbury 《International journal of geographical information science》2013,27(7):673-697

Small-scale spatial events are situations in which elements or objects vary in such a way that temporal dynamics are intrinsic to their representation and explanation. Some of the clearest examples involve local movement, from conventional traffic modeling to disaster evacuation where congestion, crowding, panic, and related safety issues are key features. We propose that such events can be simulated using new variants of pedestrian model, which embody ideas about how behavior emerges from the accumulated interactions between small-scale objects. We present a model in which the event space is first explored by agents using ‘swarm intelligence’. Armed with information about the space, agents then move in unobstructed fashion to the event. Congestion and problems over safety are then resolved through introducing controls in an iterative fashion, rerunning the model until a ‘safe solution’ is reached. The model has been developed to simulate the effect of changing the route of the Notting Hill Carnival, an annual event held in west central London over 2 days in August each year. One of the key issues in using such simulation is how the process of modeling interacts with those who manage and control the event. As such, this changes the nature of the modeling problem from one where control and optimization is external to the model to one where it is intrinsic to the simulation.  相似文献   

Maze or Labyrinth: Identifying PTSD Stressors in the Built Space of Zaatari and Calais Refugee Camps     

Samuel N. Chambers  W. Jake Jacobs  Casey Lindberg 《The Professional geographer》2013,65(4):552-565

In this study, we analyze the structure of paths within two refugee camps (Calais, France, and Zaatari, Jordan) as a preliminary study examining posttraumatic stress disorder (PTSD) stressors within such camps. We do so through a geospatial analysis of intersections and viewsheds within the camps. Given that anxiety, and the avoidance associated with it, is a primary characteristic of PTSD, refugees might prefer built environments with fewer stressors or anxiety triggers. This study provides a baseline for analyzing basic stressors in a way to test whether design supports anxiety-based avoidance. The planned structure of Zaatari shows a high prevalence of spatial PTSD stressors with lower visibility and a more maze-like structure than that of Calais. This suggests a coping strategy taken by refugees to produce a more suitable and less stressing labyrinth structure. To properly treat PTSD in geographical space, we suggest a need for consideration of this in the planning, function, and very existence of refugee camps and settlements.  相似文献   

Coleoptera and microbe biomass in Antarctic Dry Valley paleosols adjacent to the Inland Ice: Implications for Mars     

William C. Mahaney  Kris M. Hart  Shane S. O'Reilly  Christopher C.R. Allen  James M. Dohm  Ronald G.V. Hancock  Brian P. Kelleher  Michael W. Milner 《Planetary and Space Science》2012,60(1):386-398

Bulk paleosol samples collected from a Middle to Early Miocene moraine in the New Mountain area of the Dry Valleys, Antarctica, yielded Coleoptera exoskeletons and occasional endoskeletons showing considerable diagenetic effects along with several species of bacteria, all lodged in a dry-frozen but salt-rich horizon at shallow depth to the land surface. The till is at the older end of a chronologic sequence of glacial deposits, thought to have been deposited before the transition from wet-based to cold-based ice (～15 Ma), and hence, entirely weathered in contact with the subaerial atmosphere. It is possible, though not absolutely verifiable, that the skeletons date from this early stage of emplacement having undergone modifications whenever light snowmelt occurred or salt concentrations lowered the freezing temperature to maintain water as liquid. Correlation of the Coleoptera species with cultured bacteria in the sample and the likelihood of co-habitation with Beauveria bassiani found in two adjacent, although younger paleosols, leads to new questions about the antiquity of the Coleoptera and the source of N and glucose from chitinase derived from the insects. The skeletons in the 831 section may date close to the oldest preserved chitin (Oligocene) yet found on Earth. While harsh Martian conditions make it seemingly intolerable for complex, multicellular organisms such as insects to exist in the near-surface and subaerially, life within similar cold, dry paleosol microenvironments (Cryosols) of Antarctica point to life potential for the Red Planet, especially when considering the relatively diverse microbe (bacteria and fungi) population.  相似文献   

Environmental controls on the δ18O composition of freshwater calcite and aragonite in a temperate,lowland river system: significance for palaeoclimatic studies     

R. Waghorne  J.D.R. Hancock  I. Candy 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(4):576-583

The oxygen and carbon isotopic composition of surficial carbonates is a key technique for reconstructing past environments and climates. The understanding of modern isotopic systems is, however, a vital first step before applying these techniques to fossil examples. In this study the δ¹⁸O of aragonite shells from three different freshwater mollusc species and the δ¹⁸O of tufa stromatalites are analysed and compared to the modern temperature and isotopic regime of the river system in which they form (the river Gipping, Sproughton, southeast England). In all cases this range of carbonates appear to form in isotopic equilibrium with the δ¹⁸O of modern waters at water temperatures experienced during the summer months. It is, therefore, likely that in Quaternary interglacial deposits of the UK and western Europe the δ¹⁸O of fossil freshwater molluscs and tufa stromatolite carbonates will provide an indication of past summer temperature regimes and not mean annual temperatures, as has been previously suggested. The paper concludes by discussing the implications of this study for the isotopic analysis of interglacial deposits in Britain and western Europe.  相似文献   

Increased risk of non-seasonal and body immersion recreational marine bathers contacting indicator microorganisms of sewage pollution     

Bradley G  Hancock C 《Marine pollution bulletin》2003,46(6):791-794

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Book review     

Petr Jakeš  Marck Wolf 《Earth, Moon, and Planets》1995,71(1-2):175-176

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Seasat altimeter sensor file algorithms     

Hancock  D. Forsythe  R. Lorell  J. 《Oceanic Engineering, IEEE Journal of》1980,5(2):93-99

The Seasat altimeter is designed to measure three parameters important to oceanography: height of the spacecraft above the ocean surface (h), significant wave height (H_{1/3}), and ocean backscatter coefficient (sigmadeg) from which surface winds may be inferred. Since the measurement process is indirect, and the measurement environment is complicated by many factors affecting the instrument readings, corrections to the raw data are needed before they are used to compute geophysical parameters. These corrections are accomplished by the Seasat altimeter sensor file algorithms. The purpose of this paper is to describe these algorithms, why they are needed, how they are implemented, and their evaluation using in-flight data.  相似文献