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   

Editor' note     

Kevin Gallo 《国际地球制图》2013,28(4)

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New AVHRR instrument arrives in 1998     

Kevin Gallo 《国际地球制图》2013,28(1)

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Constrained free space diagrams: a tool for trajectory analysis     

Kevin Buchin  Joachim Gudmundsson 《International journal of geographical information science》2013,27(7):1101-1125

Time plays an important role in the analysis of moving object data. For many applications it is not sufficient to only compare objects at exactly the same times, or to consider only the geometry of their trajectories. We show how to leverage between these two approaches by extending a tool from curve analysis, namely the free space diagram. Our approach also allows us to take further attributes of the objects like speed or direction into account. We demonstrate the usefulness of the new tool by applying it to the problem of detecting single file movement. A single file is a set of moving entities, which are following each other, one behind the other. Our algorithm is the first one developed for detecting such movement patterns. For this application, we analyse demonstrate the performance of our tool both theoretically experimentally.  相似文献   

Bowman: a novel for lovers of geography     

Kevin M. Dunn  Dennis N. Jeans 《The Australian geographer》2013,44(1):117-121

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A Review of “Transport Geographies: Mobilities,Flows and Spaces”     

Kevin M. Curtin 《The Professional geographer》2013,65(2):287-289

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A Nationwide Comparison of Driving Distance Versus Straight-Line Distance to Hospitals   总被引：2，自引：0，他引：2  

Francis P. Boscoe  Kevin A. Henry  Michael S. Zdeb 《The Professional geographer》2013,65(2):188-196

Many geographic studies use distance as a simple measure of accessibility, risk, or disparity. Straight-line (Euclidean) distance is most often used because of the ease of its calculation. Actual travel distance over a road network is a superior alternative, although historically an expensive and labor-intensive undertaking. This is no longer true, as travel distance and travel time can be calculated directly from commercial Web sites, without the need to own or purchase specialized geographic information system software or street files. Taking advantage of this feature, we compare straight-line and travel distance and travel time to community hospitals from a representative sample of more than 66,000 locations in the fifty states of the United States, the District of Columbia, and Puerto Rico. The measures are very highly correlated (r ² > 0.9), but important local exceptions can be found near shorelines and other physical barriers. We conclude that for nonemergency travel to hospitals, the added precision offered by the substitution of travel distance, travel time, or both for straight-line distance is largely inconsequential.  相似文献   

The role of ejecta in the small crater populations on the mid-sized saturnian satellites   总被引：1，自引：0，他引：1  

Edward B. Bierhaus  Luke Dones  José Luis Alvarellos  Kevin Zahnle 《Icarus》2012,218(1):602-621

We find evidence, by both observation and analysis, that primary crater ejecta play an important role in the small crater (less than a few km) populations on the saturnian satellites, and more broadly, on cratered surfaces throughout the Solar System. We measure crater populations in Cassini images of Enceladus, Rhea, and Mimas, focusing on image data with scales less than 500 m/pixel. We use recent updates to crater scaling laws and their constants (Housen, K.R., Holsapple, K.A. [2011]. Icarus 211, 856–875) to estimate the amount of mass ejected in three different velocity ranges: (i) greater than escape velocity, (ii) less than escape velocity and faster than the minimum velocity required to make a secondary crater (v_min), and (iii) velocities less than v_min. Although the vast majority of mass on each satellite is ejected at speeds less than v_min, our calculations demonstrate that the differences in mass available in the other two categories should lead to observable differences in the small crater populations; the predictions are borne out by the measurements we have made to date. In particular, Rhea, Tethys, and Dione have sufficient surface gravities to retain ejecta moving fast enough to make secondary crater populations. The smaller satellites, such as Enceladus but especially Mimas, are expected to have little or no traditional secondary populations because their escape velocities are near the threshold velocity necessary to make a secondary crater. Our work clarifies why the Galilean satellites have extensive secondary crater populations relative to the saturnian satellites. The presence, extent, and sizes of sesquinary craters (craters formed by ejecta that escape into temporary orbits around Saturn before re-impacting the surface, see Dobrovolskis, A.R., Lissauer, J.J. [2004]. Icarus 169, 462–473; Alvarellos, J.L., Zahnle, K.J., Dobrovolskis, A.R., Hamill, P. [2005]. Icarus 178, 104–123; Zahnle, K., Alvarellos, J.L., Dobrovolskis, A.R., Hamill, P. [2008]. Icarus 194, 660–674) is not yet well understood. Finally, our work provides further evidence for a “shallow” size–frequency distribution (slope index of ～2 for a differential power-law) for comets a few kilometers diameter and smaller.  相似文献   

Quantification of middle and lower cloud variability and mesoscale dynamics from Venus Express/VIRTIS observations at 1.74 μm     

Kevin McGouldrick  Thomas W. Momary  Kevin H. Baines  David H. Grinspoon 《Icarus》2012,217(2):615-628

We present an analysis of VIRTIS-M-IR observations of 1.74 μm emission from the nightside of Venus. The 1.74 μm window in the near infrared spectrum of Venus is an ideal proxy for investigating the evolution of middle and lower cloud deck opacity of Venus because it exhibits good signal to noise due to its brightness, good contrast between bright and dark regions, and few additional sources of extinction beside the clouds themselves. We have analyzed the data from the first 407 orbits (equivalent to 407 Earth days) of the Venus Express mission to determine the magnitude of variability in the 1.74 μm radiance. We have also performed an analysis of the evolution of individual features over a span of roughly 5–6 h on two successive orbits of Venus Express. We find that the overall 1.74 μm brightness of Venus has been increasing through the first 407 days of the mission, indicating a gradual diminishing of the cloud coverage and/or thickness, and that the lower latitudes exhibited more variability and more brightening than higher latitudes. We find that individual features evolve with a time scale of about 30 h, consistent with our previous analysis. Analysis of the evolution and motion of the clouds can be used to estimate the mesoscale dynamics within the clouds of Venus. We find that advection alone cannot explain the observed evolution of the features. The measured vorticity and divergence in the vicinity of the features are consistent with evolution under the influence of significant vertical motions likely driven by a radiative dynamical feedback. We measure a zonal wind speed of around 65 m/s, and a meridional wind speed around 2.5 m/s by tracking the motion of the central region of the features. But we also find that the measured wind speeds depend strongly on the points chosen for the wind speed analysis.  相似文献   

Classification and delineation of groundwater–lake interactions in the Nebraska Sand Hills (USA) using electrical resistivity patterns     

Kevin M. Befus  M. Bayani Cardenas  John B. Ong  Vitaly A. Zlotnik 《Hydrogeology Journal》2012,20(8):1483-1495

Lake?Cgroundwater interactions exhibit a complex three-dimensional (3D) structure that is seldom studied. The utility of waterborne electrical resistivity (ER) surveys is explored for characterization of 3D groundwater flow and solute transport patterns for three lakes in the Nebraska Sand Hills, USA. Waterborne ER surveys, using contrasts between lake and groundwater solutes as natural tracers, are useful for inferring 3D patterns of groundwater flow and solute transport as well as classifying groundwater?Clake interactions. Three unique groundwater flow systems are interpreted under each lake from dense networks of two-dimensional (2D) waterborne ER surveys. A lateral transition from high to low ER values beneath the saline Wilson Lake expresses its flow-through regime, where groundwater salinity indicates changes from groundwater inflow to outflow. Alkali Lake ER profiles reveal a prevalent ER increase with depth over the lakebed area that is characteristic of groundwater discharge lakes. ER profiles beneath Gimlet Lake are the most resistive and indicate pockets of high ER related to fresh groundwater discharge into the lake, supporting a flow-through regime with a short flushing time. These ER patterns correctly classify groundwater?Clake interactions and provide high spatial resolution of mixing patterns for systems with varying water salinity.  相似文献