共查询到20条相似文献，搜索用时 156 毫秒
1.
Alan T. Murray Timothy C. Matisziw Tony H. Grubesic 《Journal of Geographical Systems》2007,9(2):103-117
Effective management of critical network infrastructure requires the assessment of potential interdiction scenarios. Optimization
approaches have been essential for identifying and evaluating such scenarios in networked systems. Although a primary function
of any network is the distribution of flow between origins and destinations, the complexity and difficulty of mathematically
abstracting interdiction impacts on connectivity or flow has been a challenge for researchers. This paper presents an optimization
approach for identifying interdiction bounds with respect to connectivity and/or flow associated with a system of origins
and destinations. Application results for telecommunications flow are presented, illustrating the capabilities of this approach.
相似文献
| Alan T. Murray (Corresponding author)Email: |
| Timothy C. MatisziwEmail: |
| Tony H. GrubesicEmail: |
2.
Fast direct GPS P-Code acquisition 总被引：5，自引：0，他引：5
GPS P-Code has a higher chipping rate, better accuracy, and anti-jamming property than C/A code. Traditionally, GPS P-Code acquisition depends on handover from C/A code. This potentially needs long acquisition time. Moreover, when C/A code is not available, it is no longer possible to acquire GPS P-Code through handover from C/A code. The purpose of this paper is to describe a new overlap average method to facilitate hardware design of fast direct P-Code acquisition. It allows the rapid code phase search to acquire GPS P-Code signals, and also decreases the hardware resource requirement. The small size FFT in the proposed methods is very promising for fast FPGA hardware system design using FFT cores. The simulation results and theoretical analysis are included demonstrating the overall performance of the proposed method.
相似文献
| Jing PangEmail: Phone: +1-916-2784549Fax: +1-916-2787215 |
3.
Ionospheric climatology derived from gps occultation observations made by the ionospheric occultation experiment 总被引：1，自引：1，他引：0
Paul Straus 《GPS Solutions》2005,9(2):164-173
The ionospheric occultation experiment (IOX) is a GPS occultation sensor with an ionospheric mission focus. IOX measurements of GPS L1 and L2 carrier phase during Earth limb views of setting GPS satellites are used together with the Abel transform to determine vertical profiles of electron density from which F-region peak parameters are determined. Data from a four and a half month period beginning in November 2001 are statistically binned and compared with a climatological model. To account for potential errors in interpretation that could arise from violation of the Abel transform assertion of spherical symmetry, the data are compared to both the climatology and to statistics of simulated ionospheric inversions using the climatological model. General characteristics of the climatology are reproduced by the occultation data. However, several significant discrepancies between the model and the data are observed during this near-solar maximum time period. In particular, average mid-latitude daytime densities are shown to be higher than the climatological prediction and the height of F2 layer in the post-sunset equatorial region is underestimated by up to 150 km.
相似文献
| Paul StrausEmail: Phone: +1-310-3365328Fax: +1-310-3361636 |
4.
Gianfranco Piras Kieran P. Donaghy Giuseppe Arbia 《Journal of Geographical Systems》2007,9(4):311-344
This paper presents an innovative approach to the study of regional economic dynamics within a nonlinear continuous-time econometric
framework—a generalized specification of the Lotka–Volterra system of equations. This specification, which accounts for interdependent
behavior of three industrial sectors and spillover effects of activities in neighboring regions, is employed in an analysis
of five Italian regions between 1980 and 2003. For these regions, we report estimation results, characterize the varying systems
dynamics, analyze the models’ local and global stability properties, and determine via sensitivity analyses which structural
features appear to exert the greatest influence on these properties.
相似文献
| Kieran P. DonaghyEmail: |
5.
Although not considered for the first generation of European Galileo satellites, the use of C-Band frequencies for navigation purposes may be taken into account for a future generation of Galileo. For this reason, a frequency band of 20 MHz bandwidth (5,010–5,030 MHz) has been allocated in the course of the World Radio Communications Conference 2000 held in Istanbul, Turkey. The use of C-Band navigation signals offers both advantages and drawbacks. One example is the ionospheric path delay which is inversely proportional to the (squared) carrier frequency and is therefore significantly smaller at C-Band. On the other hand, the use of C-Band frequencies results in increased attenuation effects such as free space loss or rainfall attenuation. It is therefore necessary to provide a detailed analysis of the effects of C-Band frequencies on the navigation process. In order to assess the feasibility of using C-Band frequencies, various aspects of signal propagation and satellite signal tracking at C-Band are examined in the context of this article. In particular, aspects like free space loss, atmospheric effects, foliage attenuation, code and carrier tracking performance, code noise, phase noise and multipath are discussed with respect to their performance at C-Band. In order to allow comparison with the current GPS system, the performance at C-Band is compared to the L-Band performance under similar or identical conditions. The results of this analysis will finally be discussed with respect to their impact on satellite payload and receiver design.
相似文献
| Markus IrsiglerEmail: Phone: +49-89-60043552Fax: +49-89-60043019 |
6.
The accuracy of standalone GPS positioning improved significantly when Selective Availability was turned off in May 2000. With the availability of various public GPS related products including precise satellite orbits and clocks, and ionosphere maps, a single-frequency standalone user can experience even a further improvement of the position accuracy. Next, using carrier phase measurements becomes crucial to smoothen the pseudorange noise. In this contribution, the most critical sources of error in single-frequency standalone positioning will be reviewed and different approaches to mitigate the errors will be considered. An optimal filter (using also carrier phase measurements) will be deployed. The final approach will then be evaluated in a decently long static test with receivers located in different regions of the world. Kinematic experiments have also been performed in various scenarios including a highly dynamic flight trial. The accuracy, in general, can be confirmed at 0.5 m horizontal and 1 m vertical, with static tests. Ultimate results demonstrate an accuracy close to 2 dm (95%) for the horizontal position components and 5 dm (95%) for the vertical in the flight experiment.
相似文献
| Anh Quan LeEmail: |
7.
Modeling the geologic architecture of an aquifer and visualizing its three-dimensional structure require lithologic data recorded
during well drilling. Uncertainties in layer boundaries arise due to questionable quality of drilling records, mixing during
the drilling process, which results in blurred contacts, and natural heterogeneity of the geologic materials. An approach
for modeling and visualizing the spatial distribution of aquifer units three-dimensionally based on fuzzy set theory is developed.
An indicator is defined for evaluating the possibility of aquifer existence based on fuzzy set theory and probability principles.
A specific interpolation method for aquifer 3D spatial distribution requiring only very basic borehole log data is proposed.
A 3D modeling and visualization system for aquifers is also developed, which can implement basic GIS functions, like borehole
identification and cross-section creation. The methodology developed is tested using real borehole lithology data available
for an aquifer in British Columbia, Canada.
相似文献
| D. M. AllenEmail: |
8.
Small-scale irregularities in the background electron density of the ionosphere can cause rapid fluctuations in the amplitude
and phase of radio signals passing through it. These rapid fluctuations are known as scintillation and can cause a Global
Positioning System (GPS) receiver to lose lock on a signal. This could compromise the integrity of a safety of life system
based on GPS, operating in auroral regions. In this paper, the relationship between the loss of lock on GPS signals and ionospheric
scintillation in auroral regions is explored. The period from 8 to 14 November 2004 is selected for this study, as it includes
both geomagnetically quiet and disturbed conditions. Phase and amplitude scintillation are measured by GPS receivers located
at three sites in Northern Scandinavia, and correlated with losses of signal lock in receivers at varying distances from the
scintillation receivers. Local multi-path effects are screened out by rejection of low-elevation data from the analysis. The
results indicate that losses of lock are more closely related to rapid fluctuations in the phase rather than the amplitude
of the received signal. This supports the idea, suggested by Humphreys et al. (2005) (performance of GPS carrier tracking loops during ionospheric scintillations. Proceedings Internationsl Ionospheric Effects
Symposium 3–5 May 2005), that a wide loop bandwidth may be preferred for receivers operating at auroral latitudes. Evidence from the Imaging Riometer
for Ionospheric Studies (IRIS) appears to suggest that, for this particular storm, precipitation of particles in the D/E regions
may be the mechanism that drives the rapid phase fluctuations in the signal.
相似文献
| Robert W. MeggsEmail: |
9.
Incorporating spatial variation in housing attribute prices: a comparison of geographically weighted regression and the spatial expansion method 总被引：10，自引：3，他引：7
Christopher Bitter Gordon F. Mulligan Sandy Dall’erba 《Journal of Geographical Systems》2007,9(1):7-27
Hedonic house price models typically impose a constant price structure on housing characteristics throughout an entire market
area. However, there is increasing evidence that the marginal prices of many important attributes vary over space, especially
within large markets. In this paper, we compare two approaches to examine spatial heterogeneity in housing attribute prices
within the Tucson, Arizona housing market: the spatial expansion method and geographically weighted regression (GWR). Our
results provide strong evidence that the marginal price of key housing characteristics varies over space. GWR outperforms
the spatial expansion method in terms of explanatory power and predictive accuracy.
相似文献
| Christopher BitterEmail: |
10.
Latent lifestyle preferences and household location decisions 总被引：3，自引：2，他引：1
Lifestyle, indicating preferences towards a particular way of living, is a key driver of the decision of where to live. We
employ latent class choice models to represent this behavior, where the latent classes are the lifestyles and the choice model
is the choice of residential location. Thus, we simultaneously estimate lifestyle groups and how lifestyle impacts location
decisions. Empirical results indicate three latent lifestyle segments: suburban dwellers, urban dwellers, and transit-riders.
The suggested lifestyle segments have intriguing policy implications. Lifecycle characteristics are used to predict lifestyle
preferences, although there remain significant aspects that cannot be explained by observable variables.
相似文献
| Jieping LiEmail: |
11.
Oliver Montenbruck Yago Andres Heike Bock Tom van Helleputte Jose van den Ijssel Marc Loiselet Christian Marquardt Pierluigi Silvestrin Pieter Visser Yoke Yoon 《GPS Solutions》2008,12(4):289-299
The global navigation satellite system receiver for atmospheric sounding (GRAS) on MetOp-A is the first European GPS receiver
providing dual-frequency navigation and occultation measurements from a spaceborne platform on a routine basis. The receiver
is based on ESA’s AGGA-2 correlator chip, which implements a high-quality tracking scheme for semi-codeless P(Y) code tracking
on the L1 and L2 frequency. Data collected with the zenith antenna on MetOp-A have been used to perform an in-flight characterization
of the GRAS instrument with focus on the tracking and navigation performance. Besides an assessment of the receiver noise
and systematic measurement errors, the study addresses the precise orbit determination accuracy achievable with the GRAS receiver.
A consistency on the 5 cm level is demonstrated for reduced dynamics orbit solutions computed independently by four different
agencies and software packages. With purely kinematic solutions, 10 cm accuracy is obtained. As a part of the analysis, an
empirical antenna offset correction and preliminary phase center correction map are derived, which notably reduce the carrier
phase residuals and improve the consistency of kinematic orbit determination results.
相似文献
| Oliver MontenbruckEmail: |
12.
The EUREF permanent network (EPN) is a network of continuously operating global navigation satellite system (GNSS) stations, primarily installed for reference frame maintenance. In order to ensure the long term reliability of the EPN products, a thorough station performance monitoring has been initiated and carried out in addition to the routine GNSS data management, processing and analysis. This paper addresses the main factors influencing the quality of the coordinate time series in a permanent GNSS network. Relevant examples, based on the EPN experience are given, the analysis strategy is introduced, the estimated coordinate offsets are published and the importance of this analysis for site velocity estimation is demonstrated. The results are derived from the analysis of the EPN weekly combined solutions covering the period from 1996 to 2003. Our target is the identification, interpretation and elimination of offsets and outliers present in the EPN coordinate time series in order to estimate reliable coordinates and velocities and consequently maintain a high quality kinematic reference network.
相似文献
| A. KenyeresEmail: Fax: +36-1-27374982 |
13.
The ecological fallacy (EF) is a common problem regional scientists have to deal with when using aggregated data in their analyses. Although there is a wide number of studies considering different aspects of this problem, little attention has been paid to the potential negative effects of the EF in a time series context. Using Spanish regional unemployment data, this paper shows that EF effects are not only observed at the cross-section level, but also in a time series framework. The empirical evidence obtained shows that analytical regional configurations are the least susceptible to time effects relative to both normative and random regional configurations, while normative configurations are an improvement over random ones.
相似文献
| Raúl RamosEmail: |
14.
Hierarchical spatial interaction among the Italian regions: a nonlinear relative dynamics approach 总被引：2，自引：2，他引：0
Regional interactions and spillover effects should be considered as important factors in growth analysis of regional economies.
Using modified versions of the Dendrinos–Sonis model, this paper analyses the spatial hierarchical system of Italy. The interaction
among Italian regions is considered at three different levels of spatial aggregation, the NUTS-1, NUTS-2 and NUTS-3 levels.
Compared to more popular spatial econometric approaches, the Dendrinos–Sonis model and its extensions provide greater flexibility
in the way interaction between regions is handled but the results strongly depend on the choice of a reference region.
相似文献
| Geoffrey J. D. HewingsEmail: |
15.
Reliability analysis is inseparably connected with the formulation of failure scenarios, and common test statistics are based on specific assumptions. This is easily overlooked when processing observation differences. Poor failure identification performance and misleading pre-analysis results, mainly meaningless minimum detectable biases and external reliability measures, are the consequence. A reasonable failure scenario for use with differenced GNSS observations is formulated which takes into account that individual outliers in the original data affect more than one processed observation. The proper test statistics and reliability indicators are given for use with correlated observations and both batch processing and Kalman filtering. It is also shown that standardized residuals and redundancy numbers fail completely when used with double differenced observations.
相似文献
| Andreas WieserEmail: Phone: +43-316-8736323Fax: +43-316-8736820 |
16.
Torben Schüler 《GPS Solutions》2006,10(2):108-125
Many kinematic GPS applications rely on high accuracy, which usually requires the ambiguities to be fixed. Normally, a reference station in the rover’s vicinity is needed for successful ambiguity resolution. Alternatively, a network surrounding the rover and allowing one to derive area correction parameters is needed. Unfortunately, both approaches are not feasible in certain situations. This paper is a contribution to precise kinematic positioning over long baselines. Atmospheric refraction becomes critical in the error budget, but progress has been made to use numerical weather models to derive tropospheric corrections, for instance. The spatial correlation of both ionospheric and tropospheric propagation delays is investigated in this paper and special attention is paid on the systematic error behavior of tropospheric refraction. The principles developed are applied to an extended reliability test of the ambiguities. Finally, it is demonstrated in positioning experiments that kinematic positioning retrieval with fixed ambiguities is actually possible for baselines between 150 and 300 km with an accuracy of approximately 2 cm in post-mission processing.
相似文献
| Torben SchülerEmail: Phone: +49-89-60042587Fax: +49-89-60043019 |
17.
Computer algebra solution of the GPS N-points problem 总被引：1，自引：0，他引：1
A computer algebra solution is applied here to develop and evaluate algorithms for solving the basic GPS navigation problem: finding a point position using four or more pseudoranges at one
epoch (the GPS N-points problem). Using Mathematica 5.2 software, the GPS N-points problem is solved numerically, symbolically, semi-symbolically, and with Gauss–Jacobi, on a work station. For the case of N > 4, two minimization approaches based on residuals and distance norms are evaluated for the direct numerical solution and their computational duration is compared. For N = 4, it is demonstrated that the symbolic computation is twice as fast as the iterative direct numerical method. For N = 6, the direct numerical solution is twice as fast as the semi-symbolic, with the residual minimization requiring less computation
time compared to the minimization of the distance norm. Gauss–Jacobi requires eight times more computation time than the direct
numerical solution. It does, however, have the advantage of diagnosing poor satellite geometry and outliers. Besides offering
a complete evaluation of these algorithms, we have developed Mathematica 5.2 code (a notebook file) for these algorithms (i.e., Sturmfel’s resultant, Dixon’s resultants, Groebner basis, reduced
Groebner basis and Gauss–Jacobi). These are accessible to any geodesist, geophysicist, or geoinformation scientist via the
GPS Toolbox () website or the Wolfram Information Center ().
相似文献
| Erik W. GrafarendEmail: |
18.
Artificial neural networks for predicting DGPS carrier phase and pseudorange correction 总被引：2，自引：2，他引：0
Arif Indriyatmoko Taesam Kang Young Jae Lee Gyu-In Jee Yong Beom Cho Jeongrae Kim 《GPS Solutions》2008,12(4):237-247
Artificial neural networks (ANNs) were used to predict the differential global positioning system (DGPS) pseudorange and carrier
phase correction information. Autoregressive moving average (ARMA) and autoregressive (AR) models were bounded with neural
networks to provide predictions of the correction. The neural network was employed to realize time-varying implementation.
Online training for real-time prediction of the carrier phase enhances the continuity of service of the differential correction
signals and, therefore, improves the positioning accuracy. When the correction signal from the DGPS was lost, the artificial
neural networks predicted the correction data with good accuracy for the navigation system during a limited period. Comparisons
of the prediction results using the two models are given.
相似文献
| Young Jae LeeEmail: |
19.
Hidden negative spatial autocorrelation 总被引：3，自引：3，他引：0
Daniel A. Griffith 《Journal of Geographical Systems》2006,8(4):335-355
Mostly lip service treatments of negative spatial autocorrelation (NSA) appear in the literature, although spatial scientists confront it in practice. NSA was detected serendipitously in recalcitrant empirical analyses containing a sizeable amount of global positive spatial autocorrelation (PSA) unaccounted for by standard spatial statistical models, and labeled hidden because conventional spatial statistical tools detected only PSA while giving absolutely not hint of NSA existing. The meaning of this phenomenon is explored empirically, with findings including: a better understanding of NSA, spatial filter model construction guidelines, effective illustrations of NSA, and how hidden NSA furnishes a diagnostic for model misspecification.
相似文献
| Daniel A. GriffithEmail: Phone: +1-972-8834950Fax: +1-972-8836297 |
20.
A systematic investigation of cross-validation in GWR model estimation: empirical analysis and Monte Carlo simulations 总被引：8，自引：2，他引：6
In geographically weighted regression, one must determine a window size which will be used to subset the data locally. Typically,
a cross-validation procedure is used to determine a globally optimal window size. Preliminary investigations indicate that
the global cross-validation score is heavily influenced by a small number of observations in the dataset. At present, the
ramifications of this behaviour in cross-validation are unknown. The research reported here explores the extent to which individual
and groups of observations impact optimal window size determination, and whether one can explain why some points are more
influential than others. In addition, we strive to examine the impact neighbourhood specification has on model quality in
terms of predictive capabilities and the ability of the method to retrieve spatially varying processes. The analysis is based
on several datasets and using simulated data in order to compare and validate results. The results provide some practical
guidelines for the use of cross-validation.
Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
相似文献
Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
| Antonio PáezEmail: |