排序方式： 共有61条查询结果，搜索用时 46 毫秒
11.
12.
Fast integer least-squares estimation for GNSS high-dimensional ambiguity resolution using lattice theory 总被引：4，自引：0，他引：4
GNSS ambiguity resolution is the key issue in the high-precision relative geodetic positioning and navigation applications.
It is a problem of integer programming plus integer quality evaluation. Different integer search estimation methods have been
proposed for the integer solution of ambiguity resolution. Slow rate of convergence is the main obstacle to the existing methods
where tens of ambiguities are involved. Herein, integer search estimation for the GNSS ambiguity resolution based on the lattice
theory is proposed. It is mathematically shown that the closest lattice point problem is the same as the integer least-squares
(ILS) estimation problem and that the lattice reduction speeds up searching process. We have implemented three integer search
strategies: Agrell, Eriksson, Vardy, Zeger (AEVZ), modification of Schnorr–Euchner enumeration (M-SE) and modification of
Viterbo-Boutros enumeration (M-VB). The methods have been numerically implemented in several simulated examples under different
scenarios and over 100 independent runs. The decorrelation process (or unimodular transformations) has been first used to
transform the original ILS problem to a new one in all simulations. We have then applied different search algorithms to the
transformed ILS problem. The numerical simulations have shown that AEVZ, M-SE, and M-VB are about 320, 120 and 50 times faster
than LAMBDA, respectively, for a search space of dimension 40. This number could change to about 350, 160 and 60 for dimension
45. The AEVZ is shown to be faster than MLAMBDA by a factor of 5. Similar conclusions could be made using the application
of the proposed algorithms to the real GPS data. 相似文献
13.
The efficacy of robust M-estimators is a well-known issue when dealing with observational blunders. When the number of observations
is considerably large—long time series for instance—one can take advantage of the asymptotic normality of the M-estimation
and compute reasonable estimates for the unknown parameters of interest. A few leading M-estimators have been employed to
identify the most likely functional model for GPS coordinate time series. This includes the simultaneous detection of periodic
patterns and offsets in the GPS time series. Estimates of white noise, flicker noise, and random walk noise components are
also achieved using the robust M-estimators of (co)variance components, developed in the framework of the least-squares variance
component estimation (LS-VCE) theory. The method allows one to compute confidence interval for the (co)variance components
in asymptotic sense. Simulated time series using white noise plus flicker noise show that the estimates of random walk noise
fluctuate more than those of flicker noise for different M-estimators. This is because random walk noise is not an appropriate
noise structure for the series. The same phenomenon is observed using the results of real GPS time series, which implies that
the combination of white plus flicker noise is well described for GPS time series. Some of the estimated noise components
of LS-VCE differ significantly from those of other M- estimators. This reveals that there are a large number of outliers in
the series. This conclusion is also affirmed by performing the statistical tests, which detect (large) parts of the outliers
but can also leave parts to be undetected. 相似文献
14.
Spatial patterns in thunderstorm rainfall events and their coupling with watershed hydrological response 总被引：1，自引：0，他引：1
Efrat Morin David C. Goodrich Robert A. Maddox Xiaogang Gao Hoshin V. Gupta Soroosh Sorooshian 《Advances in water resources》2006
Weather radar systems provide detailed information on spatial rainfall patterns known to play a significant role in runoff generation processes. In the current study, we present an innovative approach to exploit spatial rainfall information of air mass thunderstorms and link it with a watershed hydrological model. Observed radar data are decomposed into sets of rain cells conceptualized as circular Gaussian elements and the associated rain cell parameters, namely, location, maximal intensity and decay factor, are input into a hydrological model. Rain cells were retrieved from radar data for several thunderstorms over southern Arizona. Spatial characteristics of the resulting rain fields were evaluated using data from a dense rain gauge network. For an extreme case study in a semi-arid watershed, rain cells were derived and fed as input into a hydrological model to compute runoff response. A major factor in this event was found to be a single intense rain cell (out of the five cells decomposed from the storm). The path of this cell near watershed tributaries and toward the outlet enhanced generation of high flow. Furthermore, sensitivity analysis to cell characteristics indicated that peak discharge could be a factor of two higher if the cell was initiated just a few kilometers aside. 相似文献
15.
In this paper, a new family of explicit and implicit multistep methods is presented both for the error-controlled and uncontrolled modes. The main concept is to replace the Newton interpolation with the Hermite interpolation, where the Hermite polynomial is fitted to the function values and its derivatives. This idea is very useful in the numerical solution of problems (e.g., orbit propagation problem) where higher-order derivatives can easily be computed. In addition to the theoretical concept, the stability regions of the proposed methods are determined. The new methods are more stable than the well-known multistep numerical integrators (i.e., Adams–Bashforth and Adams–Bashforth–Moulton) in the explicit, implicit, and predictor–corrector forms. Using the second-order derivatives gives smaller error constants in the proposed method. The new integrators are numerically tested for a few examples, and the solutions are compared with those of the well-known multistep methods. Moreover, the CPU time and absolute integration error are compared in the satellite orbit propagation problem using various integration methods. The CHAMP mission, i.e., a German small-satellite mission for geoscientific and atmospheric research and applications, is considered as a case study for comparing the achievable accuracy of the proposed method with the existing method for solving the two-body problem. 相似文献
16.
Mohammad Talebi Abbas Sivandi-Pour Ghasem-Ali Ahmadi Ehsan Noroozinejad Farsangi Shamseddin Esmaeili Mohammad-Javad Banimahdi-Dehkordi Hamidreza Safizadeh Mahdieh Akbarpoor Ehsan Ebrahimi Rabe Sharifi Rad Manoochehr Fallah 《地震科学（英文版）》2022,35(2):122-137
Fault lineaments are the main input data in earthquake engineering and seismology studies. This study presents a digitally-based active fault map of the Kerman region in central-east Iran which experienced several devastating earthquakes on poorly exposed and/or not identified active faults. Using Landsat 8 data, we have carried out the image-based procedures of fault mapping, which include applying the contrast stretching technique, the principal component analysis, the color composite technique, the spectral rationing, and creating the false-color composite images. Besides, we have cross-checked the resulting map with the geological maps provided by the Geological Survey of Iran to decrease the associated uncertainties. The resulting map includes 123 fault segments, still, a part of which has been expressed in the previously compiled active-fault maps of Iran. Indeed, the new one is mapping the poorly exposed active faults, so-called secondary faults, which are able to produce strong events. These faults are primarily associated with poorly defined areas that accommodate low levels of seismicity; however, sporadic strong events are likely to occur. It has also been investigated that these kinds of faults are seismogenic and are able to produce destructive events. In total, the outcome of this study can also be jointed with seismic studies for investigating parts of the earthquake activity in central-east Iran, in particular for the fault-based approaches in impending earthquake-resistant buildings. 相似文献
17.
Farshad Ahmadi Feridon Radmaneh Mohammad Reza Sharifi Rasoul Mirabbasi 《Environmental Earth Sciences》2018,77(18):643
Accurate estimation of low flow as a criterion for different objectives in water resource management, including drought is of crucial importance. Despite the complex nature of water deficits, univariate methods have often been used to analyze the frequency of low flows. In this study, low flows of Dez River basin were examined during period of 1956–2012 using copula functions at the upstream of headbranches’ junction. For this purpose, at first 7-day series of low flow was extracted at the studied stations, then their homogeneity was examined by Mann–Kendall test. The results indicated that 7-day low flow series of Dez basin were homogenous. In the next stage, 12 different distribution functions were fitted onto the low flow data. Finally, for Sepid Dasht Sezar (SDS), Sepid Dasht Zaz (SDZ), and Tang Panj Bakhtiyari (TPB) stations, logistic distribution had the best fit, while for Tang Panj Sezar (TPS) station, GEV distribution enjoyed the best fit. After specifying the best fitted marginal distributions, seven different copula functions including Ali–Mikhail–Haq (AMH), Frank, Clayton, Galambos, Farlie–Gumbel–Morgenstern (FGM), Gumbel–Hougaard (GH), and Plackett were used for bivariate frequency analysis of the 7-day low flow series. The results revealed that the GH copula had the best fitness on paired data of SDS and SDZ stations. For TPS and TPB stations, Frank copula has had the best correspondence with empirical copula values. Next, joint and conditional return periods were calculated for the low flow series at the upstream of branches’ junction. The results of this study indicated that the risk of incidence of severe drought is higher in upstream stations (SDZ and SDS) when compared with downstream stations (TPB and TPS) in Dez basin. Generally, application of multivariate analysis allows researchers to investigate hydrological events with a more comprehensive view by considering the simultaneous effect of the influencing factors on the phenomenon of interest. It also enables them to evaluate different combinations of required scenarios for integrated management of basin and planning to cope with the damages caused by natural phenomena. 相似文献
18.
19.
20.
Hossein Saadat Robert Bonnell Forood Sharifi Guy Mehuys Mohammad Namdar Sasan Ale-Ebrahim 《Geomorphology》2008,100(3-4):453-464
The Iranian Soil and Water Research Institute has been involved in mapping the soils of Iran and classifying landforms for the last 60 years. However, the accuracy of traditional landform maps is very low (about 55%). To date, aerial photographs and topographic maps have been used for landform classification studies. The principal objective of this research is to propose a quantitative approach for landform classification based on a 10-m resolution digital elevation model (DEM) and some use of an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image. In order to extract and identify the various landforms, slope, elevation range, and stream network pattern were used as basic identifying parameters. These are extractable from a DEM. Further, ASTER images were required to identify the general outline shape of a landform type and the presence or absence of gravel. This study encompassed a relatively large watershed of 451 183 ha with a total elevation difference of 2445 m and a variety of landforms from flat River Alluvial Plains to steep mountains. Classification accuracy ranged from 91.8 to 99.6% with an average of 96.7% based upon extensive ground-truthing. Since similar digital and ASTER image information is available for Iran, an accurate landform map can now be produced for the whole country. The main advantages of this approach are accuracy, lower demands on time and funds for field work and ready availability of required data for many regions of the world. 相似文献