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1.
The IVS Intensive sessions are single-baseline, 1-h VLBI sessions carried out everyday in order to determine Universal Time
(UT1). We investigate different possibilities to improve the results of such sessions. We do this investigation by extracting
2-h single-baseline sessions from the CONT08 data set. These are analysed like normal Intensives, and the results are compared
to the results of the analysis of the full CONT08 data set. We find that tropospheric asymmetry is the major error source
for the single-baseline sessions. It is possible to improve the accuracy of the estimated UT1 either by using accurate a priori
tropospheric gradients or by estimating gradients in the data analysis. 相似文献
2.
3.
Z. Malkin 《Journal of Geodesy》2009,83(6):547-556
In this paper, a new geometry index of very long baseline interferometry (VLBI) observing networks, the volume of network
V, is examined as an indicator of the errors in the Earth orientation parameters (EOP) obtained from VLBI observations. It
has been shown that both EOP precision and accuracy can be well described by the power law σ = aV
c
in a wide range of the network size from domestic to global VLBI networks. In other words, as the network volume grows, the
EOP errors become smaller following a power law. This should be taken into account for a proper comparison of EOP estimates
obtained from different VLBI networks. Thus, performing correct EOP comparison allows us to investigate accurately finer factors
affecting the EOP errors. In particular, it was found that the dependence of the EOP precision and accuracy on the recording
data rate can also be described by a power law. One important conclusion is that the EOP accuracy depends primarily on the
network geometry and to lesser extent on other factors, such as recording mode and data rate and scheduling parameters, whereas
these factors have a stronger impact on the EOP precision. 相似文献
4.
Kamil Teke Tobias Nilsson Johannes Böhm Thomas Hobiger Peter Steigenberger Susana García-Espada Rüdiger Haas Pascal Willis 《Journal of Geodesy》2013,87(10-12):981-1001
Continuous, very long baseline interferometry (VLBI) campaigns over 2 weeks have been carried out repeatedly, i.e., CONT02 in October 2002, CONT05 in September 2005, CONT08 in August 2008, and CONT11 in September 2011, to demonstrate the highest accuracy the current VLBI was capable at that time. In this study, we have compared zenith total delays (ZTD) and troposphere gradients as consistently estimated from the observations of VLBI, Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) at VLBI sites participating in the CONT campaigns. We analyzed the CONT campaigns using the state-of-the-art software following common processing strategies as closely as possible. In parallel, ZTD and gradients were derived from numerical weather models, i.e., from the global European Centre for Medium-Range Weather Forecasts (ECMWF) analysis fields, the High Resolution Limited Area Model (European sites), the Japan Meteorological Agency-Operational Meso-Analysis Field (MANAL, over Japan), and the Cloud Resolving Storm Simulator (Tsukuba, Japan). Finally, zenith wet delays were estimated from the observations of water vapor radiometers (WVR) at sites where the WVR observables are available during the CONT sessions. The best ZTD agreement, interpreted as the smallest standard deviation, was found between GNSS and VLBI techniques to be about 5–6 mm at most of the co-located sites and CONT campaigns. We did not detect any significant improvement in the ZTD agreement between various techniques over time, except for DORIS and MANAL. On the other hand, the agreement and thus the accuracy of the troposphere parameters mainly depend on the amount of humidity in the atmosphere. 相似文献
5.
Lucia Plank Andreas Hellerschmied Jamie McCallum Johannes Böhm Jim Lovell 《Journal of Geodesy》2017,91(7):867-880
The possibility of observing satellites with the very long baseline interferometry (VLBI) technique has been discussed for several years in the geodetic community, with observations of either existing satellites of the global navigation satellite systems or of satellites dedicated to realise a space tie. Such observations were carried out using the Australian telescopes in Hobart and Ceduna which, for the first time, integrated all the necessary steps: planning the observations (automated scheduling), correlation of the data and the generation of a series of time delay observables suitable for a subsequent geodetic analysis. We report on the development of new and the adaptation of existing routines for observing and data processing, focusing on technology development. The aim was to use methods that are routinely used in geodetic VLBI. A series of test experiments of up to six hours duration was performed, allowing to improve the observations from session to session and revealing new problems still to be solved. The newly developed procedures and programs now enable more observations. Further development assumed, this bears the prospect of being directly applied to the observation of dedicated space-tie satellites. 相似文献
6.
We apply global optimization in order to optimize the referencing (and consequently the stability) of the Earth Orientation
Parameters (EOP) with respect to ITRF2005. These EOP are derived at a daily sampling from SLR data, simultaneously with weekly
station positions. The EOP referencing is carried out with minimum constraints applied weekly to the three rotations and over
core station networks. Our approach is based on a multi objective genetic algorithm, a particular stochastic global optimization
method, the reference system effects being the objectives to minimize. We thus use rigorous criteria for the optimal weekly
core station selection. The results evidence an improvement of 10% of the stability for Polar Motion (PM) series in comparison
to the results obtained with the network specially designed for EOP referencing by the Analysis Working Group of the International
Laser Ranging Service. This improvement of nearly 25 μas represents 50% of the current precision of the IERS 05 C04 PM reference
series. We also test the possibility of averaging the weekly networks provided by our algorithm (the Genetically Modified
Networks—GMN) over the whole time period. Although the dynamical nature of the GMN is clearly a key point of their success,
we can derive such a global mean core network, which could be useful for practical applications regarding EOP referencing.
Using this latter core network moreover provides more stable EOP series than the conventional network does. 相似文献
7.
Very long baseline interferometry (VLBI) tracking of satellites is a topic of increasing interest for the establishment of space ties. This shall strengthen the connection of the various space geodetic techniques that contribute to the International Terrestrial Reference Frame. The concept of observing near-Earth satellites demands research on possible observing strategies. In this paper, we introduce this concept and discuss its possible benefits for improving future realizations of the International Terrestrial Reference System. Using simulated observations, we develop possible observing strategies that allow the determination of radio telescope positions in the satellite system on Earth with accuracies of a few millimeters up to 1–2 cm for weekly station coordinates. This is shown for satellites with orbital heights between 2,000 and 6,000 km, observed by dense regional as well as by global VLBI-networks. The number of observations, as mainly determined by the satellite orbit and the observation interval, is identified as the most critical parameter that affects the expected accuracies. For observations of global positioning system satellites, we propose the combination with classical VLBI to radio sources or a multi-satellite strategy. Both approaches allow station position repeatabilities of a few millimeters for weekly solutions. 相似文献
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Very Long Baseline Interferometry (VLBI) plays a unique and fundamental role in the maintenance of the global (terrestrial
and celestial) reference frames, which are required for precise positioning in many research areas such as the understanding
and monitoring of global changes, and for space missions. The International VLBI Service for Geodesy and Astrometry (IVS)
coordinates the global VLBI components and resources on an international basis. The service is tasked by the International
Association of Geodesy (IAG) and International Astronomical Union (IAU) to provide products for the realization of the Celestial
Reference Frame (CRF) through the positions of quasars, to deliver products for the maintenance of the terrestrial reference
frame (TRF), such as station positions and their changes with time, and to generate products describing the rotation and orientation
of the Earth. In particular, VLBI uniquely provides direct observations of nutation parameters and of the time difference
UT1-UTC. This paper summarizes the evolution and current status of the IVS. It points out the activities to improve further
on the product quality to meet future service requirements. 相似文献
10.
We assess the accuracy of some indirect approaches to invariant point (IVP), or system reference point, determination of satellite
laser ranging (SLR) and very long baseline interferometry (VLBI) systems using both observed and simulated survey data sets.
Indirect IVP determination involves the observation of targets located on these systems during specific rotational sequences
and by application of geometrical models that describe the target motion during these sequences. Of concern is that most SLR
and VLBI systems have limited rotational freedom thereby placing constraint on the reliability of parameter estimation, including
the IVP position. We assess two current approaches to IVP analysis using survey data observed at the Yarragadee (Australia)
SLR and the Medicina (Italy) VLBI sites and also simulated data of a large rotationally constrained (azimuth-elevation) VLBI
system. To improve reliability we introduce and assess some new geometric conditions, including inter-axis, inter-circle and
inter-target conditions, to existing IVP analysis strategies. The error component of a local tie specifically associated with
the indirect determination of SLR and VLBI IVP is less than 0.5 mm. For systems with significant rotational limits we find
that the inter-axis and inter-circle conditions are critical to the computation of unbiased IVP coordinates at the sub-millimetre
level. When the inter-axis and inter-circle geometric conditions are not imposed, we retrieve biased vertical coordinates
of the IVP (in our simulated VLBI system) in the range of 1.2–3.4 mm. Using the new geometric conditions we also find that
the axis-offset estimates can be recovered at the sub- millimetre accuracy (0.5 mm). 相似文献
11.
Tests have been performed to assess the utility of surface meteorological measurements for improved geodetic performance. Three types of a priori met data are considered: a default global, seasonal model; local met measurements collected at a subset of the test network; output values from a NOAA Forecast Systems Lab assimilation model. A variety of configurations for handling tropospheric parameters in the geodetic solutions is also considered. We find no geodetic advantage in using the measured met data under any test scenario. Differences among the three types of a priori information are generally insignificant, although biases can be introduced when some troposphere parameters are not adjusted. Even when using only a relatively small network (<100 km) the differences remain minor. When observing sessions are reduced from 24 h to 6 h the increased error due to measurement noise obscures all other effects except for the shortest baselines where some advantage may be achieved by not adjusting troposphere parameters for very close station pairs. 相似文献
12.
A combination procedure of Earth orientation parameters from Global Positioning System (GPS) and Very Long Baseline Interferometry
(VLBI) observations was developed on the basis of homogeneous normal equation systems. The emphasis and purpose of the combination
was the determination of sub-daily polar motion (PM) and universal time (UT1) for a long time-span of 13 years. Time series
with an hourly resolution and a model for tidal variations of PM and UT1-TAI (dUT1) were estimated. In both cases, 14-day
nutation corrections were estimated simultaneously with the ERPs. Due to the combination procedure, it was warranted that
the strengths of both techniques were preserved. At the same time, only a minimum of de-correlating or stabilizing constraints
were necessary. Hereby, a PM time series was determined, whose precision is mainly dominated by GPS observations. However,
this setup benefits from the fact that VLBI delivered nutation and dUT1 estimates at the same time. An even bigger enhancement
can be seen for the dUT1 estimation, where the high-frequency variations are provided by GPS, while the long term trend is
defined by VLBI. The estimated combined tidal PM and dUT1 model was predominantly determined from the GPS observations. Overall,
the combined tidal model for the first time completely comprises the geometrical benefits of VLBI and GPS observations. In
terms of root mean squared (RMS) differences, the tidal amplitudes agree with other empirical single-technique tidal models
below 4 μas in PM and 0.25 μs in dUT1. The noise floor of the tidal ERP model was investigated in three ways resulting in about 1 μas for diurnal PM and 0.07 μs for diurnal dUT1 while the semi-diurnal components have a slightly better accuracy. 相似文献
13.
Susanne Glaser Rolf König Dimitrios Ampatzidis Tobias Nilsson Robert Heinkelmann Frank Flechtner Harald Schuh 《Journal of Geodesy》2017,91(7):723-733
In this study, we assess the impact of two combination strategies, namely local ties (LT) and global ties (GT), on the datum realization of Global Terrestrial Reference Frames in view of the Global Geodetic Observing System requiring 1 mm-accuracy. Simulated Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) data over a 7 year time span was used. The LT results show that the geodetic datum can be best transferred if the precision of the LT is at least 1 mm. Investigating different numbers of LT, the lack of co-located sites on the southern hemisphere is evidenced by differences of 9 mm in translation and rotation compared to the solution using all available LT. For the GT, the combination applying all Earth rotation parameters (ERP), such as pole coordinates and UT1-UTC, indicates that the rotation around the Z axis cannot be adequately transferred from VLBI to SLR within the combination. Applying exclusively the pole coordinates as GT, we show that the datum can be transferred with mm-accuracy within the combination. Furthermore, adding artificial stations in Tahiti and Nigeria to the current VLBI network results in an improvement in station positions by 13 and 12%, respectively, and in ERP by 17 and 11%, respectively. Extending to every day VLBI observations leads to 65% better ERP estimates compared to usual twice-weekly VLBI observations. 相似文献
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15.
Hiroyuki Usui 《Journal of Geographical Systems》2018,20(4):387-412
The structure of road networks has been investigated in accordance with the development of GIScience. By classifying road networks into wide and narrow ones, we can define the route as the path from the route’s origin (also called the root) on a wide road network to a narrow road segment which consists of the sequence of narrow road segments arranged by ascending order of the number of steps of adjacency to its root. The length of the route can be defined with the following geometric and topological terms: the route distance, measuring the length along the route and the depth, counting the number of road segments on the route. The depth plays the important role of being a substitute for the route distance in modelling road networks as a planar graph. Since road networks clearly exhibit irregular patterns and road segment lengths are non-uniform, it is considered appropriate to adopt a stochastic approach rather than a deterministic one to analyse the route distance. However, the relationship between the route distance and its depth has not been sufficiently investigated stochastically. Therefore, the research question is how can we estimate the route distance from its depth? Based on an empirical study in the Tokyo metropolitan region, it was found that (1) the statistical distribution of the route distance can be formulated as an Erlang distribution whose parameters are its depth and the inverse of the mean length of narrow road segments, and (2) this length is constant and close to 40 m. Therefore, we can estimate the route distance from only one parameter, the depth. Also, as a practical application, accessibility to the kth depth link in terms of firefighting was evaluated because the maximum length of the extension of fire hoses is approximately 200 m. It was found that (1) even if k?≤?5, the probability that the route distance to the kth depth link is equal to or longer than 200 m ranges from 0 to 0.45; and (2) if k?≥?8, the probability is approximately 1. These indicate the limitation of the deterministic approach because, on the basis of complete grid patterns (with intervals of 40 m between intersections), k?=?5 corresponds to a distance of 200 m from wide road networks and the route to the 5th depth link can be covered with fire hoses. Moreover, it was found that the connectivity of wide road networks is higher than that of narrow ones in terms of the smaller ratio of cul-de-sacs and the larger ratio of four-way intersections. These answers contribute substantially not only to constructing a science of cities that provides a simple model and specifies the most important parameter, but also to our understanding of the structure of narrow road networks within several hundred metres of wide road networks. 相似文献
16.
Chen Wang David Miller Iain Brown Yang Jiang Marie Castellazzi 《International Journal of Digital Earth》2016,9(6):586-605
Mitigating and adapting to climate change includes a requirement to evaluate the role of future land uses in delivering robust integrated responses that are sensitive to local landscape contexts. In practice, this emphasises the need for community engagement, planning and inclusive decision-making. Community engagement may be potentially facilitated by the use of spatially explicit quantitative scenarios of land-use change in combination with interactive visualisation. This requires a coherent framework to integrate spatial data modelling, analytical capabilities and visualisation tools in a format that will also engage diverse public audiences. These challenges were explored with a case study of virtual landscapes from N-E Scotland that was used to test preferences for scenarios of future land use. Visualisations employed texture-based rendering rather than full photo-realistic rendering to facilitate interactivity and this provided additional scope for audiences to explore multiple future scenarios compared to the present landscape. Interactive voting in a virtual landscape theatre suggested preferences for visual diversity, good stewardship and perceived naturalness that should be considered in developing planned responses to change. Further investigation of preferences was conducted using interactive 3D features located within the landscape. Study findings are reviewed against objectives for inclusive engagement in the Digital Earth agenda and used to make further recommendations on the use of scenarios and visualisation tools. In particular, technical advances in user engagement need to be developed in conjunction with emerging good practice that addresses ethical, behavioural and inclusion issues so that the content is presented in as transparent and unbiased format as possible. 相似文献
17.
Studies on small-world networks have received intensive interdisciplinary attention during the past several years. It is well-known
among researchers that a small-world network is often characterized by high connectivity and clustering, but so far there
exist few effective approaches to evaluate small-world properties, especially for spatial networks. This paper proposes a
method to examine the small-world properties of spatial networks from the perspective of network autocorrelation. Two network
autocorrelation statistics, Moran’s I and Getis–Ord’s G, are used to monitor the structural properties of networks in a process of “rewiring” networks from a regular to a random
network. We discovered that Moran’s I and Getis–Ord’s G tend to converge and have relatively low values when properties of small-world networks emerge. Three transportation networks
at the national, metropolitan, and intra-city levels are analyzed using this approach. It is found that spatial networks at
these three scales possess small-world properties when the correlation lag distances reach certain thresholds, implying that
the manifestation of small-world phenomena result from the interplay between the network structure and the dynamics taking
place on the network.
相似文献
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Computer networks like the Internet are gaining importance in social and economic life. The accelerating pace of the adoption
of network technologies for business purposes is a rather recent phenomenon. Many applications are still in the early, sometimes
even experimental, phase. Nevertheless, it seems to be certain that networks will change the socioeconomic structures we know
today. This is the background for our special interest in the development of networks, in the role of spatial factors influencing
the formation of networks, and consequences of networks on spatial structures, and in the role of externalities. This paper
discusses a simple economic model – based on a microeconomic calculus – that incorporates the main factors that generate the
growth of computer networks. The paper provides analytic results about the generation of computer networks. The paper discusses
(1) under what conditions economic factors will initiate the process of network formation, (2) the relationship between individual
and social evaluation, and (3) the efficiency of a network that is generated based on economic mechanisms.
Received: 5 July 2000 / Accepted: 28 November 2000 相似文献
20.
Analysis of geophysical networks derived from multiscale digital elevation models: a morphological approach 总被引:1,自引:0,他引:1
Tay L.T. Sagar B.S.D. Hean Teik Chuah 《Geoscience and Remote Sensing Letters, IEEE》2005,2(4):399-403
We provide a simple and elegant framework based on morphological transformations to generate multiscale digital elevation models (DEMs) and to extract topologically significant multiscale geophysical networks. These terrain features at multiple scales are collectively useful in deriving scaling laws, which exhibit several significant terrain characteristics. We present results derived from a part of Cameron Highlands DEM. 相似文献