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Pace B.,e GEOS S.p.A. Centro di Geodesia Spaziale CGS | Pacione R.,e GEOS S.p.A. Centro di Geodesia Spaziale CGS | Sciarretta C.,e GEOS S.p.A. Centro di Geodesia Spaziale CGS | Bianco G.,Centro Of Geodesia Spaziale Cgs
International Association of Geodesy Symposia | Year: 2016

Tropospheric refraction is one of the major error sources in satellite-based positioning. The delay of radio signals caused by the troposphere ranges from 2 m at the zenith to 20 m at low elevation angles, depending on pressure, temperature and humidity along the path of the signal transmission. If the delay is not properly modelled, positioning accuracy can degrade significantly. Empirical tropospheric models, with or without meteorological observations, are used to correct these delays but they cannot model tropospheric variations exactly since they are limited in accuracy and spatial resolution resulting in up to a few decimetres error in positioning solutions. The present availability of dense ground based Global Navigation Satellite System (GNSS) networks and the state of the art GNSS processing techniques enable precise estimation of Zenith Tropospheric Delays (ZTD) with different latency ranging from Near Real-Time (NRT) to post-processing. We describe a method for computing ZTD correction fields interpolating, through Ordinary Kriging, the residuals between GNSS-derived and model-computed ZTD at continuously operating GNSS stations. At a known user location, the correction which is added to the modelled- ZTD value can be obtained through a bi-linear interpolation with the four nearest grid points surrounding it. The performance of the method has been evaluated over a 1-year period at 25 European stations belonging to the EUREF and IGS network. It is found that such an empirical tropospheric model can be improved when considering tropospheric corrections coming from ground based GNSS network. © Springer International Publishing Switzerland 2015. Source


Fey A.L.,U.S. Naval Observatory | Gordon D.,NASA | Jacobs C.S.,Jet Propulsion Laboratory | Ma C.,NASA | And 29 more authors.
Astronomical Journal | Year: 2015

We present the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30 years of Very Long Baseline Interferometry observations. ICRF2 contains precise positions of 3414 compact radio astronomical objects and has a positional noise floor of ∼40 μas and a directional stability of the frame axes of ∼10 μas. A set of 295 new "defining" sources was selected on the basis of positional stability and the lack of extensive intrinsic source structure. The positional stability of these 295 defining sources and their more uniform sky distribution eliminates the two greatest weaknesses of the first realization of the International Celestial Reference Frame (ICRF1). Alignment of ICRF2 with the International Celestial Reference System was made using 138 positionally stable sources common to both ICRF2 and ICRF1. The resulting ICRF2 was adopted by the International Astronomical Union as the new fundamental celestial reference frame, replacing ICRF1 as of 2010 January 1. © 2015. The American Astronomical Society. All rights reserved.. Source


Pacione R.,Centro Of Geodesia Spaziale Cgs | Pace B.,Centro Of Geodesia Spaziale Cgs | Vedel H.,Danish Meteorological Institute | De Haan S.,Royal Netherlands Meteorological Institute | And 2 more authors.
Advances in Space Research | Year: 2011

In this article we present two methods for combination of different Global Navigation Satellite Systems (GNSS) Zenith Total Delay (ZTD) time-series for the same GNSS site, but from different producers or different processing setups. One method has been setup at ASI/CGS, the other at KNMI. Using Near Real-Time (NRT) ZTD data covering 1 year from the E-GVAP project, the performance of the two methods is inter-compared and validation is made against a combined ZTD solution from EUREF, based on post-processed ZTDs. Further, validation of the ASI combined solutions is made against independent ZTDs derived from radiosonde, Numerical Weather Prediction (NWP) model and Very Long Baseline Interferometry (VLBI) ZTD. It is found that the two combined solutions perform quite similar, with a bias from -0.17 mm to 1.52 mm and a standard deviation from 1.60 mm to 3.82 mm. Compared with respect to EUREF post-processed solutions, the NRT combined solutions shows a small but positive bias which could be due to a different way of dealing with phase ambiguities in the data reduction process. Further, it is found that the ASI combined solution compares better to both radiosonde, NWP model and VLBI ZTDs than the individual time-series upon which it is based. It is concluded that the combined NRT solutions appear a promising tool for rapid control of the NRT ZTDs produced today by a number of Analysis Centres (ACs) across Europe for use in meteorology. It is known that the NRT processing is prone to certain types of errors rarely seen in post-processing. These errors can lead to a large number of ZTDs from a given AC having correlated errors, which can do serious damage if the data are used in Numerical Weather Prediction, even if it is a rare occurrence. Identification and blocking of such data is therefore a goal in the NRT GNSS data processing and validation. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. Source


Heinkelmann R.,Deutsches Geodatisches Forschungsinstitut DGFI | Bohm J.,Vienna University of Technology | Bolotin S.,NVI, Inc. | Engelhardt G.,Bundesamt fur Kartographie und Geodasie BKG | And 7 more authors.
Journal of Geodesy | Year: 2011

Time-series of zenith wet and total troposphere delays as well as north and east gradients are compared, and zenith total delays (ZTD) are combined on the level of parameter estimates. Input data sets are provided by ten Analysis Centers (ACs) of the International VLBI Service for Geodesy and Astrometry (IVS) for the CONT08 campaign (12-26 August 2008). The inconsistent usage of meteorological data and models, such as mapping functions, causes systematics among the ACs, and differing parameterizations and constraints add noise to the troposphere parameter estimates. The empirical standard deviation of ZTD among the ACs with regard to an unweighted mean is 4.6 mm. The ratio of the analysis noise to the observation noise assessed by the operator/software impact (OSI) model is about 2.5. These and other effects have to be accounted for to improve the intra-technique combination of VLBI-derived troposphere parameters. While the largest systematics caused by inconsistent usage of meteorological data can be avoided and the application of different mapping functions can be considered by applying empirical corrections, the noise has to be modeled in the stochastic model of intra-technique combination. The application of different stochastic models shows no significant effects on the combined parameters but results in different mean formal errors: the mean formal errors of the combined ZTD are 2.3 mm (unweighted), 4.4 mm (diagonal), 8.6 mm [variance component (VC) estimation], and 8.6 mm (operator/software impact, OSI). On the one hand, the OSI model, i. e. the inclusion of off-diagonal elements in the cofactor-matrix, considers the reapplication of observations yielding a factor of about two for mean formal errors as compared to the diagonal approach. On the other hand, the combination based on VC estimation shows large differences among the VCs and exhibits a comparable scaling of formal errors. Thus, for the combination of troposphere parameters a combination of the two extensions of the stochastic model is recommended. © 2011 Springer-Verlag. Source


Dell'Agnello S.,National Institute of Nuclear Physics, Italy | Currie D.G.,University of Maryland College Park | Delle Monache G.O.,National Institute of Nuclear Physics, Italy | Cantone C.,National Institute of Nuclear Physics, Italy | And 20 more authors.
IEEE Aerospace Conference Proceedings | Year: 2010

Over the past forty years, Lunar Laser Ranging (LLR) to the Apollo Corner Cube Reflector (CCR) arrays has supplied almost all of the significant tests of General Relativity, and provided significant information on the composition and origin of the Moon. These arrays are the only experiment of the Apollo program still in operation. Initially the Apollo Lunar arrays contributed a negligible portion of the error budget used to achieve these results. However over the decades, the performance of the ground stations has been greatly upgraded so that the ranging accuracy has improved by more than two orders of magnitude. Now, after forty years, because of the lunar librations, the existing Apollo retroreflector arrays contribute a significant fraction of the limiting errors in the range measurements. University of Maryland (UMD) and INFN/LNF are now proposing a new approach to the Lunar Laser Ranging Array technology, the experiment MoonLIGHT. The new arrays will support ranging observations that are a factor 100 more accurate, reaching the micron level. The new fundamental physics and lunar physics that this new Lunar Laser Ranging Retroreflector Array for the 21st century (LLRRA-21) can provide, will be briefly described. The new lunar CCR housing has been built at the INFN/LNF. In the design of the new array there are three major challenges: 1) validate that the specifications of the CCR required for the new array, which are significantly beyond the properties of current CCRs, can indeed be achieved, 2) address the thermal and optical effects of the absorption of solar radiation within the CCR, reduce the transfer of heat from the hot housing to the CCR and 3) define a method of emplacing the CCR package on the lunar surface such that the relation between the optical center of the array and the center of mass of the Moon remains stable over the lunar day/night cycle. Its evolutionary design may be suitable for future GNSS constellations guaranteeing ranging accuracy improvement (the concept of a single reflector introduces no laser pulse spreading at all angles), weight and area saving (being its absolute optical cross section equal to a large number of the CCRs that will be used for the upcoming GNSS constellations). ©2010 IEEE. Source

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