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Ojha R.,United States Naval Observatory | Ojha R.,NVI, Inc. | Kadler M.,University of Bamberg | Kadler M.,Erlangen Center for Astroparticle Physics | And 32 more authors.
Astronomy and Astrophysics | Year: 2010

Context. A number of theoretical models vie to explain the γ-ray emission from active galactic nuclei (AGN). This was a key discovery of EGRET. With its broader energy coverage, higher resolution, wider field of view and greater sensitivity, the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope is dramatically increasing our knowledge of AGN γ-ray emission. However, discriminating between competing theoretical models requires quasi-simultaneous observations across the electromagnetic spectrum. By resolving the powerful parsec-scale relativistic outflows in extragalactic jets and thereby allowing us to measure critical physical properties, Very Long Baseline Interferometry observations are crucial to understanding the physics of extragalactic γ-ray objects. Aims. We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of-30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. Methods. These observations were made during 2007/2008 using the telescopes of the Australian Long Baseline Array in conjunction with Hartebeesthoek in South Africa. These data were correlated at the Swinburne University correlator. Results. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at http://pulsar.sternwarte.uni-erlangen.de/tanami/. Conclusions. We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright γ-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT. © 2010 ESO.

Francis O.,University of Luxembourg | Baumann H.,Federal Office of Metrology METAS | Volarik T.,Brno University of Technology | Rothleitner C.,University of Luxembourg | And 44 more authors.
Metrologia | Year: 2013

We present the results of the third European Comparison of Absolute Gravimeters held in Walferdange, Grand Duchy of Luxembourg, in November 2011. Twenty-two gravimeters from both metrological and non-metrological institutes are compared. For the first time, corrections for the laser beam diffraction and the self-attraction of the gravimeters are implemented. The gravity observations are also corrected for geophysical gravity changes that occurred during the comparison using the observations of a superconducting gravimeter. We show that these corrections improve the degree of equivalence between the gravimeters. We present the results for two different combinations of data. In the first one, we use only the observations from the metrological institutes. In the second solution, we include all the data from both metrological and non-metrological institutes. Those solutions are then compared with the official result of the comparison published previously and based on the observations of the metrological institutes and the gravity differences at the different sites as measured by non-metrological institutes. Overall, the absolute gravity meters agree with one another with a standard deviation of 3.1 μGal. Finally, the results of this comparison are linked to previous ones. We conclude with some important recommendations for future comparisons. © 2013 BIPM & IOP Publishing Ltd.

Jakobsson A.,Development Center | Hopfstock A.,Federal Agency for Cartography and Geodesy BKG | Beare M.,Spatial Group Ltd. | Patrucco R.,Ordnance Survey
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives | Year: 2013

This paper will introduce how quality of geo-information can be managed when the production environment is no longer inside one organization (e.g. collection of data is contracted out) or data is compiled from various sources like in case of Spatial Data Infrastructures (SDIs). The bases for quality management of reference geo-information are discussed using three viewpoints; data, process and organization and user centric viewpoints. These viewpoints can be met using ISO 19157 and ISO 19158 standards together with ESDIN developed Quality Model and Data Quality Services Framework (DQSF). Two different services are identified a Data Quality Web Service and a Data User Web Service. We discuss how these principles and services are implemented now within EuroGeographics and Ordnance Survey of Great Britain. Further development will be done during the European Location Framework (ELF) project, which is providing a single source of reference geo-information for Europe during 2013-2016.

Crossley D.J.,Washington University in St. Louis | Boy J.-P.,University of Strasbourg | Hinderer J.,University of Strasbourg | Jahr T.,Friedrich - Schiller University of Jena | And 4 more authors.
Geophysical Journal International | Year: 2014

The paper in question by Van Camp and co-authors [MVC] challenges previous work showing that ground gravity data arising from hydrology can provide a consistent signal for the comparison with satellite gravity data. The data sets used are similar to those used previously, that is, the gravity field as measured by the GRACE satellites versus ground-based data from superconducting gravimeters (SGs) over the same continental area, in this case Central Europe. One of the main impediments in this paper is the presentation that is frequently confusing and misleading as to what the data analysis really shows, for example, the irregular treatment of annual components that are first subtracted then reappear in the analysis. More importantly, we disagree on specific points. Two calculations are included in our comment to illustrate where we believe that the processing in [MVC] paper is deficient. The first deals with their erroneous treatment of the global hydrology using a truncated spherical harmonic approach which explains almost a factor 2 error in their computation of the loading. The second shows the effect of making the wrong assumption in the GRACE/hydrology/surface gravity comparison by inverting the whole of the hydrology loading for underground stations. We also challenge their claims that empirical orthogonal function techniques cannot be done in the presence of periodic components, and that SG data cannot be corrected for comparisons with GRACE data. The main conclusion of their paper, that there is little coherence between ground gravity stations and this invalidates GRACE comparisons, is therefore questionable. There is nothing in [MVC] that contradicts any of the previous papers that have shown clearly a strong relation between seasonal signals obtained from both ground gravity and GRACE satellite data. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Weise A.,Friedrich - Schiller University of Jena | Kroner C.,Physikalisch - Technische Bundesanstalt | Kroner C.,Helmholtz Center Potsdam | Abe M.,Helmholtz Center Potsdam | And 9 more authors.
Journal of Geodynamics | Year: 2012

Time variable gravity field models derived from the satellite mission GRACE have been demonstrated to be consistent with water mass variations in the global hydrological cycle. Independent observations are provided by terrestrial measurements. In order to achieve a maximum of reliability and information gain, ground-based gravity observations may be deployed for comparison with the gravity field variations derived from the GRACE satellite mission. In this context, the data of the network of superconducting gravimeters (SG) of the 'Global Geodynamics Project' (GGP) are of particular interest. This study is focused on the dense SG network in Central Europe with its long-term gravity observations. It is shown that after the separation and reduction of local hydrological effects in the SG observations especially for subsurface stations, the time-variable gravity signals from GRACE agree well with the terrestrial observations from the SG station cluster. Station stability of the SG sites with respect to vertical deformations was checked by GNSS based observations. Most of the variability can be explained by loading effects due to changes in continental water storage, and, in general, the stability of all stations has been confirmed. From comparisons based on correlation and coherence analyses in combination with the root mean square (RMS) variability of the time series emerges, that the maximum correspondence between the SG and GRACE time series is achieved when filtering the GRACE data with Gaussian filters of about 1000. km filter length, which is in accordance with previous publications. Empirical Orthogonal Functions (EOF) analysis was applied to the gravity time series in order to identify common characteristic spatial and temporal patterns. The high correspondence of the first modes for GRACE and SG data implies that the first EOF mode represents a large-scale (Central European) time-variable gravity signal seen by both the GRACE satellites and the SG cluster. © 2011 Elsevier Ltd.

Abe M.,Helmholtz Center Potsdam | Kroner C.,Physikalisch - Technische Bundesanstalt | Forste C.,Helmholtz Center Potsdam | Petrovic S.,Helmholtz Center Potsdam | And 8 more authors.
Geophysical Journal International | Year: 2012

The GRACE (Gravity Recovery and Climate Experiment) satellite mission provides global time-series of the Earth's gravity field. In view of limited resolution and noise from the GRACE data, various filtering techniques have been developed to extract an optimal signal. There is no conclusion on the best filter method so far, however. On the other hand, terrestrial gravity observations from superconducting gravimeters (SGs) provide variations of the gravity field with very high accuracy and time resolution, but only at single points. The aim of this study is to compare GRACE-derived temporal gravity variations with gravity time-series within a network of six Central European SG stations. Empirical orthogonal functions (EOF) analysis was applied to detect common signal characteristics over a 3 yr period (2004-2006). rms Differences between the time-series of several GRACE solutions amount to 60 per cent of the rms variability of the individual data sets. The rms differences between the SG and GRACE time-series are about 70 per cent of the rms value of the SG observations. The best agreement between SG and GRACE is obtained when using a Gaussian filter with filter lengths of 800-1250 km for the GRACE data. With the EOF analysis, a common regional signal can be deduced from all gravity data sets. Nevertheless, differences in the first EOF among the GRACE solutions were up to 40 per cent, and differences of up to 50 per cent were found between the SG-based terrestrial and the GRACE-based satellite observations. © 2012 GFZ German Research Centre for Geosciences Potsdam Geophysical Journal International © 2012 RAS.

Wziontek H.,Federal Agency for Cartography and Geodesy BKG | Wilmes H.,Federal Agency for Cartography and Geodesy BKG | Bonvalot S.,Bureau Gravimetrique International
International Association of Geodesy Symposia | Year: 2012

The steadily growing number of absolute gravimeters and absolute gravity measurements all over the world emphasizes the demand of an overview about existing locations, observations, instruments and institutions involved. As a contribution to the International Gravity Field Service (IGFS), a relational database was designed and implemented in a joint development of BKG and BGI and is in operational status now. Two objectives are aimed at: With freely available meta-data and contact details, the database should give an overview about existing stations and observations, serve as a platform for multidisciplinary cooperation and allow the coordination of forthcoming measurements. Among contributing groups or within international projects, an exchange of gravity values and processing details is possible. The database will function as a data inventory, assuring long term availability of the data. Prospectively, the database will be the foundation for a future international gravity reference system and will serve as a pool for geophysical interpretation of absolute gravity observations on a global scale. © Springer-Verlag Berlin Heidelberg 2012.

Wang Q.,Central South University | Wang Q.,Helmholtz Center Potsdam | Xu G.,Helmholtz Center Potsdam | Petrovic S.,Helmholtz Center Potsdam | And 4 more authors.
Advances in Space Research | Year: 2011

A regional tropospheric model can be constructed using surveys from GPS ground networks. Using this model the tropospheric delays of a kinematic station within the region can be interpolated. However, such a model is generally not suitable for an airborne platform high above the ground networks. In this paper, a method of constructing a regional tropospheric model for airborne GPS applications is described. First, the kinematic station in the air is projected onto the ground. Then the tropospheric delays at projected point are interpolated from those of the ground networks. Finally, the tropospheric delays at projected point are extended upward to the airborne platform using pressure and temperature gradients and humidity exponential function. For validation of this method, the data of airborne campaign carried out by BKG in cooperation with GFZ and BGR 2008 in the northern Alps are used. The results show that GPS kinematic positioning precision in height component can be improved using this method. © 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. 25.

Wiatr T.,Federal Agency for Cartography and Geodesy BKG | Suresh G.,Federal Agency for Cartography and Geodesy BKG | Gehrke R.,Federal Agency for Cartography and Geodesy BKG | Hovenbitzer M.,Federal Agency for Cartography and Geodesy BKG
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives | Year: 2016

Copernicus is an European system created for Earth observation and monitoring. It consists of a set of Earth observation satellites and in-situ sensors that provide geo-information that are used, through a set of Copernicus services, for applications related to the environment and global security. The main services of the Copernicus programme address six thematic areas: land, marine, atmosphere, climate change, emergency management and security. In Germany, there is a national service team of Copernicus service coordinators, who are responsible for the national development of the Copernicus services and for providing user-specific information about the Copernicus processes. These coordinators represent the contact points for all the programmes and services concerning their respective Copernicus theme. To publish information about Copernicus, national conferences and workshops are organised. Many people are involved in planning the continuous process of bringing the information to public authorities, research institutes and commercial companies. The Federal Agency for Cartography and Geodesy (Bundesamt für Kartographie und Geodäsie, BKG) is one such organisation, and is mainly responsible for the national land monitoring service of Copernicus. To make use of the freely available data from the Copernicus programme, the Federal Agency for Cartography and Geodesy is currently developing new applications and projects in the field of remote sensing and land monitoring. These projects can be used by other public authorities as examples on how to use the Copernicus data and services for their individual demands and requirements. Copernicus data and services are currently not very commonly used in the daily routine of the national mapping agencies, but they will soon be.

Suresh G.,Federal Agency for Cartography and Geodesy BKG | Gehrke R.,Federal Agency for Cartography and Geodesy BKG | Wiatr T.,Federal Agency for Cartography and Geodesy BKG | Hovenbitzer M.,Federal Agency for Cartography and Geodesy BKG
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives | Year: 2016

Land cover information is essential for urban planning and for land cover change monitoring. This paper presents an overview of the work conducted at the Federal Agency for Cartography and Geodesy (BKG) with respect to Synthetic Aperture Radar (SAR) based land cover classification. Two land cover classification approaches using SAR images are reported in this paper. The first method involves a rule-based classification using only SAR backscatter intensity while the other method involves supervised classification of a polarimetric composite of the same SAR image. The LBM-DE has been used for training and validation of the SAR classification results. Images acquired from the Sentinel-1a satellite are used for classification and the results have been reported and discussed. The availability of Sentinel-1a images that are weather and daylight independent allows for the creation of a land cover classification system that can be updated and validated periodically, and hence, be used to assist other land cover classification systems that use optical data. With the availability of Sentinel-2 data, land cover classification combining Sentinel-1a and Sentinel-2 images present a path for the future.

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