W. Alef, H. Rottmann, D. Graham, S. Tingay, A. Deller , A. Bertarini , W. Schlüter , J. Ihde &ndash
DiFX Software Correlator for Geodesy
VLBI correlators can be realised either in hardware (MK II, MK III, MK IV, VLBA, ...) or software, running on general purpose computers (MK I).
The development of truly digital VLBI data recording (Mark 5, K5...) and of inexpensive commodity computers have led to a renaissance of software correlators. The DiFX (Distributed FX) correlator has been verified already for astronomy. At the MPIfR in Bonn in collaboration with the BKG and the University of Bonn teh DiFX correlator is being verified for geodesy. First results will be presented.
V. Tesmer, P. Steigenberger, M. Rothacher, B. Meisel &ndash
Homogeneously reprocessed VLBI and GPS GGOS-D height time series
This investigation compares homogeneously reprocessed VLBI (OCCAM6.1e@DGFI) and GPS (Bernese5.1@GFZ and @TUM) long-term height series from 1994 to 2007. The VLBI and GPS data analysis used fully adapted state-of-the-art models (like VMF1 and a priori zenith delay from ECMWF) in both software packages. They are analyzed in terms of long term non-linear behaviour, mean annual and harmonic annual signals as well as atmospheric loading coefficients (estimated via linear regression using local ECMWF pressure). The similarity between both techniques is very good, which is assumed to be due to the adapted models and consistent reprocessing of both series. Besides, the height time series of all 161 suitable GPS stations are investigated concerning spatial correlation of their mean annual behaviour.
M. Sacher, J. Ihde, G. Liebsch &ndash
EVRF07 - A new Realization of the European Vertical Reference System
1994 the work at the Unified European Levelling Network was resumed after a break of 10 years under the name UELN-95. After four years work the results were handed over under the name UELN-95/98 to the participating countries. At the EUREF Symposium 2000 in Tromsų the UELN 95/98 solution was used as base for a first definition of the European Vertical Reference System (EVRS).
Since 2005 the need for a new realization of the EVRS has been repeated recognized. The new realization EVRF07 contains new levelling data of 14 European countries. Contrary to the EVRF2000 with only one datum point in the Netherlands the new EVRF07 datum is realized by 13 datum points distributed over Europe. The measurements have been reduced to the epoch 2000 using the land uplift model NKG2005LU. The adjusted geopotential numbers are in the zero tidal system.
P. Steigenberger, V. Tesmer &ndash
Impact of different troposphere modeling on GPS- and VLBI-derived parameters
Co-located sites of the space-geodetic techniques GPS and VLBI allow for a comparison of the common parameters determined by these two independent techniques. Long-time series of homogeneously reprocessed GPS and VLBI data computed by TU Munich/TU Dresden (GPS) and DGFI (VLBI) covering the time interval from 1994 till 2005 provide the basis for our comparisons of different troposphere mapping functions and hydrostatic a priori delays as regards station coordinates and trosphere zenith delays.
P. Steigenberger, M. Rothacher, M. Fritsche, A. Rülke, R, Dietrich &ndash
PDR GPS satellite orbits
High-precision Global Positioning System (GPS) satellite orbits are one of the core products of the International GNSS Service (IGS). Since the establishment of the IGS in 1994, the quality and consistency of the IGS orbits has steadily been improved by advances in the modeling of GPS observations. However, due to these model improvements and reference frame changes, the time series of operational orbits are inhomogeneous and inconsistent. This problem can only be overcome by a complete reprocessing starting with the raw observation data. The quality of reprocessed GPS satellite orbits for the time period 1994-2005 will be assessed in this contribution. Orbit fits show that the internal consistency of the orbits could be improved by a factor of about two in the early years.
L. Zenner, Th. Gruber &ndash
Impact of Atmospheric & Oceanic Uncertainties on GRACE De-Aliasing Products
High-frequency, time varying mass redistributions in the ocean and atmosphere have a significant impact on GRACE and are therefore eliminated in the standard GRACE de-aliasing process. Until now the atmosphere and ocean model used in the de-aliasing process are regarded as error free, although it is well known from various studies that data from atmospheric and oceanic models have large uncertainties. In order to get insight into the importance of uncertainties in these models, we perform a full error propagation of the atmospheric and oceanic parameters up to the gravity field.
The paper summarizes the standard GRACE de-aliasing process and presents the mathematical model applied for the error propagation. The effect of uncertainties in the atmospheric and oceanic input parameter (temperature, surface pressure, specific humidity, geopotential, ocean bottom pressure) on the gravity field potential coefficients will be shown in several scenarios.
U. Schäfer, G. Liebsch, H.-U. Schirmer, J. Ihde, A.V. Olesen, H. Skourup, R. Forsberg, H. Pflug, J. Neumeyer &ndash
Improving gravity field modelling in the German-Danish border region by combining airborne, satellite and terrestrial gravity data
Um die regionale Schwerefeld- und Geoidmodellierung im Grenzbereich zwischen Deutschland und Dänemark zu verbessern, insbesondere im Bereich der Nord- und Ostsee, wurden seit Oktober 2006 mehrere fluggravimetrische Vermessungen durchgeführt.
Die Ergebnisse dieser Befliegungen mit einer Gesamtstrecke von mehr als 25000 km tragen dazu bei, die Datenbasis in dieser Region zu verbessern, indem noch vorhandene Datenlücken geschlossen werden und die bisherigen Daten besser verifiziert werden können. Die mit den Daten bestimmten Geoidmodelle im Küstenbereich, sind die Grundlage für eine zuverlässige Realisierung der Höhenreferenzsysteme und die Bereitstellung von entsprechenden Modellen für wissenschaftliche und praktische Anwendungen.
Nach eine Analyse der abgeleiteten Schwereanomalien an Kreuzungspunkten ergibt sich eine Genauigkeit der fluggravimetrischen Vermessung von besser als 2 mgal für den größten Teil des Untersuchungsgebietes.
Die Anforderungen die sich aus einer Kombination der fluggravimetrischen Beobachtungen mit anderen, räumlich sehr unterschiedlich verteilten Schwerefeldfunktionalen ergeben (aus Satellitenschwerefeldmissionen, Altimetrie, terrestrischer und Seegravimetrie), werden in der Auswertetechnologie berücksichtigt und dargestellt. Es werden die numerischen Ergebnisse von räumlich hochauflösenden Schwerefeld- und Geoidmodellierungen unter Verwendung verschiedener Kombinationsmethoden präsentiert.
T. Klügel, U. Schreiber, W. Schlüter, A. Velikoseltsev &ndash
Improved measurements of short-periodic Polar Motion by the Wettzell G Ring Laser
Extremely precise inertial rotation sensors being attached to the Earth allow the measurement of the absolute rotation rate of the Earth and the orientation of the instantaneous rotation axis in an Earth fixed frame with high temporal resolution. The world's most precise inertial rotation sensor, the "G" ring laser at the Geodetic Observatory Wettzell operating since 2001, has undergone a technical upgrade in 2006. The former resolution of 10-8 with respect to the Earth rotation rate could be further improved giving more insight to signals affecting inertial Earth rotation measurements. This implies a better determination of the retrograde diurnal polar motion terms, also known as Oppolzer terms, that cannot be directly be measured by the geodetic space techniques. The recent detection limit for periodic signals in the diurnal to semi-diurnal frequency band is 0.2 milliseconds for length-of-day variations or 0.5 milliarcseconds (2.5 nanorad) for polar motion.
H. Wziontek, H. Wilmes, J. Ihde, S. Bonvalot &ndash
AGrav: An International Database for Absolute Gravity Measurements
At BKG a dedicated database was developed which covers the specific requirements of the own absolute gravity data and applications. New absolute gravity observations are stored directly in this database whereas older observations are reprocessed and then included in the database in a sequential procedure.
The growing number of absolute gravimeters and observations at global scale reflects the high importance of this type of observations e.g. for the combination with the new satellite gravity field missions, in applications related to global change processes or for the combination of gravity data with time series of height determinations. This made obvious that also for the international community there is the demand for an overview about gravity stations, observations, instruments and institutions involved in this work. It is shown that the BKG absolute gravity database can fulfil the needs for a better information about the data which will lead to a better cooperation between the groups and institutions involved.
As a common activity of BKG and "Bureau Gravimétrique International" (BGI) the web-based database for absolute gravity data was installed which is open for metadata and data from all institutions of IAG. A first goal of this database is to collect metadata as a basis for future cooperation and coordination. Furthermore, an exchange of gravity values and processing details between contributing groups is possible. Prospectively, the database will contribute to the realisation of an international absolute gravity reference and serve as a basis for geophysical interpretation of absolute gravity on a global scale.
T. Klügel, H. Wziontek, M. Abe, C. Kroner &ndash
Computation of Atmospheric Gravity Changes from 3-dimensional Numerical Weather Models
Beside the tides of the solid Earth and the ocean, mass fluctuations in the atmosphere are the main signal constituent in precise gravity measurements. In order to separate other parts of the signal, this effect is routinely eliminated by multiplying the local air pressure record with a constant or a varying factor being determined by linear regression, and substracting the product from the gravimeter time series. In this way a major part of the atmospheric noise is eliminated. However, air mass variations which are not apparent in the local air pressure, like seasonal vertical mass shifts under constant surface pressure or distant pressure anomalies, are not covered by this technique although affecting the gravimeter. The remaining atmospheric noise may superimpose other signals of interest like annual tides or accelerations due to polar motion.
The physical model is based on 3-dimensional weather models of regional and global scale, where the Newtonian attraction of each element on the instrument site is computed and summed up component-wise. The problem of the low temporal resolution of usually 6 hours and the improper point-mass assumption in the near field is solved by a cylindrical local model, where the attraction in each layer is computed analytically on the basis of the local air pressure with high temporal resolution.
The results show differences up to 2 microgal as compared to the linear regression method. The 3-dimensional atmospheric correction now allows the identification of other systematic components in the residual gravity signal like hydrological effects, and an improved determination especially of long period tidal waves. In addition the horizontal components are very useful for the attraction correction of tiltmeters, where the local air pressure is not suitable.
C. Schwatke, W. Bosch &ndash
Erkennung und Verfolgung von Eddies
Vor allem in den starken westlichen Randströmungen kommt es zur Ausbildung von Wirbeln (Eddies), die mit etwa 50-200km Durchmesser bis zu mehreren Monaten erhalten bleiben . Eddies sind Energieträger, konzentrieren Nahrung und dominieren die örtlichen Strömungen. Sie sind deshalb wichtig für Klimauntersuchungen, Fischfang und Schiffsroutenberatung. Die Satellitenaltimetrie ist grundsätzlich in der Lage, Eddies zu erkennen und zu verfolgen - allerdings sollte die raum-zeitliche Abtastung der Meeresoberfläche durch Kombination verschiedener Missionen verbessert werden. Methoden zur Erkennung und Verfolgung von Eddies wurden entwickelt und Ergebnisse geeigneter Algorithmen werden vorgestellt und mit unabhängigen Daten der ARGO-Floatern verglichen.
W. Bosch, R. Savcenko &ndash
Profile der Meerestopographie
Seit den entscheidenden Verbesserungen der Schwerefeldmodelle durch GRACE ist es möglich, signifikante Strukturen der Meerestopographie durch die "geodätische" Methode, d.h. Subtraktion von Meereshöhen und Geoidundulation zu schätzen. Hier wird ein Verfahren vorgestellt, das a) die Meereshöhenprofile entlang der Altimeterspuren nutzt und dadurch eine Interpolation auf ein Gitter vermeidet, das b) Restfehler der Schwerefeldmodelle ("striping") durch geeignete Filterung beseitigt und c) eine konsistente Filterung der Meereshöhenprofile durchführt. Die instantanen Profile der Meerestopographie werden mit unabhängigen Modellen der mittleren Meerestopographie verglichen und zeigen bis auf erhebliche offsets (verursacht durch die level-of-no-motion Annahme) sehr gut Übereinstimmung.
R. Savcenko, W. Bosch &ndash
Globale Modelle der Meeresgezeiten - ein Vergleich
FES2004, GOT4.7, TPXO7.1 und EOT08a sind aktuelle globale Gezeitenmodelle, die miteinander verglichen werden. Die Differenzen einiger Partialtiden erreichen insbesondere im Küstenbereich Amplituden bis zu 15 cm. Aber auch im offenen Ozean sind großräumige Muster mit Amplitudendifferenzen von 1-2 cm erkennbar. Vergleiche mit Küstenpegeln sind wegen unterschiedlicher Auflösung problematisch.
C. Stummer, Th. Gruber, J. Bouman, S. Rispens &ndash
GOCE Gradiometry - A Guide for Users
The GOCE High-level Processing Facility (HPF) is in charge of the production of the final GOCE level 2 products. The calibrated and corrected gravity gradients form one of these products. Several aspects have to be considered, when using these gradients.
E.g. it is important that the nine gradient components can not be measured with equal accuracies. The reason is that the GOCE gradiometer consist of six accelerometers, which only have two high sensitive axes and one less sensitive axis, each. Moreover, the error spectra of the gradients show that an accuracy of a few mE/sqrt(Hz) (E=Eötvös; 1E=10-9/s2) can only be reached within the measurement bandwidth of the gradiometer from 5 to 100 mHz. It is also important to keep in mind that for GOCE several reference frames play a role. The gradients are measured in the Gradiometer Reference Frame (GRF). This reference frame is instrument-fixed and differs by a few degrees from the Local Orbital Reference Frame (LORF), which is defined by the instantaneous direction of the velocity vector. Besides the gravity gradients themselves the product contains also additional information, like corrections to the gradients due to temporal gravity variations. This information can be used to gain the sought-after static gravity field information from GOCE. The paper summarizes the important aspects of gradiometry with GOCE and introduces in detail the content of the gravity gradients product.