Thomas Klügel

Federal Agency for Cartography and Geodesy, Frankfurt, Hesse, Germany

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Publications (43)25.25 Total impact

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    ABSTRACT: The German Antarctic Receiving Station (GARS) O’Higgins at the northern tip of the Antarctic Peninsula is a dual purpose facility for earth observation and has existed for more than 20 years. It serves as a satellite ground station for payload data downlink and telecommanding of remote sensing satellites as well as a geodetic observatory for global reference systems and global change. Both applications use the same 9 m diameter radio antenna. Major outcomes of this usage are summarised in this paper. The satellite ground station O’Higgins (OHG) is part of the global ground station network of the German Remote Sensing Data Centre (DFD) operated by the German Aerospace Centre (DLR). It was established in 1991 to provide remote sensing data downlink support within the missions of the European Remote Sensing Satellites ERS-1 and ERS-2. These missions provided valuable insights into the changes of the Antarctic ice shield. Especially after the failure of the on-board data recorder, OHG became an essential downlink station for ERS-2 real-time data transmission. Since 2010, OHG is manned during the entire year, specifically to support the TanDEM-X mission. OHG is a main dump station for payload data, monitoring and telecommanding of the German TerraSAR-X and TanDEM-X satellites. For space geodesy and astrometry the radio antenna O’Higgins significantly improves coverage over the southern hemisphere and plays an essential role within the global Very Long Baseline Interferometry (VLBI) network. In particular the determination of the Earth Orientation Parameters (EOP) and the sky coverage of the International Celestial Reference Frame (ICRF) benefit from the location at a high southern latitude. Further, the resolution of VLBI images of active galactic nuclei (AGN), cosmic radio sources defining the ICRF, improves significantly when O’Higgins is included in the network. The various geodetic instrumentation and the long time series at O’Higgins allow a reliable determination of crustal motions. VLBI station velocities, continuous GNSS measurements and campaign-wise absolute gravity measurements consistently document a vertical rate of about 5 mm/a. This crustal uplift is interpreted as an elastic rebound due to ice loss as a consequence of the ice shelf disintegration in the Prince Gustav Channel in the late 1990s. The outstanding location on the Antarctic continent and its year-around operation make GARS O’Higgins in future increasingly attractive for polar orbiting satellite missions and a vitally important station for the global VLBI network. Future plans call for the development of an observatory for environmentally relevant research. That means that the portfolio of the station will be expanded including the expansion of the infrastructure and the construction and operation of new scientific instruments suitable for long-term measurements and satellite ground truthing.
    Polar Record 10/2014; · 0.98 Impact Factor
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    ABSTRACT: Global Navigation Satellite Systems (GNSS) are important contributors to the realization of the International Terrestrial Reference System (ITRS). For the combination of different space geodetic techniques, terrestrial measurements between the corresponding reference points are necessary. Discrepancies between these so-called local ties on the one hand and the coordinate differences derived from space techniques on the other hand are a major limitation for the realization of the ITRS nowadays. In the past, these discrepancies have often been attributed to inaccurate terrestrial measurements. This paper shows that a major part of the differences can be explained by systematic GNSS-specific errors, if a global data analysis is simulated. One of the most important error sources for GNSS are interactions of the antenna with its immediate vicinity, primarily multipath. At the Geodetic Observatory Wettzell (Germany), up to six GNSS permanent sites are operated in parallel at a distance of only a few meters. This antenna array is ideal to study the impact of local effects on the various GNSS observables and linear combinations. Comparisons of solutions obtained from different GNSS observables reveal cm-level discrepancies. Individual receiver antenna calibrations have an impact on the estimated station positions on the level of several millimeters. As other error sources dominate, their application does not lead to an improvement in all cases.
    Reference Frames for Applications in Geosciences, IAG Symposia; 01/2013
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    ABSTRACT: We report the progress in the technology of fabrication of large ring lasers that has resulted in an increase in instrumental rotation sensitivity by as much as a factor of 3, to {delta}{Omega} = 1.2 Multiplication-Sign 10{sup -11} rad s{sup -1} Hz{sup -1/2}, which makes the domain of changes in the angular velocity of Earth's rotation, {Delta}{Omega}/{Omega} Almost-Equal-To 10{sup -9}, accessible to a local rotation sensor. New studies show that the largest contribution to the observed deviation in sensor performance with respect to the computed shot noise limit is caused by the micro-seismic background activity of the Earth. Our efforts have been concentrated on the improvement of sensor stability, including correction of drift effects, which are caused by the aging of the laser gas, fixing scale factor instabilities induced by atmospheric pressure variations, and minimising the temperature variations resulting from corresponding adiabatic expansion and compression of the local air around the instrument. To achieve this, we have recently introduced a pressure-stabilising vessel with dimensions slightly larger than the ring laser apparatus, such that it encloses the entire structure. By monitoring the optical frequency in the ring laser cavity continuously and stabilising the scale factor in a closed loop system with the pressure-stabilising vessel, it has become possible to extend the range of sensor stability from the short term (1 - 3 days) to well into the mid-term regime (>40 days), and possibly even well beyond that. Once a sufficiently long timeseries of the ring laser data has been recorded, we will be able to define the range of temporal stability in more detail. The extension of the regime of stability gives access to geophysical signals at frequencies substantially lower than previously observable with ring lasers. (laser applications and other topics in quantum electronics)
    Quantum Electronics 11/2012; 42(11). · 0.82 Impact Factor
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    ABSTRACT: Superconducting gravimeters (SGs) measure temporal variations of the Earth's gravity field with very high precision. A superconducting spherical test mass is kept in a constant position in a very stable magnetic field of a superconducting coil. The electrical current that has to be applied to keep this constant position is continuously measured and is an expression of variations in the gravity field. SGs have traditionally been used in geodetic applications, such as the acquisition of Earth tides for deriving elastic parameters, the assessment of gravity variations due to polar motion, the detection of seismically induced oscillations of the earth. Oceanic, atmospheric and hydrological mass displacements in the surroundings of an SG have often been considered as disturbing components of the measurements that have to be reduced for geodetic applications. Some studies in recent years, however, have shown that the disturbing signal component in SG time series can inversely be used as the signal of interest, turning a SG into a hydrological monitoring device. Being sensitive to water mass changes in their surroundings, SGs provide unique measurements of total water storage variations (sum of storage variations in the snow cover, the unsaturated soil, and the groundwater) at local scales of several hundreds of meters, not accessible by other observation techniques. In this study, we investigate the relationship between local hydrology and gravity for the SG located in a highly seasonal climate at the Geodetic Observatory TIGO in Concepción, Chile. SG time series are compared to the gravimetric response calculated by a geodetic model using soil moisture measurements to a depth of 2.6 meters and a Digital Elevation Model for an area of 2 km around the SG. The results show that variations in moisture and gravimetric response are related to the topography and the depth of analysis. A large residual SG signal gives indication of important water storage variations in the deeper unsaturated zone and the groundwater.
    European Geosciences Union: General Assembly, Vienna; 04/2012
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    ABSTRACT: After that the tilt of the large RLG's platform caused by the Earth tide and the diurnal polar motion of the Earth were detected by Schreiber et al.(2003) and Schreiber et al.(2004), the G-RLG located in fundamental station Wettzell has been improved dramatically and its sensitivity now reaches the level of less than 1*10-8. Meanwhile the diurnal polar motion model and tilt model are improved and fully consistent with the IERS convention 2010. All these above improvements allow us to investigate the largest diurnal and semi-diurnal signals caused by the Geographic Latitude Variations(GLV)in the residual of the raw data after the diurnal polar motion and Earth tide tilt reductions. The estimated amplitudes are compared with the well-modeled GLV values, and the detectability of GLV by large RLG is discussed at the end.
    04/2012;
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    ABSTRACT: The CONT11 campaign was a 15-days period of continuous Very Long Baseline Interferometry (VLBI) measurements performed in the second half of September 2011. One of the main goals of this campaign was to provide time series of Earth Orientation Parameters (EOP) with highest possible temporal resolution. For this purpose, the VLBI data from CONT11 have been analysed with the Vienna VLBI Software (VieVS) in order to determine the EOP with hourly resolution. We compare the obtained sub-daily fluctuations of polar motion and Universal Time (UT1) with the corresponding modelled variations that take into account the effects of ocean tides and thermal tides in the atmosphere. Furthermore, the EOP series are compared to observations acquired by the Wettzell "G" ring laser gyroscope during CONT11, and a combined VLBI - ring laser solution is calculated in an attempt to obtain EOP of even higher accuracy. The results are put into context to the findings from previous continuous VLBI campaigns, like CONT02, CONT05, and CONT08.
    04/2012;
  • A. Gebauer, U. Schreiber, T. Klügel
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    ABSTRACT: Ring lasers are absolute rotation sensors and allow the observation of geophysical signals over a broad spectral range. The large ring laser "G" operating in an underground facility at the geodetic observatory Wettzell (Germany) was constructed to monitor Earth rotation and actually achieves a relative stability of 5 · 10-8 over an integration time of about 30 minutes. While variations in Earth rotation occur at periods ranging from hours to days up to 1 year, the G ring laser records signals in the seismic frequency band as well, thus covering more than 10 decades. In the seismic band teleseismic events, marine microseisms and locally induced rotations caused by wind are clearly detectable. Both latter effects occur in a similar frequency range and are denoted as noise in seismology. When combining the observed rotations with data from collocated instruments, i.e. a three-axis seismometer and several high resolution tiltmeters, an extensive data set of rotations, translations and horizontal accelerations are used to study the components of the different wave fields. While the partly coherent microseisms is present in all sensors allowing the detection of the source direction using a combination of seismometer and ring laser data, the local wind strongly affects the ring laser, but is weakly visible in the tiltmeter, and nearly absent in the seismometer data. This points to a strong contribution of horizontally polarized waves of Love type producing rotations around the sensitive axis of the ring laser. As the "G" ring laser yields only one component of rotation, a three axis rotational sensor is currently developed on the basis of fiberoptic gyroscopes. This instrument is by far less sensitive than the "G"-ring, however, it will be sensitive enough to record surface waves of mid-size earthquakes. When combining this instrument with a seismometer or tiltmeters, the complete deformation matrix of three translational and three rotational motions is accessible.
    04/2012;
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    ABSTRACT: The German Antarctic Receiving Station (GARS) O'Higgins is the only place on the Antarctic continent where the different geodetic techniques VLBI, GNSS, absolute gravity and tide gauge measurements are deployed. The combination of the data yield reliable results of crustal motions and mass changes and provide a good base for interpretations with respect to postglacial rebound processes. While GNSS observations at two IGS sites yield continuous time series, the VLBI experiments using the 9 m radio telescope and gravity measurements using a FG5 absolute gravimeter are performed campaign-wise. Sea level observations using a pressure and a radar gauge and meteorological measurements complement the data set. The data from all geodetic techniques point to a vertical uplift of the region. The resulting displacements obtained by the geometric techniques VLBI and GNSS coincide within the range of error and yield uplift rates of 4-5 mm/y. The absolute gravity measurements performed in 1997 and 2011 show a decrease in gravity by roughly 16 microgal, which is consistent with the observed geometrical uplift rates. A pressure gauge is in operation since 1999. However, a continuous time series is not available, since floating ice shifted or even destroyed the installation several times. In order to obtain absolute, space referenced sea level data, an additional radar gauge referenced by a GNSS antenna has been installed in 2011. Although this installation is operated campaign-wise, the well defined reference will allow the determination of long term trends in future.
    04/2012;
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    Obras y Proyectos, 12, 71-78.; 01/2012
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    ABSTRACT: Data from the Wettzell ring laser gyroscope are combined with Very Long Baseline Interferometry observations in order to estimate polar motion and Universal Time with hourly resolution. The combination is done at the normal equation level. Data from the period 1 May to 14 October, 2010, are used. We find that the impact of the ring laser data is normally relatively small since presently the accuracy of VLBI is about one order of magnitude better than the accuracy of the ring laser measurements. However, in cases when the accuracy of VLBI is of the order of 1 mas or worse the ring laser improves the accuracy of the estimated parameters, especially for y-pole and Universal Time. For the whole period, the combination on average improves y-pole by 16% and Universal Time by 12% compared to when using only VLBI data.
    Journal of Geodynamics 01/2012; 62:69-73. · 2.97 Impact Factor
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    Obras y proyectos. 12/2011;
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    ABSTRACT: We demonstrate a 16 m(2) helium-neon ring laser gyroscope with sufficient sensitivity and stability to directly detect the Chandler wobble of the rotating Earth. The successful detection of both the Chandler and the annual wobble is verified by comparing the time series of the ring laser measurements against the "C04 series" of Earth rotation data from the International Earth Rotation and Reference System Service.
    Physical Review Letters 10/2011; 107(17):173904. · 7.73 Impact Factor
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    Dresdner Grundwassertage, Dresden, Germany; 01/2011
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    ABSTRACT: Rotation sensors exploiting the Sagnac effect have many advantages, which give rise to a broad range of applications in which such sensors are irreplaceable. By vastly increasing the size, and consequently scale factor and sensitivity of ring lasers this group has achieved relative Earth rotation measurement resolution of 2 × 10−8. Today the real-time measurement delivered by large ring lasers provides data complementary to VLBI observations [7]. The application of optical sensors in seismology is an entirely new field, which is constantly evolving with the increased demand for quality data and analysis. Both ring lasers and fiber optic gyros can be employed for various seismological measurements, providing rotational information, which cannot be obtained via traditional seismometers. This paper presents an overview of current large ring laser development status, as well as insight into new research areas where both ring lasers and fiber optic gyros can help to obtain information of great value.
    10/2010; 1(4):291-296.
  • André Gebauer, Ulrich Schreiber, Thomas Klügel
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    ABSTRACT: Large laser gyroscopes allow the observation of the global rotation rate of the Earth and provide a direct reference to the instantaneous axis of rotation with high temporal resolution. This measurement method is independent and complementary to the VLBI technique, because it does not depend on external reference objects. Periodic signals from geophysical processes are analyzed in the measured datasets. Based on changes of the G ring laser hardware (see contribution Schreiber et al., this session) the instrumental sensitivity and stability improved significantly. Thus more geophysical processes both on global and local scale become visible. The time series of the measurements also contain irregular transient signals of different origin and magnitude. Several studies were carried out to identify the origin of these signals. First studies showed that the contribution of barometric loading is too small to account for the observed rotational signals. Then the effect of wind load on a local scale was studied. A detailed Finite-Element (FE) Model was developed with a dimension of about 10 km x 10 km and a minimum height of about 2 km. The topography is derived from a digital terrain model (DTM) of 25 m spatial resolution. Depending on the topography and land use the measured wind force loads the model. The results yields rotations in comparable order of magnitude as the observed ring laser data. The talk outlines the current state of ring laser data treatment.
    05/2010;
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    ABSTRACT: Over the last decade, ring lasers have found their way back into the research laboratories. By scaling them up in size, they have gained several orders of magnitude over their commercial counterparts, both in sensitivity and stability. Unlike the established space geodetic techniques SLR/LLR and VLBI, ring lasers can be operated autonomous and continuously. While a single ring laser component already provides direct access to the instantaneous axis of rotation of the Earth, it is also susceptible to local perturbations both with respect to platform rotation and instrumental tilt caused by local wind load for example. These instrumental coupling issues are addressed in more detail in a separate paper (Gebauer et al.) in this conference. Currently the laser gyro G at the Geodetic Observatory Wettzell (Germany) can resolve rotation rates as small as 1 pico-rad/s requiring an integration time of less than 2 hours. This opens the door for the research of high frequency variations in Earth rotation. Over the last year we have improved the ring laser technology by as much as a factor of 3 in sensitivity, which makes the domain of ∆Omega/Omega ≈ 10e-9 of Earth rotation accessible to a local rotation sensor. Currently it appears that the micro-seismic background activity of the Earth causes the major part from the observed deviation of the sensor performance with respect to the computed shot noise limit. Recent efforts concentrated on the improvement of the sensor stability against drift effects caused by the aging of the laser gas, scale factor instabilities induced by atmospheric pressure variations and the corresponding temperature changes from adiabatic expansion and compression of the local air around the instrument. Over the last year have introduced a pressure stabilizing vessel enclosing the entire ring laser structure. By monitoring the optical frequency in the ring laser cavity continuously and stabilizing the scale factor in a closed loop system, it became possible to extend the range of sensor stability from the short term (1-3 days) to at least well into the mid-term regime (>40 days) and possible even well beyond that. Once a sufficiently long time-series from ring laser data has become available, we will be able to define the range of temporal stability in more detail. The extension of the regime of stability gives access to geophysical signals at frequencies substantially lower than previously observable with ring lasers. This talk outlines this recent progress in Sagnac interferometry and presents the new data.
    AGU Fall Meeting Abstracts. 05/2010;
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    ABSTRACT: The reference point stability of the receiving systems of space geodetic techniques is a fundamental aspect when regarding the stability of global reference frames. This comprises geometric aspects like local ground movements and distortions of the antenna structure as well as electrical aspects like antenna phase centers and cable delays. The ground motions at the Geodetic Observatory Wettzell are observed by a local geodetic network at regular intervals. The deformation of the radiotelescope tower is measured continuously since many years using a vertical strainmeter, which gives rise to seasonal height variations of a few millimeters. The distortion of the antenna tower in the vicinity of the reference point as a consequence of different load cases due to varying antenna azimuth and elevation positions, or thermal strains has been investigated in detail within a project of the Geodetic Institute of the Technical University Karlsruhe using tachymeter, laser tracker and tiltmeter observations over three month. The measured distortions, expressed as translations of the reference point, are far below 1 mm and proof the high stability of the antenna tower.
    05/2010;
  • 01/2010;
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    ABSTRACT: In hydrology, the appropriate estimation of the water storage term in the water balance equation is challenging. Temporal gravity data are influenced by local water storage changes (WSC) and hence, can be a direct measure of the status change of the hydrological system. In this context, we will focus on the use of temporal gravimeter measurements as an integral signal for hydrological application, by exemplarily evaluating the use of temporal gravity measurements for hydrological model calibration. A simple conceptual model based on the HBV model is used to estimate local WSC in the snow, soil, unsaturated saprolite and saturated aquifer storage. The model was calibrated based on the GLUE method against superconducting gravimeter (SG) data on the one hand and several groundwater and/or soil moisture data on the other. These different calibrated models are compared by performing a split sample test and validated by comparing the results to totally independent estimated WSC from a state-of-the-art lysimeter. The model better reproduce the temporal behavior of the gravity response than the temporal variation of the groundwater and/or soil moisture. In general, the uncertainty band of the different behavioral model sets is larger for models calibrated with groundwater and/or soil moisture data than for the model calibrated with SG data. Comparing the modeled hydrological gravity response to the SG residuals for the whole study period both signals show not only similarities in terms of amplitude, long, seasonal and short-term variations, but are also strongly related to general weather conditions of the Bavarian Forest. This study identifies advantages and limitations of the use of temporal gravity observations in the context of hydrological model calibration. Comparing the SG to groundwater and soil moisture data shows the problem of the `representativeness' of single point measurements. Terrestrial gravity measurements have a much larger sphere of influence but it is difficult to unambiguously identify the source of the gravimeter signal. Nonetheless, temporal gravity data can improve water balance studies and might be especially useful in elevated areas, like upslope areas, because until now no adequate measurement technique is available to estimate local WSC for these areas, far away from the river.
    General Assembly European Geosciences Union; 01/2010
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    T. Klügel, H. Wziontek
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    ABSTRACT: The Newtonian attraction of the atmosphere is a major source of noise in precise gravimetric measurements. A major part of the effect is eliminated using local air pressure records and constant admittance factors. However, vertical mass shifts under constant surface pressure or distant pressure anomalies are not covered by this technique although they affect the gravimeter. In order to improve the atmospheric correction and to evaluate the horizontal components of attraction as well, the Newtonian attraction is computed based on the spatial density distribution derived from three-dimensional weather models.Operational models from the German Weather Service (DWD) of various scales were used, supplemented by a global data set from the European Centre of Medium Weather Forecast (ECMWF) for comparison. The low temporal resolution and the improper point-mass assumption in the near field are tackled by a cylindrical local model by computing the attraction analytically based on local air pressure records with high temporal resolution.It is shown that a height of at least 50km and global coverage is required to meet a threshold of 1nm/s2. Neglecting the upper atmosphere leads to an overestimation of the seasonal gravity signal. At distances greater than 10° the time consuming three-dimensional computation can be replaced by a two-dimensional surface pressure approach without significant error.The results show differences up to 20nm/s2 as compared to the linear regression method. The three-dimensional atmospheric correction significantly reduces noise in the time series, giving more insight into other signals such as hydrological effects or deformation processes.
    Journal of Geodynamics 12/2009; 48(3):204-210. · 2.97 Impact Factor