Axel Nothnagel

THE SECOND REALIZATION OF THE INTERNATIONAL CELESTIAL REFERENCE FRAME BY VERY LONG BASELINE INTERFEROMETRY

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.

IVS and its important role in the maintenance of the global reference systems

Abstract VLBI technique plays a primary role in the maintenance of global reference frames. The International VLBI Service for Geodesy and Astrometry (IVS) was established in February 1999 in order to support VLBI programs for geodetic, geophysical and astrometric research and operational activities. IVS coordinates the observations, the data flow, the correlation, the data analysis and the technology developments. Today not only science, research and development are making use of the results but also practical applications are dependent on IVS products. Thus, acting within the frame of International Association of Geodesy (IAG) and the International Astronomical Union (IAU), IVS has to guarantee provision of the required results on a regular, timely basis including Earth orientation parameters, position and velocity vectors. The paper will summarize the activities of the service and will give a prospective for the next few years, based on the experiences of the first IVS General Meeting.

European VLBI for crustal dynamics

Abstract Regular geodetic VLBI observations in the European network of fixed radio telescopes have been carried out since the late eighties at an average rate of six sessions per year. From these data site coordinates, baseline length changes and station velocity vectors have been derived with steadily increasing accuracy. The overall picture of the observed present-day site motions agrees quite well with the pattern of tectonic motions inferred from the geotectonic setting of Central Europe and the western Mediterranean. Interesting details are emerging for horizontal motions of the three stations in Italy which are strongly affected by the complex interactions between the different tectonic regimes in this area. The results for vertical components are also improving with increasing length of the observational record, allowing to place upper bounds on the relative vertical motions of the sites. The geodetic VLBI network operations receive supportive funding by the European Union under the current Framework Programmes, which include Research and Training Networks.

Crustal motion results derived from observations in the European geodetic VLBI network

Geodetic VLBI observations have been performed with the European geodetic VLBI network since early 1990 on a regular basis. The purpose of these observations is to determine crustal motion in Europe and to establish a stable reference frame for other space geodetic techniques. Over the years the size of the network and the number of participating stations has steadily increased. Today, the network extends from the island of Sicily in the south to the island of Spitsbergen/Svalbard in the north and from the Iberian peninsula in the west to the Crimean peninsula in the east. The area covered by the network is affected by two main geodynamic processes which are post-glacial rebound effects in the northern part, and the evolution of the Alps-Apennines orogenic systems in the southern part. With nearly 10 years of VLBI observations the determination of crustal motion in Europe is carried out with high accuracy. Baseline measurements are achieved with an accuracy of a few parts per billion. We compare the evolution of baseline lengths and topocentric station displacements with geophysical models. Strain rates in Europe on a large scale are determined from the results of the VLBI analysis.

Improved area-based deformation analysis of a radio telescope’s main reflector based on terrestrial laser scanning

Abstract The main reflectors of radio telescopes deform due to gravitation when changing their elevation angle. This can be analyzed by scanning the paraboloid surface with a terrestrial laser scanner and by determining focal length variations and local deformations from best-fit approximations. For the Effelsberg radio telescope, both groups of deformations are estimated from seven points clouds measured at different elevation angles of the telescope: the focal length decreases by 22.7 mm when tilting the telescope from 90 deg to 7.5 deg elevation angle. Variable deformations of ± 2 mm are detected as well at certain areas. Furthermore, a few surface panels seem to be misaligned. Apart from these results, the present study highlights the need for an appropriate measurement concept and for preprocessing stepswhen using laser scanners for area-based deformation analyses. Especially, data reduction, object segmentation and laser scanner calibration are discussed in more detail. An omission of these steps would significantly degrade the deformation analysis and the significance of its results. This holds for all sorts of laser scanner based analyses.

Recent crustal movements observed with the European VLBI network: geodetic analysis and results

Abstract Purely European geodetic Very Long Baseline Interferometry (VLBI) has been performed at regular intervals since January 1990. Meanwhile 10 VLBI sites in Europe from Sicily to Spitzbergen and the Iberian pensinsula to the Crimean peninsula contribute to the observation series. The observations are used to determine recent crustal movements in these parts of Europe. Baseline measurements are achieved with an accuracy of 2.0 mm plus an additional term depending on the baseline length of less than 1 part-per-billion. Significant horizontal crustal movements are determined with relative accuracies in the range of 10–40%, significant vertical crustal movements are determined with relative accuracies in the range of 30–40%. The determined crustal movements form thus a sound basis for interpretations in a geophysical and geological context. This article concentrates on a description of the geodetic VLBI data analysis, the analysis models used and the results for crustal movements.

Supernova 1987A: radiosphere resolved with VLBI five days after the neutrino burst

Following the detection1 of radio emission from SN1987A in the Large Magellanic Cloud, we conducted very-long-baseline inter-ferometry (VLBI) observations with an interferometer composed of a NASA Deep Space Network antenna (DSS42) at Tidbinbilla, Australia and the antenna of the Hartebeesthoek Radio Astronomy Observatory, South Africa2,3. We did not detect any emission from the supernova above a level of ˜20% of the supernovas total flux density, although signals were detected from our two calibrator sources with amplitudes roughly equal to those determined in earlier VLBI observations. We infer that we resolved the supernovas radiosphere and estimate, for an epoch 5.2 days after the neutrino burst4,5, a lower bound on the radiospheres radius of 1.2 mas. Given the photometric data from the supernova6,7, a distance to the Large Magellanic Cloud of 50 ± 5 kpc (ref. 8), and an apparent expansion velocity that varied systematically with time from 18–16 x 103 km s–1 (refs 9 and 10), as estimated from the blue-shifted Hα absorption lines on the days preceding our observations, we conclude that 5.2 days after the neutrino burst the supernovas radiosphere was at least 2.5 times larger than the inferred blackbody photosphere, and at least as large as the Hα line-forming region.

Estimation of Focal Length Variations of a 100-m Radio Telescope’s Main Reflector by Laser Scanner Measurements

AbstractDue to gravitation, the main reflector of a radio telescope underlies a deformation that causes a change in focal length depending on the variations of the elevation angle of the telescope. To estimate these gravity dependent deformations of the main reflector of the 100-m radio telescope at Effelsberg, Germany, this study proposes a measurement concept based on a laser scanner being mounted upside down on the subreflector. The measurements that have been performed at seven different elevations between 90 and 7.5° are used to estimate the focal length variation of the main reflector parameterized by a rotational paraboloid. To guarantee reliability of the adjustment, this study uses an orthogonal distance regression (ODR) rather than a classical least squares adjustment in a Gauss-Helmert model and formulates the independence of the focal length estimation from the absolute position and orientation of the main reflector in space as a requirement for a reliable adjustment approach. This investigati...

Initial Results Obtained with the First TWIN VLBI Radio Telescope at the Geodetic Observatory Wettzell

Geodetic Very Long Baseline Interferometry (VLBI) uses radio telescopes as sensor networks to determine Earth orientation parameters and baseline vectors between the telescopes. The TWIN Telescope Wettzell 1 (TTW1), the first of the new 13.2 m diameter telescope pair at the Geodetic Observatory Wettzell, Germany, is currently in its commissioning phase. The technology behind this radio telescope including the receiving system and the tri-band feed horn is depicted. Since VLBI telescopes must operate at least in pairs, the existing 20 m diameter Radio Telescope Wettzell (RTW) is used together with TTW1 for practical tests. In addition, selected long baseline setups are investigated. Correlation results portraying the data quality achieved during first initial experiments are discussed. Finally, the local 123 m baseline between the old RTW telescope and the new TTW1 is analyzed and compared with an existing high-precision local survey. Our initial results are very satisfactory for X-band group delays featuring a 3D distance agreement between VLBI data analysis and local ties of 1 to 2 mm in the majority of the experiments. However, S-band data, which suffer much from local radio interference due to WiFi and mobile communications, are about 10 times less precise than X-band data and require further analysis, but evidence is provided that S-band data are well-usable over long baselines where local radio interference patterns decorrelate.

The goals, achievements, and tools of modern geodesy

Friedrich Robert Helmert (1843-1917) defined geodesy as the science “of measurements and mappings of the Earth’s surface”. Over time, this definition of geodesy has been extended, mainly as a consequence of technological developments allowing geodesy to observe the Earth on global scales with high accuracy. Today, geodesy is the science of determining the geometry, gravity field, and rotation of the Earth and their evolution in time. This understanding of modern geodesy has led to the definition of the “three pillars of geodesy”, namely (1) Geokinematics, (2) Earth Rotation and (3) the Gravity Field (see Figure 1.1 on page 4). These three pillars are intrinsically linked to each other, and they jointly change as a consequence of dynamical processes in the Earth system as a whole. The changes in Earth’s shape (including the surface of the water and ice bodies), i.e. the geokinematics, are the result of dynamic processes in the solid Earth and its fluid envelope, affecting mass distribution and angular momentum, and thus changing the gravity field and Earth rotation. Traditionally, geodesy has been a service science, providing an important utility to other sciences and many applications. This aspect has remained unchanged, and a principal tool and output of geodesy is a reference frame allowing the determination of the position of points relative to each other. But geodesy has developed into a science that can no longer satisfy this service aspect without encompassing and monitoring the whole Earth system, its kinematic and dynamics. As an additional benefit, geodesy is increasingly forced not only to “measure” the geokinematics, gravity field, and rotation, but also to “model” these quantities on the basis of mass transport and dynamics. The instruments (or measurement tools) are of crucial importance in geodesy. They in essence define the scope of the problems, which may be addressed by geodesy. Before the advent of the space age the geometrical aspects were studied mainly by measuring angles and time (time-tagging of the observations). In the best