Advanced LIGO: Context and Overview
Advanced LIGO
Gravitational waves offer a remarkable opportunity to see the universe from a new perspective,
giving access to astrophysical insights which are available in no other way. The initial LIGO
gravitational wave detectors have started observations, and are already providing data which are
being interpreted to establish new upper limits on gravitational-wave flux.
The sensitivity of the initial LIGO instruments is such that it is perfectly possible that discoveries
will be made. If they succeed, there will be a strong demand from the community to improve the
sensitivity allowing more astrophysical information to be recovered from the signals. If no
discovery is made, there will be no lesser urgency to improve the sensitivity of the instrument to
the point where there is a general consensus that gravitational waves will be detected often and
with a high signal-to-noise ratio. The development of the next generation of instrument must be
pursued aggressively to make the transition from the initial to the Advanced detector in a timely
way – after the complete science run of the initial detector, but as quickly as possible thereafter.
The Advanced LIGO detector upgrade meets these requirements for an instrument that will
establish a gravitational-wave astronomy. It is more than ten times more sensitive, and over a
much broader frequency band, than initial LIGO. It can see a volume of space more than a
thousand times greater than initial LIGO, and extends the range of compact masses that can be
observed by a factor of four or more.
This proposal to build Advanced LIGO has grown out of the LIGO Scientific Collaboration and has
broad support both nationally and internationally from that community. A closely coordinated
community R&D program, exploring the instrument science and building and testing prototype
subsystem elements, has brought the design to a highly refined state. The LIGO Laboratory will
lead and coordinate the fabrication and construction of the instruments, with the continued strong
participation of the community.
Advanced LIGO can lead the gravitational-wave field to maturity.
The LIGO Mission
LIGO was approved by the National Science Foundation to directly observe gravitational waves
from cosmic sources, and to open the field of gravitational wave astronomy. The program and
mission of the LIGO Laboratory is to:
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observe gravitational wave sources;
develop advanced detectors that approach and exploit the facility limits on interferometer
performance;
operate the LIGO facilities to support the national and international scientific community;
provide data archiving for the LIGO data and contribute computational resources for the
analysis of data
develop the software infrastructure for data analysis and participate in the search and
analysis
and support scientific education and public outreach related to gravitational wave astronomy.
LIGO is envisioned as a new capability contained in a set of facilities and not as a single
experiment. The LIGO construction project has provided the facilities that support the scientific
instrumentation, and the initial set of laser interferometers to be used in the first LIGO scientific
observation periods.
The facilities include the buildings and vacuum systems at the two observatory sites. The two
observatories are located at Hanford, Washington and Livingston, Louisiana. The performance
requirements on the LIGO facilities were intended to accommodate the initial interferometers and
future interferometer upgrades and replacements, and possible additional interferometers with
complementary capabilities. The requirements on the LIGO facilities were intended to permit
future interferometers to reach levels of sensitivity approaching the ultimate limits of groundbased interferometers, limited by reasonable practical constraints on a large facility at a specific
site.
This proposal is for the second generation of instruments to be installed in the LIGO
infrastructure, and is expected to bring the science of gravitational radiation from a discovery
mode to a mode of astrophysical observation.
LIGO Detector Scientific Goals
The scientific program for LIGO is both to test relativistic gravitation and to open the field of
gravitational wave astrophysics. More precise tests of General Relativity (and competing theories)
will be made. A brand new field of astronomy, based on observations using a completely new
information carrier: the gravitational field, will be enabled.
Initial LIGO represents an advance over all previous searches of two or three orders of
magnitude in sensitivity and in bandwidth. Its reach is such that, for the first time, foreseeable
signals due to neutron-star binary “inspirals” from the Virgo Cluster (15 Mpc distant) would be
detectable. At this level of sensitivity, it is plausible, though not guaranteed, that the first
observations of gravitational waves will be made. If signals are not observed with initial LIGO, we
will have set challenging upper limits on gravitational wave flux, far beyond the capability of any
previously existing technology.
The Advanced LIGO interferometers proposed here promise an improvement over initial LIGO in
the limiting sensitivity by more than a factor of 10 over the entire initial LIGO frequency band. It
also increases the bandwidth of the instrument to lower frequencies (from ~40 Hz to ~10 Hz) and
allows high-frequency operation due to its tunability. This translates into an enhanced physics
reach that, during its first several hours of operation will exceed the integrated observations
of the 1 year LIGO Science Run. These improvements will enable the next generation of
interferometers to study sources not accessible to initial LIGO, and to make detections with a
signal to noise ratio allowing the extraction of detailed astrophysical information. For example, the
Advanced LIGO detectors will be able to see inspiraling neutron binaries made up of two 1.4 MO•
neutron stars to a distance of 300 Mpc, some 15x further than the initial LIGO, and giving an
event rate some 3000x greater. Neutron star – black hole (BH) binaries will be visible to 650 Mpc;
and coalescing BH+BH systems will be visible to cosmological distance, to z=0.4.
The existence of gravitational waves is a crucial prediction of the General Theory of Relativity, so
far unverified by direct observation. Although the existence of gravitational radiation is not a
unique property of General Relativity, that theory makes a number of unambiguous predictions
about the character of gravitational radiation. These can be verified by observations with LIGO
providing there are sufficiently high signal to noise detections. These include probes of strongfield gravity associated with black holes, the spin character of the radiation field, and the wave
propagation speed.
The gravitational wave “sky” is entirely unexplored. Since many prospective gravitational wave
sources have no corresponding electromagnetic signature (e.g., black hole interactions), there
are good reasons to believe that the gravitational-wave sky will be substantially different from the
electromagnetic one. Mapping the gravitational-wave sky will provide an understanding of the
universe in a way that electromagnetic observations cannot. As a new field of astrophysics it is
quite likely that gravitational wave observations will uncover new classes of sources not
anticipated in our current thinking.
Detector Design Fundamentals
The effect of a propagating gravitational wave is to deform space in a quadrupolar form. The
effect alternately elongates space in one direction while compressing space in an orthogonal
direction and vice versa, with the frequency of the gravitational wave. A Michelson interferometer
operating between freely suspended masses is ideally suited to detect these antisymmetric
distortions of space induced by the gravitational waves; the strains are converted into changes in
light intensity and consequently to electrical signals via photodetectors.
Limitations to the sensitivity come from two sources: extraneous forces on the test masses, and
from a limited ability to sense the response of the masses to the gravitational wave strain. The
thermally excited motion of the test mass and the suspension is a fundamental limitation, intrinsic
to the way in which the measurement is performed; this influence is managed through the
selection of low-mechanical-loss materials and designs which capitalize on them. Seismic motion
causes forces on the mirrors due to the direct coupling through the isolation and suspension
system, a technical noise source which is minimized through design; and due to the time-varying
mass distribution near the mass (the Newtonian background).
Sensing limitations arise most fundamentally due to the statistical nature of the laser light used in
the interferometry, and the momentum transferred to the test masses by the photons (linking the
sensing and stochastic noise limitations to sensitivity). Technical noise sources which limit the
ability to sense are frequency noise and intensity fluctuations in the laser light, and scattered light
which adds random phase fluctuations to the light, can also mask gravitational signals.
In the limit, valid for LIGO, that the instrument is short compared with the gravitational strain
wavelength, longer arms give larger signals. In contrast, most competing noise sources remain
constant with length; this motivates the 4km baseline of the Observatories. More generally, the
scientific capability of LIGO is defined within the limits imposed by the physical settings of the
interferometers and by the facility design, by the design of the initial detectors and ultimately by
future interferometers designed to progressively exploit the facility capabilities.
Although the rates for gravitational wave sources have large uncertainty, a linear improvement in
sensitivity linearly improves the distance searched for detectable sources. This increases the
detection rate by the cube of the sensitivity improvement.
The Observatories
LIGO Facility Scientific Capability
The LIGO facility design envisaged a progression of increasingly sensitive interferometers
capable of extending the physics reach of the observatories. In the design of the observatories,
LIGO incorporated critical design features into its facilities in order to optimize LIGO’s ultimate
performance capabilities. These features include a building foundation and infrastructure which
provides a clean, quiet environment for the instruments; a 4km long “L” UHV beam tube system
that brings scattered light and index fluctuations due to residual gas to a negligible level; and a
system of large vacuum chambers and pumping subsystems capable of providing a flexible
envelope for a wide range of detector designs, and delivering a vacuum quality that complements
the beam tube subsystem. Advanced LIGO requires no changes in this infrastructure to meet its
scientific goals.
LIGO Hanford Observatory
LIGO Hanford Observatory (LHO), located on the U.S. Department of Energy Hanford site in
eastern Washington, comprises 5 major experimental halls for the interferometer spread over 5
miles. 1.2-m diameter ultrahigh vacuum tubing connects these halls. Three support buildings
house laboratories, offices, and an amphitheater, and two additional buildings are associated with
maintenance and operations. Approximately 90,000 square feet of this space is under tight
environmental control to minimize contamination of sensitive equipment. The physical plant has
been designed to provide a low vibration environment similar to the surrounding undeveloped
shrub-steppe environment.
LHO houses two interferometers with arm lengths of 4 km and 2 km. The 4-km equipment is
installed in vacuum chambers in the corner station and the two end stations on each arm. The 2km equipment uses vacuum chambers in the corner station and the two midstations situated
halfway down each arm. The two interferometers share 2 km of beam tube along each arm. The
beam tube can eventually accommodate up to 5 interferometer beams and the current station
buildings can accommodate up to 3 interferometers to accommodate future growth.
Figure 1 The LIGO Hanford Observatory (LHO) in aerial view. The 4-km interferometer
arms are shown with the 5 main buildings shown along the orthogonal arm layout.
LIGO Livingston Observatory
Figure 2 The LIGO Livingston Observatory (LLO) corner region in aerial view
The LIGO Livingston Observatory, located in pine forests between Baton Rouge and New
Orleans, Louisiana, is the site of a single 4-km laser interferometer gravitational wave detector.
Construction of its physical facilities, scaled to accommodate one interferometer, is complete. The
beam tube dimensions are identical to those at LHO.
Initial LIGO
Status of the LIGO Construction Project
The NSF Cooperative Agreement of May 1992 initiated LIGO Construction and Construction
Related Research and Development. The Project schedule and cost estimates were reviewed by
the NSF during September 1994 and presented to the National Science Board in November
1994. The total funding established by the Board for Construction and Construction Related R&D
were $272.1 million and $20.0 million, respectively. In addition, the NSF provided $68.7 million for
Operations through September 30, 2001 covering the period of Installation and Commissioning.
The LIGO construction effort is effectively complete, on cost and close to schedule.
Initial LIGO Detector Commissioning
Installation is complete for the three interferometers. The commissioning of the instruments
continues, with most subsystems completely operational and in use in the first science runs.
Several subsystems are coming on-line in preparation for the next science run. The instruments
have shown a steady improvement in sensitivity, at all frequencies in the planned observation
band, during the commissioning process. An example of progress is seen in Figure 3Error!
Reference source not found.. All of the instruments have made very significant progress toward
the requirements proposed for initial LIGO through a process of identification, tuning, and
incremental changes in the detector hardware as needed. The present limits to performance are
understood through a combination of measurement and system modeling, and measures to bring
the contributors to an acceptable level are underway or in preparation.
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Strain (1/ √Hz)
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LHO 4k strain curve evolution
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Dec 29, 01: E7-recombined
Jun 20, 02: pre-S1
Sep 9, 02: S1
Dec 26, 02
LIGO science requirement
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Frequency (Hz)
Figure 3 This figure shows the progression in the strain sensitivity as a function of frequency for
the Hanford 4-km interferometer over the year from December 2001 to December 2002. The most
recent noise level is a factor 20 above the final specification over a broad frequency range; many
of the sources of excess noise have been identified and are being addressed.
Future improvements to reach the design sensitivity will involve some modifications to the
electronics and to the mechanical infrastructure, and optimization of control systems and filters.
Substitute recycling mirrors (with an optimized radius of curvature for the observed mirror optical
properties) will be installed during favorable opportunities. It is anticipated that this series of
improvements in performance will largely take place between the upcoming second science run,
and the third science run, planned for the Fall of 2003.
The LIGO Science Runs
The observatories are now interleaving continued commissioning with science runs. The timing of
the runs is designed to balance the competing demands for improvements to the performance of
the machines (in sensitivity and duty cycle) with the need to observe.
The first science run, S1, took place from 23 August through 8 September 2002, and involved the
three LIGO interferometers, the UK-German GEO-600 interferometer, with some overlap in
observing with the Japanese TAMA detector. The LIGO instruments operated in their design
configuration, and accumulated roughly 100 hours of observation in triple coincidence between
the two sites during the S1 run.LIGO Scientific Collaboration (LSC, please see below) “upper
limit” groups are undertaking the search for chirp signals from binary inspirals, periodic signals
from neutron stars, burst signals from e.g., supernovae and gamma ray bursts, and from a
possible stochastic noise background. The data analysis pipelines utilize a variety of
sophisticated filtering techniques – templates, time-frequency analysis, inter-interferometer
correlations, and use of the auxiliary and environmental data channels, as examples. Data are
also being correlated with relevant optical data and, in the case of supernovae, with neutrino
signals. Results from the analysis of S1 data, providing new upper limits to the gravitational wave
flux, are being prepared for publication.
The second science run, S2, is planned to take place from 14 February 2003 until 14 April 2003.
The objective for this run is to improve the upper limits on gravitational wave flux by at least an
order of magnitude compared to the S1 run. The present sensitivity of the instruments (January
2003) enables this goal. The third science run, S3, is planned for late 2003, following a significant
commissioning interval, and will represent a transition to a mode where observation dominates
over commissioning at the Observatories. We plan to intersperse science runs with periods
dedicated to detector improvements that will be coordinated with the LSC. The overall goal is to
obtain at least one year of integrated data with an RMS strain sensitivity of h ~ 10-21 (integrated
noise level in a 1 kHz band) within the first three years of observation (ending in 2006). Once that
goal is achieved and the physics results have been obtained, installation of improved
interferometers can commence.
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LIGO Scientific Collaboration
A fundamental goal of LIGO has been to become a true national facility available to the scientific
community. In order to accomplish this, LIGO has broadened the participation to include the
community of scientists interested in participating in the LIGO research program. In order to
achieve this goal, the LIGO Scientific Collaboration (LSC)1 was formed and has now been
functional for several years. There are some 470 members, from 38 institutions in both the US
and internationally.
The Advanced LIGO design, both in basic conception and in the detailed R&D, is very much a
product of the LSC (with a strong LIGO Laboratory element). The technical working groups have
been and continue to be central to the advancement of the design, and this proposal is made with
the strong support of the many participating institutions in the LSC.
In addition, LIGO has been organized such that the search for astrophysical signals and
interpretations will be performed through the LSC. The LSC consists of both LIGO Laboratory
scientists and those from collaborating groups. The collaboration members committed to "LIGO
I", the initial LIGO detector science runs, will be responsible for the science in this beginning
phase of observation. This group in LIGO is well defined and presently consists of 111 LIGO
Laboratory scientists and 134 scientists from collaborating institutions. The LSC is organized so
as to provide “equal scientific opportunity” to all members whether they are from within LIGO
Laboratory or the broader LSC. It is growing steadily and will remain open to new members over
the coming years. It is worth noting that the LSC has significant international participation,
including collaborating groups from India, Russia, Germany, U.K, Japan and Australia. The
international partners are involved in all aspects of the LIGO research program.
Preparation tasks for the runs are organized within the LSC, LSC members participate in the data
taking runs, and the analysis of the data is coordinated through the LSC proposal driven process.
The full LSC collaboration meets twice yearly in an extended meeting, and various working
groups meet more frequently. The LSC has produced White Papers that outline the plans for
technical development of LIGO and for science data analysis. A publication policy and a
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http://www.ligo.org
conference committee are active, as well as the other functions necessary to make it a ‘full
service’ organization.
LIGO is available to all interested researchers through participation in the LSC, an open
organization. To join, a research group defines a research program with the LIGO Laboratory
through the creation of a Memorandum of Understanding (MOU) and relevant attachments2. The
group then presents its program to the LSC. When the group is accepted into the LSC it becomes
a full scientific partner in LIGO.
Exploiting the LIGO Capability: Advanced LIGO
As noted above, LIGO is designed to evolve and to support improvements in gravitational wave
detectors. A natural time for an upgrade to the instruments can be foreseen once the initial LIGO
goal of one year of integrated observation has been met. The initial LIGO infrastructure is well
designed to deliver its planned performance, and small advances in sensitivity at higher
frequencies may be possible with e.g., modest increases in the input laser power. However, a
large improvement will require an upgrade of the entire detector in a coordinated fashion. The
Advanced LIGO instrument fulfills our requirements of a significant step forward in sensitivity, and
can be delivered on a time scale that meshes with the end of the Initial LIGO observation plan.
Overview of Advanced LIGO
The sensitivity goals for the Advanced LIGO LIGO detector systems are chosen to enable the
advance from plausible detection to likely detection. These sensitivity goals require an instrument
limited only by fundamental noise sources over a very wide frequency range. To achieve this
sensitivity, almost every aspect of the interferometer must be revised from the initial LIGO design.
The system described below is the reference concept that is the basis for structuring the R&D
program and the detailed studies of system tradeoffs performed as R&D results define the
feasible parameters.
The basic optical configuration is a power-recycled and signal-recycled Michelson interferometer
with Fabry-Perot “transducers” in the arms. Using the initial LIGO design as a point of departure,
this requires the addition of a signal-recycling mirror at the output “dark” port, and changes in the
RF modulation and control systems. This additional mirror allows the gravitational wave induced
sidebands to be stored or extracted (depending upon the state of “resonance” of the signal
recycling cavity), and leads to a tailoring of the interferometer response according to the
character of a source (or specific frequency in the case of a fixed-frequency source).
To improve the quantum-limited sensitivity, the laser power is increased from the initial LIGO
value of 10 W to ~200 W. The conditioning of the laser light follows initial LIGO closely, with a
ring-cavity mode cleaner and reflective mode-matching telescope.
Whereas initial LIGO uses 25-cm, 11-kg, fused-silica test masses, the test mass optics for
Advanced LIGO are larger in diameter (~30 cm) to reduce thermal noise contributions and more
massive (~40 kg) to keep the radiation reaction noise to a level comparable to the suspension
thermal noise; sapphire is the baseline material for the test masses. Compensation of the thermal
lensing in the test mass optics (due to absorption in the substrate and coatings) is added to
handle the much-increased power – of the order of 1 MW in the arm cavities.
The test mass is suspended by fused silica fibers, in contrast to the steel wire sling suspensions
used in initial LIGO. The resulting suspension thermal noise is anticipated to be less than the
radiation pressure noise (in broad-band observation mode) and to be comparable to the
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http://www.ligo.org/mou/mou.html
Newtonian background (“gravity gradient”) at 10 Hz. The complete suspension has four pendulum
stages, contributing to the seismic isolation and providing multiple points for actuation.
The seismic isolation system is built on the initial LIGO piers and support tubes but otherwise is a
complete replacement, required to bring the seismic cutoff frequency from 40 Hz (initial LIGO) to
10 Hz. RMS motions (frequencies less than 10 Hz) are reduced by active servo techniques. The
result is to render the seismic noise negligible at all observing frequencies. Through the
combination of the seismic isolation and suspension systems, the required control forces on the
test masses will be reduced by many orders of magnitude in comparison with initial LIGO,
reducing also the probability of non-gaussian noise in the test mass.
The overall performance of Advanced LIGO is dominated at most frequencies by the quantum
noise of sensing the position of the test masses, with a contribution at mid-frequencies from the
internal thermal noise of the test mass. This design, with modest enhancements after it enters
scientific use, should take this interferometer architecture to its technical endpoint; it is as
sensitive as one can make an interferometer based on familiar technology: a Fabry-Perot
Michelson configuration using room temperature transmissive optics. Further advances will come
from R&D that is just beginning, such as the exploration of cryogenic optics and suspensions,
purely reflective optics, and changes in the readout to one which fully exploit our nascent
understanding of the quantum nature of the measurement (e.g., ’speed meters’). These later
developments will be timely for instruments to be developed in the second decade of this century.
Advanced LIGO R&D 2002-2006
During 2002 through 2006, most LIGO Laboratory detector R&D will be directed at the challenges
posed by the Advanced LIGO design. This R&D program is a significant part of the larger R&D
program of the LIGO Scientific Collaboration. The LSC program has been developed in a
collaborative manner and is coordinated through the LSC Working Groups and by the LIGO
Laboratory.
The R&D program currently underway, supported by the NSF under cooperative agreement PHY0107417, is designed to support this construction proposal. This proposal requests that
construction funding commence at a time when the major R&D issues are satisfactorily resolved,
and the subsystems ready for fabrication.
The activities carried out prior to construction funding include small-scale fundamental research
motivated by Advanced LIGO. Examples of this are studies of core optic substrate absorption,
measurement of mechanical losses in suspension fibers, and studies of candidate photodetectors
for the gravitational wave readout. The R&D program also includes large-scale prototypes of
subsystems such as full-scale seismic isolation systems, full-scale suspensions, and full-scale
core optics. In order to carry out this research, in most cases these components must be fully
engineered to the realistic requirements and configuration of Advanced LIGO. In order to study
the performance and control of a suspension subsystem, the prototype studied must represent
the Advanced LIGO design down to details such as suspension fiber material, bonding technique,
and control electronics design and component selection and physical layout. This kind of rigorous
full-scale development program is needed to reduce risks prior to defining and committing to a
construction project.
Advanced LIGO Schedule and Cost
The Advanced LIGO fabrication and construction schedule grows out of the tightly coordinated
R&D program currently underway. The objectives in establishing the schedule are to
• Allow the initial LIGO instruments to be fully exploited, and in particular to ensure the
commitment to a full integrated year of observation with in initial LIGO instruments
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Allow a complete R&D cycle, with extensive testing of final designs, before committing to
fabrication
Bring the Advanced LIGO instruments on-line as quickly as possible to meet the
demands of the community for the observing capability of the Advanced LIGO detector.
The schedule is based on our extensive experience with the design, fabrication, construction, and
commissioning of the Initial LIGO detectors.
The highlights of the schedule are
• Receipt of funding for the fabrication and construction project in early 2005, with some
advance funding for critical long-lead items in early 2004
• Delivery of first interferometer hardware to the observatory staging facilities in mid-2006
• Decommissioning of initial LIGO at the LIGO Livingston Observatory in early 2007, and
simultaneous start of installation of Advanced LIGO there
• Decommissioning of initial LIGO at the LIGO Hanford Observatory in late 2007, and
simultaneous start of installation of Advanced LIGO there
• Both observatories in commissioning by mid-2008
• Both observatories in operation by late 2009
The cost estimate developed for this proposal was performed at the lowest feasible level, given
the present level of development in the WBS, in a bottom-up manner. Most subsystems have
been costed at a level of detail comparable to initial LIGO; several have not achieved the maturity
in R&D to allow this level of detail, but carry contingency appropriate to the basis. The techniques
used were the same methods used to estimate initial LIGO construction costs, though our cost
experience in initial LIGO substantially improved the input knowledge base for the new estimate.
Contingency was estimated using the formal graded approach to assessing technical, cost and
schedule risk that was used in initial LIGO.
The joint United Kingdom/German GEO Project3 has proposed to provide a capital investment in
this construction project. The UK proposal is for approximately $11.5 million. They propose to
apply these resources to providing the suspension subsystem, including suspension assemblies,
their controls, and installation and commissioning. The German proposal is for the design and
fabrication of the pre-stabilized laser subsystem, and the value of the contribution is planned to
be valued at XXXX. The GEO Project is a full partner in Advanced LIGO, participating at all levels
in the effort.
With the GEO capital contribution, the required US Advanced LIGO costs are $ XXXXX K in FY
2002 $. Escalating this sum to the approximate mid-point of Advanced LIGO construction (2006),
using the average inflation rate quoted by the US Department of Labor for the last 6 years (2.4%),
yields a total request to the NSF for constructing Advanced LIGO of $ XXXXX K.
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http://www.geo600.uni-hannover.de/