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The Fundamental Station Wettzell -current status and future plans

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					                                 The Fundamental Station Wettzell
                                  -current status and future plans-
                             Wolfgang Schluter1 and Reiner Dassing
                         Bundesamt fur Kartographie und Geodasie (BKG)
                      Fundamentalstation Wettzell, D-93444 Kotzting, Germany

                                Richard Kilger and Ulrich Schreiber
                 Forschungseinrichtung Satellitengeodasie der TU-Munchen (FESG)
                     Fundamentalstation Wettzell, D-93444 Kotzting, Germany
                                                Abstract
              The current status of the Fundamental Station Wettzell is described. The objec-
          tives are pointed out and the change from the satellite observation station to a geode-
          tic observatory, the Fundamental Station (FS) Wettzell with its space techniques such
          as SLR/LLR, VLBI, GPS, GLONASS and PRARE and with its facilities for local
          measurements such as a gravitymeter, seismometer, watervapor radiometer and its
          time and frequency system is described. The contribution of the FS-Wettzell within
          the international activities is summarized. The development of the Transportable
          Integrated Geodetic Observatory (TIGO) embeded in the work of Wettzell and the
          development of the laser gyroscopes "CII" and "G" for the monitoring of the Earth
          rotation parameter DUT1 will outline future instrumentations and activities.

    1. Fundamental Stations - General Remarks
         The objectives of Fundamental Stations, which are coordinated within the Interna-
    tional Space Geodetic network (ISGN) are to
          represent the Earth and the Earth motion in space,
          de ne and realise the terrestrial reference frame (ITRF),
          maintain the terrestrial reference frame by observing variations with respect to the
          extraterrestrial reference frame (ICRF).
    The main important characteristics for Fundamental Stations are therefore
          permanency, to provide observations on a long term basis,
          complementary, to carry out a broad spectrum of observations with di erent sensi-
          tivities,
          redundancy, to employ various, independant techniques for controlling
    To make use of the bene t of all techniques, collocated at a fundamental station with their
    advantages and various sensitivities, precise local ties have to be observed with the means
    of the classical survey. The local excentricities between the reference points of the various
    systems is the key for the combination.
1
    schlueter@wettzell.ifag.de
2. From the Satellite Observation Station to the Fundamental Station
   Wettzell
     In the period from 1970 to 1980 the satellite observation station Wettzell was estab-
lished by the Institut fur Angewandte Geodasie (former name of BKG), in cooperation
with the Technical University of Munich and the Deutsche Geodatische Forschungsinsti-
tut (DGFI) on behalf of the former Sonderforschungsbereich (SFB) Satellitengeodasie,
which was supported by the DFG, the German Research Foundation. The station was
equipped with a 1st and 3rd generation SLR system, a ballistic camera (BKM75) and
Doppler receivers for the observation of the TRANSIT-satellites and GEOS-C. End of the
70ies, beginning of the 80ies more emphasis has been placed to the collocation of space
techniques and to the employment of in situ observations towards the establishment of
a Fundamental Station. The two major steps have been the implementation of the 20m
radiotelescope dedicated for the geodetic VLBI and the replacement of the 3rd gen. SLR
system by the Wettzell Laser Ranging System (WLRS) as a dual purpose system for SLR
and LLR. Today, the equipment consists of
      20m Radiotelescope for geodetic VLBI,
      the WLRS for SLR/LLR (a modular transportable system MTLRS for SLR is avail-
      able),
      microwave receivers for the observation of GPS-, GLONASS-satellites and PRARE
      (the Doppler observations have been stopped in 1992),
      in situ techniques such as
         { a time and frequency system,
         { a superconducting gravitymeter and a seismometer,
         { meteorological sensors for air temperature, pressure and humidity and a water
            vapor radiometer.
The infrastructure from the rst installations to the establishment of the Fundamental
Stations has been completely changed the wooden barracks which housed the equipment
and the o ces have been replaced by a modern laboratory, housing the workshops and the
bureaus of the employers ( gure 2). The responsible agencies operating the Fundamental
Station Wettzell are the Bundesamt fur Kartographie and Geodasie and the Forschung-
seinrichtung Satellitengeodasie of the Technische Universitat Munchen (FESG) on behalf
of the Forschungsgruppe Satellitengeodasie (FSG).

3. 20m Radiotelescope for geodetic VLBI
     The Construction of the 20m Radiotelescope has been carried out in the period from
1980 to 1983. The design goals have been for geodetic VLBI which requires a stable
invariant point as the reference point of the antenna. The speci cations therefor are:
      Antenna - Cassegrain type with
        { mainre ector 20m (diameter)
        { subre ector 2.70m (diameter)
      2-Axis Mount (Alt-Az)
        { azimuth +/- 270 degree,
    Figure 1. Satellite Observation Station Wettzell 1976 and Funda-
    mental Station Wettzell 1993




    Figure 2.     20m Radiotelescope and Operators Room

        { elevation 0-90 degree,
        { pointing precission 20 arcsec.
      The 20m radiotelescope of Wettzell is mainly employed for the realisation and the
maintenance of the global reference frames (ITRF resp. ICRF). Most experiments are
dedicated to the determination of the Earth orientation parameter and to the determi-
nation of the crustal movements. Only a few observations are dedicated for astronomical
applications. The regular experiments are:
      IRIS A, NEOS A + B for the determination of the Earth orientation parameters,
      (from 1993 to 1998 (Dec.) 936 experiments (24h) have been successfully performed),
      IRIS S, for the determination of the Earth orientation parameters as IRIS A, in-
      cluding stations in South Africa and South America (since 1986 more than 121
      experiments with 24 hours of observations have been carried out),
      INTENSIVE for the rapid determination of the DUT1 parameter (since 1984 more
      than 3742 sessions have been observed at rst with Westford/USA and now with
      Greenbank/USA),
      EUROPE as a regional crustal deformation experiment in Europe (since 1988 more
      than 52 Experiments have been observed),
      NASA Experiments (X-ATL, ... GLOBAL, etc.) for Crustal Movements studies for
      Research and Development etc. (so far 184 Experiments have been conducted).
    Figure 3.      The WLRS-Building and the 75cm Telescope

In the next future the FS-Wettzell will participate in the CORE-Project. In addition
to the INTENSIVE experiments a comparable experiment is in the discussion between
the BKG and the GSI (Geographical Survey Institute of Japan) employing the K4 data
aquisition terminal on the baseline Wettzell-Tsukuba.

4. Wettzell Laser Ranging System (WLRS)
     The WLRS is a dual pupose Laser ranging system, capable to range to arti cial
satellites and to the Moon. It has been designed and constructed in the period from 1986
to 1989. The operations have been taken over from the 3rd gen. SLR-system after a two
years testphase in the year 1991. WLRS enables to carry out SLR observations during
day and night time. Three shifts allow to operate the WLRS for 24 hours a day at 350
days per year. The technical speci cations are:
       75cm optical telescope for transmit and receive the laser pulse and for guiding the
       target,
       Nd:YAG-Laser
          { wavelength 532nm and 1064nm (two color-capability),
          { pulse duration 80ps (single and semi train pulse mode),
          { pulse energy 360mJ by 10 or 20Hz repetition rate and 30mJ by 50Hz repetition
            rate,
       Multichannel Receiving System
          { Micro Channel Plate Photomultiplier MCP F4129f
          { Avalanche Photo Diodes for 532nm and 1064nm wavelengths (Geiger mode)
          { Streak Camera Hamamatsu C2909
     The precission in ranging to the LAGEOS is 4-6mm (normalpoints).
     A summary of the observed satellite passes is given in the gure 5. It has to be
pointed out, that the number of observed passes increased from year to year. The reason
is the automatisation, the gain of experiences and the increasing number of satellites. In
1998 the maintenance of the telescope and the replacement of the Nd:YAG-laser was the
reason of the lower number of observed satellites. Lunar tracking is not yet a routinely
procedure.
     The technical re nements are concentrated to a replacement of the Control Unit with
a new control computer and event timer. The old CAMAC event timer will be replaced
    Figure 4. The Nd:YAG-Lasersystem and the Multichannel Detec-
    torsystem




    Figure 5.      The number of observed satellite passes since 1990

by the newly developed picosecond event timer from Dassault/F which will provide a
resolution of better than 6ps. This will be evident for the two color application of the
WLRS capability. In order to improve the Lunar ranging capability and especially the
local seeing, adaptive optics will be employed.

5. Microwave Systems
     Since the very beginning the FS-Wettzell is engaged in the use of the satellite naviga-
tion technology for geodetic applications. Up to the year 1992 Doppler observations have
been carried out in the frame of the TRANSIT-network. First experiments employing the
GPS technology, especially testing the pseudo random noise modulation, have been car-
ried out in the years 1978 and 1979 by the NTS 1 and 2 (Navigation Technology Satellite)
time transfere experiments. A permanent GPS-receiver for geodetic applications has been
installed in the year 1987, since that time a few hour window with more than 3 satellites in
common view was observable all over Europe. The rst receiver was a TI4100, replaced by
a MINIMAC 2816AT, by a ROGUE and at last by a TURBO ROGUE receiver. Today an
ensemble of four GPS-receivers (TURBO ROGUE, TRIMBLE SSI 4000, ASHTECH Z12
and Z18) is in operation. In addition a 3SNAVIGATION receiver and the Ashtech Z18
are capable to track the GLONASS satellites. Beside the GPS- and GLONASS -receivers
a PRARE is employed for continuously tracking. The FS-Wettzell provides a core station
for the IGS-Network, a permanent reference station for the European (EUREF) and for
the German (GREF) reference network. More than 30 receivers are remote controlled
from the FS-Wettzell. The following stations have to be mentioned:
       O'Higgins, Lhasa and Reykjavik within the IGS-Network,
                                                                                                 GPS - files
                                                 1 PPS                                           weekly
                                             Distribution                    Data
                                                                                                 Clock - values
                                              Amplifier                    Collector
                                                                                                 monthly
                                                                              PC
                                                                                                 BIPM
                                1 PPS      UTC (Wettzell)        1 PPS
                                                                               Counter
                                            Master Clock           Start
                                                                                  Stop

                                          Phase Microstepper
               GPS - TTR6
                                         5 MHz
                   5 MHz                                 5 MHz                  1 PPS
                                                                  Clock
   Systems      Distribution       Cs - Standard
                                                                  Modul
                 Amplifier                                                               1 PPS
                                                                                                 SLR
                                                                                                 Clock
                                                         5 MHz    Clock         1 PPS
                                   Cs - Standard
                                                                  Modul
                                                                                         1 PPS
                                                                                                 VLBI
                                                                                                 Clock
                                                         5 MHz    Clock         1 PPS
                                   Cs - Standard
                                                                  Modul
                                                                                         1 PPS
                                                                                                 Rogue
                                                                                                 Clock
                                                         5 MHz    Clock         1 PPS
                                   Cs - Standard
                                                                  Modul                  1 PPS
                                                                                                 TIGO
                                                                                                 Clocks
                                                         5 MHz    Clock         1 PPS
                                   Cs - Standard
                                                                  Modul



     VLBI         5 MHz
                               5 MHz                5 MHz         Clock         1 PPS
  SLR / LLR    Distribution             H - Maser
                Amplifier                                         Modul
      GPS

                                                    5 MHz         Clock         1 PPS
                                        H - Maser
                                                                  Modul


                                                    5 MHz         Clock         1 PPS
                                        H - Maser
                                                                  Modul




         Figure 6. Diagram of the Time and Frequency System, the H-Maser
         EFOS 3, X-tal-, Rb- and CS-standards

              Ankara, Nicosia, So a, Hofn, Zelenchukskaja and more within the EUREF- network
              20 stations within the German reference network GREF
In Addition to the GPS-observations GLONASS observations were carried out at the sta-
tions Ankara employing a 3SNAVIGATION and at Reykjavik an ASHTECH Z18 receiver.
remotely controlled from FS-Wettzell (IGEX).

6. In situ observations
     In addition to the space techniques SLR/LLR, VLBI and mirowave techniques some
basic local information is required to support the global observations. At the FS-Wettzell
in situ observations are carried out as basic services:
       Time and Frequency
       The time and frequency system has to provide the epoch for the space observations
       with respect to a globally available timescale such as UTC(BIPM) or UTC(USNO)
       and the reference frequency e.g. for VLBI. At the FS Wettzell a local timescale
       UTC(IfAG) is generated by a clock ensemble of 5 Cs-primary standards. The clocks
       are continuously compared to UTC(USNO) and UTC(BIPM) via GPS-time trans-
       feres carried out in cooperation with the time keeping laboratories on behalf of the
       the BIPM as common view observations. The precise frequency on the short term
       basis is derived from an ensemble of 3 H-Masers. All frequency standards drive
       clocks which are internally compared. The observations are reported to the BIPM
       on a weekly resp. on a monthly basis. The BIPM published the results in the
       Circular T monthly.
       Gravity observations
       The variations of gravity as a result of tidal e ects, ground water uctuations etc.
       is observed by continuous monitoring of the gravity employing a superconducting
       gravity meter. The absolute gravity is obtained through the observations with an
       absolute gravity meter periodically.
   Figure 7. Super Conducting Gravity Meter, Water Vapor Radiome-
   ter and Seismometer




   Figure 8.     The Transportable Integrated Geodetic Observatory (TIGO)

    Meteorological Data Recording Systems
    For most of the space techniques refraction corrections have to be derived preferable
    from ground based local observations as the temperature, pressure and humidity,
    rainfall, wind direction and speed. Such data are recorded and monitored at the FS-
    Wettzell and stored every 15 minutes. Observations with a water vapor radiometer
    are carried out continuously to obtain the wet pass delay, routinely derived for the
    zenith direction.
    Seismometer STS-2
    Taking the advantage of the infrastructur of a FS, support could be given to the mon-
    itoring of the seismicity. At the FS-Wettzell a 3 component broadband seismometer
    has been installed, which is supporting the regional German Seismic Network. The
    monitoring of the seismicity is used for the correlation of seismic events with geodetic
    results.

7. Future Developments
   Two major developments have to be mentioned:
    Transportable Integrated Geodetic Observatory (TIGO)
    TIGO is developed as a transportable FS, which will improve the ISGN preferably
    in the southern hemisphere. TIGO employes VLBI, SLR and Microwave techniques
    (GPS) as well as insitu observations. The design of TIGO is very much in uenced
    from the experiences made at the FS-Wettzell.
Figure 9.    The "CII"-Ringlaser

Laser gyroscopes. Developments are carried out to make use of Laser gyroscopes for
the monitoring of the Earth rotation. The advantage of such a gyroscope is that
   { the short term variations of the earth rotation (DUT1) will monitored,
   { the results will be available in "near real time",
   { no global network is required to observe the variations and
   { the absolute reference frame is realized by the gyroscope itself.
In a rst step a prototype for the monitoring of the Earth rotation has been de-
veloped and built in close cooperation of the BKG, FESG and the University of
Canterbury/NZ, the so called CII-ring (size: 1mx1m). Experiences have been gained
concerning the technical lay out and the feasibility, which encouraged to built a large
ring of 4x4m. The large ring calles "G" (Grossring) will provide the sensitivity to
monitor the variations of DUT1 on the short term basis.

				
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