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Developments with Absolute Gravimetry

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The FG5 Gravimeter. Light max. No Light /2. Free falling. mass. Laser. Free fall : the FG5 absolute gravimeter. Why do we need both AG and CGPS? ... – PowerPoint PPT presentation

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Title: Developments with Absolute Gravimetry


1
Developments with Absolute Gravimetry
  • Simon D.P. Williams, Trevor F. Baker,
  • Daniel McLaughlin

2
Carter reports on sea level monitoring
recommended the use of Absolute Gravity (AG) as a
complimentary (and independent) technique to CGPS
for VLM monitoring.
3
Free fall the FG5 absolute gravimeter
Free falling mass
Interferometer
  • At present, the fringes are produced from the
    light of a He-Ne laser (red light _at_ 633 nm)
  • Free fall on 20 cm during 0.2 s
  • 640.000 fringes, with frequency sweeping from 0
    to 6 MHz.
  • In practice, one takes 1 fringe / 1000
  • The time intervals between the occurrence of
    each fringe are measured by a Rb oscillator (or
    Cs if available)

Laser
The FG5 Gravimeter
4
Why do we need both AG and CGPS?
  • AG is absolute whereas CGPS is still really
    relative to the reference frame
  • CGPS is less expensive per site and more robust
  • CGPS is dependent on the US DoD and the IGS
    (cost?)
  • AG is sensitive to mass changes as well as land
    movement (good and bad)
  • AG is therefore a good calibration method for the
    more abundant GPS. It will also be useful for the
    new gravity missions.
  • Other geodetic tools also important e.g SLR,
    VLBI, DORIS

5
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7
In Europe, various AG groups have measured at the
following sites near to tide gauges
Sweden, Spain, Ukraine, Russia, Poland,
Latvia, Croatia, Greece, Iceland, Norway,
Germany, France, UK, Belgium..
8
AG Measurements at UK Tide Gauges
In the UK we have concentrated on AG measurements
at 3 UK tide gauges, considered to be the core UK
gauges, Newlyn, Lerwick and Aberdeen
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10
Methodology
  • Measure for at least 3 days at each site, at
    least once per year (hopefully!)
  • Data from each day are processed separately and
    correction made for solid-earth tides,
    ocean-loading effects, atmospheric pressure and
    polar motion.
  • Single admittance factor and local pressure data
    are used to correct for atmospheric pressure.

11
Intercomparisons
12
Intercomparisons
13
Results
14
Post-glacial rebound
15
Conclusions
  • AG measurement spanning 5-10 years can be used to
    measure vertical land movements at tide gauges.
  • Absolute Gravity is a useful complimentary
    technique to CGPS.
  • Good site selection is essential to AG
    measurement success.
  • Given a few more years of measurement, results
    can be used to test competing GIA models.
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