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Developments with Absolute Gravimetry

Developments with Absolute Gravimetry. Simon D.P. Williams, Trevor F. Baker, Daniel McLaughlin. “Carter reports” on sea level monitoring recommended the use of Absolute Gravity (AG) as a complimentary (and independent ) technique to CGPS for VLM monitoring.  Light max. /2.  No Light.

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Developments with Absolute Gravimetry

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  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.  Light max. /2  No Light 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 @ 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 relativeto 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. 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…..

  6. 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

  7. 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.

  8. Intercomparisons…

  9. Intercomparisons…

  10. Results

  11. Post-glacial rebound

  12. 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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