H'P' Plag

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Constraining the Tie Between the Origin of ITRF
and the Center of Mass of the Earth System Can
Absolute Gravimetery help?
H.-P. Plag Nevada Bureau of Mines and Geology
and Nevada Seismological Laboratory University of
Nevada, Reno
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Introduction (1)?
Studies of regional to global-scale processes
require observations of changes in Earth's shape
with respect to the Center of Mass of the
(complete) Earth system (CM), for example, global
sea level changes local sea level scenarios,
post-glacial rebound, regional mountain building,
sediment basins.
ITRF97 - ITRF2000
Space-geodetic techniques measure geometry with
respect to a Reference Frame Origin (RFO).
The RFO is currently not well constrained to the
CM, uncertainty of the order of 2 mm/yr.
ITRF2000 - ITRF2005
-2 mm/yr
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Introduction (2)?
Space-geodetic techniques measure geometry with
respect to a Reference Frame Origin (RFO).
Absolute gravimeters measure gravity changes with
respect to the Center of Mass of the (complete)
Earth system (CM).
Gravity changes measured by a gravimeter are
caused by height changes and mass changes.
Wahr et al. (1995) Collocated observations give
information on current mass changes.
Plag et al. (2008) In the absence of current
mass changes, collocated observations give
information on a bias of height changes.
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Plag et al., 2008
Observations

• Some close to large concurrent mass changes
• Most on Northern hemisphere, i.e. Europe
• and North America

• 37 gravity stations
• Not all of sufficient quality
• Some without continuous GPS

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Plag et al., 2008
Regression Results

• Exclude delta gt 5.00
• Remaining stations 26
• Regression results
• Correlation coefficient
• -0.97 lt -0.93 lt -0.84
• Regression line
• g -(0.885-0.464)-(1.529-0.127)u

• This gives
• - bias in Z Component
• 0.57 mm/yr
• - alpha 0.64 mm/(nms-2)
• (compared to 0.65 mm/(nms-2) from
• numerical PGR studies)?

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Conclusions
Regression results support the claim that
collocated absolute gravity measurements
constrain the tie between RFO and CM.

• Recommendation collocated absolute gravity
  measurements with the goal to constrain RFO to CM
  tie
• - Collocate permanent absolute gravimeters with
  core geometric sites
• - Number of core stations about 40 (GGOS 2020)?
• - Global distribution
• Condition
• - Sites with small on-going mass changes

• Required
• - Coordination of geometric and gravimetric
  networks

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Conclusions
Circles Core stations Stars Superconducting
and absolute gravity stations