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The Implications for Higher-Accuracy Absolute Gravity Measurements for
NGS and its GRAV-D Project
Vicki Childers, Daniel Winester, Mark Eckl, Dru Smith, Daniel Roman
National Geodetic Survey
NGS Gravity Program (Pre-GRAV-D)
For NAVD 88 Orthometric Heights
1976-77
1980’s
vulcan.wr.usgs.gov
etc.usf.edu
NGS Gravity Program (Pre-GRAV-D)
Absolute Gravity Measurements
1980’s to Present
1995 to Present
Table Mountain Geophysical Observatory
Superconducting Gravimeter & FG5 Absolute Gravimeter
NGS GRAV-D Project
GRAV-D: Gravity for the Redefinition of the American Vertical Datum -> New datum by 2022
• Comprised of two parts:– Gravity field “Snapshot”
baseline: Airborne gravity survey of all US-held territories
– Temporal geoid change monitored for datum updates
Role of Terrestrial Gravity in GRAV-D
• New gravity tie for each airborne survey (absolute – A10)
A10 Absolute Gravimeter
Role of Terrestrial Gravity in GRAV-D
• New gravity tie for each airborne survey (absolute – A10)
• Re-survey problem areas identified by airborne data (relative, absolute – A10)
• Monitor long-term geoid change via periodic re-measurement (relative, abs - A10 & FG5)
• TMGO Intercomparisons for abs gravimeters• Geoid Slope Validation Surveys: Proof of
Concept (Gravity for ortho hgts)
How Would NGS Use a More Accurate Gravimeter?
• Long-term monitoring of local or regional temporal geoid change
• Replace FG5 (better speed, more portability, indoor and outdoor deployment, more stations per time) and A10 in all work (relative meters too!)
• Deployment in less quiet and remote areas• Improved accuracy assessment for FG5s through
intercomparisons
Assuming….
Assuming….
• Significant improvement to tides and ocean-loading corrections code to have accurate measurements at time intervals of 4 hours or less.– Total uncertainty ascribed to earth tide, ocean
loading, and polar tide correctors is > 1 μGal (Technical Protocol for 8th ICAG-2009)
• An efficient method of determining vertical gravity gradient
The AOSense Atom Interferometric Absolute Gravimeter
Mark KasevichAOSense, Inc.
1991 Light-pulse atom interferometer
Falling rock Falling atom
• Distances measured in terms of phases (t1), (t2) and (t3) of optical laser field at position where atom interacts with laser beam
• Atomic physics processes yield a ~ [(t1)-2(t2)+(t3)]
• Determine trajectory curvature with three distance measurements (t1), (t2) and (t3)
• For curvature induced by acceleration a, a ~ [(t1) - 2(t2) + (t3)]
Kinematic model for sensor operation
Why superb sensors?
• Atom = near perfect inertial reference.
• Laser/atom interactions register relative motion between atom and sensor case.
• Sensor accuracy derives from the exceptional stability of optical wavefronts.
Sensor Case
Atoms
Gravimeter
Laser
AOSenseAOSense408-735-9500AOSense.comSunnyvale, CA
AOSense Commercial Compact Gravimeter
Commercial cold atom gravimeter
• Noise < 0.7 g/Hz1/2
• 10 Gal resolution• > 12 Hz update rate• Shipped 11/22/10• First commercial
atom optic sensor
AOSenseAOSense408-735-9500AOSense.comSunnyvale, CA
Sensor output
(blue) Instrument output(red dashed) model
Interferometer fringe
AOSenseAOSense408-735-9500AOSense.comSunnyvale, CA
Next Generation Instrument (in development)
• Fieldable• Improved noise performance• Improved accuracy• Improved vibration control
AOSenseAOSense408-735-9500AOSense.comSunnyvale, CA
AOSense, Inc.
AOSenseAOSense408-735-9500AOSense.comSunnyvale, CA
• Founded in 2004 to develop cold-atom sensors (Brent Young CEO).
• Core capability is design, fabrication and testing of navigation and gravimetric sensors based on cold-atom technologies.
• Staff of 40
• 20k sq. ft. R&D space (clean rooms, assembly, testing)