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)