The Implications for Higher-Accuracy Absolute Gravity Measurements for NGS and its GRAV-D Project

1. 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

2. NGS Gravity Program (Pre-GRAV-D) 1976-77 1980’s etc.usf.edu vulcan.wr.usgs.gov For NAVD 88 Orthometric Heights

3. NGS Gravity Program (Pre-GRAV-D) 1995 to Present Superconducting Gravimeter & FG5 Absolute Gravimeter 1980’s to Present Table Mountain Geophysical Observatory Absolute Gravity Measurements

4. 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

5. Role of Terrestrial Gravity in GRAV-D • New gravity tie for each airborne survey (absolute – A10) A10 Absolute Gravimeter

6. 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)

7. 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….

8. 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

9. The AOSense Atom Interferometric Absolute Gravimeter Mark Kasevich AOSense, Inc.

10. 1991 Light-pulse atom interferometer

11. Kinematic model for sensor operation 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)]

12. Why superb sensors? Gravimeter Laser • 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 408-735-9500 AOSense.com Sunnyvale, CA AOSense

13. AOSense Commercial Compact Gravimeter • Commercial cold atom gravimeter • Noise < 0.7mg/Hz1/2 • 10 mGal resolution • > 12 Hz update rate • Shipped 11/22/10 • First commercial atom optic sensor 408-735-9500 AOSense.com Sunnyvale, CA AOSense

14. Sensor output (blue) Instrument output (red dashed) model Interferometer fringe 408-735-9500 AOSense.com Sunnyvale, CA AOSense

15. Next Generation Instrument (in development) • Fieldable • Improved noise performance • Improved accuracy • Improved vibration control 408-735-9500 AOSense.com Sunnyvale, CA AOSense

16. AOSense, Inc. • 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) 408-735-9500 AOSense.com Sunnyvale, CA AOSense