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) 1976-77 vulcan.wr.usgs.gov 1980’s For determination of NAVD 88 orthometric heights on bench marks Maintain a national network of absolute gravity stations Establish and maintain the Table Mountain Geophysical Observatory and constrain drift in the superconducting gravimeter housed there Enter into cooperative projects with both domestic and international entities on topics involving temporal change in gravity or heights (e.g. hydrology, GIA, tectonics, subsidence) GRAV-D: to replace the current vertical datum with a gravimetric geoid etc.usf.edu For NAVD 88 Orthometric Heights

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

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 NGS Gravity program has been recently revitalized by the GRAV-D project, a project designed to replace the national vertical datum by 2022. The primary aspect of the program is an airborne gravity survey over the entire country. Terrestrial gravimetry supports this program in several ways: Create a new absolute gravity tie for each airborne survey at the sensor location (use A10 now) Perform relative surveys in regions where airborne data indicate a problem surface data set or an area of temporal change in the gravity field (i.e. subsidence, tectonic motion, GIA). In support of a program to monitor long term change to the geoid. As part of a series of proof of concepts surveys for the GRAV-D Project called the Geoid Slope Validation Survey. Absolute gravimeter Intercomparisons held at NGS’ Table Mountain Geophysical Observatory in Longmont, CO.

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) NGS Gravity program has been recently revitalized by the GRAV-D project, a project designed to replace the national vertical datum by 2022. The primary aspect of the program is an airborne gravity survey over the entire country. Terrestrial gravimetry supports this program in several ways: Create a new absolute gravity tie for each airborne survey at the sensor location (use A10 now) Perform relative surveys in regions where airborne data indicate a problem surface data set or an area of temporal change in the gravity field (i.e. subsidence, tectonic motion, GIA). In support of a program to monitor long term change to the geoid. As part of a series of proof of concepts surveys for the GRAV-D Project called the Geoid Slope Validation Survey. Absolute gravimeter Intercomparisons held at NGS’ Table Mountain Geophysical Observatory in Longmont, CO.

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…. The New Atomic Interferometer sensor being developed promises increased accuracy while being field portable. How would we use this? Geoid change, particularly as deployed regionally in areas of tectonic or hydrologic change. Replace FG5 work above (better speed, more portability, indoor and outdoor deployment, more stations per time). If Caveat A is not met, then might as well continue with FG5. Deployment in remote areas (e.g. by helicopter or pack animal on mountain tops or canyons) either for filling in control or for temporal change. Will re-introduce cooperation with outside researchers. Coastlines work: sea level change, NGS Coastal, elevation changes at gauges or shorelines. Economical method to bring gravity control to islands. If Caveat B is met, then it can serve as validation for GRAV-D ala GSVS.

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 Caveat A: Requires major improvement in tides and ocean-loading corrections code, to have corrections to sub-microGal level of data collected over four hours or less. Otherwise we are left with a more portable version of A-10. Caveat B: Requires an efficient method of determining vertical gravity gradient; either from internal calculations or deploying g-meter in raised position.

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

1991 Light-pulse atom interferometer

Kinematic model for sensor operation Falling rock Falling atom Determine trajectory curvature with three distance measurements (t1), (t2) and (t3) For curvature induced by acceleration a, a ~ [(t1) - 2(t2) + (t3)] 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)]

Why superb sensors? AOSense 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. Atoms Gravimeter Laser Sensor Case AOSense 408-735-9500 AOSense.com Sunnyvale, CA

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

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

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

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