Advances and Best Practices in Airborne Gravimetry from the U.S. GRAV-D Project

Theresa M. Damiani1, Vicki Childers1, Sandra Preaux2, Simon Holmes3, and Carly Weil2

U.S. National Geodetic SurveyData Solutions and TechnologyEarth Resources Technology

Program critical to U.S. National Geodetic Surveys (NGS) mission to define, maintain, and provide access to the U.S. National Spatial Reference System

Gravity for the Redefinition of the American Vertical DatumOfficial NGS policy as of Nov 14, 2007Re-define the Vertical Datum of the USA as a gravimetric geoid by 2022 (at current funding levels)

Airborne Gravity SnapshotAbsolute Gravity TrackingTarget: 2 cm accuracy orthometric heightsEGU Conference*What is GRAV-D?4/2013

EGU Conference

RequirementsTo achieve the target 1-2 cm accuracy of the geoid will require:GRACE and GOCEHighly accurate (1 mGal) airborne gravity data across the nationImproved terrestrial gravity dataAccurate residual terrain modelingGeoid theory and spectral data blending

Re-evaluate sources of error in airborne gravity methods: collection (3 slides) and processing (3 slides).

After five years and > 27% of the country surveyed, significant improvements have been made: Case Study: 2008 Alaska Survey (6 slides).

Data Collection Best PracticesRemove Gravity Tie Bias UncertaintyMeasurements at Aircraft Parking Spot:Absolute Gravity (Micro-g LaCoste A-10)Vertical Gravity Gradient (G-meter and G-pod)

A-10G-meter w/ AliodG-pod

Data Collection Best PracticesGravimeter very close to center of gravity of aircraftNavigation Grade IMU, mounted on top of TAGSMultiple High-rate GNSS receivers on aircraft (GPS/GLONASS)Lever Arm between instruments with surveying equipment

Micro-g LaCoste TAGS GravimeterNovAtel SPAN-SEw/ Honeywell IRS IMU

Data Collection Quality Control>5 years, 14 operators, and 7 aircraft: Requires standardized checklists, worksheets, instructions, logbooks; Test FlightsQuality Control Guidelines: Troubleshooting Guides, Operating Specifications, and Visualization Tools

Gravity Processing AdvancesPast (1960s through 1980s):Low & slow flights (low altitude, low velocity)Less computation power resulted in use of small angle approximations and dropped terms in gravity correction equationsDesired < 10 mGal error, biases ok

GRAV-D:High altitude, high velocity, desire as close to 1 mGal as possibleRecognition of Offlevel Correction LimitationsBetter FilteringDiscrete DerivativesGPS and IMU research for positioning, aircraft heading/attitude calculations, and inputs to gravity correctionsStill Ongoing!

Gravity Processing Advances Example: Eotvos CorrectionHarlan 1968 - defines r and in terms of latitude, longitude and ellipsoidal height - 1st order approximation drops all terms

U.S. Latitudes: 30 to 50 degrees N; Europe Latitudes: 35 to 55 degrees NLow & SlowLow & FastHigh & Fast

Case Study: Alaska 2008http://www.ngs.noaa.gov/GRAV-D/data_products.shtmlCrossover differences of same 202 points for all versionsAirborne gravity compared with EGM2008 at altitude

Product VersionYearGravity SoftwarePositioningAeroGrav2008AeroGravGPS-onlyNewton (no IMU)2012Newton v1.2GPS-onlyNewton (with IMU)2012Newton v1.2GPS+IMU

Crossover Difference MapsAeroGravNewton (no IMU)Newton (IMU)

Crossover StatisticsFrom 2008 to 2012:65.0% Decrease in RangeMean about the same (within error range)61.5% Decrease in Standard DeviationIncreased Internal Consistency of Airborne Data, solely due to data processing advances

Difference with respect to EGM2008AeroGravNewton (no IMU)Newton (IMU)NGSTerrestrialGravity

Create three GRAV-D airborne gravity ellipsoidal harmonic models (with EGM2008 outside the area) out to n=2159. Inside the survey area, compare airborne models with increasing n from 360 to 2159 with EGM2008 (always n=2159)

This modeling is for evaluation purposes only.High-frequency Spectral AnalysisModel 1:AeroGravModel 2:Newton(no IMU)Model 3:Newton(IMU)n=2159GRAV-Dn=2159EGM2008EGM2008N=2159GRAV-Dn=360GRAV-Dn=361GRAV-Dn=362

55 km27 km18.5 km14 km11 km9 kmn170011.75 kmChilders et al., 1999Estimated Resolutionn145013.8 km2008 to 2012Improvement

Thank YouAirborne Gravity Data Products Portal:http://www.ngs.noaa.gov/GRAV-D/data_products.shtml

More information:http://www.ngs.noaa.gov/GRAV-D

Contacts:Dr. Theresa Damiani theresa.damiani@noaa.govGRAV-D Program Manager, Dr. Vicki Childers vicki.childers@noaa.gov

Green = Blocks Available for Download

*Full funding was estimated at approximately $5.5M / year. Although full funding was not approved, partial funding was approved in 2010 at $3M/year. As such, the initially hoped for 2018 target date will almost certainly not be met.

Current best target for completion of airborne surveys and implementation of the new vertical datum is is 2022 (updated October 2010).*