Advances and Best Practices in Airborne Gravimetry from the U.S. GRAV-D Project Theresa M. Damiani 1, Vicki Childers 1, Sandra Preaux 2, Simon Holmes 3, and Carly Weil 2 1.U.S. National Geodetic Survey 2.Data Solutions and Technology 3.Earth Resources Technology

Program critical to U.S. National Geodetic Survey’s (NGS’) mission to define, maintain, and provide access to the U.S. National Spatial Reference System Gravity for the Redefinition of the American Vertical Datum Official NGS policy as of Nov 14, 2007 Re-define the Vertical Datum of the USA as a gravimetric geoid by 2022 (at current funding levels) Airborne Gravity Snapshot Absolute Gravity Tracking Target: 2 cm accuracy orthometric heights EGU Conference2 What is GRAV-D? 4/2013

Requirements To achieve the target 1-2 cm accuracy of the geoid will require: – GRACE and GOCE – Highly accurate (1 mGal) airborne gravity data across the nation – Improved terrestrial gravity data – Accurate residual terrain modeling – Geoid 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 Practices Remove Gravity Tie Bias Uncertainty Measurements at Aircraft Parking Spot: – Absolute Gravity (Micro-g LaCoste A-10) – Vertical Gravity Gradient (G-meter and “G-pod”) Parking spot ID A-10 G-meter w/ Aliod “G-pod”

Data Collection Best Practices Gravimeter very close to center of gravity of aircraft Navigation Grade IMU, mounted on top of TAGS Multiple High-rate GNSS receivers on aircraft (GPS/GLONASS) Lever Arm between instruments with surveying equipment Micro-g LaCoste TAGS Gravimeter NovAtel SPAN-SE w/ Honeywell µIRS IMU

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

Gravity Processing Advances Past (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 equations – Desired < 10 mGal error, biases ok GRAV-D: – High altitude, high velocity, desire as close to 1 mGal as possible – Recognition of Offlevel Correction Limitations – Better Filtering – Discrete Derivatives – GPS and IMU research for positioning, aircraft heading/attitude calculations, and inputs to gravity corrections – Still Ongoing!

Gravity Processing Advances Example: Eotvos Correction Harlan defines r and ω in terms of latitude, longitude and ellipsoidal height - 1 st order approximation drops all terms <1 mgal to get an overall error <10 mgal Acceleration of a moving object in a rotating reference system Coriolis Centrifugal Variation in rotation rate Relative acceleration Vertical Acceleration Eötvös Correction

U.S. Latitudes: 30 to 50 degrees N; Europe Latitudes: 35 to 55 degrees N Low & Slow Low & Fast High & Fast

Case Study: Alaska Product VersionYearGravity SoftwarePositioning “AeroGrav”2008AeroGravGPS-only Newton (no IMU)2012Newton v1.2GPS-only Newton (with IMU)2012Newton v1.2GPS+IMU Crossover differences of same 202 points for all versions Airborne gravity compared with EGM2008 at altitude

Crossover Difference Maps AeroGrav Newton (no IMU) Newton (IMU)

Crossover Statistics From 2008 to 2012: – 65.0% Decrease in Range – Mean about the same (within error range) – 61.5% Decrease in Standard Deviation Increased Internal Consistency of Airborne Data, solely due to data processing advances

Difference with respect to EGM2008 AeroGravNewton (no IMU)Newton (IMU) NGS Terrestrial Gravity

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 Analysis Model 1: AeroGrav Model 2: Newton (no IMU) Model 3: Newton (IMU) n=2159 GRAV-D n=2159 EGM2008 N=2159 GRAV-D n=360 GRAV-D n=361 GRAV-D n=362

55 km27 km18.5 km14 km11 km9 km n≈ km Childers et al., 1999 Estimated Resolution n≈ km 2008 to 2012 Improvement

Thank You Airborne Gravity Data Products Portal: – More information: – Contacts: – Dr. Theresa Damiani – GRAV-D Program Manager, Dr. Vicki Childers Green = Blocks Available for Download