Advances and Best Practices in Airborne Gravimetry from the U. S 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 Survey Data Solutions and Technology Earth Resources Technology

What is GRAV-D? 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 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). 4/2013 EGU Conference

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 G-meter w/ Aliod A-10 “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 Acceleration of a moving object in a rotating reference system Centrifugal Variation in rotation rate Coriolis Relative acceleration Harlan 1968 - defines r and ω in terms of latitude, longitude and ellipsoidal height - 1st order approximation drops all terms <1 mgal to get an overall error <10 mgal Vertical Acceleration Eötvös Correction

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

Case Study: Alaska 2008 Product Version Year Gravity Software http://www.ngs.noaa.gov/GRAV-D/data_products.shtml Product Version Year Gravity Software Positioning “AeroGrav” 2008 AeroGrav GPS-only Newton (no IMU) 2012 Newton v1.2 Newton (with IMU) GPS+IMU Crossover differences of same 202 points for all versions Airborne gravity compared with EGM2008 at altitude

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

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 NGS Terrestrial Gravity Newton (no IMU) AeroGrav Newton (IMU)

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

2008 to 2012 Improvement Childers et al., 1999 Estimated Resolution n≈1450 13.8 km n≈1700 11.75 km 55 km 27 km 18.5 km 14 km 11 km 9 km

Thank You Airborne Gravity Data Products Portal: More information: 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.gov GRAV-D Program Manager, Dr. Vicki Childers vicki.childers@noaa.gov Green = Blocks Available for Download