Advances and Best Practices in Airborne Gravimetry from the

U.S. GRAV-D ProjectTheresa M. Damiani1, Vicki Childers1, Sandra

Preaux2, Simon Holmes3, and Carly Weil2

1.U.S. National Geodetic Survey2.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

heightsEGU Conference 2

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 IDA-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-SEw/ 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 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

• Acceleration of a moving object in a rotating reference system

rωωrdt

ωd

dt

rdω2

dt

rda

2

2

Coriolis CentrifugalVariation 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 & FastHigh & Fast

Case Study: Alaska 2008

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 GPS-only

Newton (with IMU) 2012 Newton v1.2 GPS+IMU

• Crossover differences of same 202 points for all versions

• Airborne gravity compared with EGM2008 at altitude

Crossover Difference MapsAeroGravNewton (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)

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 Analysis

Model 1:AeroGrav

Model 2:Newton(no IMU)

Model 3:Newton(IMU)

n=2159

GRAV-Dn=2159

EGM2008

EGM2008N=2159

GRAV-Dn=360GRAV-Dn=361GRAV-Dn=362

55 km 27 km 18.5 km 14 km 11 km 9 km

n≈170011.75 km

Childers et al., 1999Estimated Resolutionn≈145013.8 km

2008 to 2012Improvement

Thank You• Airborne 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.gov– GRAV-D Program Manager,

Dr. Vicki Childersvicki.childers@noaa.gov

Green = Blocks Available for Download