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
rωωrdtωd
dtrdω2
dtrda 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
[email protected]– GRAV-D Program Manager,
Dr. Vicki [email protected]
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