1. 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

2. 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

3. 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).

4. 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”

5. 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

6. 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

7. 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!

8. 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

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

10. Case Study: Alaska 2008 Product Version Year Gravity Software
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

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

12. 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

13. Difference with respect to EGM2008
NGS
Terrestrial
Gravity
Newton (no IMU)
AeroGrav
Newton (IMU)

14. 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 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

15. 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

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