1. Airborne Gravity Processing 101 Sandra Preaux sandra.preaux@noaa.gov 571-344-5792

2. Outline What we have: –The Measurement Systems –The Data What we want: Gravity How we get there: –Conversion From Raw Measurements –A Note About Time –Correcting for Airplane Motion –Correcting for Sensor Orientation –Tying into an Absolute Reference –Filter

3. IMU GPS Base Station Or 2Or 3 GPS Antenna Positioning System – GPS in Gravimeter – GPS in IMU – GPS Base Station Orientation System – IMU – Gravimeter Platform Gravity – Main sensor in the Gravimeter Gravimeter Absolute Gravity Tie

4. NRL Meter S93 in NOAA 52 in Florida for GLS05 Survey, May 2005 NRL Meter S93 in NOAA 52 in Alabama For GLS06, January 2006

5. The Data Position –Latitude, Longitude, Ellipsoidal Height –And/or GPS X, Y, Z –Velocity and Acceleration Orientation –Pitch, Roll, Yaw –Platform Cross and Long Acceleration Gravimeter –Spring Tension, Beam Position, Cross Coupling Meta Data –Lever Arm between GPS antenna and Gravimeter –Vertical Distance from Gravimeter to Tie Spot –Gravity at the Tie Spot

6. What do we want from this data? Gravity, g g=Gravitation-the Vertical Component of Centrifugal Acceleration Units: 1 gal = 1 cm/s 2 = 10 3 mgal 1 mgal = 10 -5 m/s 2 At the Equator: C=3373 mgal We want errors in g of 1 to 2 mgal or less www.dinosaurtheory.com/solution.html

7. How do we get there? 1.Combine ST, BV, and CC with various manufacturer scale factors to get g raw 2.Translate positions from the GPS antenna to the sensor position 3.Synchronize position and gravity measurements 4.Remove Accelerations due to Airplane motion 5.Correct for slight deviation of sensor from level 6.Correct for Instrument Drift and Tie to Absolute Measurement 7.Filter

8. Synchronizing the Data More than 95% of the Measured Beam Velocity is due to Aircraft Motion A Cross-Covariance Analysis finds the best delay for each 4 minute window during the flight to 0.01 seconds

9. Airplane Motion Full acceleration of the airplane in a rotating reference system Historically –r, ω and their derivatives are defined in terms of latitude, longitude and height –The resulting vertical acceleration is then subtracted from g raw –Common simplifications reduce computation time with errors smaller than 1 mgal for low altitude, slow flight Coriolis http://www.webweaver.nu/clipart/earth2.shtml Centrifugal

10. Eötvös Correction East Depends on: Speed Direction Latitude Altitude The Vertical Component of: Coriolis Term Centrifugal of Airplane West

11. Vertical Acceleration 2 nd Derivative of Height Mostly less than 10,000 mgal May have spikes to 50,000 mgal during data collection May have spikes to 100,000s mgal offline

12. Off Level If the gravity meter is off level, g measured < g Two ways to correct –Derive off level angle from platform horizontal accelerations –Use the difference in the sum of accelerations between the airplane and the meter to derive the correction

14. 1.Combine ST, BV, and CC with various manufacturer scale factors to get g raw 2.Translate positions from the GPS antenna to the sensor position 3.Synchronize position and gravity measurements 4.Remove Accelerations due to Airplane motion 5.Correct for slight deviation of sensor from level 6.Correct for Instrument Drift and Tie to Absolute Measurement 7.Filter How do we get there?

16. Thank You