Geophysics-based method of locating a stationary earth object
View Patent ↗A geophysics-based method for determining the position of a stationary earth object uses the periodic changes in the gravity vector of the earth caused by the sun- and moon-orbits. Because the local gravity field is highly irregular over a global scale, a model of local tidal accelerations can be compared to actual accelerometer measurements to determine the latitude and longitude of the stationary object.
1. A geophysics-based method for fixing a position of a stationary earth object, comprising:
a) providing a model that predicts tidal acceleration at a plurality of locations on the earth as a function of time and location relative to a reference time in an earth-fixed frame comprising orthogonal axes,
b) measuring an acceleration in at least one direction of the stationary earth object at an actual location for a period of time relative to the reference time with a sensor package in a sensor frame comprising orthogonal axes,
c) transforming the sensor frame using an estimated rotation matrix,
d) estimating a position of the object to input to the model to provide a predicted acceleration at the estimated position,
e) comparing the measured acceleration in the transformed frame to the predicted acceleration in the earth-fixed frame with a pattern recognition algorithm to provide a goodness-of-fit metric for the estimated position, and
f) repeating steps b) to e) at least once to provide the position fix for the actual location of the stationary earth object.
2. The method of claim 1 , wherein the earth-fixed frame comprises a North-East-Down coordinate system.
3. The method of claim 1 , wherein the position estimating of step d) comprises estimating a latitude and a longitude of the object.
4. The method of claim 1 , wherein the period of time is greater than 2 hours.
5. The method of claim 1 , wherein the period of time is greater than 12 hours.
6. The method of claim 1 , wherein the sensor package comprises at least one accelerometer.
7. The method of claim 6 , wherein the at least one accelerometer has a sensitivity of better than 1 μG for frequencies of less than 23 μHz.
8. The method of claim 6 , wherein the at least one accelerometer has a sensitivity of better than 0.1 μG for frequencies of less than 23 μHz.
9. The method of claim 6 , wherein the at least one accelerometer has a sensitivity of better than 10 nG for frequencies of less than 23 μHz.
10. The method of claim 6 , wherein the at least one accelerometer has a dynamic range of greater than positive to negative 1 G relative to the local gravity vector.
11. The method of claim 1 , further comprising measuring at least one environmental condition at the actual location to input to the model.
12. The method of claim 1 , wherein the pattern recognition algorithm comprises a matched filter algorithm.
13. The method of claim 12 , wherein the goodness-of-fit metric comprises with a correlation coefficient.
14. The method of claim 1 , wherein step f) is repeated until the method is unable to resolve further changes in the goodness-of-fit metric.
15. The method of claim 1 , wherein the one of the orthogonal axes of the sensor frame comprises a gravimetric “down” axis.
16. The method of claim 3 , wherein an initial latitude estimation is determined by gyrocompassing.
17. The method of claim 1 , wherein an initial position estimation is determined by an inertial measurement unit calculation relative to a previous known location.
18. The method of claim 1 , wherein the at least one direction comprises two perpendicular directions.
19. The method of claim 1 , wherein the at least one direction comprises three perpendicular directions.
20. The method of claim 1 , wherein the sensor package comprises a gimbaled platform.
21. The method of claim 1 , wherein the sensor package comprises a strap-down platform.
22. The method of claim 6 , wherein the at least one accelerometer comprises a gravimeter.
23. The method of claim 6 , wherein the at least one accelerometer comprises an inertial accelerometer.
24. The method of claim 6 , wherein the at least one accelerometer is based on microelectromechanical systems technology.