Method and system for compensating for soft iron magnetic disturbances in a heading reference system

1. A method for compensating for soft iron magnetic disturbances in a heading reference system, said system comprising a heading gyro disposed along a heading measurement axis and a magnetometer coupled to said heading gyro for providing an accurate heading relative to magnetic north; said method comprising the steps of:

providing an iterative control loop for controlling an output heading value corresponding to true magnetic north, and providing a compensated output heading reference value to said heading reference system from said iterative control loop for compensating for soft iron magnetic disturbances above a predetermined acceptable threshold value, said iterative control loop being configured to perform the steps of:

reading and processing a value of data from said magnetometer and said heading gyro for providing a change in each of said values from an immediately previous reading of said values;

comparing said change in magnetometer value with said change in gyro value since said immediately previous reading; determining whether any difference in said compared values exceeds an expected gyro drift over a period since said immediately previous reading; and

providing a new heading value comprising said gyro change in value plus the immediately previous heading value as an output to said gyro when said difference exceeds said expected gyro drift over said period since said immediately previous reading, said predetermined acceptable threshold value comprising said expected gyro drift; whereby said gyro heading is capable of maintaining said accurate heading in the face of said soft magnetic disturbances.

2. The compensation method in accordance with claim 1 wherein said iterative control loop being further adapted to use said magnetometer reading for said heading value when said difference in said compared values does not exceed said expected gyro drift over said period.

3. The compensation method in accordance with claim 2 further comprising the step of iteratively repeating said steps performed by said iterative control loop for subsequent readings of said magnetometer and gyro data values for maintaining said accurate heading in the face of said soft magnetic disturbances.

4. The compensation method in accordance with claim 1 further comprising the step of iteratively repeating said steps performed by said iterative control loop for subsequent readings of said magnetometer and gyro data values for maintaining said accurate heading in the face of said soft magnetic disturbances.

5. The compensation method in accordance with claim 1 further comprising the step of periodically correcting said gyro heading utilizing data provided from said magnetometer.

6. The compensation method in accordance with claim 1 wherein said compensation method further comprises blending heading gyro measurements with magnetometer measurements, said predetermined acceptable threshold value comprising a gyro error covariance.

7. The compensation method in accordance with claim 6 wherein said blending comprises using a Kalman filter to blend said heading gyro measurements with said magnetometer measurements.

8. The compensation method in accordance with claim 1 wherein said heading gyro comprises a triaxial heading gyro comprising three gyro components each being disposed along a different heading measurement axis, and said magnetometer comprises a triaxial magnetometer comprising three magnetometer components each being disposed along a different heading measurement axis.

9. The compensation method in accordance with claim 1 further comprising the step of correcting said magnetometer for the effect of soft iron magnetic disturbances utilizing data provided from said heading gyro.

10. The compensation method in accordance with claim 9 wherein correcting said magnetometer uses the heading gyro change plus the last magnetometer reading for the magnetometer value.

11. The compensation method in accordance with claim 1 further comprising periodically recalibrating the heading gyro using said magnetometer and at least one accelerometer.

12. The compensation method in accordance with claim 1 wherein said heading reference system comprises an aircraft heading reference system.

13. The compensation method in accordance with claim 12 wherein said aircraft heading reference system comprises an integrated standby unit.

14. The compensation method in accordance with claim 1 wherein said heading reference system comprises a vehicle inertia system.

15. The compensation method in accordance with claim 1 wherein said heading reference system comprises an air data and attitude heading reference system.

16. The compensation method in accordance with claim 1 wherein said heading reference system comprises a primary air data and attitude heading reference system.

17. The compensation method in accordance with claim 1 wherein said heading reference system comprises a secondary air data and attitude heading reference system.

18. The compensation method in accordance with claim 1 wherein said heading reference system comprises a primary inertial navigation system.

19. The compensation method in accordance with claim 1 wherein said heading reference system comprises a secondary inertial navigation system.