Correction of gyroscopic measurements for directional drilling

1. A method of estimating a directional parameter of a downhole component, comprising:

deploying a rotatable borehole string in a borehole, the borehole string including the downhole component, the downhole component being rotatable with the borehole string, the downhole component including a gyroscope device and a magnetometer device;

collecting gyroscope measurement data from the gyroscope device and oscillating magnetic field measurement data from the magnetometer device during rotation of the gyroscope device and the magnetometer device with the borehole string; and

estimating, by a processor, the directional parameter of the downhole component, wherein the estimating includes correcting the gyroscope measurement data to remove an oscillating signal component from the gyroscope measurement data using the oscillating magnetic field measurement data, the oscillating signal component from the gyroscope measurement data and the oscillating magnetic field measurement data related to the rotation of the borehole string.

2. The method of claim 1 , wherein the directional parameter is a true north azimuth.

3. The method of claim 1 , wherein the gyroscope measurement data include x-y gyroscope measurement data, and the oscillating magnetic field measurement data include x-y magnetic field measurement data, wherein a plane defined by x and y is perpendicular to a longitudinal axis of the downhole component.

4. The method of claim 3 , wherein the correcting includes demodulating the x-y gyroscope measurement data using the x-y magnetic field measurement data, and wherein the estimating includes calculating a x-y earth rate.

5. The method of claim 4 , wherein the demodulating includes removing the oscillating signal component from the x-y gyroscope measurement data using a demodulator.

6. The method of claim 3 , wherein the gyroscope measurement data include z-gyroscope measurement data, wherein z is perpendicular to the plane defined by x and y.

7. The method of claim 6 , wherein the estimating includes calculating a z-earth rate using the z-gyroscope measurement data.

8. The method of claim 3 , wherein the x-y gyroscope measurement data and the x-y magnetic field measurement data are acquired while rotating the downhole component slower than about 300 degree/min.

9. The method of claim 3 , further comprising collecting x-y-accelerometer measurement data.

10. The method of claim 9 , wherein the correcting includes demodulating the x-y-accelerometer measurement data using the x-y magnetic field measurement data, and wherein the estimating includes calculating a x-y earth rate.

11. The method of claim 10 , wherein the estimating includes calculating a true north azimuth using the x-y earth rate, an inclination and a latitude.

12. The method of claim 10 , wherein the estimating includes calculating a z-earth rate using z-gyroscope measurement data, wherein z is perpendicular to the plane defined by x and y.

13. The method of claim 12 , wherein the estimating further includes using a filter, wherein the filter calculates a true north azimuth using the x-y earth rate, the z-earth rate and an inclination.

14. The method of claim 13 , wherein the inclination is determined using z-accelerometer measurement data.

15. The method of claim 14 , wherein the filter is a Kalman filter.

16. The method of claim 1 , wherein the downhole component is a rotary steering assembly comprising a sleeve rotating with the borehole string at a rate that is less than a rotational rate of the borehole string, wherein the sleeve comprises the gyroscope device and the magnetometer device.

17. A system for estimating a directional parameter of a downhole component, comprising:

a rotatable borehole string, the borehole string comprising the downhole component, the downhole component being rotatable with the borehole string, the downhole component including a gyroscope device and a magnetometer device; and

a processor configured to perform:

collecting gyroscope measurement data from the gyroscope device and oscillating magnetic field measurement data from the magnetometer device during rotation of the gyroscope device and the magnetometer device with the borehole string; and

estimating the directional parameter of the downhole component, wherein the estimating includes correcting the gyroscope measurement data to remove an oscillating signal component from the gyroscope measurement data using the oscillating magnetic field measurement data, the oscillating signal component of the gyroscope measurement data and the oscillating magnetic field measurement data related to the rotation of the borehole string.

18. The system of claim 17 , wherein the downhole component includes a rotary steering assembly having a sleeve configured to rotate with the borehole string at a rate that is less than a rotational rate of the borehole string, and the gyroscope device and the magnetometer device are located in the sleeve.

19. The system of claim 18 , wherein the sleeve includes a plurality of pads extendable to control a direction of a drill bit of the borehole string.

20. The system of claim 17 , wherein the correcting includes:

demodulating the gyroscope measurement data using the oscillating magnetic field measurement data and wherein estimating includes calculating an x-y earth rate, wherein a plane defined by x and y is perpendicular to a longitudinal axis of the downhole component.

21. The system of claim 20 , wherein the downhole component includes an accelerometer device, and the processor is configured to perform: collecting accelerometer measurement data from the accelerometer device, and correcting the accelerometer measurement data by demodulating the accelerometer measurement data using the oscillating magnetic field measurement data.

22. The system of claim 20 , wherein the estimating includes a Kalman filter, wherein the Kalman filter calculates a true north azimuth using the x-y earth rate, a z-earth rate and an inclination.

23. The system of claim 20 , further comprising a demodulator configured to remove the oscillating signal component from the gyroscope measurement data.

24. The system of claim 17 , wherein the estimating includes calculating a true north azimuth of the downhole component.