Rear axle center locating

1. A rear axle center locating system comprising:

a tractor comprising:

a rear axle having a center;

a global positioning system (GPS) antenna offset from the rear axle;

inertial measurement units; and

a position acquisition unit comprising:

a processing unit; and

a non-transitory computer-readable medium containing instructions to direct the processing unit to:

determine a geographic location of the GPS antenna based upon signals received by the GPS antenna; and

determine a geographic location of the center of the rear axle based upon the geographic location of the GPS antenna and combined data from the inertial measurement units.

2. The rear axle center locating system of claim 1 , wherein the instructions are to direct the processing unit to determine a roll and pitch of the tractor based upon the combined data from the inertial measurement units and to determine the geographic location of the center of the rear axle based upon the geographic location of the GPS antenna and the determined roll and pitch.

3. The rear axle center locating system of claim 1 further comprising a wheel encoder to output signals indicating rotation and yaw of wheels of the tractor, wherein the instructions are to direct the processing unit to determine the geographic location of the center of the rear axle based upon a prior geographic location of the center of the rear axle and the signals indicating rotation and yaw of the wheels of the tractor.

4. The rear axle center locating system of claim 1 , wherein the tractor comprises a roof and wherein the inertial measurement units are supported by the roof.

5. The rear axle center locating system of claim 4 further comprising a camera supported by the roof, wherein one of the inertial measurement units is part of the camera.

6. The rear axle center locating system of claim 5 further comprising a GPS unit supported by the roof, wherein the GPS unit comprises both the GPS antenna and another of the inertial measurement units.

7. The rear axle center locating system of claim 4 further comprising a GPS unit supported by the roof, wherein the GPS unit comprises both the GPS antenna and one of the inertial measurement units.

8. The rear axle center locating system of claim 1 , wherein the instructions are to direct the processing unit to combine the data from the inertial measurement units using a Kalman filter.

9. The rear axle center locating system of claim 1 further comprising:

wheels; and

an actuator to adjust steering of the wheels, wherein the instructions are to direct the processing unit to output control signals to the actuator to adjust the steering of the wheels based upon the determined geographic location of the center of the rear axle.

10. The rear axle center locating system of claim 1 , wherein the non-transitory computer-readable medium and the processing unit are carried by the tractor.

11. A non-transitory computer-readable medium containing instructions, which when executed by a processor, cause the processor to:

acquire GPS signals indicating a position of a GPS antenna offset from a center of a rear axle of a tractor;

acquire signals from multiple inertial measurement units;

determine a geographic location of the center of the rear axle of the tractor based upon the position of the GPS antenna and the signals from a combination of the multiple inertial measurement units; and

adjust steering of wheels of the tractor based upon the determined geographic location of the center of the rear axle.

12. The medium of claim 11 , wherein the instructions which when executed by the processor, cause the processor to determine a roll and pitch of the tractor based upon the signals from a combination of the multiple inertial measurement units and to determine the geographic location of the center of the rear axle of the tractor based upon the position of the GPS antenna and the determined roll and pitch of the tractor.

13. The medium of claim 11 , wherein the instructions which when executed by the processor, cause the processor to to determine the geographic location of the center of the rear axle based upon a prior geographic location of the center the rear axle and the signals indicating rotation and yaw of wheels of the tractor.

14. The medium of claim 11 , wherein the tractor comprises a roof and wherein the inertial measurement units are supported by the roof.

15. The medium of claim 14 further comprising a camera supported by the roof, wherein one of the inertial measurement units is part of the camera.

16. The medium of claim 15 further comprising a GPS unit supported by the roof, wherein the GPS unit comprises both the GPS antenna and another of the inertial measurement units.

17. The medium of claim 14 further comprising a GPS unit supported by the roof, wherein the GPS unit comprises both the GPS antenna and one of the inertial measurement units.

18. The medium of claim 11 , wherein the instructions are to direct the processing unit to combine the data from the inertial measurement units using a Kalman filter.

19. A method comprising:

acquiring GPS signals indicating a position of a GPS antenna offset from a center of a rear axle of a tractor;

acquiring signals from multiple inertial measurement units;

determining a geographic location of the center of the rear axle of the tractor based upon the position of the GPS antenna and the signals from a combination of the multiple inertial measurement units; and

adjusting the steering of wheels of the tractor is based upon the determined geographic location of the center of the rear axle.