Injected Alignment for an INS System from Trackmodel Data in a Race Car Environment

T. Ford, J. Hamilton, M. Bobye

Abstract:	In order to provide meaningful results, an inertial system (INS) that uses a strapdown inertial measurement unit (IMU) requires that the orientation relationship between the body frame of the IMU, as defined by the sensor axis of the IMU, and the computational frame be determined. This relationship, the alignment of the inertial system, is often derived with a combination of the measured accelerations of the IMU compared to the gravity vector, and the angular rate measured by the IMU compared to the earth rate. The computation requires that the system does not experience any specific forces except gravity, is not physically rotating except from earth rotation, and that gyro bias in the unit is small compared to the earth rate. In a race car, the robustness of the system requires that the alignment of the IMU can be obtained even when the system is moving at a high rate and around corners such that the unit can sense significant non-vertical specific forces and motion induced rotation. The standard method for alignment under these circumstances could easily give roll and pitch errors of 45 degrees or more. This type of initial error leads to non-linear errors in the Kalman filter estimators, which causes the filter to take a prohibitively long time to estimate its system errors well enough to make the system useful. For this environment, another method for determining the body attitude is required. The NASCAR race tracks all are parameterised with surface models consisting of contiguous triangles. These are used in the NovAtel Inc. receivers to aid the differential GPS positioning solution that is subsequently transformed to screen coordinates and used to annotate the race cars for the TV broadcasts. Information from the same triangles can be used to orient the body frame of the IMU in the race car provided the relationship between the vehicle and body frames are known and that the velocity vector of the vehicle is known from GPS. The accuracy of the roll and pitch measurements can be determined to +/- 2 degrees, and the heading of the unit can be determined to 5 degrees with this method. In this paper, the alignment problem is discussed in detail. The effect of large alignment errors in the system is illustrated, and two approaches for using track model data to compute the system alignment are discussed. Results that show the effect of the injected alignment are presented.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	509 - 519
Cite this article:	Ford, T., Hamilton, J., Bobye, M., "Injected Alignment for an INS System from Trackmodel Data in a Race Car Environment," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 509-519.
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