Fusion Filter Algorithm Enhancements For a MEMS GPS/IMU

A low cost solid state GPS/IMU navigation unit has been developed that incorporates measurements from a GPS, MEMS gyros and accelerometers, and fluxgate magnetometers to provide a complete navigation solution at a high output rate. The Crossbow Technology, Inc. AHRS500 family of inertial sensor products provides standalone solutions for: • Vertical gyro applications • Attitude and heading reference system (AHRS) applications • Full navigation GPS/IMU applications. Firmware inside the AHRS500’s onboard processors produces calibrated angular rate measurements, calibrated acceleration measurements, calibrated magnetometer measurements, and the estimated navigation state which includes body attitude (roll, pitch, heading), local level horizontal navigation frame position (latitude, longitude, and altitude) and velocity at a high output rate. The algorithm used to estimate the navigation state is an Extended Kalman Filter (EKF) trajectory correction approach in which the inertial accelerometers and gyros propagate the state trajectory made up of the position, velocity, and body attitude, and the supporting sensors (GPS and magnetometers) provide ECEF position and velocity, and earth magnetic field measurements which the filter uses to calculate corrections to the trajectory state and estimate inertial sensor errors. This fusion of multiple sensors into an EKF allows for a wide variety of sensor characterizations including bias, scale factor, and unit mounting misalignment. In addition to the inertial sensor characterization, the magnetometer sensed earth magnetic field disturbances from hard-iron and soft-iron ferrous material effects are estimated and accounted for directly in the filter. Under static conditions, the attitude and heading errors are less than 0.1 degrees, and under dynamic flight tests when compared to a high accuracy INS system (Litton LN-100G), the attitude and heading errors are shown to be less than 0.5 degrees. The position and velocity estimates directly follow the GPS accuracy level when the GPS is providing a low GDOP solution. When the GPS accuracy level drops due to satellite occlusion, the combined solution maintains the accuracy level when compared to the INS and smoothes over GPS estimate error nonlinearities. If the GPS drops out all together, the navigation solution remains stable and will stay within an error of 1 meter in 60 seconds.