Inertial Reference Frame
Earth-Centered Inertial Frame, ECI Frame

IMU gyroscope measures angular rotation rate in radians per second or degrees per second bias and drift

bias when the sensor is stationary, the rate measurement is non-zero. this means that the bias the measurements are off-center.

drift when we calculate the angles of the current orientation, by looking at the rate measurements, the bias in those measurements lead to drift in the current position angles.

gyro bias causes gyro drift

Ways to eliminate bias and drift:

• Extended Kalman Filter

accelerometer measures linear acceleration in 3 axes: x,y,z

four accelerations: linear, gravity, coriolis, centripetal

measured acceleration = linear and gravity - coriolis + centripetal

coriolis and centripetal accleration depends on earth's rotation, velocity, and position and are fine measurements relevant only to high-precision sensors.

by looking at acceleration due to gravity, we can estimate roll and pitch roll, phi, φ pitch, theta, θ

integrate acceleration to velocity integrate velocity to position

accelerometer also has bias which leads to drift in velocity and position

Extended Kalman Filter can be used to eliminate bias

accelerometer also has noise, due to vibration

Low-pass filter can be used to eliminate noise.

Note: do NOT use a low-pass filter on gyroscope readings