This paper presents the comparison of the WB-3 ultra-miniaturized inertial measurement unit (EMU), developed by our group, with the commercial EMU IntertiaCube by InterSense and the Vicon motion capture system. WB-3 is provided with a 32-bit microcontroller and 9-axis inertial sensors (miniaturized MEMS accelerometer, gyroscope and magnetometer). We implemented an Extended Kalman Filter (EKF) based on quaternion for the sensor fusion to retrieve the attitude. A new methodology for the automatic estimation of the measurement covariance matrix on the EKF is presented (R-Adaptive Algorithm). The preliminary results showed that WB-3 performance is comparable with Vicon. The R-Adaptive Algorithm improves the attitude reconstruction in case of non-static condition compared with the commercial IMU.