Determining the R Matrix in EKF for the IMU

we are currently working on developing an AUV in which we utilize the pixhawk for its IMU then using those sensor reading in a SLAM Algorithm.

but an integral part of the EKF Slam or any slam algorithm is to get the R matrix which is the noise in the measurments.

and i cannot find to find any documentation regarding that matter.
the only thing related to sensor noise i found is the resolution of the IMU Sensor BMI055 , but nothing on the other sensor, and of course the pixhawk integrates the 2 imu units, so this changes the data specified in the datasheet.

so does anyone have any experience in that matter?