Why has not got every axis its own covariance? For example i need tune covariance term of accelerometer. I calculated the SNR of each axis. X-axis accelerometer has got very noise and Y and Z axis is clear. In this case i have to tune it using SNR of X-axis accelerometer. But it will affect all axis. Am i understant correctly? If i am true, i think something is wrong. Can you explain why has not got every axis its own covariance?