Error in EKF Measurement Model Docs

[Maximilian Nitsch]

Hi,

I think I maybe found multiple (sign) errors in the EKF measurent model:
https://ahrs.readthedocs.io/en/latest/filters/ekf.html

In for example in the first row of h(qhat_t), the sign before gy * (qw * qz+qx * qy) has to be gy * (qx * qy-qw * qz).
I have calculated this both manually and with the Symbolic Toolbox of MATLAB and come to the same result.

You can check my calculation by defining your gravity vector g = [gx gy gz]^T and mutliply from left with rotation matrix C(qhat).

Is this a calculation error or have I overlooked something?

Best regards
Max

[Mario Garcia]

Hi Max,

Thanks for reviewing the documentation. The line in the documentation is, on my opinion, still correct, as it is the product of the transposed rotation matrix C. That is why the following element has also opposed signs (gz*(qx*qz−qw*qy))

In the package it can also be confirmed at line 1198 of the script ekf.py. There we are using the transposed rotation matrix, as it states in the documentation.

In the end, one could not even see the effects of this product, because after multiplying the transposed rotation matrix C with the gravitational acceleration vector g=[gx, gy, gz]=[0, 0, 1], it yields only elements multiplying gz, Even changing between NED or ENU configurations (they just flip the sign of the resulting vector)

Do you think my conclusion is correct? If not, please let me know so that I can fix any further problem. Thanks!

[Mario Garcia]

Maybe I should mention in the documentation that we are transposing the rotation matrix because we are comparing the vectors in the sensor frame (as opposite to the global frame) and, thus, an opposite rotation is needed.

[Mario Garcia]

I think the issue at hand was cleared and, without further feedback, I proceed to close it. As mentioned, a clarification will be added to the documentation of the EKF.