Angular velocity has different sign than attitude quaternion about ukf HOT 1 CLOSED

Hi Brian,

Thanks for the kind words – I hope you’re able to find it useful.

There are a set of comments in ‘state.h’ describing the composition of the state vector – the attitude quaternion is defined in such a way as to describe a rotation from the North/East/Down frame to the vehicle body frame.

So, if the vehicle was heading directly West, a magnetic field vector of say [1, 0, 0] in the NED frame would end up as [0, 1, 0] in the body frame – a rotation of 90 degrees clockwise, looking down along the Z-axis – and the corresponding quaternion is [0.707, 0, 0, 0.707].

However, the angular velocity around the Z-axis required to spin the vehicle counter-clockwise from the North direction to the West direction is negative, which is the opposite sign to the imaginary part of the quaternion which describes that orientation.

I must confess I got myself confused about coordinate systems a number of times developing the filter, and in hindsight this might not have been the most intuitive choice of attitude representation, so let me know if this explanation makes sense!

Cheers,
Daniel

from ukf.