In Simultaneous Localization and Mapping: Part I, the Extended Kalman Filter is described on page 5. I'm confused about where it says "$w_k$ are additive, zero mean uncorrelated Gaussian motion disturbances with covariance $Q_k$".
To state an assumption: I think that $w_k$ is a single motion disturbance at time k, although I'm not positive about this.
My question is, given that covariance is the measure of how much two random variables change together, what are the two random variables for $Q_k$?
Some additional thoughts:
I considered the possibility that this could be the covariance between the wheels, but there could be three wheels, and covariance is only between two variables. (Am I wrong about this?) I also considered that it might be between the x and y coordinates, but what about the angles?