Find the measurement covariance matrix, $R$, for the optical flow sensor.
$ \dot{x} = f(x, u)$
$ y = h(x) $
$ \dot{\hat{x}} = f(\hat{x}, u)$
$ \hat{y} = h(\hat{x}) $
$ r = y - \hat{y} $
$ S = E[rr^T] = HPH^T + R $
$S = cov(r)$ - can compute using a finite sample equation
$R = S - HPH^T$