[ros-users] Q about covariance matrices in nav_msgs/Odometry

Hi,

I am trying to create a nav_msgs/Odometry publisher, to be used by
the robot_pose_ekf node. My robot only give me 3 numbers: x,y and theta
while the nav_msgs/Odometry message needs covariance and many other data.

My question is what variables correspond to the covariance matrices
in nav_msgs/Odometry? how should I go about calculating the matrices from
what I have. Any pointer to more reading materials would be appreciated.

Thanks,

-Andy