Thanks Wim,

A bit more research shows that something like below would work since my
robot only move along xy plane and rotate around z axis. But I am still
totally lost on what cov_x, cov_y and rcov_z really mean and how to compute
them. Any suggestion / pointer will be appreciated.

msg.pose.covariance = {cov_x, 0, 0, 0, 0, 0,
                        0, cov_y, 0, 0, 0, 0,
                        0, 0, 99999, 0, 0, 0,
                        0, 0, 0, 99999, 0, 0,
                        0, 0, 0, 0, 99999, 0,
                        0, 0, 0, 0, 0, rcov_z}

Best,

-Andy