[ros-users] non-fully-qualified frame_id

Folks,

I am getting this warning:

[ WARN] [14.657000000]: Message from [/gazebo_1270159292170067578] has a non-fully-qualified frame_id [base_footprint].
Resolved locally to [/base_footprint]. This is will likely not work in multi-robot systems. This message will only
print once.

Which I did not care about before, since I had just one robot in simulation. But now I need two or more and it does not
work indeed.

This is the line of code that sends the transform (from gazebo_ros_diffdrive):

transform_broadcaster_.sendTransform(tf::StampedTransform(base_link_to_odom,ros::Time::now(), "odom", "base_footprint"));

Can anybody explain what this warning means how would I go about fixing it? I couldn't find anything on the wiki that
mentions this.

Thanks,
Anton