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I think you already answered the question yourself. It's because you are running a robot_localization node. That node takes odom and imu topics, in your case probably /husky_velocity_controller/odom and /imu/data and computes a transform from the odom frame to the base_link frame, which it then publishes on the TF tree. The best way to see this is to inspect the TF tree (either using view_frames or on the terminal using ros-blessed). You will see who (which node) is publishing the odom->base_link transform, and it will be ekf_localization_node if what I'm saying is correct.