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Hi, I ran into this same issue while also enabling multi robot simulation for ridgeback.

I had the correct tf prefixes for ekf_localization however I was still seeing the same /base_link error caused by the missing namespace/tf prefix.

I was able to fix this by modifying the velocity controller's base_frame_id and odom_frame_id.

<rosparam param="ridgeback_velocity_controller/base_frame_id" subst_value="True">$(arg tfpre)/base_link</rosparam>
<rosparam param="ridgeback_velocity_controller/odom_frame_id" subst_value="True">$(arg tfpre)/odom</rosparam>

In addition to the nre_simmultihusky repo, there is shehancaldera's more recent multi husky repo. I found both useful for figuring out the tf tree for ridgeback.

The top portion of my tf tree looks like this:

• map
  • r0_tf/odom
    • r0_tf/base_link
      • r0_tf/chassis_link
      • r0_tf/mid_mount
  • r1_tf/odom
    • r1_tf/base_link
      • r1_tf/chassis_link
      • r1_tf/mid_mount

Each translation is handled by the following broadcaster:

+-----------------+--------------------+------------------------------------------------------+
| Parent          | Child              | Broadcaster                                          |
+-----------------+--------------------+------------------------------------------------------+
| map             | r0_tf/odom         | r0/map_odom_broadcaster (static_transform_publisher) |
+-----------------+--------------------+------------------------------------------------------+
| r0_tf/odom      | r0_tf/base_link    | r0/ekf_localization                                  |
+-----------------+--------------------+------------------------------------------------------+
| r0_tf/base_link | r0_tf/chassis_link | r0/robot_state_publisher                             |
+-----------------+--------------------+------------------------------------------------------+