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I have [..] no model of the robot

and

How would I tell the robot where the ultrasonics sensors are?

A 'model' is actually one way to indicate to a big part of the infrastructure where various bits and pieces on your robot are, as that would cause (fi) the robot_state_publisher to start broadcasting TF frames. The publications representing the ultrasonic measurements would then include the name of their TF frame in the msg header, which other nodes can then use to transform the measurements in whatever local coordinate frame they require.

In ROS, we model robots with URDF.

An alternative to this would be to just publish the TF transforms yourself, without using URDF and the robot_state_publisher. That you could do with the static_transform_publisher and the proper arguments.

I would recommend just modelling your robot with URDF, as it would also allow RViz to visualise all sorts of things. Personally I would not want to work on a ROS robot without at least a URDF.

I have [..] no model of the robot

and

How would I tell the robot where the ultrasonics sensors are?

A 'model' is actually one way to indicate to a big part of the infrastructure where various bits and pieces on your robot are, as that would cause (fi) the robot_state_publisher to start broadcasting TF frames. The publications representing the ultrasonic measurements would then include the name of their TF frame in the msg header, which other nodes can then use to transform the measurements in into whatever local coordinate frame system they require.

In ROS, we model robots with URDF.

An alternative to this would be to just publish the TF transforms yourself, without using URDF and the robot_state_publisher. That you could do with the static_transform_publisher and the proper arguments.

I would recommend just modelling your robot with URDF, as it would also allow RViz to visualise all sorts of things. Personally I would not want to work on a ROS robot without at least a URDF.