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Every update step you could find the transform of the target frame in your robot vehicle frame, and then extract the x linear distance to the target, and the yaw rotation (see http://answers.ros.org/question/246991/get-rotation-between-two-frames/ for a recent example), and give your differential drive a linear x command proportional to the x distance and an angular z command proportional to the yaw. Ideally your differential drive accepts a cmd_vel Twist message and that linear x and angular z go straight into it.