I am trying to follow '8. PR2 Navigation Stack Demo' in pr2_simulator/Tutorials/BasicPR2Controls.

After launching pr2 with rviz by typing

• roslaunch ~/pr2navtutorial.launch
• roslaunch pr2navigationglobal rvizmovebase.launch

In 'move_base.rviz', I can see a (green) path is planned between the init and goal pos selected by me. But PR2 does not move to follow the path as described in the tutorial. The error messages from the command line are as follows:

[ERROR] [1405347027.289110503, 80.577000000]: Extrapolation Error: Unable to lookup transform, cache is empty, when looking up transform from frame [/odomcombined] to frame [/map] [ERROR] [1405347027.289213486, 80.577000000]: Global Frame: /odomcombined Plan Frame size 126: /map [ERROR] [1405347027.289307676, 80.577000000]: Could not get local plan

Any idea to fix this problem?