no laser scan received after starting amcl [closed]

edit

I've built a map using gmapping and I would like to localize my pr2 in that map. I do the following:

roscore
roslaunch pr2_gazebo world1.launch 
rosrun map_server map_server map.yaml

Then to run amcl I do:

rosrun amcl amcl scan:=base_scan _odom_frame:=odom_combined use_map_topic:=true

After that I get:

No laser scan received
Verify that data is being published on the /base_scan topic.

When I visualize laser data in rviz using fixed frame: base_laser_link and target frame: odom_combined, I am able to see laser data on the base_scan topic.

A subset of my tf tree looks like this:

/odom_combined
      |
/base_footprint
      |
/base_link
    / 
/base_laser_link

I don't understand, why is it saying that no laser scan is received?

Is it because there is no map frame attached to odom_combined? But I thought amcl is supposed to publish the transform: map -> odom

Here is output from rxconsole:

Node: /amcl
Time: 83.407000000
Severity: Warn
Location: /tmp/buildd/ros-fuerte-navigation-1.8.3/debian/ros-fuerte-navigation/opt/ros/fuerte/stacks/navigation/amcl/src/amcl_node.cpp:checkLaserReceived:540
Published Topics: /rosout, /tf, /amcl_pose, /particlecloud, /amcl/parameter_descriptions, /amcl/parameter_updates

No laser scan received (and thus no pose updates have been published) for 83.407000 seconds.  Verify that data is being published on the /base_scan topic.