Hi, I am using two nodes: the first is a stage from stage_ros, the second is a controller for the robot that I wrote in python. I started using the nodes separately, launching the first from a launch file:

 <launch>
    <node name="stage" pkg="stage_ros" type="stageros"
    args="$(find stage_ros)/world/willow-erratic.world"/>
</launch>

The second from terminal with "rosrun ...". In this way all works fine. If I try to insert the second node in the launch file modifying the launch file in this way

<launch>
    <node name="stage" pkg="stage_ros" type="stageros"
    args="$(find stage_ros)/world/willow-erratic.world"/>

    <node name="controller2" pkg="simplebot" type="controller2.py"/>
</launch>

and now I have some problems with the tf transformation that was working perfectly before. Here is the error that I get when I launch the new launch file:

tf.Exception: Lookup would require extrapolation into the past.  Requested time 4.900000000 but the earliest data is at time 1444231224.668656111, when looking up transform from frame [target] to frame [base_laser_link]. canTransform returned after 5.00914 timeout was 5.

Here is the code that can have something to do with the transformation:

self.sub = rospy.Subscriber('/base_scan', LaserScan, self.laser_callback, queue_size=1)
....
time = rospy.Time(0)
self.tf_listener.waitForTransform('/base_laser_link','/target', time, rospy.Duration(5.0))
(self.t_pos, self.t_ang) = self.tf_listener.lookupTransform('/base_laser_link','/target', time)
....
def position_callback(self, data):
    ...
    #send the target position to tf
    self.br.sendTransform(
        (self.x_target, self.y_target, 0.0), (0.0, 0.0, 0.0, 1.0),
        rospy.Time.now(), '/target', '/odom')

So the problem is that I don't understand why it was working launching the two nodes separately and why is not working with the two nodes in the same launch file