Add gps sensor to robot_pose_ekf but 0 message output

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Hello,

I followed the tutorial on how to add a gps sensor to robot_pose_ekf (http://ros.org/wiki/robot_pose_ekf/Tutorials/AddingGpsSensor), but the filter is not outputing any message. My gps messages are published on the /vo topic. Here's the output when I run wtf.py.

Input:
 * Odometry sensor
   - is NOT active
   - received 0 messages
   - listens to topic 
 * IMU sensor
   - is NOT active
   - received 0 messages
   - listens to topic /imu_data
 * Visual Odometry sensor
   - is active
   - received 124 messages
   - listens to topic /gps_meas
Output:
 * Robot pose ekf filter
   - is active
   - sent 0 messages
   - pulishes on topics /robot_pose_ekf/odom_combined and /tf

I mapped the /vo topic to /gps_meas.

And here's an example of my gps message when I rostopic echo /gps_meas:

header: 
  seq: 10261
  stamp: 
    secs: 1354823750
    nsecs: 691171604
  frame_id: base_footprint
child_frame_id: base_link
pose: 
  pose: 
    position: 
      x: 0.0
      y: 0.0
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: 0.0
      w: 1.0
  covariance: [1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0]
twist: 
  twist: 
    linear: 
      x: 0.0
      y: 0.0
      z: 0.0
    angular: 
      x: 0.0
      y: 0.0
      z: 0.0
  covariance: [1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0]