navsat_transform_node outputs zero pose on /odometry/gps

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Hi all,

I'am trying to get robot_localization to work with imu and GPS (no real odometry) using the navsat_transform_node node with Gazebo.

I have my first ekf node, running with imu0 only as input, to obtain the odom frame. Then I try to fuse GPS information using the navsat_transform_node, but this node only outputs zero odometry on /odometry/gps:

header: 
   seq: 194
   stamp: 
   secs: 40
   nsecs: 200000000
   frame_id: "odom"
   child_frame_id: ''
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: [1.0099999999999998, -8.788794008375919e-25, 0.0, 0.0, 0.0, 0.0, -8.271806125530277e-25, 1.0099999999999998, -1.6940658945086007e-21, 0.0, 0.0, 0.0, 6.776263578034403e-21, 0.0, 1.01, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
twist: 
  twist: 
     linear: 
        x: 0.0
        y: 0.0
        z: 0.0
    angular: 
       x: 0.0
       y: 0.0
       z: 0.0
 covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

Is there something obvious that I am missing?

My launch file for the robot_localization components is the following

<launch>

<rosparam command="load" file="$(find symeter2)/config/ekf_config.yaml" />

<!-- Launch Symeter 2 gui-->
<node pkg="symeter2_gui" type="symeter2_gui" name="symeter2_gui" output="screen" />

<!-- Launch the robot_localization node -->
<node pkg="robot_localization" type="ekf_localization_node" name="ekf_se_odom" clear_params="true">
    <rosparam param="initial_state">[0.0,  0.0,  0.0,
                             0.0,  0.0,  0.0,
                             0.0,  0.0,  0.0,
                             0.0,  0.0,  0.0,
                             0.0,  0.0,  0.0]</rosparam>

</node>

<node pkg="robot_localization" type="ekf_localization_node" name="ekf_se_map" clear_params="true">
    <remap from="odometry/filtered" to="odometry/filtered_map"/>
</node>

<node pkg="robot_localization" type="navsat_transform_node" 
    name="navsat_transform" respawn="true" output="log">

    <rosparam param="datum">[49.9, 8.9, 0.0, map, base_link]</rosparam>

    <remap from="/imu/data" to="/symeter2/imu0" />
    <remap from="/gps/fix" to="/symeter2/fix/" />
    <remap from="/odometry/filtered" to="/odometry/filtered" />


</node>

   </launch>

and the ekf_config.yaml file is the following:

ekf_se_odom:

  frequency: 30
  sensor_timeout: 1
  two_d_mode: false
  transform_time_offset: 0.0
  transform_timeout: 0.0
  print_diagnostics: true
  debug: false
  publish_tf: true
  publish_acceleration: false

  map_frame: map
  odom_frame: odom
  base_link_frame: base_link
  world_frame: odom

  imu0: symeter2/imu0
  imu0_config: [false, false, false, #x, y, z
            true,  true,  true,  #roll, pitch, yaw
            false, false, false, #vx, vy, vz,
            true,  true, true,  #vroll, vpitch, vyaw
            true,  true,  true]  #ax, ay, az
  imu0_nodelay: false
  imu0_differential: false
  imu0_relative: false
  imu0_queue_size: 10
  imu0_remove_gravitational_acceleration: true

  use_control: false

  process_noise_covariance ...