magnetic field sensor in gazebo?

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You can use the gazebo_ros_p3d plugin to retrieve ground truth pose data. Depending on your needs you might want to increase gaussian noise, or write your own plugin based on gazebo_ros_p3d (if you need a more sophisticated error model than gaussian noise).

  <controller:gazebo_ros_p3d name="quadro_groundtruth_sim" plugin="libgazebo_ros_p3d.so">
    <alwaysOn>true</alwaysOn>
    <updateRate>100.0</updateRate>
    <bodyName>quadrotor_base</bodyName>
    <topicName>ground_truth_pose</topicName>
    <gaussianNoise>0.0</gaussianNoise>
    <frameName>map</frameName>
    <interface:position name="my_box_p3d_position_iface" />
  </controller:gazebo_ros_p3d>