How to configure properly robot_localization with IMU, Odometry and GPS data

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Hello everyone! I'm new to ROS, and I'm working with robot_localization package (2D mode) on Melodic. I'm using an IMU, Odometry and GPS data. I've configured robot_localization using two EKF and navsat_transform_node.

• The first EKF has odom as world_frame, and IMU and Odometry as subscribers;
• The second EKF has map as world_frame, and IMU, Odometry and /odometry/gps as subscribers;
• Navsat_transform_node has GPS, IMU and /odometry/filtered_map as subscribers.

I'm using IMU sensor from my smartphone, through the Android app Ros Sensors Driver.

I was wondering if I were setting right all the nodes: /odometry/filtered looks like it's working, but I was noticing that there is no difference in the results if I use only the odometry and the Imu, without the gps data... How is it possible? I've also noticed that the covariance matrix of /odometry/filtered is diverging fairly quickly, and I don't know how to reduce this divergence. Can anyone give me some advice? Here are the launch files.

<launch>

<param name="/use_sim_time" value="true" />
<node pkg="tf2_ros" type="static_transform_publisher" name="bl_imu" args="-0.06 0 0.1376 0.385 0.018 -0.010 base_link imu_link" />
<node pkg="tf2_ros" type="static_transform_publisher" name="bl_gps" args="0.18 0.13 0.1476 0 0 0 base_link gps_link" />
<node name="ekf_localization_node" pkg="robot_localization" type="ekf_localization_node" clear_params="true">

  <param name="frequency" value="30"/>
  <param name="sensor_timeout" value="0.1"/>
  <param name="two_d_mode" value="true"/>

  <param name="map_frame" value="map"/>
  <param name="odom_frame" value="odom"/>
  <param name="base_link_frame" value="base_link"/>
  <param name="world_frame" value="odom"/>

  <param name="transform_time_offset" value="0.0"/>
  <param name="transform_timeout" value="0.0"/>

    <param name="odom0" value="/wolf/odom"/>

    <param name="imu0" value="/wolf/imu"/>

    <rosparam param="odom0_config">[false, false, false,
                                    false, false,  true,
                                     true,  true,  true,
                                    false, false, false,
                                    false, false, false]</rosparam>
    <rosparam param="imu0_config">[false, false, false,
                                    true,  true, true,
                                   false, false, false,
                                    true,  true,  true,
                                    true,  true,  true]</rosparam>
    <param name="odom0_differential" value="false"/>
    <param name="imu0_differential" value="false"/>

    <param name="odom0_relative" value="false"/>
    <param name="imu0_relative" value="true"/>

    <param name="imu0_remove_gravitational_acceleration" value="true"/>

    <param name="print_diagnostics" value="true"/>

    <param name="odom0_queue_size" value="10"/>
    <param name="imu0_queue_size" value="10"/>

  <param name="debug"           value="false"/>
  <param name="debug_out_file"  value="debug_ekf_localization.txt"/>

  <rosparam param="process_noise_covariance">[0.05, 0,    0,    0,    0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0.05, 0,    0,    0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0,    0.06, 0,    0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0,    0,    0.03, 0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0,    0,    0,    0.03, 0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0,    0,    0,    0,    0.06, 0,     0,     0,    0,    0 ...