/odometry/gps and /odometry/filtered output is drifting

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Hi, I am using robot localization package along with only GPS and IMU sensor to find out the position,

Below is my launch file

    <launch>
       <node pkg="tf2_ros" type="static_transform_publisher" name="bl_imu" args = "0 0 0 0 0 0 1 base_link imu">
    </node>

   <node pkg="tf2_ros" type="static_transform_publisher" name="bl_gps" args = "0 0 0 0 0 0 1 base_link gps_link">
    </node>

  <node pkg="robot_localization" type="navsat_transform_node" name="navsat_transform_node" respawn="true" output="screen">
    <rosparam command="load" file="$(find robot_localization)/params/navsat_transform_template.yaml" />
      <!-- Placeholders for input remapping. Set your topic names as the "to" values.-->
     <remap from="/imu/data" to="/imu/data" />
    <remap from="/gps/fix" to="/fix" />
    <remap from="/odometry/filtered" to="/odometry/filtered"/>
  </node>

   <node pkg="robot_localization" type="ekf_localization_node" name="ekf_odom" clear_params="true" output="screen">
      <rosparam command="load" file="$(find robot_localization)/params/ekf_template.yaml" />

  </node>
</launch>

my YAML configuration file is as below,

   frequency: 30
    sensor_timeout: 0.1

    two_d_mode: true
    transform_time_offset: 0.0
    transform_timeout: 0.0
    print_diagnostics: true
    debug: false

    debug_out_file: /path/to/debug/file.txt.
    publish_tf: true
    publish_acceleration: false

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

   imu0: imu/data
     imu0_config: [false, false, false,
              false, false, true,
              false, false, false,
              false, false, true,
              false, false, false]

    imu0_nodelay: false
    imu0_differential: false
    imu0_relative: true
    imu0_queue_size: 10
    imu0_pose_rejection_threshold: 0.8                 # Note the difference in parameter names
    imu0_twist_rejection_threshold: 0.8                #
    imu0_linear_acceleration_rejection_threshold: 0.8  
    imu0_remove_gravitational_acceleration: true


    odom0: /odom
   odom0_config: [false, false, false,
               false, false, false,
               true, true, false,
               false, false, true,
               false, false, false]
    odom0_queue_size: 10
    odom0_nodelay: false
    odom0_differential: false
    odom0_relative: true
    odom0_pose_rejection_threshold: 5
    odom0_twist_rejection_threshold: 1

odom1: /odometry/gps
odom1_config: [true, true, false,
               false, false, false,
               false, false, false,
               false, false, false,
               false, false, false]

odom1_differential: false
odom1_relative: true
odom1_queue_size: 10
odom1_pose_rejection_threshold: 2
odom1_twist_rejection_threshold: 0.2
odom1_nodelay: false

    imu0_remove_gravitational_acceleration: true

    use_control: false
    stamped_control: false.
    control_timeout: 0.2

    control_config: [true, false, false, false, false, true]
    acceleration_limits: [1.3, 0.0, 0.0, 0.0, 0.0, 3.4]
    deceleration_limits: [1.3, 0.0, 0.0, 0.0, 0.0, 4.5]
    acceleration_gains: [0.8, 0.0, 0.0, 0.0, 0.0, 0.9]
    deceleration_gains: [1.0, 0.0, 0.0, 0.0, 0.0, 1.0]

    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,    0,    0,    0,    0,
                               0,    0,    0,    0,    0,    0,    0.025, 0,     0,    0,    0,    0,    0,    0,    0,
                               0,    0,    0,    0,    0,    0,    0,     0.025, 0,    0,    0,    0,    0,    0,    0,
                               0,    0,    0 ...