Getting linear acceleration from mavlink_ros (APM2.6)

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Hello,

I am working on a team that hopes to implement a SLAM algorithm on a quadrotor using an xv-11 LIDAR and IMU data from an APM2.6. So far, I have the scan data coming in over a serial to bluetooth adapter, and I have mavlink_ros running to pull in the IMU data from ROS at 25Hz. I had to change some parameters on the arudpilot using QGroundControl to get this 25Hz rate. I was hoping to combine the scans with the IMU and use laser_scan_matcher to provide odometry for gmapping. However, when I run "rostopic echo fcu/imu" (the imu data coming in from the ardupilot), I get the following:

 header: 
  seq: 4206
  stamp: 
    secs: 1395075512
    nsecs: 568997836
  frame_id: fcu
orientation: 
  x: 0.0453795122836
  y: -0.0137595356653
  z: 0.00616009589894
  w: 0.998856059831
orientation_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
angular_velocity: 
  x: -0.00247164955363
  y: -0.000945815816522
  z: 0.000395094510168
angular_velocity_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
linear_acceleration: 
  x: 0.0
  y: -0.0
  z: -0.0
linear_acceleration_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

I have a couple questions: Are the covariance fields needed? Also, I assume laser_scan_matcher will be needing the linear_acceleration fields. Does anyone know how to get the linear accelerations from the APM? Is there a parameter on the APM that I need to change to get this?

Thank you for any help in this regard.