Calculate covariance matrix from x,y,z data of /odom

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I've recorded a bag file from a custom robot (in real world) that does not provide covariane matrix and I want to use /odom to feed an ekf, but covariance matrix is 0. How can I calculate it?

It's a sample of /odom:

pose: 
  pose: 
    position: 
      x: 0.082
      y: 0.507
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: -0.789272088731
      w: 0.614043622188
  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]
twist: 
  twist: 
    linear: 
      x: 0.104
      y: 0.0
      z: 0.0
    angular: 
      x: 0.0
      y: 0.0
      z: 0.0663225115758
  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]