Well, a few days ago I installed SITL on my Ubuntu computer with this tutorial http://dev.ardupilot.com/wiki/setting-up-sitl-on-linux, all was OK but when I tried to run a mission, the copter starts to fly and so on, but there is a moment when the console shows:

APM: EKF Variance

and obviously the copter flays bad, it doesn't follow the path, I don't know what the problem is. Any ideas ?

Screenshots:

At the beginning:

When the mission was executed:

Thanks in advance.