I wrote a patch for the "release-2-1-patches" branch in order to add Kurt Konolige's motion model to model_position.c (which is already included in the "release-2-0-patches" branch).
See also: http://www.nabble.com/Adding-noise-to-stage-position-measurement-td7560550.html
Gaussian motion noise
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fixed/patches in release-2-1-patches