Bruce Schumm has written a program to calculate covariance matrices with which to smear tracks. The covariance matrix is built up from a description of the detector which includes the geometry, the material, and the expected measurement uncertainties of the tracking detectors.
1.) get the code
> wget http://www.slac.stanford.edu/~schumm/lcdtrk20011204.tar.gzImage Removed
> tar -zxvf lcdtrk20011204.tar.gz