I. Introduction
Tracking in the Muon Spectrometer of the ATLAS experiment [1] at the Large Hadron Collider is done with monitored drift tubes (MDTs) [2] designed to provide good space resolution and to ensure robust and reliable operation for many years. The barrel spectrometer consists of three measuring stations immersed in a toroidal field of Tm. Each station provides six or eight measured points with an average accuracy of less than 100 m per point. The momentum resolution of the spectrometer is limited by the MDT measurement accuracy to about 10% at a momentum of 1 TeV/c and by multiple scattering to about 2.5% for muons of momentum up to 250 GeV.