  In the upcoming LHC Run 3, starting in 2021, the upgraded Time Projection
Chamber (TPC) of the ALICE experiment will record minimum bias Pb-Pb collisions
in a continuous readout mode at an interaction rate up to 50 kHz. This
corresponds to typically 4-5 overlapping collisions during the electron drift
time in the detector. Despite careful tuning of the new quadruple GEM-based
readout chambers, which fulfill the design requirement of an ion backflow below
1%, these conditions will lead to space-charge distortions of several
centimeters that fluctuate in time. They will be corrected via a calibration
procedure that uses the information of the Inner Tracking System (ITS), which
is located inside, and the Transition Radiation Detector (TRD) and
Time-Of-Flight system (TOF), located around the TPC, respectively. By using
such a procedure the intrinsic track resolution of the TPC of a few hundred
micrometers can be restored.
  The required online tracking algorithm for the TRD, which is based on a
Kalman filter, is presented. The procedure matches extrapolated ITS-TPC tracks
to TRD space-points utilizing GPUs. Subsequently these global tracks are
refitted neglecting the TPC information. The residuals of the TPC clusters to
the interpolation of the refitted tracks are used to create a map of
space-charge distortions. Regular updates of the map compensate for changes in
the TPC conditions. The map is applied in the final reconstruction of the data.
First performance results of the tracking algorithm will be shown.
