Alignment of the Atlas Inner Detector tracking system

The ATLAS experiment is equipped with a charged particle tracking system built on three subdetectors, which provide high precision measurements made from a fine detector granularity. The pixel and microstrip subdetectors, which use the silicon technology, are complemented with the transition radiation tracker. The alignment of the ATLAS Inner Detector tracking system requires the determination of its almost 36000 degrees of freedom. From the tracking point of view, the alignment parameters should be known with few microns accuracy. This permits to attain an optimal measurement of the parameters of the charged particles trajectories, thus enabling ATLAS to achieve its ambitious physics goals. The implementation of the alignment software, its framework and the data flow will be discussed, including the selection of an alignment and calibration stream at the ATLAS Event Filter stage. The results obtained on the recent computing challenges, where large scale simulation samples have been used in order to mimic the off-line ATLAS operation, will be presented. Special attention will be payed on the alignment results obtained from the real cosmic ray data taking. Using those samples one has derived the first ever alignment constants set for the commissioned ATLAS Inner Detector. These constants are ready to be used for the reconstruction of the first LHC collisions.