At low rigidities (<10 GV), we have implemented a new track fitting algorithm based on the Kalman filtering technique. It more accurately accounts for energy losses and multiple scattering by charged particles, see C. Höppner, S.  Neubert, B. Ketzer, and S. Paul, Nuclear Inst. and Methods in Phys. Res., Sect.

The nine tracker layers independently measure the charge $|Z|$ of cosmic rays. The ionization energy losses deposited in the silicon, see Figure 1, are proportional to $Z^{2}$ and this is measured with both the x- and y- side strips.

Test beam data are crucial in understanding the detector performance details. Figure 1 shows the comparison between data and the Monte Carlo simulation of the inverse rigidity measured by the tracker for 400 GeV/c test beam protons.

The y coordinate provides better accuracy by design, in which the readout strips have much smaller pitch compared to the strip pitch in the x coordinate. We present the improvement in the accuracy of determination of y-coordinate, which is the most important for the determination of momentum (or rigidity).

In AMS, for all Z, the largest systematic error in the determination of the fluxes at the highest energies is due to the uncertainty of the absolute rigidity scale.  The AMS tracker was aligned using the CERN SPS test beam.

We have improved the track finding algorithm. The new algorithm uses cellular automation for finding the track segments and then builds the tracks, as illustrated in Figure 1. This improves track finding efficiency and rejection of spurious hits in the detector.