Studies of systematic uncertainties of polarization estimation for experiments with WASA detector at COSY

In November 2010, the azimuthally symmetric WASA detector and the polarized proton beam of COSY, have been used to collect a high statistics sample of $\vec{p}p\to pp\eta$ reactions in order to determine the analyzing power as a function of the invariant mass spectra of the two particle subsystems. Here, we show studies of the influence of the beam and target characteristics such as location and direction on the determination of the polarization.