The paper presents an analysis of the main factors limiting the accuracy of positioning using GNSS-measurements (especially vertical component), methods of creating models corrective phase variations centers GPS-antenna. Assessing the impact of the relative and absolute calibrations, types of domes and the elevation masks of processing GNSS-measurements performed. The study included 10 treatment vector obtained by a two-day measurements at 13 points of a European network of permanent stations EUREF. Vectors have different lengths, elevation, type of equipment used and the geometry of the satellite constellation. Dependence of changes of azimuth and altitude variations of the components of the phase center of the antenna defined (PCV - Phase Center Variations). The paper shows that to achieve millimeter accuracy prerequisite is the use of absolute calibration of phase centers of the antennas, specify the exact type of antenna, such as the dome and the mask in elevation.