Conventional phase center calibration is carried out in anechoic chamber, when in the outfield, the calibration values may be not useful because of the phase errors arose by multipath. In the paper, we present a model of antenna receiving signal; the model is improved by adding antenna model and the characterizations of right hand circular polarization and left hand circular polarization. Based on the model, we analysis the impact of up-to-down ratio on phase errors. As an example, we use a choke antenna to assess the performance of the simulation results. When the phase center variations tolerance is 2 mm, in order to mitigate the impact of phase center variations on high precision GNSS by using calibration values, the up-to-down ratios must be all higher than 15 dB.
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