Abstract: This paper deals with an important error source for GPS positioning caused by the phase center offset of each receiver/antenna involved.Furthermore,it is also investigated to find out the calibration method and processing procedures,which provide the basic parameters for the average phase center.If the calibration could be made before putting the receiver/antenna sets into a surveying campaign,and also the calibration results are employed into the field observation and data processing,it is obvious that more accurate positioning results can be obtained,say better than(3mm+k^*d),where d is the distance of the baseline in km.,and k is the scale factor in ppm.depended on the mathematical model and the ephemeris type used in the processing.On the other hand,the problem which involves the use of multiple type of GPS receiver/antenna sets in a common observation session and computational run has been solved.