Global Navigation Satellite Systems (GNSS) are not only widely used for precise positioning, navigation and timing but also for establishing of terrestrial reference frames for geospatial applications, such as land and water management. The quality of GNSS carrier phase measurements depends on the knowledge about the location of the exact electrical reception point of the GNSS receiver antenna, also known as phase center. Because the location of this receiving point varies with the direction of the incoming satellite signal, phase center corrections (PCC), including a phase center offset (PCO) and phase center variations (PCV), have to be taken into account. These corrections are determined by a calibration of the antennas either in an anechoic chamber using artificially generated signals or in the field by use of a robot and real GNSS signals. The frequency dependent PCC are published in the IGS Antenna Exchange format (ANTEX). In order to take the benefits from the higher quality of the newer frequencies (like GPS L5) and satellite systems (e.g. Galileo or Beidou) so that multi-GNSS measurements can be processed, PCC have to be provided also for these signals. In this contribution, the calibration procedure developed at the Institut für Erdmessung (IfE) is presented. The robot model as well as the data acquisition and analysis is shown. Furthermore, the estimation process of the PCC using spherical harmonics is explained in details. We show, that an absolute GNSS receiver antenna calibration using a robot and real signals can successfully be carried out at the Institut für Erdmessung (IfE). The results underline an overall good repeatability with an RMS for the difference patterns of different calibrations smaller than two millimeters. It is shown that the L5 patterns significantly vary from L2, so that specific calibration values are needed. In addition, the concept of a joint estimation approach of same frequencies (like GPS L1 and Galileo L1) and its difference to the "classical" approach of frequency and system dependent pattern is presented. It can be seen, that differences up to 5.5 mm are present, if the joint estimated PCC are compared to the "classical" EL1X PCC. This underlines the demand of not only frequency but also GNSS specific PCC.