Global Navigation Satellite Systems (GNSS) receivers are an essential sensor for modern global positioning, navigation, and precise timing applications. High precision, i.e., geodetic GNSS positioning applications are based on carrier-phase measurements, where understanding the signal electrical reception characteristics, i.e., receiver antenna phase center corrections (PCCs), is critical. With the main goal of determining the PCC models of GNSS receiver antennas, only a few antenna calibration systems are in operation or under development worldwide. The International GNSS Service (IGS) publishes type-mean PCC models for almost all geodetic-grade GNSS antennas. However, the type-mean models are not perfect and do not fully reflect the signal reception properties of individual GNSS receiver antennas. Published relevant scientific research has shown that the application of individual PCC models significantly improves the accuracy of GNSS positioning. In this paper, the automated GNSS receiver antenna calibration system, recently developed at the Faculty of Geodesy of the University of Zagreb in Croatia, is shortly presented. The developed system is an absolute field calibration system based on the utilization of a Mitsubishi MELFA RV-4FML-Q 6-axis industrial robot. During calibration, the robot performs precise antenna under test (AUT) rotations and tilting. The antenna PCC modelling is based on time-differenced double-difference carrier-phase observations and spherical harmonics (SH) expansion. Our early antenna calibration results, for the Global Positioning System (GPS) L1 frequency, show a sub-millimeter repeatability of the estimated PCC model and a sub-millimeter agreement with the IGS approved Geo++ GmbH values.