# Experimental Validation of a GNSS Receiver Antenna Absolute Field Calibration System
## 4 Conclusions  
This research builds upon previous studies and further substantiates that the newly developed GNSS receiver antenna robotic calibration system, designed and implemented de novo at the Laboratory for Measurements and Measuring Technique, University of Zagreb--Faculty of Geodesy, Croatia, delivers meaningful and reliable dual-frequency GPS calibration results. With the goal of extensively evaluating the antenna calibration system at LMMT, three independent validation tests were conducted. To acquire data for validation and analysis purposes, four calibration campaigns were carried out, within which six distinct GNSS receiver antennas were individually calibrated using the LMMT robotic calibration system. An independent accuracy analysis at the PCC pattern level was performed by comparing LMMT calibration results with Geo++ GmbH results. In line with that, Geo++ GmbH and LMMT individual dual-frequency PCC models for the TRM57971.00 NONE antenna (S/N:30739001) were compared and PCC differences calculated. An estimated overall agreement between the dual-frequency GPS (L1 and L2) antenna PCC patterns determined by Geo++ GmbH and LMMT was achieved, with an RMS deviation of up to 0.45 mm. These experimental results are in line with previous research and further prove the compatibility of LMMT and Geo++ GmbH calibration facilities at the dual-frequency GPS level. Furthermore, a short-baseline validation test was conducted to evaluate how the application of different PCC models transfers to the spatial domain and thus reflects on geodetic-grade positioning in relative positioning mode, and to determine if individual antenna calibration at the LMMT calibration facility results in higher baseline accuracy when compared to type-mean antenna PCC models provided by IGS. Research results showed that a significant difference in short-baseline accuracies, with respect to the antenna phase center models applied (LMMT individual and IGS type-mean), was achieved. The developed GNSS receiver antenna robotic calibration system at LMMT results in approximately 2.5 times higher baseline accuracy compared to IGS type-mean models. Moreover, in the last validation test, the impact of the receiver antenna PCC model (LMMT individual and IGS type-mean) on GNSS/GPS network solution was evaluated. Because of the different PCC models applied, differences in station positions reach 4 mm in terms of total spatial (3D) offset. The developed GNSS receiver antenna robotic calibration system at LMMT resulted in approximately 23% higher network realization accuracy with respect to the use of IGS type-mean models. The obtained results are in line with the established scientific consensus and confirm the practical importance and significance of GNSS receiver antenna individual absolute calibration in the context of high-accuracy GNSS positioning. Individual calibration is particularly important to users dealing with the realization of national, regional, and global reference networks, geodynamics, monitoring, engineering projects, etc. Multi-GNSS is undoubtedly the standard in contemporary practical and scientific applications requiring high-accuracy satellite positioning. Moreover, the need for high-precision multi-GNSS receiver antenna phase center models will increase in future. The advancement of the precise point positioning (PPP) technique is expected to achieve a level of accuracy where the accuracy of type-mean antenna models will become a limiting factor. Therefore, future research at LMMT will primarily focus on further calibration system developments in terms of providing multi-GNSS and multi-frequency calibration results to the antenna community. Also, the system will be developed following the activities of the IGS Antenna Working Group (IGS AWG) and the development of the new ANTEX 2.0 file format. Conceptualization, A.T. and M.Z. ; methodology, A.T. and M.Z. ; software, A.T.; validation, A.T. and M.Z. ; formal analysis, A.T. and M.Z. ; investigation, A.T., M.Z., K.S. and K.S. ; resources, A.T., M.Z., K.S. and K.S.