# Experimental Validation of a GNSS Receiver Antenna Absolute Field Calibration System
## 1 Introduction
Given the fact that, in partnership with the University of Hannover, Institute of Geodesy (Institut fur Erdmessung--IfE), it developed the first robotic GNSS receiver antenna calibration system in 2000 and has contributed to the IGS ANTEX since then, the German company Geo++ GmbH is globally regarded as a leading authority in the field [21]. Furthermore, other robotic systems have been developed or are under further development at IfE, Germany [22; 23; 24], Geoscience Australia in Canberra, Australia [25], National Geodetic Survey (NGS) in Virginia, USA [26], GNSS Research Center of the Wuhan University in China [10; 27; 28], the Institute of Geodesy and Photogrammetry at ETH Zurich in Switzerland [18; 19; 29], the Institute of Geodesy and Civil Engineering of the University of Warmia and Mazury in Olsztyn, Poland [20], and Topcon Positioning System in Concordia, Italy [30]. Recent publications and results of the IGS \"RingCalVal\" global campaign, which includes almost all the mentioned institutions, have shown that current contributors to the official IGS ANTEX file have more variability in terms of calibration results than previously appreciated [24]. Moreover, considering constant developments and modernizations of GNSS [31], the main current issues [17], the importance of this open research area to the international scientific antenna community, and the high level of interest in it, the development of a new robotic calibration system is highly desirable. Additionally, a significant scientific and practical value of a new calibration system to the Republic of Croatia lies in the continued development of the Croatian Positioning System (CROPOS) [32]. Therefore, due to such a strong impetus, in Croatia, at the Laboratory for Measurements and Measuring Technique (LMMT), University of Zagreb--Faculty of Geodesy, in 2023, the development of a new GNSS receiver antenna absolute field calibration system began [17; 33; 34; 35]. In [PERSON] et al. [17], the authors initially presented the newly developed absolute field calibration system at LMMT in Croatia, along with the underlying calibration methodology and PCC estimation model. They also published preliminary antenna calibration results supported only by a simple repeatability analysis and validation through comparison with independent calibration results obtained by Geo++ GmbH in Germany. Analyses have been conducted on the PCC pattern level and only for the GPS L1 frequency (frequency code G01 consistent with ANTEX). In summary, the obtained results have shown themselves to be extremely promising and demonstrated the potential and feasibility of the developed absolute field calibration system. However, a comprehensive validation and investigation of the influence of LMMT calibration results on GNSS positioning has not been conducted. Therefore, this study builds upon previous research efforts and advances the developed calibration system to dual-frequency GPS calibrations. The primary goals of this research are to conduct a more comprehensive and in-depth evaluation of the accuracy of dual-frequency GPS calibration results on the pattern level with respect to absolute calibrations obtained by Geo++ GmbH and to extensively validate LMMT dual-frequency GPS calibration results in the spatial (coordinate) domain, i.e., to investigate how the application of LMMT PPC models reflects on geodetic-grade GNSS positioning. The structure of this paper is as follows. In Section 2 a concise theoretical overview of the standard PCC model and description of the LMMT antenna calibration system are provided. Furthermore, applied methodological strategies are elaborated and conducted experimental validation tests are explained. Section 3 details the obtained experimental research results, providing a thorough analysis and discussion. Lastly, Section 4 assesses the research objectives, synthesizes the key insights, and provides the concluding remarks.