# Preliminary Results of an Astri/UWM EGNSS Receiver Antenna Calibration Facility
## 2 Antenna Calibration Techniques  
Methodologies for calibrating GNSS antennas have been developed since the 1980s. So far, different antenna calibration techniques have been established. Generally, they can be divided into absolute and relative or field and laboratory procedures [22]. Relative PCCs are created when a calibrated GNSS antenna phase center position is measured relative to a reference antenna (typically Dorne Margolin T, assuming that the reference antennas PCVs are zero). Absolute methods include scenarios where PCVs are determined independently of the reference antenna during calibration. A field procedure is based on real satellite signal tracking [23; 24]. A laboratory procedure is carried out in an anechoic chamber and uses simulated GNSS signals [3]. Besides classification based on the calibration method, PCC models are categorized into individual and mean modes. An individual antenna PCC model is created when a specific antenna/ radome is calibrated. In mean-type calibration, several antennas/radomes of the same type are calibrated and the final PCC model is created through the combinations of all results into single individual file. The absolute field calibration procedure developed in Hannover by IfE and Geo++ [25] uses a robot that tilts and rotates the antenna. In this methodology, homogeneous coverage of observation is ensured, even for negative elevations, and real satellite signals are tracked during the calibration. The majority of errors and biases are mathematically canceled by using a short base (<10 m) during calibration and observing the creation of time differences of single difference (TDSD). Nevertheless, GNSS antenna calibration is still a challenging issue. Depending on the method, the GNSS antenna PCCs may differ in the order of several millimeters [1; 23; 24; 2]. In [23], the results of absolute calibration provided by three calibration centers (two in-field and one in anechoic chamber) were compared (Geo++, NGS, and University of Bonn). The results proved that the differences in PCCs can reach up to 5 mm in the case of low elevation corrections (up to 20\({}^{\circ}\)). Consequently, the University of Warmia and Mazury in Olsztyn, in cooperation with Astri Polska, started a project aimed at the development and implementation of a receiver antenna field calibration facility for a multi-frequency and multi-system GNSS. The applied approach is based on the \"Hannover\" idea [24; 25; 26], where the PCO and PCV results of an absolute test antenna are determined by the analysis of the observable TDSD (one satellite only). In comparison to the \"Hannover\" concept, some innovations are introduced with our approach. The first innovation is a different antenna rotation point definition. In the \"Hannover\" concept, during GNSS antenna calibration, the antenna is rotated around a fixed point, i.e., a nominal phase center, which is usually a GPS L1 frequency MPC. As MPCs are different for different antennas and frequencies, we decided to define our antenna rotation point as fixed for all calibrations, regardless of the antenna type or signal frequency. As a result, we have the same stable fixed rotation point in each calibration, but the antennas are not rotating around their nominal phase center. Appropriate corrections should be applied in order to achieve the effect of rotating around a nominal phase center. These corrections, which are the function of difference between the nominal phase center for a specific frequency and the rotation point position, as well as the SV position, are calculated during a post-processing stage. One of the crucial issues in a PCV determination procedure is robot and GNSS time synchronization. The tilt and rotation angles must be recorded with a GNSS timestamp. To achieve this synchronization, a National Marine Electronics Association local time zone (NMEA ZDA) message containing a coordinated universal time (UTC) timestamp transmitted by one of the receivers was proposed. This message appears about 20 ms after a full GPS second.