# Preliminary Results of an Astri/UWM EGNSS Receiver Antenna Calibration Facility
## 5 Data Collection  
To obtain a precise and reliable PCC model, it is critical to distribute the observations over the entire antenna's hemisphere in a dense and uniform manner. Some additional  
Figure 4: The idea of TDSD for GNSS receiver antenna calibration purposes.  
observations are also recommended for negative elevation angles. Meeting these requirements would not be possible without a high-precision robot that can flexibly rotate and tilt the antenna during the calibration process. In the Astri/UWM calibration procedure, it takes less than 2 h to collect the required number of observations, and, in consequence, to derive a PCC model from one calibration session. To verify the obtained results and determine the model's internal accuracy, it is recommended to carry out at least three calibration sessions, which altogether takes about 5 h (Figure 5).  
Figure 5 presents coverage of observations after 1 h and 40 min of observation (1 calibration) and after 5 h of observations (3 calibrations). The red dots indicate observations in a static mode (without antenna tilting and rotating). It is thereby clear that even long static measurements cannot provide appropriate coverage of the observations. Only the use of the robot together with the proper rotation scenario ensures sufficient number of observations for calibration purposes (blue dots in Figure 5).  
In this paper, the results of the initial calibration experiments carried out to verify the applied algorithms and the implemented calibration procedure are presented. Two models of GNSS antennas were chosen to validate the obtained PCCs models, namely, LEIAR10/NONE and TRM115000.00/NONE (Figure 6). Several institutes have previously calibrated these antennas and the results have been published in the jigs05.atx, jigs08.atx and jigs14.atx files of the IGS ([[https://ftp.epncb.oma.be/ftp/station/general/igs14.atx](https://ftp.epncb.oma.be/ftp/station/general/igs14.atx)]([https://ftp.epncb.oma.be/ftp/station/general/igs14.atx](https://ftp.epncb.oma.be/ftp/station/general/igs14.atx)); access date: 5 July 2021).  
Two experiments were conducted using the two aforementioned antenna types in order to test the repeatability of calibration results and determine the external accuracy of the generated models in comparison to the results available in the jigs14.atx file. Both antennas were calibrated in three sessions using 1-s measurement intervals. The data collection for each experiment was completed after reaching full observational coverage for the test antenna in three calibration sessions (about five hours). Detailed information concerning the calibration measurements can be found in Table 1.  
Figure 5: Observations coverage on test antenna hemisphere in case of static antenna (red) and robot rotated and tilted one (blue): (**a**) after 1 h 40 min (1 calibration); (**b**) after 5 h (3 calibrations).  
As seen in Table 1, during both calibrations, we used the same type of receiver (Topcon Net-G3A) on both stations (reference robot), as recommended by the currently operating calibration facilities. Moreover, we used the same types and lengths of the wires for connecting receivers with the external clock and antennas to minimize the hardware delay effect.