## 1 Introduction  
The antenna is a crucial part of the Global Navigation Satellite System (GNSS) ground receiver setup. It is responsible for receiving the signals transmitted by GNSS satellites, transforming the electromagnetic energy into electric currents, and forwarding it to the GNSS receiver. In an ideal scenario, the phase pattern of an antenna is constant, which introduces the same phase delay or advance in all directions. In reality, this phase pattern is irregular and depends on the antenna characteristics, environment, and receiving signal frequency [1]. The estimated satellite range then refers to an imaginary phase center. This imaginary phase center is actually not a single point, or a point at all, as it depends on the frequency of the signal and line of sight to the satellite. To relate this phase center to a physical reference point on the antenna, the Antenna Reference Point (ARP), two steps are used. The first part is a correction in the line of sight to an intermediate point, called the Antenna Phase Center (APC). This is the so-called Phase Center Variation (PCV). The PCV is the projection onto the line of sight of the difference between the actual phase center and the APC. It is therefore a correction based on the elevation and azimuth angle between the actual phase center and the satellite. Representing this correction, the PCV is usually given as a table in elevation and azimuth angle, with different tables for each frequency. The second part, called the Phase Center Offset (PCO), is the offset between the ARP and APC. This is illustrated in Figure 1. The ARP is typically defined as the intersection of the vertical symmetry axis with the bottom of the antenna [2], which is a clearly identifiable physical point on the antenna. The definition of the APC is more open. It is common practice to use a different APC for each frequency, though it is also possible to use a single APC for all frequencies. It is also common practice to choose the APC such that the PCVs become small numbers (as far as this is possible). The PCV and PCO are related: change one, and you should change the other. A good operational practice is to define the APC as the point for which the mean of the PCVs is zero. Since the PCVs are typically different for each frequency, setting the mean of the PCVs to zero would lead to a different APC for each frequency. This can be considered as a Mean Phase Center. The advantage of this definition is that users may choose to ignore the PCV correction, and only use the larger PCO correction. The magnitude of the PCV typically ranges from several mm to a few cm [3]. The PCO can be several centimeters in the vertical direction, and a few mm in the horizontal direction. The problem with the mean phase center definition of the APC is that, apart from the antenna characteristics and environmental factors (multipath), also the cutoff angle and processing strategy (weighting of observations) influence the APC definition [4]. In this paper, we do not always follow the mean phase center definition for the APC. We see the APC more as an intermediate point, that often has no practical value at all. For GNSS meteorology, the absolute APC, and thus the absolute PCO, has no practical value, only the differential (between frequency) PCO matters. Unlike positioning, for GNSS meteorology, it is more important to apply the PCV than the absolute PCO corrections. The same is true for deformation monitoring as long as we do not change instruments. The standard format for PCVs and PCOs is the IGS Antenna Exchange Format (ANTEX). To specify antenna characteristics, individual antennas need to be calibrated. One distinguishes between relative calibrations obtained from a base-rover setup and absolute calibrations obtained in the field with a robotic arm [5; 6; 7] or in an anechoic chamber [8] using artificial GNSS signals.