# GPSMAP 66 sr: Assessment of Its GNSS Observations and Centimeter-Accurate Positioning
## 3 Assessment of Observation Quality and Antenna Calibration
### Antenna Phase-Center Calibration  
Phase-center offsets (PCOs) and variations (PCVs) were determined for the GPSMAP 66 sr by field calibration. We used the DRB2 rotational device (Figure 9), which enables observations in four azimuthal orientations per minute but does not perform antenna tiltings [11]. The GPSMAP 66 sr was mounted in a holder in an upright position. We selected an antenna reference point (ARP) to which the PCO refer and defined a north orientation: the device was oriented toward the north when the display pointed south (Figure 10).  
Figure 8: Single-difference G5/E5 residuals of a short, known baseline between geodetic-grade equipment and GPSMAP 66 sr. Epoch-wise differential receiver clock errors were estimated from ambiguity-fixed observations and removed for all the observations. Residuals of those three satellites with temporary quarter- or half-cycle ambiguities are shown in colors. All other GPS/Galileo residuals are shown in gray.  
Figure 7: Distribution of single-epoch double-difference fractional cycle ambiguities of a short, known baseline between geodetic-grade equipment and GPSMAP 66 sr.  
The observations in the various azimuthal orientations enable the determination of PCV for the complete upper hemisphere, and they also help to mitigate carrier-phase multipath effects on the horizontal PCO components. However, the height components may still be affected by local multipath.  
The local reference station equipped with Sepentrio PolaRx5 receiver and JavRingAnt_DM JVDM antenna served as reference. The height difference between the reference antenna and Garmin was determined by precise leveling. The calibration requires ambiguity fixing in the short baseline between reference antenna and antenna to be calibrated and, thus, were limited to the dual-frequency signals of GPS/Galileo. Since the baseline length between the reference antenna and Garmin amounted to just a few meters, all atmospheric influences were eliminated and, therefore, did not degrade the calibration results. The calibration session lasted 16 h.  
Figure 11 lists PCO and depicts PCV results for GPS/Galileo. The major components to be taken into account for precise positioning are the vertical offsets of almost 14 cm above ARP and the eccentric position of the Garmin antenna with around 1 cm offset to the west. They are also shown in Figure 10. All other calibration components are fairly small: the north offset does not exceed 1 mm, and all PCVs stay below 1 cm. For most precise applications, it will be sufficient to apply corrections for the PCO components only. The full calibration results are available in ANTEX format [12] as Supplementary Material to this paper.  
Figure 10: Depiction of the necessary specifications with regard to antenna phase center (APC) calibration: position of the antenna reference point (ARP) and azimuthal orientation of the device toward north. The mean APCs as determined by calibration are also shown.  
Figure 9: GPSMAP 66 sr in its holder on top of the DRB2 rotational device, which enables observations in four azimuthal orientations per minute.