# GPSMAP 66 sr: Assessment of Its GNSS Observations and Centimeter-Accurate Positioning
## 3 Assessment of Observation Quality and Antenna Calibration
### Integer Property of Estimated Ambiguities  
Double-difference carrier-phase ambiguities must possess integer property to make the very high accuracy of the carrier-phase observations available to positioning without any degradation and, thus, to centimeter-accurate positioning results. In contrast to geodetic-grade receivers, which always produce carrier-phase observations with the property of integer ambiguities, other equipment, e.g., smartphones, can fail in this respect [2].  
Double-difference (DD) carrier-phase observations involve simultaneous GNSS observations of two receivers. In order to test the GPSMAP 66 sr with respect to the integer property of its carrier-phase ambiguities, we processed a 24 h dataset of a short, known baseline (7 m) between a geodetic-grade Septentrio PolaRx5 and a GPSMAP 66 sr. The data were collected in a roof-top environment with little multipath disturbances. Antenna phase-center corrections were applied (Section 3.3), and the observations were processed in baseline mode.  
Further analysis was conducted based on DD residuals of the carrier-phase observations. Due to the very short length of the baseline, they are free of atmospheric effects and not affected by orbit errors. Thus, the information content of the DD residuals comprises carrier-phase multipath, noise, and eventually, differential instrumental delays, which may prevent integer ambiguities.  
The property of integer ambiguities can be detected best by a statistical analysis of fractional cycle DD residuals. In the case of integer property, their distribution shows a clear maximum at 0.0 cycles. Distributions without a clear maximum indicate that no integer ambiguity fixing can be performed with this signal. In the case of the GPSMAP 66 sr, the GPS and Galileo carrier-phase observations clearly prove their integer ambiguity property, whereas the GLONASS R1 signal does not, which is probably caused by the FDMA technique used for GLONASS signals (Figure 7). Similar results had been found for selected smartphones in recent years [5], but compared with the smartphones, the GPSMAP 66 sr produces smaller deviations of the GPS/Galileo residuals from zero, which is suspected to be caused by its higher-quality antenna.  
Figure 6: Carrier-phase noise and multipath of the GPSMAP 66 sr as determined from the observation residuals of a short baseline of two such receivers (RMS values).  
When processing several long sessions of GPSMAP 66 sr carrier-phase observations, we found that some G5/E5 ambiguities were not integers but seem to be a quarter- or half-cycle off the full integer (Figure 8). In the RINEX observation files, these observations had not been flagged accordingly.  
When processing a static baseline, this results in an ambiguity fixing of all carrier-phase observations except those with quarter- or half-cycle offsets. As long as only very few observations are affected by this problem, static positioning is not degraded. However, in precise kinematic baseline processing, such errors cause incorrect ambiguity fixing and, consequently, incorrect positioning results. Due to these incorrect carrier-phase observations, we were not able to produce correct and reliable centimeter-accurate kinematic results.