# GPSMAP 66 sr: Assessment of Its GNSS Observations and Centimeter-Accurate Positioning
## 4 Precise Positioning Results
### Relative Carrier-Phase Positioning with Respect to VRS  
Shorter observation times for centimeter-accurate positioning can be expected in baseline mode. This requires a local reference station or, in our case, a regional network of consciously operating reference stations. The network we used is operated by the Saxonian State Survey Department (Staatsbetrieb Geobasisinformation und Vermessung Sachsen (GeoSN)), with interstation distances of roughly 50 km. This department offers real-time correction data and also runs a postprocessing service SAPOS-GPPS PrO for RINEX files from rover sites [23]. Nowadays, these services are free of charge.  
The postprocessing service computes virtual reference station (VRS) observations for the approximate user position (usually a few m off the final user position) and afterward runs a baseline processing between VRS and rover observations. More on this technique can be found, e.g., in [24]. The resulting coordinates of this service refer to the geodetic reference system ETRS89.  
SAPOS-GPPS-PrO maintains a database of antenna phase-center corrections, which, of course, did not include the GPSMAP 66 sr antenna. The service operators agreed to help by adding our calibration results (Section 3.3) to this database. Afterward, we could send the slightly modified Garmin RINEX files to the processing service and obtained positioning results. The modification of the RINEX files consisted of two changes to the RINEX header records: we added the antenna name and the vertical antenna height between marker and ARP.  
This relative carrier-phase positioning was tested on five geodetic control points established by GeoSN. We selected control points with minimal or little obstructions and, thus, suitable for precise GNSS positioning. The primary station marks were buried 80 cm down. However, the stations come with a stone monument as surface marks for ordinary use. We mounted the Garmin receiver in its holder on top of a 2 m pole (Figure 17). This setup allows the determination of vertical eccentricities to better a few mm and of the horizontal eccentricities better 1 to 2 cm.  
Figure 16: PPP coordinate errors (RMS values) as a function of session length for the static observations under adverse environmental conditions.  
At each station, we collected observations for more than 1 h. The RINEX files were cut into files of 15 min of observations, four files per station. Each file was sent to the SAPOS-GPPS PrO processing service, and in all cases, we received in return ambiguity-fixed solutions with quality indicator \"high\". We compared these results with the ETRS89 station coordinates from the GeoSN database. These coordinates possess the accuracy levels: horizontal <2 cm and vertical +\(-\)3 cm.  
In a first comparison (Figure 18a), we determined the coordinate errors with respect to the provided station coordinates. All deviations stayed within +\(-\)3 cm. The main causes of these deviations are the limited accuracy of the given ETRS89 station coordinates, eccentricity errors of our setup, especially in the horizontal components, and GNSS positioning errors. With these trial data, we were not able to separate these three contributions from another, but nevertheless, we can conclude that centimeter-accurate positioning can be achieved.  
In order to get more insight into the GNSS positioning errors, we also compared the 15 min session results with the mean ETRS89 positions determined with 1 h of observations (Figure 18b). Since the eccentricities remained unchanged, the 15 min session results are only independent with respect to GNSS. The horizontal deviations now clearly demonstrate sub cm accuracy. The vertical deviations are slightly larger.  
Figure 17: Receiver setup at geodetic control points on top of a 2 m pole.  
We conclude that, in relative carrier-phase positioning mode with session lengths of many minutes, the GPSMAP 66 sr is capable of reliably providing coordinate results on the centimeter level.