# GPSMAP 66 sr: Assessment of Its GNSS Observations and Centimeter-Accurate Positioning
## 4 Precise Positioning Results
### Dual-Frequency GPS/Galileo Precise Point Positioning
They can be determined from triple-frequency GPS observations from a set of globally distributed GNSS reference stations, as described in [21]. For each of our observation days, we selected 10 globally distributed sites of the IGS (CIBG, DAV1, DGAR, KOUR, MAL2, MKEA, NKLG, NYA2, PNGM, and WTZS) and computed G1/G5--G1/G2 clock corrections, with a temporal resolution of 5 min. We applied them to the CODE GPS Block IIF clock corrections prior to the G1/G5 PPP processing. The first correction mainly affects the contribution of the code observations to a PPP solution. For the carrier phases, any constant biases are absorbed by the real number ambiguities. Thus, in our applications with at least 30 min of static observations, the first correction has only little effect. The second correction is of importance for GPS Block IIF satellites only, which provide less than half of all dual-frequency GPS/Galileo signals used in our PPP positioning. In a first PPP trial, we conducted long-term observations in several sessions of altogether 100 h at a roof-top site without any signal obstructions and with little multipath disturbances. The GPSMAP 66 sr was mounted on the tribrach and holder to guarantee horizontal and vertical eccentricities in the order of a few millimeters (Figure 12). An external power supply was used to enable a session length of longer than 24 h.
* The datasets were divided into sessions of various lengths from 30 min (\(n=200\)) to 16 h (\(n=5\)). The PPP results were compared with ITRF 2014 station coordinates from  
Figure 12: GPSMAP 66 sr mounted on tribrach and holder for long-term observations in roof-top environment. earlier GNSS determination with geodetic-grade equipment. RMS values of the deviation in north/east/height are presented in Figure 13. As usual with PPP solutions without ambiguity fixing, so-called float solutions, the north component is determined best, cf. [22]. East and height components show lower accuracies in the sessions up to 4 h. In the longer sessions, the RMS values of the east component become closer to those of the north component. The height component remains worse. With half an hour of observations, the accuracies are on a few 10 cm levels (RMS). However, after several hours, the horizontal components reach 1 cm levels. The RMS values of the height component remain on the few cm levels. We conclude that, with this kind of Garmin receiver, ITRF coordinates can be determined in PPP mode on the accuracy (RMS) level of 10 cm after about 1 h, 5 cm after a few hours, and a few centimeters after many hours of static observations at a favorable site. A second PPP trial was performed under adverse conditions caused by limited sky visibility. Due to vegetation and a building close by, large portions of the sky were obstructed (Figure 14). Additionally, the satellite visibility gap around the north celestial pole due to satellite orbit inclination angles of nominal \(55^{\circ}\) (GPS) and \(56^{\circ}\) (Galileo) contributes to the reduced number of available GNSS signals. The average number of dual-frequency observations above an elevation mask angle of 10 degrees amounted to 6.5. This is considerably lower than the average number of eight, which was demonstrated for the unobstructed site (cf. Figure 3). In addition, some signals penetrated the foliage, which causes signal disturbances, which, in turn, cause, e.g., an increased number of cycle slips. Figure 14: Limited sky visibility at the adverse observation site due to signal obstructions and the satellite visibility gap around the north celestial pole. Figure 13: PPP coordinate errors (RMS values) as a function of session length for the static, roof-top observations. The antenna setup was very much simplified to just fixing the device to the hand strap of a trekking pole (Figure 15). We expect that the eccentricities of the ARP are stable on the level of several centimeters, with larger uncertainties for the horizontal components, as compared with the vertical component.