# GPSMAP 66 sr: Assessment of Its GNSS Observations and Centimeter-Accurate Positioning
## 3 Assessment of Observation Quality and Antenna Calibration
### Observation Quality  
GNSS observations of the Garmin GPSMAP 66 sr (software v3.30) consist of code, carrier phase, Doppler, and C/N0 on one (GLONASS) or two frequencies (GPS, Galileo, QZSS). However, the datasets are not complete, but the tracking channels seem to be assigned to certain kinds of satellites and frequencies with different priorities. Based on long-term static observations at a roof-top site without any signal obstructions, we determined the completeness of the observations as percentages of the available GNSS signals (Figure 2). GNSS satellites that were set to an unhealthy status were excluded. QZSS satellites were also discarded since the sample collected at the central European site was too small to obtain meaningful results. The highest percentage of complete observation datasets was obtained for the first frequency of GPS. Above an elevation mask angle of 30 degrees, the observations are virtually complete. However, around 25% of the satellites elevated between 10 and 20 degrees were not observed. The missing number of observations of the second GPS frequency G5 reaches around 25% for almost all elevation angles. Here, it has to be taken into account that in 2021 only about half of the satellites in the present GPS constellation are able to transmit G5 signals. Thus, the overall number of dual-frequency GPS observations amounts to less than 50% of the available G1 observations (Figure 3). Figure 2: Completeness of GPSMAP 66 sr GNSS observations as function of elevation angle and GNSS signal. The number of Galileo observations on both frequencies is identical (Figures 2 and 3), but their completeness is very much elevation-dependent. On average, only about 60% of the available signals were observed. The maximum number of simultaneously observed Galileo signals never exceeded seven satellites, even if more satellites were far above the horizon. The settings of the GNSS chip seem to restrict the number of Galileo observations, although they could contribute very much to high-accurate dual-frequency observations. GLONASS single-frequency observations are also incomplete, with only about 50% of the available signals being observed. Due to frequent interruptions of the signals' tracking, which results in observation gaps and cycle slips, GLONASS observations could only be considered in some of the quality further assessment steps and were not taken into account in Figure 3.  
Figure 3 compares the average number of available signals in space with the number of actually observed signals. Presently (mid-2021), around 15 GPS/Galileo signals are available on G1/E1 and 11 signals on G5/E5. The GPSMAP 66 sr (v3.30), however, observes on average only 12 satellites on the first frequency and 8 satellites on the second. Due to the continuously changing satellite distribution, the number of dual-frequency observations can occasionally drop below five, even at sites without any signal obstructions. Under less favorable conditions, the number of observed satellites often falls short of the minimum number of required satellites. This occurs more often for G5/E5 or dual-frequency observations, compared with G1/E1 single-frequency observations. The recorded signal strength values C/N\({}_{0}\) show the typical elevation dependence known from the patch and other helix antennas (Figure 4). Maximum signal levels are reached for elevation angles larger than 40 deg. Below elevation angles of 20 deg, the signal strength decreases rapidly with decreasing elevation angle. On average, GPS signal strength levels exceed those of Galileo and GLONASS signals. G5/E5 signal levels are higher than the corresponding G1/E1 signals. Figure 3: Average number of available GPS/Galileo signals and actual GPSMAP 66 sr observations per epoch at a mid-latitude site (Dresden) without any signal obstructions in mid-2021, elevation mask angle 10 deg.