## 3 Results
### Preliminary Data Analysis: Measurement Availability, Signal Strength, and Geometry
the maximum number of Galileo satellites for the smartphone was 4; for the TDCP case, only a few discontinuities were observed for uBBox, while for the smartphone case, large discontinuities were observed, and even in case of Doppler availability, CP is often missing. Regarding BeiDou, the available Doppler and CP measurements are similar for uBBox and Xiaomi; only small differences can be noted. Specifically, satellite C30 is tracked by uBBox and not by Xiaomi, and satellites C02 and C05 are tracked by Xiaomi and not by uBBox. C02 and C05 are both geostationary (GEO) satellites, highlighting a uBBox shortage in tracking such satellites, while C30 is a medium Earth orbit (MEO) satellite and is probably not tracked by Xiaomi due to the inherent selection criteria of the device. Xiaomi is not able to store CP for C02, C05, and C08, while CP is discontinuous for C19 and C20. C02, C05, and C08 are all characterized by low \(C/N_{0}\) values, about 25 dB-Hz, inhibiting the storage of reliable CP measurements; C08 is an inclined geosynchronous orbit (IGSO) satellite. CP discontinuities of C19 and C20 are related to frequent drops in their C/N0 values below 25 dB-Hz. The number of epochs where Doppler is present and CP is missing is significantly smaller in uBBox. An analysis of the measurements stored by Xiaomi, panels (c), (e), (g), and (i), indicates that a measurement selection criterion appears to be implemented in the device, privileging GPS and BeiDou measurements over Glonass and Galileo ones. This behavior is probably due to the limited number of channels available for tracking. An additional analysis of measurement availability is carried out by comparing the availability percentages of Doppler and CP measurements of two GPS satellites, G02 and G06, for the three considered devices. In Figure 3a, the measurement availability for G02 is shown, while Figure 3b shows that for G06. These particular satellites have been selected because CP from G02 is continuous for all three devices, while for G06, some discontinuities in CP are evident for uBBox and Xiaomi, as shown in panels (a), (b), and (c) of Figure 2. The graphs in Figure 3 confirm what emerges from Figure 2, relative to satellites G02 and G06. Specifically, the availability of Doppler and CP measurements of G02 is 100% for all the considered devices, with only a few gaps for CP with Xiaomi, which has 99.9% availability. Regarding the G06 satellite, Doppler and CP availabilities are 99.9% for Novatel; they are 99.8% and 97.8%, respectively, for uBBox; and they are 92.2% and 83.1%, respectively, for Xiaomi. The behavior of \(C/N_{0}\) is shown in Figure 4, which is structured like the previously described Figure 2. In the panels, the \(C/N_{0}\) values of the tracked satellites are displayed with respect to time; in black, the mean \(C/N_{0}\) values for each GNSS and device are shown too. Considering the GPS case, panels (a), (b), and (c), the Novatel device shows larger \(C/N_{0}\) values than uBBox and Xiaomi, with mean \(C/N_{0}\) values equal to 44.7 dB-Hz, 41.3 dB-Hz, and 37.7 dB-Hz, respectively. This could be related to the different gain antennas characterizing the three devices and to the way \(C/N_{0}\) is computed by each device. The \(C/N_{0}\) behavior for Novatel seems more regular with values never below 25 dB-Hz, while the uBBox receiver and the smartphone were able to track very weak signals with \(C/N_{0}\) values as low as 15 dB-Hz. For uBBox and Xiaomi, the \(C/N_{0}\) behavior is more fluctuating, with lower values for Xiaomi. Figure 3: Availability percentage of Doppler and CP measurements for satellite G02, panel (**a**), and for satellite G06, panel (**b**). Mask angle and \(C/N_{0}\) limit not applied. Considering the Glonass case, in panels (d) and (e), more satellites are tracked by uBlox than by Xiaomi; moreover, \(C/N_{0}\) values for Xiaomi are lower, e.g., satellite R09 has \(C/N_{0}\) values between 40 and 50 dB-Hz for uBlox (purple dots in panel (d)) and between 30 and 40 for Xiaomi (yellow dots in panel (e)).