## 3 Results
### Preliminary Data Analysis: Measurement Availability, Signal Strength, and Geometry  
Velocity estimation is strongly affected by the number of available measurements, by the geometry, and by the measurements' quality. Before the results are presented in the velocity domain, a preliminary analysis of the data is performed. In detail, the following aspects are investigated:  
* Availability of raw Doppler shift and CP measurements;
* \(C/N_{0}\) behavior;
* Availability and geometry of Doppler and TDCP measurements considering mask angle and \(C/N_{0}\) limit. As evidenced by Figure 1, the location where the test took place has no significant obstacles, and the multipath level is low, so it is a typical open-sky scenario; consequently, a large number of measurements is expected. The availability of Doppler and CP observables is shown in Figure 2 for each device, GNSS, and satellite ID. In this analysis, all the tracked measurements are considered, without considering the mask angle and \(C/N_{0}\) limit. Specifically, panels in the first row refer to GPS satellites, panels in the second row to Glonass, panels in the third row to Galileo, and panels in the fourth row to BeiDou. Moreover, panels in the first column refer to the Novatel device, panels in the second column to uBlox, and panels in the third column to Xiaomi. Only one panel is present in the first column, i.e., panel (a), because the Novatel device is able to acquire only GPS signals. In each panel, a blue dot indicates the availability of both Doppler and CP measurements for a certain epoch, while a red dot indicates the availability of only Doppler measurements and the unavailability of CP measurements in the considered epoch. It never happens that CP is available and Doppler is not. Figure 1: Panel (**a**): location of the test; the equipment is placed on point P. Panel (**b**): equipment deployment. Panel (**c**): multipath probability distribution for the uBlox receiver. From an analysis of the panels (a), (b), and (c), it is evident that the tracked GPS satellites are roughly the same, except for G11, which is tracked by uBlox but not by Novatel and Xiaomi. There are no red dots for Novatel, while they are present for uBlox and even more for Xiaomi. This highlights that, in the GPS case, the CP observable is continuous for the high-grade device, while discontinuities appear for the high-sensitivity device and, above all, for the smartphone chip. This issue reverberates on the availability of a TDCP solution, which is based on time-differenced CP measurements. The comparison with the geodetic receiver in terms of the other GNSSs is not possible. An analysis of the panels (d) and (e), regarding Glonass measurements from uBlox and Xiaomi, respectively, indicates that the tracked satellites are significantly different: measurements from R02, R10, R11, and R19 satellites are present for uBlox and not for Xiaomi. Glonass Doppler measurements for Xiaomi are often insufficient for standalone velocity estimation, and CP is often missing even when Doppler is available, causing a long unavailability of TDCP Glonass-only velocity. A very similar consideration can be made for Galileo measurements, by analyzing panels (f) and (g): for Doppler shift, the uBlox device was able to track up to 11 satellites, while  
Figure 2: Availability of Doppler and CP observables (mask angle and \(C/N_{0}\) limit not applied). Each panel refers to a single device and to a single GNSS. Panels in the first row, (**a–c**), refer to GPS; panels in the second row, (**d,e**), to Glonass; panels in the third row, (**f,g**), to Galileo; and panels in the fourth row, (**h,i**), to BeiDou. The panel in the first column, (**a**), refers to the Novatel device; panels in the second column, (**b,d,f,h**), to uBlox; and panels in the third column, (**c,e,g,i**), to Xiaomi.