## 2 GNSS Signal Characteristics
### Phase-Code Differences  
The gradual accumulation of phase errors cannot be ignored when high-precision positioning is carried on using measurements from a smartphone. The phase-code combination between satellite and receiver in meters is given directly as [8,16]:  
\[L_{i}-P_{i}=-2\frac{f_{1}^{2}}{f_{i}^{2}}I-\lambda_{i}N_{i}+B_{L_{i}}^{r}-B_{P_ {i}}^{r}-B_{L_{i}}^{s}+B_{P_{i}}^{s}+\varepsilon_{L_{i}}-\varepsilon_{P_{i}} \tag{3}\]  
where the subscript \(i\) denotes frequency, \(L\) denotes raw phase observation scaled to distance, \(P\) denotes raw code observation, \(f_{i}\) is the frequency of \(L_{i}\), \(I\) is the ionospheric delay on frequency \(L_{1}\), \(\lambda_{i}\) is the wavelength of \(f_{i}\), \(N_{i}\) is the integer ambiguity of \(L_{i}\) in cycle, the symbols \(B_{P_{i}}^{r}\), \(B_{L_{i}}^{r}\), \(B_{P_{i}}^{s}\), and \(B_{L_{i}}^{s}\) are code and phase hardware delay of receiver and satellite, respectively, and the \(\varepsilon\) terms are unmodeled errors including code and phase multipath effect and observation random noise. Part of ionospheric delay in Equation (3) can be eliminated with IGS final global ionospheric map (GIM) products, and the difference value \(D_{i}\) is written as:  
\[D_{i}=L_{i}-P_{i}+2\frac{f_{1}^{2}}{f_{i}^{2}}I=-\lambda_{i}N_{i}+(B_{L_{i}}^{ r}-B_{P_{i}}^{r})-(B_{L_{i}}^{s}-B_{P_{i}}^{s})+\varepsilon_{L_{i}}- \varepsilon_{P_{i}} \tag{4}\]  
The value of \(D_{i}\) is mainly affected by integer ambiguity, code and phase hardware delay of receiver and satellite, multipath effect, and measurement noise. Therefore, the \(D_{i}\) values always fluctuate within a fixed range, and the trends of \(D_{i}\) values of the Huawei Mate30 and geodetic receiver are shown in Figure 4.  
From Figure 4d, the trends of \(D_{i}\) values of GPS/BDS/GLONASS in the geodetic receiver are in line with the expectation. In Figure 4a, the significant change of \(D_{i}\) value mainly concentrates in the L5 band, and the changing trends of \(D_{i}\) value of GPS in L1 band are similar to that of the geodetic receiver. This is different from the experimental result of the Huawei P20 in [8], and it indicates that the quality of GPS carrier phase observations of the Huawei Mate30 has been improved. However, in Figure 4b, we clearly found that all of the BDS carrier phase observations in the Huawei Mate30 were affected by the gradual accumulation of phase errors, and the trends correspond to a change that ranged from \(-1.203\) to \(-1.548\) cm/s, with a mean value of \(-1.401\) cm/s. In addition, when the difference value between the carrier phase observations (in meters) and the pseudorange observations reached the threshold of 50 m, the carrier phase observations (in meters) were consistent with the pseudorange by means of cycle slip. In Figure 3b, taking BDS pseudo random noise (PRN) 04 and PRN 16 for example, the extra cycle slips of PRN 04 and PRN 16 were detected in 3640 epochs and 2890 epochs, which was the same as the time when the carrier phase observations (in meters) were consistent with pseudorange in Figure 4b. The phenomenon of cycle slip after reaching the threshold did not happen in other similar experiments [8,16]. For GLONASS in Figure 4c, a large number of cycle slips, except for the R6 satellite, make carrier phase observations and pseudorange observations coincide with each other. However, there seems to be no accumulated deviation from the \(D_{i}\) value in GLONASS carrier phase observations.