## 3 Results
### Quality Check of GNSS Observations
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Receiver**} & \multirow{2}{*}{**Station**} & \multicolumn{2}{c}{**Integrity**} & \multirow{2}{*}{**MPI (m)**} & \multirow{2}{*}{**MP2 (m)**} & \multirow{2}{*}{**CSR**} & \multicolumn{2}{c}{**SNR1**} & \multicolumn{2}{c}{**SNR2**} \\  & & **(\%)** & & & & & **(dB-Hz)** & **(dB-Hz)** \\ \hline \multirow{8}{*}{Geodetic} & Est1 & 89 & 0.27 & 0.31 & 2.11 & 42.48 & 35.94 \\  & Est2 & 86 & 0.39 & 0.26 & 15.63 & 46.44 & 35.45 \\  & Est3 & 84 & 0.20 & 0.19 & 0.44 & 42.69 & 40.59 \\  & Est4 & 86 & 0.25 & 0.31 & 0.39 & 44.31 & 40.51 \\  & Est5 & 95 & 0.33 & 0.41 & 0.09 & 44.58 & 41.48 \\  & Est6 & 84 & 0.19 & 0.25 & 0.21 & 42.68 & 41.84 \\  & CULC1 & 97 & 0.34 & 0.40 & 0.22 & 46.68 & 42.14 \\  & ISL1 & & & & & 42.00 & None \\  & ISL2 & & & None & & 41.40 & \\  & ISL3 & & & & & 42.96 & data \\  & CULC2 & 100 & 0.34 & 0.42 & 0.21 & 46.49 & 42.20 \\ \hline \multirow{2}{*}{Low-cost} & LCSF & \multicolumn{2}{c}{None} & \multirow{2}{*}{41.14} & None \\  & CULC3 & 100 & 0.37 & 0.38 & 0.23 & 46.20 & 42.05 \\ \hline \multirow{2}{*}{Low-cost} & LCDF & 78 & 0.65 & 0.45 & 1.76 & 40.28 & 40.17 \\  & CULC4 & 95 & 0.35 & 0.39 & 0.53 & 46.04 & 42.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of quality check of the GNSS observations results. Figure 5: Integrity. Red line: 95% of integrity [36]. Figure 8 shows the number of cycle slips per 1000 observations (CSR) where the recommended value by the IGS is 1 (1000/(o/slps)), which is represented by the red line [36]. It is evident that EST2 has the highest amount of loss in the carrier phase with 15.63 (1000/(o/slps)), with EST1 and LCDF also exceeding the recommended value. None of the evaluated stations show a performance of 100% in the quality indicators. This is generally related to the station environment, receiver type, and operating time. The stations in which the quality check was performed have three acceptable variables out of six (MP1, MP2, SNR1, SNR2, CSR, INTEGRITY). Based on the standard proposed by the IGS (Figure 9), the EST3 and EST6 stations have the best performance, complying with five of the six variables analyzed. Figure 6: Multipath effect (MP1 and MP2). Red line: maximum value of 0.30 m for the multipath effect [36]. Figure 7: Signal-to-noise ratio (SNR). Red line: 36 dB-Hz according to [41]. Analysis of Probable Error Circle (CEP) and Vertical Positioning Accuracy (EPV) of the GNSS Observations  
The analysis of the CEP and the EPV was carried out considering the norm for geodetic works in Mexican territory. In the PPP method, it is found that not every station with dual-frequency receivers complies with the regulations to establish stations in the national geodetic network classifications, while only stations EST1, EST3 and EST5 comply with the regulations without using the GGM10 model (Table 6 and Figure 10). It is inferred that the PPP technique in urban areas may not be feasible, since the limit exceeds that established by regulations for vertical control. On the other hand, if the GGM10 model is not used (Table 6), the accuracy achieved for the vertical component can be utilized for the vertical control (EST1, EST3 and EST5). In the rest of the cases, the obtained values for PPP-EPV with GGM10 geoid are outside of the reference parameter, with the single frequency receivers obtaining the worst values (Table 6). The obtained results are correlated with the low accuracy achieved in the PPP method by using single frequency receivers. The low-cost dual-frequency receivers have a similar performance to geodetic receivers; however, they have better performance than the low-cost and geodetic single-frequency receivers, as evident by CEP and EPV values. When not affected by the environment of an urban area, the low-cost dual-frequency receiver can be considered for the densification of the national reference geodetic network [15]. Low-cost single-frequency receivers may be feasible to utilize depending upon the environment and the observation time.