# Time to First Fix Robustness of Global Navigation Satellite Systems: Comparison Study
## 3 Results
### Time to First Fix  
The TTFF is the main target parameter of this study. For its calculation, we measure the time between the cold start and the generation of the first GGA (global positioning system fix data) NMEA sentence with a fix. As a starting point, the TTFF achieved without attenuation is presented in Figure 6. This graph illustrates the variability in the results due to the different geometries of the satellites. In this sense, Galileo is the GNSS most affected by this because, at the time of the test, its constellation was not yet completed. Please note that the statistical values of the data depicted in Figure 6 are shown in the first column of results in Table 2.  
Figure 5: SNR versus attenuation for the different GNSSs.  
The results of the TTFF as a function of attenuation for the different GNSSs are presented in Figure 7. In addition, the corresponding statistics (mean, standard deviation, minimum, and maximum) are shown in Table 2. It should be noted that, in order to be able to derive graphs and statistics that include all cases under test, if no fix is achieved until the end of the recording, the TTFF is set to 10 min. However, the use of this upper limit for the TTFF gives rise to an artificial reduction in the standard deviation for high attenuation levels.  
The outcome data reveal that GPS is the GNSS that provides the lowest and most stable TTFF for all attenuation levels up to 20 dB. GLONASS behaves in a similar way to GPS until 8 dB, but its performance degrades for higher attenuation. On the other hand, BeiDou and Galileo present TTFF performances that are significantly affected by attenuation.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**TTFF**} & \multirow{2}{*}{**GNSS**} & \multicolumn{8}{c}{**Attenuation (dB)**} \\ \cline{3-11}  & & **0** & **2** & **4** & **6** & **8** & **10** & **12** & **14** & **16** & **18** & **20** \\ \hline \multirow{4}{*}{Mean (s)} & BeiDou & 50.6 & 55.6 & 64.7 & 80.7 & 118 & 187 & 234 & 349 & 439 & 517 & 586 \\  & Galileo & 80.9 & 82.2 & 90.7 & 116 & 164 & 241 & 301 & 390 & 473 & 530 & 583 \\  & GLONASS & 23.6 & 24.2 & 25.4 & 31.7 & 42.0 & 70.3 & 96.9 & 178 & 256 & 330 & 470 \\  & GPS & 31.9 & 30.5 & 30.6 & 30.7 & 31.3 & 32.6 & 33.9 & 36.8 & 39.8 & 43.3 & 51.3 \\ \hline \multirow{4}{*}{Standard deviation (s)} & BeiDou & 12.4 & 26.5 & 49.8 & 66.1 & 70.3 & 88.1 & 100 & 127 & 128 & 95.8 & 27.8 \\  & Galileo & 52.9 & 50.5 & 58.2 & 77.0 & 108 & 138 & 149 & 140 & 107 & 88.7 & 39.2 \\  & GLONASS & 4.62 & 5.30 & 7.79 & 28.7 & 65.3 & 96.4 & 126 & 182 & 202 & 203 & 175 \\  & GPS & 5.92 & 4.97 & 4.78 & 4.78 & 4.20 & 2.82 & 2.39 & 3.06 & 4.11 & 6.08 & 12.5 \\ \hline \multirow{4}{*}{Minimum (s)} & BeiDou & 38.5 & 41.8 & 45.1 & 48.0 & 53.6 & 76.3 & 117 & 167 & 198 & 273 & 447 \\  & Galileo & 28.2 & 30.5 & 32.2 & 32.4 & 42.4 & 73.0 & 83.1 & 113 & 247 & 273 & 404 \\  & GLONASS & 18.1 & 18.1 & 18.1 & 18.1 & 18.6 & 20.1 & 20.1 & 25.8 & 30.5 & 43.9 & 63.0 \\  & GPS & 17.4 & 17.4 & 17.4 & 17.4 & 18.1 & 21.9 & 27.4 & 32.1 & 33.0 & 34.9 & 37.4 \\ \hline \multirow{4}{*}{Maximum (s)} & BeiDou & 154 & 225 & 384 & 589 & 600 & 600 & 600 & 600 & 600 & 600 \\  & Galileo & 327 & 327 & 327 & 380 & 422 & 547 & 600 & 600 & 600 & 600 & 600 \\  & GLONASS & 46.3 & 49.5 & 72.7 & 210 & 600 & 600 & 600 & 600 & 600 & 600 \\  & GPS & 42.0 & 35.2 & 35.3 & 35.2 & 40.4 & 40.9 & 43.5 & 46.5 & 60.0 & 70.8 & 119 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Statistical results for the TTFF.  
Figure 6: TTFF along the 4 days of the test for the different GNSSs without attenuation.