# System Proposal for Mass Transit Service Quality Control Based on GPS Data
## 3 Preliminary  
### Global Positioning System (GPS)  
The positioning system used in our proposal is GPS. This system allows the location of mobile objects (airplanes, ships, cars, etc.) to be determined, thus enabling their route to be monitored. To provide geolocation information, GPS uses a constellation of satellites that emit a signal that is received and processed by the receivers built into mobile devices. When a GPS receiver is able to receive signals from at least four different satellites, a three-dimensional geolocation is obtained (latitude, longitude and altitude). In addition to position data, the GPS receiver also provides the time that the data were received; this information comes from the atomic clock on the GPS satellite, which is encoded and sent with the signal emitted by the satellite. The receiver also provides data indicating how many satellites were used to obtain the data (data quality) or the time elapsed since they were obtained (data age). In a conventional GPS receiver, position data errors vary depending on various factors, with errors of up to 100 m in the early days of civilian use, when the intentional degradation named Selective Availability (SA) was active [28]. There are positioning systems based on GPS that provide very precise measurements, such as Differential GPS, which achieves accuracy of less than 2 m. The errors in GPS data are due to different causes. A first source of error is found in the GPS system itself: in the errors that occur in the satellite orbits, caused by drift from true time by the atomic clocks on the satellites and by the geometric configurations of the satellites \"visible\" to the GPS receiver. Since its inception, GPS performance has gradually improved; Table 1 presents some performance metrics for the system from a study conducted in 2013 by the University of Texas, Austin [29]. Among other performance metrics analysed by this study, this table shows the metrics related to Signal-in-Space (SIS), User Range Error (URE), Accuracy, Position Service Availability, and Position Accuracy. AOD refers to Age of Data, which is provided for each GPS reading and indicates the time elapsed since the last position was calculated. A second cause of error is the propagation medium, i.e., refraction in the ionosphere and troposphere, and errors produced by the same signal reaching the receiver by two or more different paths because it has bounced off buildings or natural elements; this error is called multipath error. A final cause of error in position data is due to errors in the GPS receivers. Our proposed system uses conventional GPS receivers installed in vehicles, and the accuracy of the data that they provide depends on the satellite constellation that they are able to view at the time that the position and \begin{table}
\begin{tabular}{c c} \hline \hline
**Metric** & **Value** \\ \hline \multirow{4}{*}{SIS URE Accuracy} & \(\leq\)7.8 m 95\% Global average URE during normal operations over all AODs \\  & \(\leq\)6.0 m 95\% Global average URE during normal operations at zero AOD \\  & \(\leq\)12.8 m 95\% Global average URE during normal operations at any AOD \\  & \(\leq\)30 m 99.94\% Global average URE during normal operations \\  & \(\leq\)30 m 99.79\% Worst-case single point average URE during normal operations \\ \hline \multirow{4}{*}{Position Service Availability} & \(\geq\)99\% Horizontal, average location \\  & \(\geq\)99\% Vertical, average location \\  & \(\geq\)90\% Horizontal, worst-case location \\  & \(\geq\)90\% Vertical, worst-case location \\ \hline \multirow{4}{*}{Position Accuracy} & \(\leq\)9 m 95\% Horizontal, global average \\  & \(\leq\)15 m 95\% Vertical, global average \\  & \(\leq\)17 m 95\% Horizontal, worst site \\  & \(\leq\)37 m 95\% Vertical, worst site \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A selection of GPS performance metrics. geometry of the constellation is calculated. In an ideal scenario, with constellations of more than 7 satellites an accuracy of less than 2 m can be obtained.