## 3 Design of Mobile Unit (MU)
### GNSS (GPS) Subsystem Design
#### 3.2.1 Static Positioning  
The static accuracy of selected GPS modules is presented in this section. Portable devices carried by airport personnel (or attached to other objects, e.g., vehicles) usually have an unknown antenna orientation with respect to satellites. For this reason, two test cases were evaluated: GPS antenna pointing up (best case) and down (worst case). The accuracy results led to the selection of the GPS module and its application in the mobile unit. The investigated modules for the chosen antenna selection and its orientation are shown in Figures 6 and 7. All cumulative histograms show a percentage of accuracy with respect to a reference point (with known location). The static positioning error (calculated for a 10 m circle) for the downward antenna orientation is about 15% worse than for the upward antenna orientation for the module mod_3. For module mod_4, the difference increases to almost 20%. The most interesting result occurs for the mod_2 module: for the upward antenna orientation, 100% of the positions are within the 7 m circle, but for the downward orientation, only 15% are within the 10 m variation (Tables 5-9). The conclusions are the following: module 2 (mod_2) is superior, but only for upward pointing antennas. Therefore, this module should be considered for use in fixed orientation cases (e.g., vehicle). Modules 3 and 4 (mod_3 and mod_4) should be considered for portable operation. \begin{table}
\begin{tabular}{c c} \hline \hline
**Abbreviation** & **Chipset** \\ \hline mod\_1 & MediaTek MT3329 (Pololu Robotics and Electronics, \\ mod\_2 & Las Vegas, NV, USA) \\ \hline mod\_3 & \\ mod\_4 & MediaTek MT3318 (Pololu Robotics and Electronics, \\ mod\_5 & Las Vegas, NV, USA) \\ mod\_6 & \\ \hline mod\_7 & SIRFstarIII (SiRF Commercial Company, \\ mod\_8 & San Jose, CA, USA) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Selected GPS receivers. Figure 6: Static positioning error: internal antenna heading upwards. Cases with an external microstrip antenna (passive, but with a larger area than the built-in internal antenna) and an external active antenna (with approx. 20 dB preamplifier) were also tested. The results (cumulative histograms of positioning error) are shown in Figures 8 and 9. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Module** & **mod\_3** & **mod\_4** \\ \hline \multirow{3}{*}{Positioning error [m]} & min. & 0.24 & 0.73 \\  & avg. & 14.4 & 2.86 \\  & max. & 93.1 & 4.92 \\ \hline \multicolumn{3}{c}{\(N\) of measurements} & 4004 & 580 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Static location errors for internal antenna heading upwards. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Module** & **mod\_4** & **mod\_5** & **mod\_2** \\ \hline \multirow{3}{*}{Positioning error [m]} & min. & 0.30 & 4.03 & 13.2 \\  & avg. & 18.8 & 26.1 & 53.0 \\  & max. & 67.1 & 26.7 & 72.9 \\ \hline \multicolumn{3}{c}{\(N\) of measurements} & 3887 & 3481 & 3870 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Static location errors for external microstrip antenna heading upwards. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Module** & **mod\_1** \\ \hline \multirow{3}{*}{Positioning error [m]} & min. & 13.2 \\  & avg. & 53.0 \\  & max. & 72.9 \\ \hline \multicolumn{3}{c}{\(N\) of measurements} & 8535 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Static location errors for external microstrip antenna heading downwards. Figure 7: Static positioning error: internal antenna heading downwards. It can be clearly seen that the active antenna (with preamplifier) improves the positioning accuracy at the cost of higher power consumption (see next section). The maximum average positioning error occurs for modules mod_5 (built-in antenna down) and mod_1 (microstrip antenna down); its value is more than 50 m, which is an unacceptable result. The best average accuracy is obtained with the mod_3 module (antenna down)--less than 10 m.  
Modules mod_3 and mod_4 have the best performance in static measurements.