# ADS-B Like UTM Surveillance Using APRS Infrastructure
## 2 Materials and Methods  
The proposed hierarchical UTM [4] requires a surveillance system infrastructure including ground infrastructure and OBU for UAVs. The regional UTM (RUTM) and national UTM (NUTM) shown in Figure 1 are proposed for UAV surveillance management under local government and CAA in Taiwan [9]. The APRS has the widest coverage range of the proposed ADS-B like solutions for UTM.  
The APRS is commonly used by HAMs and volunteer rescue groups. In most Asian countries, a specific frequency of 144.64 MHz is assigned for public use. The transceiver station for APRS is called i-Gate. The APRS i-Gates relay messages via the Internet to any destination. There are more than 200 i-Gates distributed over Taiwan. It is very convenient to use APRS as an ADS-B Like solution for UAVs. Since an APRS at 144.64 MHz is noisy from many irregular users, to avoid communication traffic jamming, the UTM project has applied a public specific frequency of 144.61 MHz for UAV  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Device** & **4G/LTE** & **LoRa** & **X-Bee** & **APRS** \\ \hline Character & Public & Professional & & \\ \hline Coverage & Except remote & 0–25 miles into air & & \\ \hline Range and Altitude & \textless{}500 feet AGL & \textless{}5000 feet AGL & & \\ \hline Relay Sta. & Exist & Need to add & & \\ \hline Price & \textgreater{}650 US & \textless{}100 US & & \\ \hline System & Available & Need construction 1000–1500 US/each & & \\ \hline Data rate & Video & Data & & \\ \hline Weight & \textless{}300 g & \textless{}200g & & \\ \hline Voltage & & & 5–12 Vdc & \\ \hline Power & & \textless{}1 W & & \\ \hline Cloud & Direct & Relay onto via Gateways & & \\ \hline Legal radio & Legal, authorize to system providers & Legal, need application & & \\ \hline Frequency by NCC regulation & 902–928 MHz & 920–925 MHz & 920–928 MHz & 144–146 MHz \\ \hline Technique for UTM & \multicolumn{2}{c}{Mature and ready} & \multicolumn{2}{c}{Need license and setting} \\ \hline \hline \end{tabular}
\end{table}
Table 1: ADS-B Like characteristics comparison.  
Figure 1: The hierarchical UTM under construction in Taiwan [4; 11].  
OBUs. The reason for selecting 144.61 MHz is to avoid frequent conflict from other users. From our preliminary tests [6], each i-Gate uses the 6 dBi antenna can cover a 40 km radius range operation for an APRS with 0.5 W radio power. It is particularly useful for high altitude fixed wing UAVs flying into remote and mountain territories.  
In the hierarchical UTM development, the APRS OBU is designed and fabricated using a 'LightAPRS' module [16] to integrate with Pixhawk flight controller (FC) [17] for UAVs. The ground transceiver station (GTS) is designed with an off-the-shelf KENWOOD TM721G radio frequency receiver, \"TNC-pi\" [18] (terminal node controller) and Raspberry pi to relay messages onto the Internet to the UTM Cloud database. Figure 2 shows the proposed ADS-B Like system structure using APRS. The TNC-pi demodulates audio signal into digital signal, and then passes it to Raspberry Pi (RPi), a single board computer with Linux OS, for further processing and storing onto the UTM Cloud. The UTM cloud is constructed with a database to manipulate all UAV surveillance data to regional UTM (RUTM) operation centers [4].