# ADS-B Like UTM Surveillance Using APRS Infrastructure
## 2 Materials and Methods
### APRS On-Board Unit (OBU)  
The APRS OBU is fabricated with a LightAPRS module with a specific firmware design shown in Figure 3. The firmware starts the process of assigning the UAV ID, pilot ID, audio frequency, call-sign SSID, and symbol code. The ADS-B Like surveillance data format is specified including: [Heading(5); UAV_ID(6); Pilot_ID(6); Lat.(9); Long.(10); Alt.(4); 6 DoF(Vx, Vy, Vz, \(\alpha\), \(\beta\), \(\gamma\)) (36); V(6); A(6); Tail(2)]. The Heading and Tail in surveillance data are for distinguishing the transmitter from the other HAM users. The ADS-B Like surveillance data not only contains position tracking but also includes 6 DoF flight control data at 90 Bytes. The data format is designed with the purpose of collecting UAV flight performance into Big Data for further study and analysis [4; 6]. The original data ending with the UAV operation current is collected and arranged in AX.25 format [19]. Then, the digital data is modulated into audio frequency using frequency shift keying (AFSK) for transmission by the built-in RF module in LightAPRS at 144.61 MHz, which is distinguished from HAM users at 144.64 MHz.  
The data format of the ADS-B Like solution for UTM surveillance is shown in Table 2. For APRS OBUs, the flight control data (Vx, Vy, Vz, \(\alpha\), \(\beta\), \(\gamma\), voltage, current) are from the Pixhawk FC, and the position data (latitude, longitude, and altitude) are from a built-in GPS module in LightAPRS. Note that the three-axis flight speed Vx, Vy, and Vz are based on the inertial north-east-down (NED) frame. The UART port is used for the connection between LightAPRS and Pixhawk FC, and the MAVLink protocol [20] is used to communicate with the Ardupilot flight control software in the Pixhawk for requesting flight data and setting the time interval.  
Figure 2: ADS-B like system structure using APRS.  
_Aerospace_**2020**, \(7\), 100  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Data** & **Bytes** & **Data** & **Bytes** \\ \hline Heading & 5 & Tailing & 2 \\ \hline UAV ID & 6 & Pitch \(\alpha\) & 6 \\ \hline Pilot ID & 6 & Yaw \(\beta\) & 6 \\ \hline Latitude & 9 & Roll \(\gamma\) & 6 \\ \hline Longitude & 10 & Voltage & 6 \\ \hline Altitude & 4 & Current & 6 \\ \hline V\({}_{\rm x}\) & 6 & V\({}_{\rm z}\) & 6 \\ \hline V\({}_{\rm y}\) & 6 & & \\ \hline \multicolumn{3}{c}{Data unit and format} \\ \hline Data & Unit & format \\ \hline Latitude & deg. & \begin{tabular}{c} xx.xxxxxN \\ e.g., 22.90454N \\ \end{tabular} \\ \hline Longitude & deg. & \begin{tabular}{c} xx.xxxxxE \\ e.g., 120.27418E \\ \end{tabular} \\ \hline Altitude & feet & \begin{tabular}{c} Integer \\ e.g., 0050 \\ \end{tabular} \\ \hline V\({}_{\rm x}\), V\({}_{\rm y}\), V\({}_{\rm z}\) & m/s & \begin{tabular}{c} xx.xx \\ e.g., 2.22 \\ \end{tabular} \\ \hline \(\alpha\), \(\beta\), \(\gamma\) & deg. & \begin{tabular}{c} xx.xx \\ e.g., -152.3 \\ \end{tabular} \\ \hline Voltage & volt & \
\begin{tabular}{c} xx.xx \\ e.g., 12.42 \\ \end{tabular} \\ \hline Current & Amp. &
\begin{tabular}{c} xx.xx \\ e.g., 10.52 \\ \end{tabular} \\ \hline \end{tabular}
\end{table}
Table 2: ADS-B Like surveillance data format.  
Figure 3: Firmware flowchart of APRS OBU.