# ADS-B Like UTM Surveillance Using APRS Infrastructure
## 2 Materials and Methods
### Ground Transceiver Station  
The core of the APRS ground transceiver station (GTS), also known as i-Gate, is built by the RPi single board computer. It is programmed by using Python to control data processing, and then relays data to UTM Cloud via Internet. The flowchart of programs in RPi is shown in Figure 4.  
The APRS transmits data in audio message. Thus, the TCN-Pi recovers audio messages from the RF receiver back into digital data first, and then transmits digital data to RPi through the serial port as  
b'\xc@vx0@vx82\xa@vx0b4\x9a\x88\x9a'\x884\xac'\x86\x8\x94\xaar'\xae\x92\x88\x8\x8 ab@b\xae\x8\x8\x8\  
Figure 5, to fly from CJCU campus to a nearby area. Since the flight control data cannot be acquired from the flight controller, the APRS OBU transmits the position data and the hard-code flight control data to UTM cloud in this test. The APRS OBU is mounted on the top of the UAV. In addition, LoRa OBU, another ADS-B like system, is also included for the test. The antenna of APRS GTS is fixed on the rooftop of a building on the CJCU campus as shown in Figure 6. From the UTM controller center, whenever the UAV takes off to fly, its position will be shown clearly on the UTM surveillance display as shown in Figure 7.  
Figure 8 shows the flight test around CJCU campus with a flight track record of about 4.7 km at 55 m (AGL) altitude at 3-4 m/sec cruise speed. The maximum distance between the UAV and APRS GTS is about 1.4 km. The APRS OBU reports 90 byte of surveillance data about every 10 s. In this test, all of the received surveillance data in GTS have a complete 90 bytes. The test result shows that the APRS ADS-B like system works well under different UAV flight directions. To test the APRS ADS-B like coverage, a series of flight test are conducted from the near site about 4 km to the remote site about 40 km away from CJCU GTS. The DIY 550 quad-rotor UAV--equipped with GPS, Pixhawk FC, APRS OBU, and 433 MHz telemetry--is adopted for these tests. The usage of 433 MHz telemetry is for uploading the flight mission to Pixhawk. The right hand side in Figure 9 shows the 550 quad-rotor UAV, assembled by: 550 mm carbon airframe, 4108-390 kV motor, 30A ESC, 1555 propeller, and 65 25C 5800 mah LiPo battery. The purpose for these tests was to examine: (1) the effective range of all communication devices; (2) complete 90 byte surveillance data; (3) data stability and continuity; (4) UAV attitude by 6 DoF. Test case 2:  
In this test, the test site was located 37.63 km away from CJCU GTS with an altitude of 90 m above MSL. The UAV fle  
Figure 8: Flight surveillance test near CJCU campus. Figure 9: Remote test at 37.6 km away from CJCU GTS for APRS ADS-D like. the UAV icon is displayed on the UTM surveillance system in the left hand side leaving six points behind as its past track. The 90 byte surveillance data from APRS OBU are collected as follows (Supplementary Materials):  
2020-02-21 18:14:53 028 4.83  
@@@**U00005O0000523.23106N120.36976E0184  
1.28 -0.71 0.05 -4.2 -50.5 2.6 24.37 8.53##  
The data begins with date, time, data sequence number 028, and 4.83 for OBU voltage. The data starts from heading \"@@***\", UAV ID \"U00005\", Pilot ID \"O00005\", surveillance data, and tailing by \"##\" to clearly distinguish from other APRS users. The real flight data is shown in Table 3. The UAV ID and Pilot ID are connecting to the management information system (MIS) from CAA [9]. Figures 10-13 show the surveillance data in statistics in accordance with the data sequence number. Figure 10 shows the actual data receiving intervals. The varying intervals are due to a delay in fetching data from Pixhawk FC. The delay time according to radio traffic and congestion is minor in the tests. The interval is measured by the difference of receiving time for successive surveillance data in APRS GTS. The maximum fluctuation is about 4 s, and is in the acceptable range for UTM operation. Figure 11 shows the UAV altitude tracking above MSL. Figure 12 shows the power supply status along the flight.