From Semi-Autonomous to Fully Autonomous: Our Flight Control Architecture

As a pilot, there's one thing I'm most afraid of: the vehicle going out of control in autonomous mode and not being able to switch to manual fast enough. That's why the architecture was built in two tiers: semi-autonomous and fully autonomous. The switch is a single toggle on the transmitter. I've got its position memorized, and I still double-check it before every takeoff.

Semi-autonomous: Loiter

Emergency interventions are on me, through the FlySky FS-i6X transmitter. To reduce my workload, Loiter mode is active. If I'm not touching the sticks, the vehicle just holds its position. No drift. If there's wind, GPS position hold takes over; in our tests in the school yard, deviation never exceeded half a meter even in a 4 m/s crosswind.

Fully autonomous: Guided

During the actual competition mission (target detection, alignment, landing), the vehicle is put into Guided mode. Pilot input is entirely passive. Every flight decision comes from the image-processing algorithm running on the Pi 5. The Pixhawk just executes the command. I'm on standby, hand on the switch.

Manual override

A dedicated switch on the transmitter can cancel the mission. If autonomous behavior heads somewhere unwanted, I flip the switch; the vehicle drops into Loiter and I take over. In our first SITL tests we saw the override respond late; we went back and revised the MAVLink message priorities, and response time is now under half a second.

Failsafe protocols

• RTL: automatic return-to-launch when the battery hits a critical level.
• Signal loss: when the transmitter signal drops, the Pixhawk switches to Loiter and an RTL countdown starts.
• GPS loss: Loiter isn't possible, Althold (altitude hold) takes over, and I switch to manual control.
• Audible warning: GPS lock status and low voltage are reported via the buzzer.
• Safety button: acts as a mechanical lock before the vehicle is armed, preventing the motors from spinning up by accident.

GPS and telemetry

We raised the Holybro M10 GPS on a carbon-fiber mast, away from the motors' magnetic noise. It provides centimeter-accurate data for autonomous navigation and Loiter. Telemetry runs on 915 MHz, the transmitter on 2.4 GHz. The two bands are kept deliberately separate, no interference. Last season another team apparently had both systems on the same band, and it made for a bad day in the field for them; we took note when we heard about it.