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MAVLink Proxy

The MAVLink proxy is the agent’s core service. It connects to the flight controller over serial UART, parses MAVLink frames, and distributes them to every other part of the system: the GCS over WebSocket, scripts via the IPC socket, the cloud relay, and the TUI. The proxy runs as the ados-mavlink systemd service.

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Flight controller auto-detection

When mavlink.serial_port is empty in config (the default), the proxy scans serial ports in this order:
  1. Ports listed in the board profile’s uart_paths (e.g., /dev/ttyS0, /dev/ttyAMA0)
  2. USB serial devices at /dev/ttyACM* and /dev/ttyUSB*
For each candidate port, it opens the connection at the configured baud rate and listens for MAVLink heartbeat messages. The first port that sends a valid heartbeat within 5 seconds wins. If no FC is found, the proxy retries every 10 seconds. This handles cases where the FC boots slower than the companion computer or gets power-cycled mid-flight.

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Serial connection

SettingConfig keyDefault
Portmavlink.serial_portAuto-detect
Baud ratemavlink.baud_rate57600
System IDmavlink.system_id1
Component IDmavlink.component_id191
Common baud rates by firmware:
FirmwareDefault baud
ArduPilot57600 or 921600
PX4115200 or 921600
Betaflight115200
For the best telemetry rates, use 921600 baud. At 57600, you are limited to roughly 60 messages per second. At 921600, you can push 500+ messages per second, which matters for high-rate attitude data and parameter downloads.

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Multi-client distribution

The proxy is a one-to-many bridge. A single serial connection feeds multiple consumers: 
Flight Controller (UART)
       │
  ados-mavlink (proxy)
       │
       ├── /run/ados/mavlink.sock    (IPC, binary MAVLink)
       ├── /run/ados/state.sock      (IPC, JSON telemetry 10Hz)
       ├── WebSocket :8765            (GCS, browser clients)
       └── ados-cloud                 (MQTT relay to cloud)

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IPC sockets

Two Unix domain sockets carry data between services: /run/ados/mavlink.sock (binary): raw MAVLink frames with a 4-byte little-endian length prefix before each frame. Any service or script can connect and receive the full MAVLink stream. /run/ados/state.sock (JSON): a parsed telemetry summary published at 10 Hz. Contains attitude, position, speed, battery, GPS, and armed state as JSON. Used by the health service, cloud relay, and TUI.

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WebSocket endpoint

The proxy exposes a WebSocket endpoint at port 8765 (configurable via mavlink.endpoints). This is the primary way ADOS Mission Control and other browser-based GCS tools connect to the drone. The WebSocket carries raw binary MAVLink frames, wire-compatible with the SITL bridge tool. No encoding changes, no JSON wrapping. The GCS decodes MAVLink in the browser using the same CRC and payload tables as pymavlink. 
mavlink:
  endpoints:
    - type: websocket
      host: "0.0.0.0"
      port: 8765
      enabled: true

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Companion heartbeat

The proxy sends a 1 Hz companion heartbeat to the flight controller with MAV_TYPE_ONBOARD_CONTROLLER. This prevents ArduPilot’s GCS failsafe from triggering when the companion computer is the only MAVLink endpoint. Without this heartbeat, ArduPilot assumes the GCS has disconnected after 5 seconds and can trigger RTL or land depending on failsafe settings.

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Stream requests

On connection, the proxy requests data streams from the FC:
  • SYS_STATUS (battery, health)
  • ATTITUDE (roll, pitch, yaw)
  • GLOBAL_POSITION_INT (GPS lat/lon/alt)
  • GPS_RAW_INT (fix type, satellites, HDOP)
  • VFR_HUD (airspeed, groundspeed, heading)
  • RC_CHANNELS (RC input values)
  • HEARTBEAT (armed state, flight mode)
Stream requests are re-sent every 30 seconds to handle FC reboots and MAVLink buffer stalls. The initial request fires immediately on connection.

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Reconnection

If the serial connection drops (FC power cycle, USB disconnect, cable fault), the proxy:
  1. Closes the old connection cleanly (prevents file descriptor leaks)
  2. Waits 2 seconds
  3. Restarts auto-detection from the beginning
  4. Re-requests all data streams once reconnected
The IPC sockets and WebSocket stay open during reconnection. Clients see a gap in telemetry but do not need to reconnect themselves. Optional message signing for security. When enabled, the proxy signs outgoing messages with a shared secret and rejects unsigned incoming messages. 
mavlink:
  signing:
    enabled: true
    key: "your-shared-secret-key"
Both the agent and the FC must use the same signing key. If they mismatch, all communication silently fails. Test signing on the bench before field use.

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State IPC format

The JSON telemetry on /run/ados/state.sock looks like this (published at 10 Hz): 
{
  "timestamp": 1713264000.123,
  "fc_connected": true,
  "fc_port": "/dev/ttyS0",
  "fc_baud": 921600,
  "armed": false,
  "mode": "STABILIZE",
  "attitude": {"roll": 0.02, "pitch": -0.01, "yaw": 178.5},
  "position": {"lat": 12.9716, "lon": 77.5946, "alt_msl": 920.0, "alt_agl": 0.0},
  "speed": {"groundspeed": 0.0, "airspeed": 0.0},
  "battery": {"voltage": 22.8, "current": 0.5, "remaining": 98},
  "gps": {"fix": 3, "satellites": 14, "hdop": 0.9}
}
From the CLI: 
$ ados mavlink
Connected: True
Port:      /dev/ttyS0
Baud:      921600
From the REST API: 
curl http://localhost:8080/api/status \
  -H "X-ADOS-Key: your_key"
From the TUI, press m for the MAVLink screen.