MAVLink Connection Setup

Establish a MAVLink UDP connection from MATLAB to a PX4 or ArduPilot flight controller, with MATLAB acting as a ground control station (GCS). This skill encodes the correct protocol sequence and heartbeat pattern that agents consistently get wrong.

When to Use

• User wants to connect MATLAB to a PX4 or ArduPilot autopilot via UDP
• User is building a ground control station in MATLAB
• User needs to set up MAVLink communication with SITL or networked autopilot
• User asks about heartbeat exchange or client discovery
• User references mavlinkio, mavlinkdialect, or sendudpmsg

When NOT to Use

• User wants to read/set parameters, upload missions, or download logs over an

already-established connection — these work well without this skill

• User is working with Simulink MAVLink blocks (different workflow)
• User only wants to parse a .ulg log file offline — use ulogreader directly
• User asks about MAVLink message serialization/deserialization only

Workflow

Follow this exact sequence. The order matters — skipping or reordering steps causes silent failures.

1. Create the dialect

dialect = mavlinkdialect("common.xml", 2);

Use "common.xml" for both PX4 and ArduPilot (covers all standard messages). Use "ardupilotmega.xml" only if you need ArduPilot-specific extension messages. The 2 specifies MAVLink protocol version 2.

Available dialects: common.xml, ardupilotmega.xml, standard.xml, minimal.xml

2. Create the MAVLink IO interface

mavlink = mavlinkio(dialect, 'SystemID', 255, 'ComponentID', 1);

• SystemID 255, ComponentID 1 is the standard GCS identity
• Do NOT create mavlinksub(mavlink, "HEARTBEAT") for connection verification.

mavlinkio has a built-in heartbeat subscriber that feeds listClients(). Creating a manual HEARTBEAT subscriber is redundant and wasteful.

3. Connect UDP transport

connect(mavlink, "UDP", LocalPort=14550);

All transport options are name-value pairs. Never use positional arguments.

Name-value options: LocalPort (default 0), ConnectionName (default "Connection#")

4. Build the GCS heartbeat message

`createmsg` signature: `createmsg(dialect, msgName)` — the dialect object is always the first argument, message name string is second. Do NOT pass the mavlinkio object to createmsg.

hbMsg = createmsg(dialect, "HEARTBEAT");
hbMsg.Payload.type(:) = dialect.enum2num("MAV_TYPE", "MAV_TYPE_GCS");
hbMsg.Payload.autopilot(:) = dialect.enum2num("MAV_AUTOPILOT", "MAV_AUTOPILOT_INVALID");
hbMsg.Payload.base_mode(:) = 0;
hbMsg.Payload.custom_mode(:) = 0;
hbMsg.Payload.system_status(:) = 0;  % GCS has no vehicle state

CRITICAL: Always access fields via `msg.Payload.fieldname(:)` — never msg.fieldname(:). The message struct has a .Payload sub-struct that contains all protocol fields. Writing hbMsg.type(:) = ... fails because type is not a top-level field — it lives at hbMsg.Payload.type.

CRITICAL: Always use `(:)` indexing on payload field assignments. Writing msg.Payload.type = 6 (without (:)) silently replaces the wire type (uint8) with double, producing corrupted MAVLink packets. The (:) preserves the original data type.

5. Start periodic heartbeat

There are two workflows depending on whether the autopilot is already broadcasting:

Workflow A: Auto-discovery (autopilot already broadcasting heartbeats)

If PX4 SITL configured to broadcast mavlink messages, the autopilot's heartbeats arrive automatically. Poll listClients first, then send heartbeats back to the discovered client:

% Wait for autopilot to appear
timeout = 10;
tic;
discovered = false;
while toc < timeout
    clients = listClients(mavlink);
    if height(clients) > 1
        discovered = true;
        break;
    end
    pause(0.5);
end

if discovered
    % Use SystemID/ComponentID from listClients output
    remoteClient = clients(clients.SystemID ~= 255, :);  % exclude local GCS
    autopilot = mavlinkclient(mavlink, remoteClient.SystemID, remoteClient.ComponentID);
    hbTimer = timer('ExecutionMode', 'fixedRate', 'Period', 1, ...
        'TimerFcn', @(~,~) sendmsg(mavlink, hbMsg, autopilot));
    start(hbTimer);
end

Workflow B: Manual initiation (autopilot not yet broadcasting)

If the autopilot requires GCS heartbeats before it will respond, use sendudpmsg with the autopilot's listening port. For PX4 SITL, find this in the build log: [mavlink] ... on udp port <SITL_PORT> remote port 14550. Ask the user for this port if not known.

sitlHost = "172.x.x.x";  % IP of SITL instance (use "ip a" in WSL to find it)
sitlPort = 18570;         % PX4 SITL listening port (from SITL build log "udp port" line)
hbTimer = timer('ExecutionMode', 'fixedRate', 'Period', 1, ...
    'TimerFcn', @(~,~) sendudpmsg(mavlink, hbMsg, sitlHost, sitlPort));
start(hbTimer);

• sendudpmsg(io, msg, host, port) sends to a specific UDP endpoint — use when

the client is not yet discovered

• sendmsg(io, msg, client) sends to a discovered client — use after listClients

shows the autopilot

• MAVLink standard heartbeat rate is 1 Hz
• Do NOT use port 14550 as the remote port — that is the GCS local port.

The autopilot's listening port is different (e.g., PX4 SITL -u port).

6. Verify client discovery (Workflow B only)

After starting heartbeat via sendudpmsg, poll until the autopilot responds:

timeout = 10;
tic;
discovered = false;
while toc < timeout
    clients = listClients(mavlink);
    if height(clients) > 1  % local GCS client is always listed
        discovered = true;
        break;
    end
    pause(0.5);
end

if discovered
    disp(clients);
else
    error("Autopilot not discovered within %d seconds.", timeout);
end

listClients(mavlink) uses the built-in heartbeat subscriber — no manual mavlinksub needed. For Workflow A, discovery is already done in Step 5.

7. Clean up when done

stop(hbTimer);
delete(hbTimer);
disconnect(mavlink);

Always stop and delete the timer before disconnecting to prevent orphaned timers.

Key Functions

Patterns

UDP Connection to PX4 SITL (Auto-Discovery)

PX4 SITL configured to broadcast to GCS port 14550 (check build log for remote port 14550). The autopilot appears in listClients automatically.

% Setup
dialect = mavlinkdialect("common.xml", 2);
mavlink = mavlinkio(dialect, 'SystemID', 255, 'ComponentID', 1);
connect(mavlink, "UDP", LocalPort=14550);

% Build GCS heartbeat
hbMsg = createmsg(dialect, "HEARTBEAT");
hbMsg.Payload.type(:) = dialect.enum2num("MAV_TYPE", "MAV_TYPE_GCS");
hbMsg.Payload.autopilot(:) = dialect.enum2num("MAV_AUTOPILOT", "MAV_AUTOPILOT_INVALID");
hbMsg.Payload.base_mode(:) = 0;
hbMsg.Payload.custom_mode(:) = 0;
hbMsg.Payload.system_status(:) = 0;

% Wait for autopilot to be discovered
timeout = 10;
tic;
while toc < timeout
    clients = listClients(mavlink);
    if height(clients) > 1
        break;
    end
    pause(0.5);
end
disp(clients);

% Start GCS heartbeat back to the discovered autopilot
remoteClient = clients(clients.SystemID ~= 255, :);
autopilot = mavlinkclient(mavlink, remoteClient.SystemID, remoteClient.ComponentID);
hbTimer = timer('ExecutionMode', 'fixedRate', 'Period', 1, ...
    'TimerFcn', @(~,~) sendmsg(mavlink, hbMsg, autopilot));
start(hbTimer);

% ... perform operations ...

% Clean up
stop(hbTimer);
delete(hbTimer);
disconnect(mavlink);

Subscribing to Specific Messages

When you need to monitor a specific message type (beyond connection verification):

% Subscribe to all messages of a type
sub = mavlinksub(mavlink, "GLOBAL_POSITION_INT");

% Subscribe to messages from a specific discovered client
remoteClient = clients(clients.SystemID ~= 255, :);
autopilot = mavlinkclient(mavlink, remoteClient.SystemID, remoteClient.ComponentID);
sub = mavlinksub(mavlink, autopilot, "ATTITUDE");

% Read the latest message(s)
msgs = latestmsgs(sub, 1);
if ~isempty(msgs)
    disp(msgs.Payload);
end

Sending Commands After Discovery

Once a client is discovered via listClients, you can use sendmsg with the client:

% After discovery, send to specific client
clients = listClients(mavlink);
if height(clients) > 1
    remoteClient = clients(clients.SystemID ~= 255, :);
    autopilot = mavlinkclient(mavlink, remoteClient.SystemID, remoteClient.ComponentID);

    % Create command message
    cmdMsg = createmsg(dialect, "COMMAND_LONG");
    cmdMsg.Payload.target_system(:) = remoteClient.SystemID;
    cmdMsg.Payload.target_component(:) = remoteClient.ComponentID;
    cmdMsg.Payload.command(:) = 400;  % MAV_CMD_COMPONENT_ARM_DISARM
    cmdMsg.Payload.param1(:) = 1;     % arm

    sendmsg(mavlink, cmdMsg, autopilot);
end

Heartbeat Timer with Error Handling

For robust applications, wrap the timer callback to prevent silent failures:

% Using sendudpmsg (pre-discovery, to known SITL endpoint)
hbTimer = timer('ExecutionMode', 'fixedRate', 'Period', 1, ...
    'ErrorFcn', @(~,evt) warning("Heartbeat error: %s", evt.Data.message), ...
    'TimerFcn', @(~,~) sendudpmsg(mavlink, hbMsg, sitlHost, sitlPort));
start(hbTimer);

Gotchas

• Port 14550 is the GCS local port, not the autopilot's port. When using

sendudpmsg, the remote port must be the autopilot's listening port (PX4 SITL -u flag), not 14550. If the autopilot is already discovered via listClients, use sendmsg(io, msg, client) instead — it routes automatically.

• Use `sendudpmsg` for pre-discovery messages, not `sendmsg`. Calling

sendmsg(io, msg, client) throws an error if the client hasn't been discovered yet. Use sendudpmsg(io, msg, host, port) for heartbeats and any pre-discovery communication.

• All message fields live under `.Payload`. Write msg.Payload.type(:) = ...,

never msg.type(:) = .... The top-level message struct contains metadata; protocol fields are always at msg.Payload.fieldname.

• Always use `(:)` on payload field assignments. msg.Payload.field(:) = value

preserves the wire type (uint8, uint16, int32, etc.). Without (:), MATLAB replaces the field with a double, producing corrupted MAVLink packets on the wire. This bug is silent — no error, no warning — and only manifests during interop.

• `createmsg(dialect, msgName)` — dialect is the first argument. Do NOT pass

mavlinkio to createmsg. The io object sends messages; the dialect creates them.

• `connect()` uses name-value pairs only. It is connect(io, "UDP", LocalPort=14550)

not connect(io, "udpin", "0.0.0.0", 14550).

• Do NOT create `mavlinksub(mavlink, "HEARTBEAT")` for connection verification.

mavlinkio already has a built-in heartbeat subscriber that populates listClients(). A manual HEARTBEAT subscriber is redundant. Only use mavlinksub for non-heartbeat message types (e.g., "GLOBAL_POSITION_INT", "ATTITUDE").

• `listClients` always includes the local GCS client. Check height(clients) > 1

to confirm a remote system was discovered, not > 0.

• `mavlinkclient(io, sysID, compID)` is a constructor, not a listing function.

It creates a handle — it does not verify the client exists. Use listClients(io) to check for discovered clients.

• The subscriber read method is `latestmsgs(sub, count)` — not read(),

receive(), or next().

• Always stop and delete timers. Orphaned timers continue running after

disconnect and can cause MATLAB instability. Use stop(t); delete(t) or wrap in onCleanup.

Conventions

• GCS identity: SystemID 255, ComponentID 1
• GCS local port (what GCS binds to): 14550
• PX4 SITL listening port: varies (check SITL build log for udp port <N>; e.g., 18570)
• ArduPilot SITL default outbound port to GCS: 14550
• Typical autopilot identity: SystemID 1, ComponentID 1
• Heartbeat rate: 1 Hz (MAVLink standard)
• Use "common.xml" dialect unless ArduPilot-specific extensions are needed

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