mavros_lite

mavros_lite_node is a small MAVROS-compatible ROS 2 bridge for the local PX4 simulator. It binds one loopback UDP port and learns the PX4 source endpoint from the first MAVLink packet. It does not use the MAVROS UAS, router, or pluginlib runtime.

The implementation is exported as the fss_mavros_lite shared library. MavrosLite is a transport-free ROS/MAVLink adapter that is constructed with an existing rclcpp::Node. It adds its publishers, subscriptions, services, and timers to that host node; constructing it never opens a socket, starts a receiver thread, or creates a second ROS node. It can be fed already-decoded mavlink_message_t values by another program. MavlinkUdpTransport is a separate optional class in the fss_mavros_lite_udp library, used by the standalone node. PX4 direct ROS transport links only fss_mavros_lite.

The node exposes the standard topic and service names below its namespace for the setpoint_raw, local_position, imu, sys_status, command, and global_position modules. The existing PX4 launch starts it at /mavros.

Parameters:

Parameter	Default	Meaning
udp.bind_port	24540	Loopback port receiving PX4 MAVLink packets
udp.remote_host	127.0.0.1	Fixed remote address; loopback only
udp.remote_port	0	Fixed PX4 port, or 0 to learn it
target_system	1	Accepted PX4 MAVLink system id
target_component	1	Target component for outgoing messages
system_id	1	MAVLink system id used by the bridge
component_id	191	MAVLink component id used by the bridge
command.ack_timeout	5.0	Seconds to wait for COMMAND_ACK

Standalone example:

ros2 run fss_px4_sim mavros_lite_node --ros-args \
  -r __ns:=/mavros -p udp.bind_port:=24540 -p target_system:=1

PX4 simulator launch transport:

# PX4 and a separate MAVROS Lite process communicate by UDP.
ros2 launch fss_px4_sim px4_rotor_sim_single.launch.py \
  px4.mavlink_transport:=udp mavros_type:=lite

# In-process behavior: PX4 owns mavros_lite and exchanges decoded messages.
ros2 launch fss_px4_sim px4_rotor_sim_single.launch.py \
  px4.mavlink_transport:=direct_ros

PX4 accepts px4.mavlink_transport values udp, direct_ros, and empty. In direct_ros mode the UDP port parameters are unused and MAVROS Lite is embedded in the PX4 process. In empty mode PX4 does not create its MAVLink module. The mavros_type launch argument defaults to disabled; use official to start the installed MAVROS node or lite to start mavros_lite_node. The latter two options require px4.mavlink_transport:=udp; otherwise launch prints a warning and starts no external MAVROS node. This argument does not modify PX4 transport parameters.

The official MAVROS branch is allowlisted to the same six plugins. Its sys_status plugin requests AUTOPILOT_VERSION; the reduced PX4 simulator does not currently implement MAV_CMD_REQUEST_AUTOPILOT_CAPABILITIES, so official MAVROS logs a timeout and falls back to default capabilities.

PX4 uses the same separation internally: MAVLINK owns the receiver, streamer, transport selection, and direct ROS bridge, while Px4MavlinkUdpTransport exclusively owns the PX4 UDP socket, byte encoding/parsing, and receive thread.

Embedding example:

#include <fss_px4_sim/mavros_lite/core.hpp>

fss_px4_sim::mavros_lite::MavrosLite::Config config;
auto lite = std::make_shared<fss_px4_sim::mavros_lite::MavrosLite>(*node, config);

lite->set_send_callback([](const mavlink_message_t &message) {
  // Deliver an outgoing ROS-generated MAVLink message to the flight stack.
});
lite->receive_message(message_from_flight_stack);

Add lite to the application's executor. Calls to receive_message() dispatch to the six Lite modules, while their outgoing messages are delivered through the callback installed by set_send_callback().