# Sending a Custom Message from MAVROS to PX4


This article has been tested against:

  • Ubuntu: 18.04
  • ROS: Melodic
  • PX4 Firmware: 1.9.0

However these steps are fairly general and so it should work with other distros/versions with little to no modifications.

# MAVROS Installation

Follow Source Installation instructions from mavlink/mavros (opens new window) to install "ROS Kinetic".


  1. We start by creating a new MAVROS plugin, in this example named keyboard_command.cpp (in workspace/src/mavros/mavros_extras/src/plugins) by using the code below:

    The code subscribes a 'char' message from ROS topic /mavros/keyboard_command/keyboard_sub and sends it as a MAVLink message.

     #include <mavros/mavros_plugin.h>
     #include <pluginlib/class_list_macros.h>
     #include <iostream>
     #include <std_msgs/Char.h>
     namespace mavros {
     namespace extra_plugins{
     class KeyboardCommandPlugin : public plugin::PluginBase {
         KeyboardCommandPlugin() : PluginBase(),
        { };
         void initialize(UAS &uas_)
             keyboard_sub = nh.subscribe("keyboard_sub", 10, &KeyboardCommandPlugin::keyboard_cb, this);
         Subscriptions get_subscriptions()
             return {/* RX disabled */ };
         ros::NodeHandle nh;
         ros::Subscriber keyboard_sub;
        void keyboard_cb(const std_msgs::Char::ConstPtr &req)
             std::cout << "Got Char : " << req->data <<  std::endl;
     }   // namespace extra_plugins
     }   // namespace mavros
    PLUGINLIB_EXPORT_CLASS(mavros::extra_plugins::KeyboardCommandPlugin, mavros::plugin::PluginBase)
  2. Edit mavros_plugins.xml (in workspace/src/mavros/mavros_extras) and add the following lines:

    <class name="keyboard_command" type="mavros::extra_plugins::KeyboardCommandPlugin" base_class_type="mavros::plugin::PluginBase">
         <description>Accepts keyboard command.</description>
  3. Edit CMakeLists.txt (in workspace/src/mavros/mavros_extras) and add the following line in add_library.

  4. Inside common.xml in (workspace/src/mavlink/message_definitions/v1.0), copy the following lines to add your MAVLink message:

      <message id="229" name="KEY_COMMAND">
         <description>Keyboard char command.</description>
         <field type="char" name="command"> </field>

# PX4 Changes

  1. Inside common.xml (in PX4-Autopilot/mavlink/include/mavlink/v2.0/message_definitions), add your MAVLink message as following (same procedure as for MAVROS section above):

      <message id="229" name="KEY_COMMAND">
         <description>Keyboard char command.</description>
         <field type="char" name="command"> </field>
  2. Remove common, standard directories in (PX4-Autopilot/mavlink/include/mavlink/v2.0).

    rm -r common
    rm -r standard
  3. Git clone "mavlink_generator" to any directory you want and execute it.

    git clone https://github.com/mavlink/mavlink mavlink-generator
    cd mavlink-generator
    python mavgenerate.py
  4. You will see a "MAVLink Generator" popup:

    • For XML, "Browse" to /PX4-Autopilot/mavlink/include/mavlink/v2.0/message_definitions/standard.xml.
    • For Out, "Browse" to /PX4-Autopilot/mavlink/include/mavlink/v2.0/.
    • Select Language C
    • Select Protocol 2.0
    • Check Validate

    Then, press Generate. You will see common, and standard directories created in /PX4-Autopilot/mavlink/include/mavlink/v2.0/.

  5. Make your own uORB message file key_command.msg in (PX4-Autopilot/msg). For this example the "key_command.msg" has only the code:

    char cmd

    Then, in CMakeLists.txt (in PX4-Autopilot/msg), include

  6. Edit mavlink_receiver.h (in PX4-Autopilot/src/modules/mavlink)

    #include <uORB/topics/key_command.h>
    class MavlinkReceiver
        void handle_message_key_command(mavlink_message_t *msg);
        orb_advert_t _key_command_pub{nullptr};
  7. Edit mavlink_receiver.cpp (in PX4-Autopilot/src/modules/mavlink). This is where PX4 receives the MAVLink message sent from ROS, and publishes it as a uORB topic.

    void MavlinkReceiver::handle_message(mavlink_message_t *msg)
    MavlinkReceiver::handle_message_key_command(mavlink_message_t *msg)
        mavlink_key_command_t man;
        mavlink_msg_key_command_decode(msg, &man);
    struct key_command_s key = {};
        key.timestamp = hrt_absolute_time();
        key.cmd = man.command;
        if (_key_command_pub == nullptr) {
            _key_command_pub = orb_advertise(ORB_ID(key_command), &key);
        } else {
            orb_publish(ORB_ID(key_command), _key_command_pub, &key);
  8. Make your own uORB topic subscriber just like any example subscriber module. For this example lets create the model in (/PX4-Autopilot/src/modules/key_receiver). In this directory, create two files CMakeLists.txt, key_receiver.cpp. Each one looks like following.


        MODULE modules__key_receiver
        MAIN key_receiver
        STACK_MAIN 2500
        STACK_MAX 4000


    #include <px4_config.h>
    #include <px4_tasks.h>
    #include <px4_posix.h>
    #include <unistd.h>
    #include <stdio.h>
    #include <poll.h>
    #include <string.h>
    #include <math.h>
    #include <uORB/uORB.h>
    #include <uORB/topics/key_command.h>
    extern "C" __EXPORT int key_receiver_main(int argc, char **argv);
    int key_receiver_main(int argc, char **argv)
        int key_sub_fd = orb_subscribe(ORB_ID(key_command));
        orb_set_interval(key_sub_fd, 200); // limit the update rate to 200ms
        px4_pollfd_struct_t fds[1];
        fds[0].fd = key_sub_fd, fds[0].events = POLLIN;
        int error_counter = 0;
            int poll_ret = px4_poll(fds, 1, 1000);
            if (poll_ret == 0)
                PX4_ERR("Got no data within a second");
            else if (poll_ret < 0)
                if (error_counter < 10 || error_counter % 50 == 0)
                    PX4_ERR("ERROR return value from poll(): %d", poll_ret);
                if (fds[0].revents & POLLIN)
                    struct key_command_s input;
                    orb_copy(ORB_ID(key_command), key_sub_fd, &input);
                    PX4_INFO("Recieved Char : %c", input.cmd);
        return 0;

    For a more detailed explanation see the topic Writing your first application.

  9. Lastly add your module in the default.cmake file correspondent to your board in PX4-Autopilot/boards/. For example for the Pixhawk 4 add the following code in PX4-Autopilot/boards/px4/fmu-v5/default.cmake:


Now you are ready to build all your work!

# Building

# Build for ROS

  1. In your workspace enter: catkin build.

  2. Beforehand, you have to set your "px4.launch" in (/workspace/src/mavros/mavros/launch). Edit "px4.launch" as below. If you are using USB to connect your computer with Pixhawk, you have to set "fcu_url" as shown below. But, if you are using CP2102 to connect your computer with Pixhawk, you have to replace "ttyACM0" with "ttyUSB0". Modifying "gcs_url" is to connect your Pixhawk with UDP, because serial communication cannot accept MAVROS, and your nutshell connection simultaneously.

  3. Write your IP address at "xxx.xx.xxx.xxx"

      <arg name="fcu_url" default="/dev/ttyACM0:57600" />
      <arg name="gcs_url" default="udp://:14550@xxx.xx.xxx.xxx:14557" />

# Build for PX4

  1. Build PX4-Autopilot and upload in the normal way.

    For example, to build for Pixhawk 4/FMUv5 execute the following command in the root of the PX4-Autopilot directory:

    make px4_fmu-v5_default upload

# Running the Code

Next test if the MAVROS message is sent to PX4.

# Running ROS

  1. In a terminal enter
    roslaunch mavros px4.launch
  2. In a second terminal run:
    rostopic pub -r 10 /mavros/keyboard_command/keyboard_sub std_msgs/Char 97
    This means, publish 97 ('a' in ASCII) to ROS topic "/mavros/keyboard_command/keyboard_sub" in message type "std_msgs/Char". "-r 10" means to publish continuously in "10Hz".

# Running PX4

  1. Enter the Pixhawk nutshell through UDP. Replace xxx.xx.xxx.xxx with your IP.

    cd PX4-Autopilot/Tools
    ./mavlink_shell.py xxx.xx.xxx.xxx:14557 --baudrate 57600
  2. After few seconds, press Enter a couple of times. You should see a prompt in the terminal as below:


    Type "key_receiver", to run your subscriber module.

    nsh> key_receiver

Check if it successfully receives a from your ROS topic.