# ROS 2 User Guide The ROS 2-PX4 architecture provides a deep integration between ROS 2 and PX4, allowing ROS 2 subscribers or publisher nodes to interface directly with PX4 uORB topics. This topic provides an overview of the architecture and application pipeline, and explains how to setup and use ROS 2 with PX4. :::note From PX4 v1.14, ROS 2 uses [uXRCE-DDS](/px4-user-guide/development/middleware/uxrce_dds.md) middleware, replacing the *FastRTPS* middleware that was used in version 1.13 (v1.13 does not support uXRCE-DDS). The [migration guide](/px4-user-guide/development/middleware/uxrce_dds.md#fast-rtps-to-uxrce-dds-migration-guidelines) explains what you need to do in order to migrate ROS 2 apps from PX4 v1.13 to PX4 v1.14. If you're still working on PX4 v1.13, please follow the instructions in the [PX4 v1.13 Docs](https://docs.px4.io/v1.13/en/ros/ros2_comm.html). ::: ## Overview The application pipeline for ROS 2 is very straightforward, thanks to the use of the [uXRCE-DDS](/px4-user-guide/development/middleware/uxrce_dds.md) communications middleware. ![Architecture uXRCE-DDS with ROS 2](/files/SzkR4JCCd6P1SuMz65SL) The uXRCE-DDS middleware consists of a client running on PX4 and an agent running on the companion computer, with bi-directional data exchange between them over a serial, UDP, TCP or custom link. The agent acts as a proxy for the client to publish and subscribe to topics in the global DDS data space. The PX4 [uxrce\_dds\_client](/px4-user-guide/development/modules_main/modules_system.md#uxrce-dds-client) is generated at build time and included in PX4 firmware by default. It includes both the "generic" micro XRCE-DDS client code, and PX4-specific translation code that it uses to publish to/from uORB topics. The subset of uORB messages that are generated into the client are listed in [PX4-Autopilot/src/modules/uxrce\_dds\_client/dds\_topics.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/dds_topics.yaml). The generator uses the uORB message definitions in the source tree: [PX4-Autopilot/msg](https://github.com/PX4/PX4-Autopilot/tree/main/msg) to create the code for sending ROS 2 messages. ROS 2 applications need to be built in a workspace that has the *same* message definitions that were used to create the uXRCE-DDS client module in the PX4 Firmware. You can include these by cloning the interface package [PX4/px4\_msgs](https://github.com/PX4/px4_msgs) into your ROS 2 workspace (branches in the repo correspond to the messages for different PX4 releases). Note that the micro XRCE-DDS *agent* itself has no dependency on client-side code. It can be built from [source](https://github.com/eProsima/Micro-XRCE-DDS-Agent) either standalone or as part of a ROS build, or installed as a snap. You will normally need to start both the client and agent when using ROS 2. Note that the uXRCE-DDS client is built into firmware by default but not started automatically except for simulator builds. :::note In PX4v1.13 and earlier, ROS 2 was dependent on definitions in [px4\_ros\_com](https://github.com/PX4/px4_ros_com). This repo is no longer needed, but does contain useful examples. ::: ## Installation & Setup The supported ROS 2 platforms for PX4 development are ROS 2 "Humble" on Ubuntu 22.04, and ROS 2 "Foxy" on Ubuntu 20.04. ROS 2 "Humble" is recommended because it is the current ROS 2 LTS distribution. ROS 2 "Foxy" reached end-of-life in May 2023, but is still stable and works with PX4. :::note PX4 is not as well tested on Ubuntu 22.04 as it is on Ubuntu 20.04 (at time of writing), and Ubuntu 20.04 is needed if you want to use [Gazebo Classic](/px4-user-guide/development/simulation/sim_gazebo_classic.md). ::: To setup ROS 2 for use with PX4: * [Install PX4](#install-px4) (to use the PX4 simulator) * [Install ROS 2](#install-ros-2) * [Setup Micro XRCE-DDS Agent & Client](#setup-micro-xrce-dds-agent-client) * [Build & Run ROS 2 Workspace](#build-ros-2-workspace) Other dependencies of the architecture that are installed automatically, such as *Fast DDS*, are not covered. ### Install PX4 You need to install the PX4 development toolchain in order to use the simulator. :::note The only dependency ROS 2 has on PX4 is the set of message definitions, which it gets from [px4\_msgs](https://github.com/PX4/px4_msgs). You only need to install PX4 if you need the simulator (as we do in this guide), or if you are creating a build that publishes custom uORB topics. ::: Set up a PX4 development environment on Ubuntu in the normal way: ```sh cd git clone https://github.com/PX4/PX4-Autopilot.git --recursive bash ./PX4-Autopilot/Tools/setup/ubuntu.sh cd PX4-Autopilot/ make px4_sitl ``` Note that the above commands will install the recommended simulator for your version of Ubuntu. If you want to install PX4 but keep your existing simulator installation, run `ubuntu.sh` above with the `--no-sim-tools` flag. For more information and troubleshooting see: [Ubuntu Development Environment](/px4-user-guide/development/getting_started/dev_env/dev_env_linux_ubuntu.md) and [Download PX4 source](/px4-user-guide/development/getting_started/building_px4.md). ### Install ROS 2 To install ROS 2 and its dependencies: 1. Install ROS 2. :::: tabs ::: tab humble To install ROS 2 "Humble" on Ubuntu 22.04: ```sh sudo apt update && sudo apt install locales sudo locale-gen en_US en_US.UTF-8 sudo update-locale LC_ALL=en_US.UTF-8 LANG=en_US.UTF-8 export LANG=en_US.UTF-8 sudo apt install software-properties-common sudo add-apt-repository universe sudo apt update && sudo apt install curl -y sudo curl -sSL https://raw.githubusercontent.com/ros/rosdistro/master/ros.key -o /usr/share/keyrings/ros-archive-keyring.gpg echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/ros-archive-keyring.gpg] http://packages.ros.org/ros2/ubuntu $(. /etc/os-release && echo $UBUNTU_CODENAME) main" | sudo tee /etc/apt/sources.list.d/ros2.list > /dev/null sudo apt update && sudo apt upgrade -y sudo apt install ros-humble-desktop sudo apt install ros-dev-tools source /opt/ros/humble/setup.bash && echo "source /opt/ros/humble/setup.bash" >> .bashrc ``` The instructions above are reproduced from the official installation guide: [Install ROS 2 Humble](https://docs.ros.org/en/humble/Installation/Ubuntu-Install-Debians.html). You can install *either* the desktop (`ros-humble-desktop`) *or* bare-bones versions (`ros-humble-ros-base`), *and* the development tools (`ros-dev-tools`). ::: ::: tab foxy To install ROS 2 "Foxy" on Ubuntu 20.04: * Follow the official installation guide: [Install ROS 2 Foxy](https://index.ros.org/doc/ros2/Installation/Foxy/Linux-Install-Debians/). You can install *either* the desktop (`ros-foxy-desktop`) *or* bare-bones versions (`ros-foxy-ros-base`), *and* the development tools (`ros-dev-tools`). ::: :::: 2. Some Python dependencies must also be installed (using **`pip`** or **`apt`**): ```sh pip install --user -U empy pyros-genmsg setuptools ``` ### Setup Micro XRCE-DDS Agent & Client For ROS 2 to communicate with PX4, [uXRCE-DDS client](/px4-user-guide/development/modules_main/modules_system.md#uxrce-dds-client) must be running on PX4, connected to a micro XRCE-DDS agent running on the companion computer. #### Setup the Agent The agent can be installed onto the companion computer in a [number of ways](/px4-user-guide/development/middleware/uxrce_dds.md#micro-xrce-dds-agent-installation). Below we show how to build the agent "standalone" from source and connect to a client running on the PX4 simulator. To setup and start the agent: 1. Open a terminal. 2. Enter the following commands to fetch and build the agent from source: ```sh git clone https://github.com/eProsima/Micro-XRCE-DDS-Agent.git cd Micro-XRCE-DDS-Agent mkdir build cd build cmake .. make sudo make install sudo ldconfig /usr/local/lib/ ``` 3. Start the agent with settings for connecting to the uXRCE-DDS client running on the simulator: ```sh MicroXRCEAgent udp4 -p 8888 ``` The agent is now running, but you won't see much until we start PX4 (in the next step). :::note You can leave the agent running in this terminal! Note that only one agent is allowed per connection channel. ::: #### Start the Client The PX4 simulator starts the uXRCE-DDS client automatically, connecting to UDP port 8888 on the local host. To start the simulator (and client): 1. Open a new terminal in the root of the **PX4 Autopilot** repo that was installed above. :::: tabs ::: tab humble * Start a PX4 [Gazebo](/px4-user-guide/development/simulation/sim_gazebo_gz.md) simulation using: ```sh make px4_sitl gz_x500 ``` ::: ::: tab foxy * Start a PX4 [Gazebo Classic](/px4-user-guide/development/simulation/sim_gazebo_classic.md) simulation using: ```sh make px4_sitl gazebo-classic ``` ::: :::: The agent and client are now running they should connect. The PX4 terminal displays the [NuttShell/PX4 System Console](/px4-user-guide/development/debug/consoles/system_console.md) output as PX4 boots and runs. As soon as the agent connects the output should include `INFO` messages showing creation of data writers: ``` ... INFO [uxrce_dds_client] synchronized with time offset 1675929429203524us INFO [uxrce_dds_client] successfully created rt/fmu/out/failsafe_flags data writer, topic id: 83 INFO [uxrce_dds_client] successfully created rt/fmu/out/sensor_combined data writer, topic id: 168 INFO [uxrce_dds_client] successfully created rt/fmu/out/timesync_status data writer, topic id: 188 ... ``` The micro XRCE-DDS agent terminal should also start to show output, as equivalent topics are created in the DDS network: ``` ... [1675929445.268957] info | ProxyClient.cpp | create_publisher | publisher created | client_key: 0x00000001, publisher_id: 0x0DA(3), participant_id: 0x001(1) [1675929445.269521] info | ProxyClient.cpp | create_datawriter | datawriter created | client_key: 0x00000001, datawriter_id: 0x0DA(5), publisher_id: 0x0DA(3) [1675929445.270412] info | ProxyClient.cpp | create_topic | topic created | client_key: 0x00000001, topic_id: 0x0DF(2), participant_id: 0x001(1) ... ``` ### Build ROS 2 Workspace This section shows how create a ROS 2 workspace hosted in your home directory (modify the commands as needed to put the source code elsewhere). The [px4\_ros\_com](https://github.com/PX4/px4_ros_com) and [px4\_msgs](https://github.com/PX4/px4_msgs) packages are cloned to a workspace folder, and then the `colcon` tool is used to build the workspace. The example is run using `ros2 launch`. :::note The example builds the [ROS 2 Listener](#ros-2-listener) example application, located in [px4\_ros\_com](https://github.com/PX4/px4_ros_com). [px4\_msgs](https://github.com/PX4/px4_msgs) is needed too so that the example can interpret PX4 ROS 2 topics. ::: #### Building the Workspace To create and build the workspace: 1. Open a new terminal. 2. Create and navigate into a new workspace directory using: ```sh mkdir -p ~/ws_sensor_combined/src/ cd ~/ws_sensor_combined/src/ ``` :::note A naming convention for workspace folders can make it easier to manage workspaces. ::: 3. Clone the example repository and [px4\_msgs](https://github.com/PX4/px4_msgs) to the `/src` directory (the `main` branch is cloned by default, which corresponds to the version of PX4 we are running): ```sh git clone https://github.com/PX4/px4_msgs.git git clone https://github.com/PX4/px4_ros_com.git ``` 4. Source the ROS 2 development environment into the current terminal and compile the workspace using `colcon`: :::: tabs ::: tab humble ```sh cd .. source /opt/ros/humble/setup.bash colcon build ``` ::: ::: tab foxy ```sh cd .. source /opt/ros/foxy/setup.bash colcon build ``` ::: :::: This builds all the folders under `/src` using the sourced toolchain. #### Running the Example To run the executables that you just built, you need to source `local_setup.bash`. This provides access to the "environment hooks" for the current workspace. In other words, it makes the executables that were just built available in the current terminal. :::note The [ROS2 beginner tutorials](https://docs.ros.org/en/humble/Tutorials/Beginner-Client-Libraries/Creating-A-Workspace/Creating-A-Workspace.html#source-the-overlay) recommend that you *open a new terminal* for running your executables. ::: In a new terminal: 1. Navigate into the top level of your workspace directory and source the ROS 2 environment (in this case "Humble"): :::: tabs ::: tab humble ```sh cd ~/ws_sensor_combined/ source /opt/ros/humble/setup.bash ``` ::: ::: tab foxy ```sh cd ~/ws_sensor_combined/ source /opt/ros/foxy/setup.bash ``` ::: :::: 2. Source the `local_setup.bash`. ```sh source install/local_setup.bash ``` 3. Now launch the example. Note here that we use `ros2 launch`, which is described below. ``` ros2 launch px4_ros_com sensor_combined_listener.launch.py ``` If this is working you should see data being printed on the terminal/console where you launched the ROS listener: ```sh RECEIVED DATA FROM SENSOR COMBINED ================================ ts: 870938190 gyro_rad[0]: 0.00341645 gyro_rad[1]: 0.00626475 gyro_rad[2]: -0.000515705 gyro_integral_dt: 4739 accelerometer_timestamp_relative: 0 accelerometer_m_s2[0]: -0.273381 accelerometer_m_s2[1]: 0.0949186 accelerometer_m_s2[2]: -9.76044 accelerometer_integral_dt: 4739 ``` ## Controlling a Vehicle To control applications, ROS 2 applications: * subscribe to (listen to) telemetry topics published by PX4 * publish to topics that cause PX4 to perform some action. The topics that you can use are defined in [dds\_topics.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/dds_topics.yaml), and you can get more information about their data in the [uORB Message Reference](/px4-user-guide/development/middleware/msg_docs.md). For example, [VehicleGlobalPosition](/px4-user-guide/development/middleware/msg_docs/vehicleglobalposition.md) can be used to get the vehicle global position, while [VehicleCommand](/px4-user-guide/development/middleware/msg_docs/vehiclecommand.md) can be used to command actions such as takeoff and land. The [ROS 2 Example applications](#ros-2-example-applications) examples below provide concrete examples of how to use these topics. ## Compatibility Issues This section contains information that may affect how you write your ROS code. ### ROS 2 Subscriber QoS Settings ROS 2 code that subscribes to topics published by PX4 *must* specify a appropriate (compatible) QoS setting in order to listen to topics. Specifically, nodes should subscribe using the ROS 2 predefined QoS sensor data (from the [listener example source code](#ros-2-listener)): ```cpp ... rmw_qos_profile_t qos_profile = rmw_qos_profile_sensor_data; auto qos = rclcpp::QoS(rclcpp::QoSInitialization(qos_profile.history, 5), qos_profile); subscription_ = this->create_subscription("/fmu/out/sensor_combined", qos, ... ``` This is needed because the ROS 2 default [Quality of Service (QoS) settings](https://docs.ros.org/en/humble/Concepts/About-Quality-of-Service-Settings.html#qos-profiles) are different from the settings used by PX4. Not all combinations of publisher-subscriber [Qos settings are possible](https://docs.ros.org/en/humble/Concepts/About-Quality-of-Service-Settings.html#qos-compatibilities), and it turns out that the default ROS 2 settings for subscribing are not! Note that ROS code does not have to set QoS settings when publishing (the PX4 settings are compatible with ROS defaults in this case). ### ROS 2 & PX4 Frame Conventions The local/world and body frames used by ROS and PX4 are different. | Frame | PX4 | ROS | | ----- | ------------------------------------------------ | ---------------------------------------------- | | Body | FRD (X **F**orward, Y **R**ight, Z **D**own) | FLU (X **F**orward, Y **L**eft, Z **U**p) | | World | FRD or NED (X **N**orth, Y **E**ast, Z **D**own) | FLU or ENU (X **E**ast, Y **N**orth, Z **U**p) | :::tip See [REP105: Coordinate Frames for Mobile Platforms](http://www.ros.org/reps/rep-0105.html) for more information about ROS frames. ::: Both frames are shown in the image below (FRD on the left/FLU on the right). ![Reference frames](/files/bqMzWYlHsl3Ail6dXrdH) The FRD (NED) conventions are adopted on **all** PX4 topics unless explicitly specified in the associated message definition. Therefore, ROS 2 nodes that want to interface with PX4 must take care of the frames conventions. * To rotate a vector from ENU to NED two basic rotations must be performed: * first a pi/2 rotation around the `Z`-axis (up), * then a pi rotation around the `X`-axis (old East/new North). * To rotate a vector from NED to ENU two basic rotations must be performed: * * first a pi/2 rotation around the `Z`-axis (down), * then a pi rotation around the `X`-axis (old North/new East). Note that the two resulting operations are mathematically equivalent. * To rotate a vector from FLU to FRD a pi rotation around the `X`-axis (front) is sufficient. * To rotate a vector from FRD to FLU a pi rotation around the `X`-axis (front) is sufficient. Examples of vectors that require rotation are: * all fields in [TrajectorySetpoint](/px4-user-guide/development/middleware/msg_docs/trajectorysetpoint.md) message; ENU to NED conversion is required before sending them. * all fields in [VehicleThrustSetpoint](/px4-user-guide/development/middleware/msg_docs/vehiclethrustsetpoint.md) message; FLU to FRD conversion is required before sending them. Similarly to vectors, also quanternions representing the attitude of the vehicle (body frame) w\.r.t. the world frame require conversion. [PX4/px4\_ros\_com](https://github.com/PX4/px4_ros_com) provides the shared library [frame\_transforms](https://github.com/PX4/px4_ros_com/blob/main/include/px4_ros_com/frame_transforms.h) to easily perform such conversions. ## ROS 2 Example Applications ### ROS 2 Listener The ROS 2 [listener examples](https://github.com/PX4/px4_ros_com/tree/main/src/examples/listeners) in the [px4\_ros\_com](https://github.com/PX4/px4_ros_com) repo demonstrate how to write ROS nodes to listen to topics published by PX4. Here we consider the [sensor\_combined\_listener.cpp](https://github.com/PX4/px4_ros_com/blob/main/src/examples/listeners/sensor_combined_listener.cpp) node under `px4_ros_com/src/examples/listeners`, which subscribes to the [SensorCombined](/px4-user-guide/development/middleware/msg_docs/sensorcombined.md) message. :::note [Build ROS 2 Workspace](#build-ros-2-workspace) shows how to build and run this example. ::: The code first imports the C++ libraries needed to interface with the ROS 2 middleware and the header file for the `SensorCombined` message to which the node subscribes: ```cpp #include #include ``` Then it creates a `SensorCombinedListener` class that subclasses the generic `rclcpp::Node` base class. ```cpp /** * @brief Sensor Combined uORB topic data callback */ class SensorCombinedListener : public rclcpp::Node { ``` This creates a callback function for when the `SensorCombined` uORB messages are received (now as micro XRCE-DDS messages), and outputs the content of the message fields each time the message is received. ```cpp public: explicit SensorCombinedListener() : Node("sensor_combined_listener") { rmw_qos_profile_t qos_profile = rmw_qos_profile_sensor_data; auto qos = rclcpp::QoS(rclcpp::QoSInitialization(qos_profile.history, 5), qos_profile); subscription_ = this->create_subscription("/fmu/out/sensor_combined", qos, [this](const px4_msgs::msg::SensorCombined::UniquePtr msg) { std::cout << "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n"; std::cout << "RECEIVED SENSOR COMBINED DATA" << std::endl; std::cout << "=============================" << std::endl; std::cout << "ts: " << msg->timestamp << std::endl; std::cout << "gyro_rad[0]: " << msg->gyro_rad[0] << std::endl; std::cout << "gyro_rad[1]: " << msg->gyro_rad[1] << std::endl; std::cout << "gyro_rad[2]: " << msg->gyro_rad[2] << std::endl; std::cout << "gyro_integral_dt: " << msg->gyro_integral_dt << std::endl; std::cout << "accelerometer_timestamp_relative: " << msg->accelerometer_timestamp_relative << std::endl; std::cout << "accelerometer_m_s2[0]: " << msg->accelerometer_m_s2[0] << std::endl; std::cout << "accelerometer_m_s2[1]: " << msg->accelerometer_m_s2[1] << std::endl; std::cout << "accelerometer_m_s2[2]: " << msg->accelerometer_m_s2[2] << std::endl; std::cout << "accelerometer_integral_dt: " << msg->accelerometer_integral_dt << std::endl; }); } ``` :::note The subscription sets a QoS profile based on `rmw_qos_profile_sensor_data`. This is needed because the default ROS 2 QoS profile for subscribers is incompatible with the PX4 profile for publishers. For more information see: [ROS 2 Subscriber QoS Settings](#ros-2-subscriber-qos-settings), ::: The lines below create a publisher to the `SensorCombined` uORB topic, which can be matched with one or more compatible ROS 2 subscribers to the `fmu/sensor_combined/out` ROS 2 topic. ```cpp private: rclcpp::Subscription::SharedPtr subscription_; }; ``` The instantiation of the `SensorCombinedListener` class as a ROS node is done on the `main` function. ```cpp int main(int argc, char *argv[]) { std::cout << "Starting sensor_combined listener node..." << std::endl; setvbuf(stdout, NULL, _IONBF, BUFSIZ); rclcpp::init(argc, argv); rclcpp::spin(std::make_shared()); rclcpp::shutdown(); return 0; } ``` This particular example has an associated launch file at [launch/sensor\_combined\_listener.launch.py](https://github.com/PX4/px4_ros_com/blob/main/launch/sensor_combined_listener.launch.py). This allows it to be launched using the [`ros2 launch`](#ros2-launch) command. ### ROS 2 Advertiser A ROS 2 advertiser node publishes data into the DDS/RTPS network (and hence to the PX4 Autopilot). Taking as an example the `debug_vect_advertiser.cpp` under `px4_ros_com/src/advertisers`, first we import required headers, including the `debug_vect` msg header. ```cpp #include #include #include using namespace std::chrono_literals; ``` Then the code creates a `DebugVectAdvertiser` class that subclasses the generic `rclcpp::Node` base class. ```cpp class DebugVectAdvertiser : public rclcpp::Node { ``` The code below creates a function for when messages are to be sent. The messages are sent based on a timed callback, which sends two messages per second based on a timer. ```cpp public: DebugVectAdvertiser() : Node("debug_vect_advertiser") { publisher_ = this->create_publisher("fmu/debug_vect/in", 10); auto timer_callback = [this]()->void { auto debug_vect = px4_msgs::msg::DebugVect(); debug_vect.timestamp = std::chrono::time_point_cast(std::chrono::steady_clock::now()).time_since_epoch().count(); std::string name = "test"; std::copy(name.begin(), name.end(), debug_vect.name.begin()); debug_vect.x = 1.0; debug_vect.y = 2.0; debug_vect.z = 3.0; RCLCPP_INFO(this->get_logger(), "\033[97m Publishing debug_vect: time: %llu x: %f y: %f z: %f \033[0m", debug_vect.timestamp, debug_vect.x, debug_vect.y, debug_vect.z); this->publisher_->publish(debug_vect); }; timer_ = this->create_wall_timer(500ms, timer_callback); } private: rclcpp::TimerBase::SharedPtr timer_; rclcpp::Publisher::SharedPtr publisher_; }; ``` The instantiation of the `DebugVectAdvertiser` class as a ROS node is done on the `main` function. ```cpp int main(int argc, char *argv[]) { std::cout << "Starting debug_vect advertiser node..." << std::endl; setvbuf(stdout, NULL, _IONBF, BUFSIZ); rclcpp::init(argc, argv); rclcpp::spin(std::make_shared()); rclcpp::shutdown(); return 0; } ``` ### Offboard Control For a complete reference example on how to use Offboard control with PX4, see: [ROS 2 Offboard control example](/px4-user-guide/robotics/ros/ros2/ros2_offboard_control.md). ## Using Flight Controller Hardware ROS 2 with PX4 running on a flight controller is almost the same as working with PX4 on the simulator. The only difference is that you need to start both the agent *and the client*, with settings appropriate for the communication channel. For more information see [Starting uXRCE-DDS](/px4-user-guide/development/middleware/uxrce_dds.md#starting-agent-and-client). ## Custom uORB Topics ROS 2 needs to have the *same* message definitions that were used to create the uXRCE-DDS client module in the PX4 Firmware in order to interpret the messages. The definition are stored in the ROS 2 interface package [PX4/px4\_msgs](https://github.com/PX4/px4_msgs) and they are automatically synchronized by CI on the `main` and release branches. Note that all the messages from PX4 source code are present in the repository, but only those listed in `dds_topics.yaml` will be available as ROS 2 topics. Therefore, * If you're using a main or release version of PX4 you can get the message definitions by cloning the interface package [PX4/px4\_msgs](https://github.com/PX4/px4_msgs) into your workspace. * If you're creating or modifying uORB messages you must manually update the messages in your workspace from your PX4 source tree. Generally this means that you would update [dds\_topics.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/dds_topics.yaml), clone the interface package, and then manually synchronize it by copying the new/modified message definitions from [PX4-Autopilot/msg](https://github.com/PX4/PX4-Autopilot/tree/main/msg) to its `msg` folders. Assuming that PX4-Autopilot is in your home directory `~`, while `px4_msgs` is in `~/px4_ros_com/src/`, then the command might be: ```sh rm ~/px4_ros_com/src/px4_msgs/msg/*.msg cp ~/PX4-Autopilot/mgs/*.msg ~/px4_ros_com/src/px4_msgs/msg/ ``` :::note Technically, [dds\_topics.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/dds_topics.yaml) completely defines the relationship between PX4 uORB topics and ROS 2 messages. For more information see [uXRCE-DDS > DDS Topics YAML](/px4-user-guide/development/middleware/uxrce_dds.md#dds-topics-yaml). ::: ## Customizing the Topic Namespace Custom topic namespaces can be applied at build time (changing [dds\_topics.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/dds_topics.yaml)) or at runtime (useful for multi vehicle operations): * One possibility is to use the `-n` option when starting the [uxrce\_dds\_client](/px4-user-guide/development/modules_main/modules_system.md#uxrce-dds-client) from command line. This technique can be used both in simulation and real vehicles. * A custom namespace can be provided for simulations (only) by setting the environment variable `PX4_UXRCE_DDS_NS` before starting the simulation. :::note Changing the namespace at runtime will append the desired namespace as a prefix to all `topic` fields in [dds\_topics.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/dds_topics.yaml). Therefore, commands like: ```sh uxrce_dds_client start -n uav_1 ``` or ```sh PX4_UXRCE_DDS_NS=uav_1 make px4_sitl gz_x500 ``` will generate topics under the namespaces: ```sh /uav_1/fmu/in/ # for subscribers /uav_1/fmu/out/ # for publishers ``` ::: ## ros2 CLI The [ros2 CLI](https://docs.ros.org/en/humble/Tutorials/Beginner-CLI-Tools.html) is a useful tool for working with ROS. You can use it, for example, to quickly check whether topics are being published, and also inspect them in detail if you have `px4_msg` in the workspace. The command also lets you launch more complex ROS systems via a launch file. A few possibilities are demonstrated below. ### ros2 topic list Use `ros2 topic list` to list the topics visible to ROS 2: ```sh ros2 topic list ``` If PX4 is connected to the agent, the result will be a list of topic types: ``` /fmu/in/obstacle_distance /fmu/in/offboard_control_mode /fmu/in/onboard_computer_status ... ``` Note that the workspace does not need to build with `px4_msgs` for this to succeed; topic type information is part of the message payload. ### ros2 topic echo Use `ros2 topic echo` to show the details of a particular topic. Unlike with `ros2 topic list`, for this to work you must be in a workspace has built the `px4_msgs` and sourced `local_setup.bash` so that ROS can interpret the messages. ```sh ros2 topic echo /fmu/out/vehicle_status ``` The command will echo the topic details as they update. ``` --- timestamp: 1675931593364359 armed_time: 0 takeoff_time: 0 arming_state: 1 latest_arming_reason: 0 latest_disarming_reason: 0 nav_state_timestamp: 3296000 nav_state_user_intention: 4 nav_state: 4 failure_detector_status: 0 hil_state: 0 ... --- ``` ### ros2 topic hz You can get statistics about the rates of messages using `ros2 topic hz`. For example, to get the rates for `SensorCombined`: ``` ros2 topic hz /fmu/out/sensor_combined ``` The output will look something like: ```sh average rate: 248.187 min: 0.000s max: 0.012s std dev: 0.00147s window: 2724 average rate: 248.006 min: 0.000s max: 0.012s std dev: 0.00147s window: 2972 average rate: 247.330 min: 0.000s max: 0.012s std dev: 0.00148s window: 3212 average rate: 247.497 min: 0.000s max: 0.012s std dev: 0.00149s window: 3464 average rate: 247.458 min: 0.000s max: 0.012s std dev: 0.00149s window: 3712 average rate: 247.485 min: 0.000s max: 0.012s std dev: 0.00148s window: 3960 ``` ### ros2 launch The `ros2 launch` command is used to start a ROS 2 launch file. For example, above we used `ros2 launch px4_ros_com sensor_combined_listener.launch.py` to start the listener example. You don't need to have a launch file, but they are very useful if you have a complex ROS 2 system that needs to start several components. For information about launch files see [ROS 2 Tutorials > Creating launch files](https://docs.ros.org/en/humble/Tutorials/Intermediate/Launch/Creating-Launch-Files.html) ## Troubleshooting ### Missing dependencies The standard installation should include all the tools needed by ROS 2. If any are missing, they can be added separately: * **`colcon`** build tools should be in the development tools. It can be installed using: ```sh sudo apt install python3-colcon-common-extensions ``` * The Eigen3 library used by the transforms library should be in the both the desktop and base packages. It should be installed as shown: :::: tabs ::: tab humble ```sh sudo apt install ros-humble-eigen3-cmake-module ``` ::: ::: tab foxy ```sh sudo apt install ros-foxy-eigen3-cmake-module ``` ::: :::: ## Additional information * [ROS 2 in PX4: Technical Details of a Seamless Transition to XRCE-DDS](https://www.youtube.com/watch?v=F5oelooT67E) - Pablo Garrido & Nuno Marques (youtube) * [DDS and ROS middleware implementations](https://github.com/ros2/ros2/wiki/DDS-and-ROS-middleware-implementations) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://px4.gitbook.io/px4-user-guide/robotics/ros/ros2/ros2_comm.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.