# ROS 2 Multi Vehicle Simulation

[XRCE-DDS](https://px4.gitbook.io/px4-user-guide/development/middleware/uxrce_dds) allows for multiple clients to be connected to the same agent over UDP. This is particular useful in simulation as only one agent needs to be started.

## Setup and Requirements

The only requirements are

* To be able to run [multi-vehicle simulation](https://px4.gitbook.io/px4-user-guide/development/simulation/multi-vehicle-simulation) without ROS 2 with the desired simulator ([Gazebo](https://px4.gitbook.io/px4-user-guide/development/simulation/sim_gazebo_gz/multi_vehicle_simulation), [Gazebo Classic](https://px4.gitbook.io/px4-user-guide/development/simulation/sim_gazebo_classic/multi_vehicle_simulation_gazebo#multiple-vehicle-with-gazebo-classic), [FlightGear](https://px4.gitbook.io/px4-user-guide/development/simulation/flightgear/multi_vehicle_flightgear) and [JMAVSim](https://px4.gitbook.io/px4-user-guide/development/simulation/jmavsim/multi_vehicle_jmavsim)).
* To be able to use [ROS 2](https://px4.gitbook.io/px4-user-guide/robotics/ros/ros2/ros2_comm) in a single vehicle simulation.

## Principle of Operation

In simulation each PX4 instance receives a unique `px4_instance` number starting from `0`. This value is used to assign a unique value to [UXRCE\_DDS\_KEY](https://px4.gitbook.io/px4-user-guide/advanced_config/parameter_reference#UXRCE_DDS_KEY):

```sh
param set UXRCE_DDS_KEY $((px4_instance+1))
```

:::note By doing so, `UXRCE_DDS_KEY` will always coincide with [MAV\_SYS\_ID](https://px4.gitbook.io/px4-user-guide/advanced_config/parameter_reference#MAV_SYS_ID). :::

Moreover, when `px4_instance` is greater than zero, a unique ROS 2 [namespace prefix](https://px4.gitbook.io/px4-user-guide/development/middleware/uxrce_dds#customizing-the-topic-namespace) in the form `px4_$px4_instance` is added:

```sh
uxrce_dds_ns="-n px4_$px4_instance"
```

:::note The environment variable `PX4_UXRCE_DDS_NS`, if set, will override the namespace behavior described above. :::

The first instance (`px4_instance=0`) does not have an additional namespace in order to be consistent with the default behavior of the `xrce-dds` client on a real vehicle. This mismatch can be fixed by manually using `PX4_UXRCE_DDS_NS` on the first instance or by starting adding vehicles from index `1` instead of `0` (this is the default behavior adopted by [sitl\_multiple\_run.sh](https://github.com/PX4/PX4-Autopilot/blob/main/Tools/simulation/gazebo-classic/sitl_multiple_run.sh) for Gazebo Classic).

The default client configuration in simulation is summarized as follows:

| `PX4_UXRCE_DDS_NS` | `px4_instance` | `UXRCE_DDS_KEY`  | client namespace      |
| ------------------ | -------------- | ---------------- | --------------------- |
| not provided       | 0              | `px4_instance+1` | none                  |
| provided           | 0              | `px4_instance+1` | `PX4_UXRCE_DDS_NS`    |
| not provided       | >0             | `px4_instance+1` | `px4_${px4_instance}` |
| provided           | >0             | `px4_instance+1` | `PX4_UXRCE_DDS_NS`    |

## Adjusting the `target_system` value

PX4 accepts [VehicleCommand](https://px4.gitbook.io/px4-user-guide/development/middleware/msg_docs/vehiclecommand) messages only if their `target_system` field is zero (broadcast communication) or coincides with `MAV_SYS_ID`. In all other situations, the messages are ignored. Therefore, when ROS 2 nodes want to send `VehicleCommand` to PX4, they must ensure that the messages are filled with the appropriate `target_system` value.

For example, if you want to send a command to your third vehicle, which has `px4_instance=2`, you need to set `target_system=3` in all your `VehicleCommand` messages.


---

# 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_multi_vehicle.md?ask=<question>
```

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.