# Multi-Vehicle Simulation with Gazebo

This topic explains how to simulate multiple UAV vehicles using [Gazebo (Gz)](../sim_gazebo_gz/index.md) and SITL.

::: info
Multi-Vehicle Simulation with Gazebo is only supported on Linux.
:::

Gazebo makes it very easy to setup multi-vehicle scenarios (compared to other simulators).

First build PX4 SITL code using:

```sh
make px4_sitl
```

Each instance of PX4 can then be launched in its own terminal, specifying a unique instance number and its desired combination of [environment variables](../sim_gazebo_gz/index.md#usage-configuration-options):

```sh
ARGS ./build/px4_sitl_default/bin/px4 [-i <instance>]
```

- `<instance>`:
  The instance number of the vehicle.
  - Each vehicle must have a unique instance number.
    If not given, the instance number defaults to zero.
  - When used with `PX4_SIM_MODEL`, Gazebo will automatically pick a unique model name in the form `${PX4_SIM_MODEL}_instance`.
- `ARGS`:
  A list of environmental variables, as described in [Gazebo Simulation > Usage/Configuration Options](../sim_gazebo_gz/index.md#usage-configuration-options).

This allows for greater flexibility and customization.

## Multiple Vehicles with ROS 2 and Gazebo

[Multiple vehicles with ROS 2](../ros2/multi_vehicle.md) are possible.

- First follow the installation instructions for [Gazebo](../sim_gazebo_gz/index.md).
- Then configure your system for [ROS 2 / PX4 operations](../ros2/user_guide.md#installation-setup).
- In different terminals manually start a multi vehicle simulation.
  This example spawns 2 X500 Quadrotors and aFPX fixed-wing.

  ::: info
  Note that in the first terminal you **do not** specify standalone mode. The first terminal will start the gz-server and the other two
  instances will connect to it.
  :::

  **Terminal 1**

  ```sh
  PX4_SYS_AUTOSTART=4001 PX4_SIM_MODEL=gz_x500 ./build/px4_sitl_default/bin/px4 -i 1
  ```

  **Terminal 2**

  ```sh
  PX4_GZ_STANDALONE=1 PX4_SYS_AUTOSTART=4001 PX4_GZ_MODEL_POSE="0,1" PX4_SIM_MODEL=gz_x500 ./build/px4_sitl_default/bin/px4 -i 2
  ```

  **Terminal 3**

  ```sh
  PX4_GZ_STANDALONE=1 PX4_SYS_AUTOSTART=4003 PX4_GZ_MODEL_POSE="0,2" PX4_SIM_MODEL=gz_rc_cessna ./build/px4_sitl_default/bin/px4 -i 3
  ```

- Start the agent:

  ```sh
  MicroXRCEAgent udp4 -p 8888
  ```

  The agent will automatically connect to all clients.

- Run `ros2 topic list` to see the topic list, which should look like this:

```sh
/parameter_events
/px4_1/fmu/in/obstacle_distance
/px4_1/fmu/in/offboard_control_mode
/px4_1/fmu/in/onboard_computer_status
/px4_1/fmu/in/sensor_optical_flow
/px4_1/fmu/in/telemetry_status
/px4_1/fmu/in/trajectory_setpoint
/px4_1/fmu/in/vehicle_attitude_setpoint
/px4_1/fmu/in/vehicle_command
/px4_1/fmu/in/vehicle_mocap_odometry
/px4_1/fmu/in/vehicle_rates_setpoint
/px4_1/fmu/in/vehicle_trajectory_bezier
/px4_1/fmu/in/vehicle_trajectory_waypoint
/px4_1/fmu/in/vehicle_visual_odometry
/px4_1/fmu/out/failsafe_flags
/px4_1/fmu/out/sensor_combined
/px4_1/fmu/out/timesync_status
/px4_1/fmu/out/vehicle_attitude
/px4_1/fmu/out/vehicle_control_mode
/px4_1/fmu/out/vehicle_global_position
/px4_1/fmu/out/vehicle_gps_position
/px4_1/fmu/out/vehicle_local_position
/px4_1/fmu/out/vehicle_odometry
/px4_1/fmu/out/vehicle_status
/px4_2/fmu/in/obstacle_distance
/px4_2/fmu/in/offboard_control_mode
/px4_2/fmu/in/onboard_computer_status
/px4_2/fmu/in/sensor_optical_flow
/px4_2/fmu/in/telemetry_status
/px4_2/fmu/in/trajectory_setpoint
/px4_2/fmu/in/vehicle_attitude_setpoint
/px4_2/fmu/in/vehicle_command
/px4_2/fmu/in/vehicle_mocap_odometry
/px4_2/fmu/in/vehicle_rates_setpoint
/px4_2/fmu/in/vehicle_trajectory_bezier
/px4_2/fmu/in/vehicle_trajectory_waypoint
/px4_2/fmu/in/vehicle_visual_odometry
/px4_2/fmu/out/failsafe_flags
/px4_2/fmu/out/sensor_combined
/px4_2/fmu/out/timesync_status
/px4_2/fmu/out/vehicle_attitude
/px4_2/fmu/out/vehicle_control_mode
/px4_2/fmu/out/vehicle_global_position
/px4_2/fmu/out/vehicle_gps_position
/px4_2/fmu/out/vehicle_local_position
/px4_2/fmu/out/vehicle_odometry
/px4_2/fmu/out/vehicle_status
/px4_3/fmu/in/obstacle_distance
/px4_3/fmu/in/offboard_control_mode
/px4_3/fmu/in/onboard_computer_status
/px4_3/fmu/in/sensor_optical_flow
/px4_3/fmu/in/telemetry_status
/px4_3/fmu/in/trajectory_setpoint
/px4_3/fmu/in/vehicle_attitude_setpoint
/px4_3/fmu/in/vehicle_command
/px4_3/fmu/in/vehicle_mocap_odometry
/px4_3/fmu/in/vehicle_rates_setpoint
/px4_3/fmu/in/vehicle_trajectory_bezier
/px4_3/fmu/in/vehicle_trajectory_waypoint
/px4_3/fmu/in/vehicle_visual_odometry
/px4_3/fmu/out/failsafe_flags
/px4_3/fmu/out/sensor_combined
/px4_3/fmu/out/timesync_status
/px4_3/fmu/out/vehicle_attitude
/px4_3/fmu/out/vehicle_control_mode
/px4_3/fmu/out/vehicle_global_position
/px4_3/fmu/out/vehicle_gps_position
/px4_3/fmu/out/vehicle_local_position
/px4_3/fmu/out/vehicle_odometry
/px4_3/fmu/out/vehicle_status
/rosout
```