# Multi-Vehicle Simulation with ROS 2

XRCE-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

# 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:

param set UXRCE_DDS_KEY $((px4_instance+1))


By doing so, UXRCE_DDS_KEY will always coincide with MAV_SYS_ID.

Moreover, when px4_instance is greater than zero, a unique ROS 2 namespace prefix in the form px4_$px4_instance is added:

uxrce_dds_ns="-n px4_$px4_instance"


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 (opens new window) 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 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.