PX4 Software-in-Loop with WSL 2#

The Windows subsystem for Linux version 2 uses a Virtual Machine which has a separate IP address from your Windows host machine. This means PX4 cannot find AirSim using "localhost" which is the default behavior for PX4.

You will notice that on Windows ipconfig returns a new ethernet adapter for WSL like this (notice the vEthernet has (WSL) in the name:

Ethernet adapter vEthernet (WSL):

   Connection-specific DNS Suffix  . :
   Link-local IPv6 Address . . . . . : fe80::1192:f9a5:df88:53ba%44
   IPv4 Address. . . . . . . . . . . : 172.31.64.1
   Subnet Mask . . . . . . . . . . . : 255.255.240.0
   Default Gateway . . . . . . . . . :

This address 172.31.64.1 is the address that WSL 2 can use to reach your Windows host machine.

Starting with this PX4 Change Request (which correlates to version v1.12.0-beta1 or newer) PX4 in SITL mode can now connect to AirSim on a different (remote) IP address. To enable this make sure you have a version of PX4 containing this fix and set the following environment variable in linux:

export PX4_SIM_HOST_ADDR=172.31.64.1

Note: Be sure to update the above address 172.31.64.1 to match what you see from your ipconfig command.

Open incoming TCP port 4560 and incoming UDP port 14540 using your firewall configuration.

Now on the linux side run ip address show and copy the eth0 inet address, it should be something like 172.31.66.156. This is the address Windows needs to know in order to find PX4.

Edit your AirSim settings file and add LocalHostIp to tell AirSim to use the WSL ethernet adapter address instead of the default localhost. This will cause AirSim to open the TCP port on that adapter which is the address that the PX4 app will be looking for. Also tell AirSim to connect the ControlIp UDP channel by setting ControlIp to the magic string remote. This resolves to the WSL 2 remote ip address found in the TCP socket.

{
    "SettingsVersion": 1.2,
    "SimMode": "Multirotor",
    "ClockType": "SteppableClock",
    "Vehicles": {
        "PX4": {
            "VehicleType": "PX4Multirotor",
            "UseSerial": false,
            "LockStep": true,
            "UseTcp": true,
            "TcpPort": 4560,
            "ControlIp": "remote",
            "ControlPortLocal": 14540,
            "ControlPortRemote": 14580,
            "LocalHostIp": "172.31.64.1",
            "Sensors":{
                "Barometer":{
                    "SensorType": 1,
                    "Enabled": true,
                    "PressureFactorSigma": 0.0001825
                }
            },
            "Parameters": {
                "NAV_RCL_ACT": 0,
                "NAV_DLL_ACT": 0,
                "COM_OBL_ACT": 1,
                "LPE_LAT": 47.641468,
                "LPE_LON": -122.140165
            }
        }
    }
}

See PX4 LockStep for more information. The "Barometer" setting keeps PX4 happy because the default AirSim barometer has a bit too much noise generation. This setting clamps that down a bit.

If your local repo does not include this PX4 commit, please edit the Linux file in ROMFS/px4fmu_common/init.d-posix/rcS and make sure it is looking for the PX4_SIM_HOST_ADDR environment variable and is passing that through to the PX4 simulator like this:

# If PX4_SIM_HOST_ADDR environment variable is empty use localhost.
if [ -z "${PX4_SIM_HOST_ADDR}" ]; then
    echo "PX4 SIM HOST: localhost"
    simulator start -c $simulator_tcp_port
else
    echo "PX4 SIM HOST: $PX4_SIM_HOST_ADDR"
    simulator start -t $PX4_SIM_HOST_ADDR $simulator_tcp_port
fi

Note: this code might already be there depending on the version of PX4 you are using.

Note: please be patient when waiting for the message:

INFO  [simulator] Simulator connected on TCP port 4560.

It can take a little longer to establish the remote connection than it does with localhost.

Now you can proceed with the steps shown in Setting up PX4 Software-in-Loop.