Setting up PX4 Software-in-Loop#

The PX4 software provides a "software-in-loop" simulation (SITL) version of their stack that runs in Linux. If you are on Windows then you can use the Cygwin Toolchain or you can use the Windows subsystem for Linux and follow the PX4 Linux toolchain setup.

If you are using WSL2 please read these additional instructions.

Note that every time you stop the unreal app you have to restart the px4 app.

  1. From your bash terminal follow these steps for Linux and follow all the instructions under NuttX based hardware to install prerequisites. We've also included our own copy of the PX4 build instructions which is a bit more concise about what we need exactly.

  2. Get the PX4 source code and build the posix SITL version of PX4:

    mkdir -p PX4
    cd PX4
    git clone --recursive
    bash ./PX4-Autopilot/Tools/setup/ --no-nuttx --no-sim-tools
    cd PX4-Autopilot
    And find the latest stable release from and checkout the source code matching that release, for example:
    git checkout v1.11.3

  3. Use following command to build and start PX4 firmware in SITL mode:

    make px4_sitl_default none_iris
    If you are using older version v1.8.* use this command instead: make posix_sitl_ekf2 none_iris.

  4. You should see a message saying the SITL PX4 app is waiting for the simulator (AirSim) to connect. You will also see information about which ports are configured for mavlink connection to the PX4 app. The default ports have changed recently, so check them closely to make sure AirSim settings are correct.

    INFO  [simulator] Waiting for simulator to connect on TCP port 4560
    INFO  [init] Mixer: etc/mixers/quad_w.main.mix on /dev/pwm_output0
    INFO  [mavlink] mode: Normal, data rate: 4000000 B/s on udp port 14570 remote port 14550
    INFO  [mavlink] mode: Onboard, data rate: 4000000 B/s on udp port 14580 remote port 14540

    Note: this is also an interactive PX4 console, type help to see the list of commands you can enter here. They are mostly low level PX4 commands, but some of them can be useful for debugging.

  5. Now edit AirSim settings file to make sure you have matching UDP and TCP port settings:

        "SettingsVersion": 1.2,
        "SimMode": "Multirotor",
        "ClockType": "SteppableClock",
        "Vehicles": {
            "PX4": {
                "VehicleType": "PX4Multirotor",
                "UseSerial": false,
                "LockStep": true,
                "UseTcp": true,
                "TcpPort": 4560,
                "ControlPortLocal": 14540,
                "ControlPortRemote": 14580,
                        "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
    Notice the PX4 [simulator] is using TCP, which is why we need to add: "UseTcp": true,. Notice we are also enabling LockStep, 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 which allows PX4 to achieve GPS lock more quickly.

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

  7. Now run your Unreal AirSim environment and it should connect to SITL PX4 via TCP. You should see a bunch of messages from the SITL PX4 window. Specifically, the following messages tell you that AirSim is connected properly and GPS fusion is stable:

    INFO  [simulator] Simulator connected on UDP port 14560
    INFO  [mavlink] partner IP:
    INFO  [ecl/EKF] EKF GPS checks passed (WGS-84 origin set)
    INFO  [ecl/EKF] EKF commencing GPS fusion

    If you do not see these messages then check your port settings.

  8. You should also be able to use QGroundControl with SITL mode. Make sure there is no Pixhawk hardware plugged in, otherwise QGroundControl will choose to use that instead. Note that as we don't have a physical board, an RC cannot be connected directly to it. So the alternatives are either use XBox 360 Controller or connect your RC using USB (for example, in case of FrSky Taranis X9D Plus) or using trainer USB cable to your PC. This makes your RC look like a joystick. You will need to do extra set up in QGroundControl to use virtual joystick for RC control. You do not need to do this unless you plan to fly a drone manually in AirSim. Autonomous flight using the Python API does not require RC, see No Remote Control below.

Setting GPS origin#

Notice the above settings are provided in the params section of the settings.json file:

    "LPE_LAT": 47.641468,
    "LPE_LON": -122.140165,

PX4 SITL mode needs to be configured to get the home location correct. The home location needs to be set to the same coordinates defined in OriginGeopoint.

You can also run the following in the SITL PX4 console window to check that these values are set correctly.

param show LPE_LAT
param show LPE_LON

Smooth Offboard Transitions#

Notice the above setting is provided in the params section of the settings.json file:

    "COM_OBL_ACT": 1

This tells the drone automatically hover after each offboard control command finishes (the default setting is to land). Hovering is a smoother transition between multiple offboard commands. You can check this setting by running the following PX4 console command:

param show COM_OBL_ACT

Check the Home Position#

If you are using DroneShell to execute commands (arm, takeoff, etc) then you should wait until the Home position is set. You will see the PX4 SITL console output this message:

INFO  [commander] home: 47.6414680, -122.1401672, 119.99
INFO  [tone_alarm] home_set

Now DroneShell 'pos' command should report this position and the commands should be accepted by PX4. If you attempt to takeoff without a home position you will see the message:

WARN  [commander] Takeoff denied, disarm and re-try

After home position is set check the local position reported by 'pos' command :

Local position: x=-0.0326988, y=0.00656854, z=5.48506

If the z coordinate is large like this then takeoff might not work as expected. Resetting the SITL and simulation should fix that problem.

WSL 2#

Windows Subsystem for Linux version 2 operates in a Virtual Machine. This requires additional setup - see additional instructions.

No Remote Control#

Notice the above setting is provided in the params section of the settings.json file:

    "NAV_RCL_ACT": 0,
    "NAV_DLL_ACT": 0,

This is required if you plan to fly the SITL mode PX4 with no remote control, just using python scripts, for example. These parameters stop the PX4 from triggering "failsafe mode on" every time a move command is finished. You can use the following PX4 command to check these values are set correctly:

param show NAV_RCL_ACT
param show NAV_DLL_ACT

NOTE: Do NOT do this on a real drone as it is too dangerous to fly without these failsafe measures.

Manually set parameters#

You can also run the following in the PX4 console to set all these parameters manually:

param set NAV_RCL_ACT 0
param set NAV_DLL_ACT 0

Setting up multi-vehicle simulation#

You can simulate multiple drones in SITL mode using AirSim. However, this requires setting up multiple instances of the PX4 firmware simulator to be able to listen for each vehicle's connection on a separate TCP port (4560, 4561, etc). Please see this dedicated page for instructions on setting up multiple instances of PX4 in SITL mode.

Using VirtualBox Ubuntu#

If you want to run the above posix_sitl in a VirtualBox Ubuntu machine then it will have a different ip address from localhost. So in this case you need to edit the settings file and change the UdpIp and SitlIp to the ip address of your virtual machine set the LocalIpAddress to the address of your host machine running the Unreal engine.

Remote Controller#

There are several options for flying the simulated drone using a remote control or joystick like xbox gamepad. See remote controllers