SITL.md

Running software-in-the-loop simulations with Silent Wings

This document describes how to run Frigatebird/ArduPlane software-in-the-loop (SITL) simulation with Silent Wings. Whether you are working on a soaring controller for a specific type of lift (e.g., a thermal) or a cross-country sailplane navigation strategy, you may find it useful to test your approach in simulation. This is especially true for long-range navigation, whose evaluation in the open world is logistically difficult and expensive. SITL testing allows you to do exactly that by compiling Frigatebird/ArduPlane code into a special binary that can run on your regular computer, receive sensor data from a simulator running on the same or different machine, and send inputs back to the simulator in order to control a simulated sailplane.

Silent Wings is a simulator that focuses specifically on sailplanes. In spite of being somewhat dated by now, as of April 2019 it still provides some of the most accurate simulations of sailplane flight and details of atmospheric phenomena that matter for it, such as thermal lifecycles, distribution of thermal and ridge lift over terrain, their dependence on weather, etc. This makes it a great tool for testing various aspects of autonomous soaring.

The following instructions have been tested on Windows 10.

Prerequisites

• Install Silent Wings and at least one scenery package for it. The 2-week demo version can be used to try out these instructions. We suggest starting with Norway scenery packages; our mission uses the Frya-Lillehammer-Starmoen map.

• Configure Silent Wings to send sensor data and receive control inputs remotely. To do so, open %USERPROFILE%\AppData\Local\VirtualStore\Program Files (x86)\Silent Wings\Silent Wings\data\options.dat and replace the "UDP Output protocol" and "Remote control protocol" sections in it with

    #---------------------
    # UDP Output protocol 
    #---------------------
    use_output_udp      = true
    output_udp_protocol = binary
    output_udp_address  = localhost
    output_udp_port     = 6060
    output_udp_rate     = 90
  
    #-------------------------
    # Remote control protocol 
    #-------------------------
    use_remote_control  = true
    remote_control_port = 6070
    remote_control_mode = UDP
  

  Also, set

    screen_refresh     = 60
  

  Note that there is another options.dat file, in C:\Program Files (x86)\Silent Wings\Silent Wings\data. It is not the one we need.

  The output_udp_rate = 90 value works well for screen_refresh = 60. When adjusting the latter, you will probably want to adjust the former as well for smoother control.

• Install Mission Planner, a ground control station (GCS) software. Other ArduPilot GCS software may work as well.

• Clone and build the SITL flavor of Frigatebird.

Running Silent Wings SITL

1. Launch Silent Wings. In Silent Wings' Flight Planner section, on the Aircraft tab choose a sailplane. Note that each sailplane model has distinct characteristics, and the Frigatebird autopilot needs a parameter (.param) file for that model in order to be aware of them. We provide a parameter file for Rolladen-Schneider LS-8b and this is the sailplane we recommend as a start.

   Set weather and air traffic conditions on the Weather and Traffic tabs respectively. On the Location tab, choose the map and starting location, and note its GPS coordinates, altitude, and heading. You will need this information when launching SITL. Then click the Fly Now! button on the bottom right. The game is going to start in a paused state. Keep it paused for now.
   

2. Launch Frigatebird SITL. To do so, open a Cygwin64 terminal, switch to your Frigatebird repository's root directory, and run

    build/sitl/bin/arduplane --model silentwings --home <lat>,<lon>,<alt>,<yaw>
   

   e.g.,

    build/sitl/bin/arduplane --model silentwings --home 60.8805696600,11.6695970300,216,145
   

   Here, <lat> and <lon> are the start location's latitude and longitude that you noted down when launching Silent Wings, with a "-" sign if latitude is S or longitude is W. However, <alt> should be the elevation of the start location's terrain above sea level, even if you chose to start in the air. To determine it, subtract the starting altitude you noted down earlier (it is AGL) from the altitde you see in the top left corner of the Silent Wings window (it is MSL) when the mission starts. The result is the <alt> that you need to pass to the SITL. Thus, note that for the mission in the above screenshot, <alt>=216, not 800. <yaw> is the initial heading you picked for the Silent Wings mission.

   TIP: We recommend start locations with near-zero terrain elevation above the sea level, because the SITL flavor Frigatebird/ArduPlane occasionally behaves quirky if the aircraft in the simulation descends below the home location's <alt>.

   At this point, the SITL should be waiting for a connection to the ground control station (Mission Planner):

   

3. Launch Mission Planner, connect a joystick, and set up the joystick in Mission Planner to configure its roll, pitch, and yaw axes (other axes will be ignored by Silent Wings). Unfortunately, Mission Planner appears to forget joystick settings when it is shut down, so this step has to be redone every time Mission Planner is restarted or the joystick is reconnected.

4. Have Mission Planner establish connections to Silent Wings and Frigatebird SITL. To do so, in Mission Planner's top right corner choose TCP, click Connect, enter the IP address where Silent Wings is running (127.0.0.1 if it is on the same machine) in the window that pops up, and enter 5760 as the port number in the next window. Wait until the Connect Mavlink window disappears, all parameters are copied over, and all calibration messages in Mission Planner's HUD disappear. You should see Mission Planner's HUD report that the GPS has lock and the airspeed sensor reads a value around 0. You may also see a "Bad AHRS" or "Bad or No Terrain Data" messages; ignore them.

   Now Frigatebird is ready to control the plane in Silent Wings.

5. Load the parameter file for the chosen Silent Wings sailplane into Frigatebird via Mission Planner. (This step needs to be done only the first time you fly a given sailplane model in Silent Wings.) To do so, go to Mission Planner's CONFIG/TUNING tab, choose Full Parameter List on the left, click Load from file on the right, locate the parameter file, and click Write Params on the right. After doing this, terminate Frigatebird SITL in the Cygwin64 terminal, close Mission Planner, and repeat steps 2 and 4.

   

6. If desired, load a mission in Mission Planner. To do so, go to Mission Planner's FLIGHT PLAN tab, click Load WP File on the right, locate the mission file, and click Write WPs. We provide a file for a test mission near Starmoen, Norway

   

7. Start flying! By now, everything is ready to commence the flight. In the Silent Wings window, hit the "p" key to unpause the simulation. You should see airspeed, altitude, and AHRS readings in Mission Planner's HUD closely match those in Silent Wings' top right and left corners. To have Silent Wings' sailplane follow the loaded mission, switch the flight mode to Auto in Mission Planner.

   

A big thanks goes to Sam Tabor and Thomas Sevaldrud of Silent Wings for their help with the development of this SITL, and to Jack Langelaan of Penn State University for providing valuable insights.