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The FlightGear protocol sends head tracking data directly to the FlightGear flight simulator via UDP packets. It’s specifically designed for FlightGear and provides native integration with the simulator’s view system.

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How It Works

The protocol sends a UDP datagram containing 6DOF tracking data to FlightGear’s property system:
  1. Packages position (X, Y, Z in meters) and rotation (heading, pitch, roll in degrees) into a binary structure
  2. Sends the data via UDP to a configurable IP address and port
  3. FlightGear reads the data and updates the view camera accordingly

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Data Structure

struct flightgear_datagram {
    double x;       // Position X (meters)
    double y;       // Position Y (meters)  
    double z;       // Position Z (meters)
    double h;       // Heading (degrees)
    double p;       // Pitch (degrees)
    double r;       // Roll (degrees)
    int status;     // Status flag (1 = active)
};

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Compatible Simulators

FlightGear

Open-source flight simulator - all versions that support UDP property input
This protocol is specifically designed for FlightGear. For other flight simulators:
  • FSX/Prepar3D: Use SimConnect
  • X-Plane: Use FreeTrack with X-Camera plugin
  • MSFS 2020: Use FreeTrack

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Setup Instructions

1

Select FlightGear protocol

In OpenTrack, go to the Output dropdown and select “FlightGear”.
2

Configure network settings

Click the settings button next to the Output dropdown.Network configuration:
  • IP Address: Where FlightGear is running
    • Default: 127.0.0.1 (same computer)
    • For networked setup: Enter FlightGear PC’s IP address
  • Port: UDP port number
    • Default: 5542
    • Must match FlightGear’s configured port
3

Configure FlightGear

Start FlightGear with UDP input enabled. Add this to your FlightGear command line or launcher settings:
--generic=socket,in,25,127.0.0.1,5542,udp,opentrack
Or in the FlightGear launcher:
  • Go to Settings → Additional Settings
  • Add: --generic=socket,in,25,127.0.0.1,5542,udp,opentrack
4

Create FlightGear protocol file

FlightGear needs a protocol definition file to parse OpenTrack data.Create a file named opentrack.xml in FlightGear’s Protocol directory:Location:
  • Windows: <FG Install>\data\Protocol\opentrack.xml
  • Linux: ~/.fgfs/Protocol/opentrack.xml or /usr/share/games/flightgear/Protocol/opentrack.xml
  • macOS: /Applications/FlightGear.app/data/Protocol/opentrack.xml
5

Protocol file contents

Add this XML content to opentrack.xml:
<?xml version="1.0"?>
<PropertyList>
  <generic>
    <input>
      <binary_mode>true</binary_mode>
      <byte_order>host</byte_order>
      
      <chunk>
        <name>x-position</name>
        <type>double</type>
        <node>/sim/current-view/x-offset-m</node>
      </chunk>
      
      <chunk>
        <name>y-position</name>
        <type>double</type>
        <node>/sim/current-view/y-offset-m</node>
      </chunk>
      
      <chunk>
        <name>z-position</name>
        <type>double</type>
        <node>/sim/current-view/z-offset-m</node>
      </chunk>
      
      <chunk>
        <name>heading</name>
        <type>double</type>
        <node>/sim/current-view/heading-offset-deg</node>
      </chunk>
      
      <chunk>
        <name>pitch</name>
        <type>double</type>
        <node>/sim/current-view/pitch-offset-deg</node>
      </chunk>
      
      <chunk>
        <name>roll</name>
        <type>double</type>
        <node>/sim/current-view/roll-offset-deg</node>
      </chunk>
      
      <chunk>
        <name>status</name>
        <type>int</type>
      </chunk>
    </input>
  </generic>
</PropertyList>
6

Start tracking

  1. Start OpenTrack and click Start
  2. Launch FlightGear with the generic protocol configuration
  3. Your head movements will control the view in FlightGear

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Configuration

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Default Settings

IP Address: 127.0.0.1
Port: 5542

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Network Configuration

For running OpenTrack and FlightGear on different computers:
1

Find FlightGear PC IP

On the computer running FlightGear, find its local IP address:
  • Windows: ipconfig in command prompt
  • Linux/Mac: ifconfig or ip addr
2

Update OpenTrack settings

In OpenTrack FlightGear settings, enter the FlightGear PC’s IP address.
3

Configure FlightGear

Update FlightGear command line to listen on all interfaces:
--generic=socket,in,25,0.0.0.0,5542,udp,opentrack
4

Check firewall

Ensure UDP port 5542 is allowed through the firewall on the FlightGear PC.

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Coordinate Transformations

OpenTrack coordinates are converted for FlightGear: 
// Position (cm to meters)
FlightData.x = -headpose[TX] * 0.01;  // Negated
FlightData.y =  headpose[TY] * 0.01;
FlightData.z =  headpose[TZ] * 0.01;

// Rotation (degrees)
FlightData.h = -headpose[Yaw];    // Heading (negated)
FlightData.p =  headpose[Pitch];  // Pitch
FlightData.r = -headpose[Roll];   // Roll (negated)

// Status
FlightData.status = 1;  // Always active when tracking

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Troubleshooting

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”Can’t bind to [IP:Port]” Error

Cause: OpenTrack cannot bind to the UDP socket. Solution:
  1. Check if another application is using the port
  2. Verify IP address format is correct (e.g., 192.168.1.100)
  3. Try changing the port number
  4. Run OpenTrack as administrator

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FlightGear Doesn’t Respond to Tracking

Symptoms: OpenTrack is running but view doesn’t move in FlightGear. Solution:
  1. Verify protocol file exists: Check opentrack.xml is in Protocol folder
  2. Check command line: Ensure --generic=socket,in,25,127.0.0.1,5542,udp,opentrack is in FlightGear launch options
  3. Match IP and port: Settings must match between OpenTrack and FlightGear
  4. Firewall: Add exception for UDP port 5542
  5. Test connectivity: Use Wireshark or tcpdump to verify UDP packets are being sent

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View Jumps or Jitters

Cause: Network latency or packet loss. Solution:
  • If networked: Use wired connection instead of WiFi
  • Reduce OpenTrack filter smoothing
  • Check for network congestion
  • Ensure FlightGear frame rate is stable

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Wrong Axis Movement

Cause: Coordinate system mismatch. Solution:
  • Verify opentrack.xml maps chunks to correct view properties
  • Check OpenTrack mapping settings
  • May need to invert specific axes in OpenTrack mapping

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FlightGear View Configuration

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View System Properties

The protocol controls these FlightGear properties:
  • /sim/current-view/x-offset-m - X position offset
  • /sim/current-view/y-offset-m - Y position offset
  • /sim/current-view/z-offset-m - Z position offset
  • /sim/current-view/heading-offset-deg - Heading offset
  • /sim/current-view/pitch-offset-deg - Pitch offset
  • /sim/current-view/roll-offset-deg - Roll offset

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Compatible View Types

Head tracking works best with these FlightGear views:
  • Cockpit View (default)
  • Helicopter View
  • Chase View
External views may have limited head tracking functionality.

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Advanced Usage

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Custom Update Rate

FlightGear’s command line specifies update rate (25 Hz in example): 
--generic=socket,in,25,127.0.0.1,5542,udp,opentrack
                    ^^-- Update rate in Hz
Adjust this to match your tracking input device framerate:
  • 25 Hz - Standard
  • 30 Hz - Smoother
  • 60 Hz - Very smooth (requires fast tracking source)

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Multiple Tracking Sources

To use multiple UDP inputs in FlightGear, use different ports: 
--generic=socket,in,25,127.0.0.1,5542,udp,opentrack \
--generic=socket,in,30,127.0.0.1,5543,udp,joystick
The FlightGear protocol is designed for dedicated FlightGear users who want native integration. For casual use or multiple simulators, consider using FreeTrack protocol with appropriate plugins.

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Technical Details

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Binary Protocol

The protocol sends binary data in host byte order:
  • 6 doubles (8 bytes each) = 48 bytes
  • 1 integer (4 bytes) = 4 bytes
  • Total packet size: 52 bytes per update

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Network Performance

At 25 Hz update rate:
  • Bandwidth: ~10 Kbps
  • Packets per second: 25
  • Latency: Typically <1ms on local network
Very low overhead, suitable for network play.