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Pegasus Simulator

Pegasus Simulator is a multirotor simulation and control framework built on top of NVIDIA Isaac Sim’s physics engine. Isaac Sim provides the underlying physics and rendering and Pegasus Simulator implements the drone flight dynamics, control algorithms, and PX4 integration.

Pegasus Sim accurately models multirotor dynamics and environment interaction, providing a physically grounded foundation for UAV simulation. Pegasus connects directly to the AirStack autonomy stack with its PX4 MAVLink integration, allowing AirStack’s higher-level planning and perception components to control a realistically simulated drone as if it were a physical vehicle.

Custom Pegasus Node in AirStack

AirStack extends the core Pegasus Simulator through a Custom Pegasus OmniGraph Node. This node serves as an Isaac Sim OmniGraph action graph wrapper around the Pegasus Sim code which can then be saved in Universal Scene Description (USD) format which are easily readable in Isaac-Sim. This design allows Pegasus-based simulations to be defined entirely in USD, enabling scenario reusability and modular composition. Users can drop the same Pegasus node into different environments or swap robots while maintaining consistent physics and control behavior.

Through this approach, AirStack leverages Pegasus to create a flexible, reusable, and realistic simulation framework for aerial robotics research and development.

Launch Configuration

Launch Configuration

At the top level of the AirStack simulation environment, a .env file controls how Pegasus and Isaac Sim are launched: 

ISAAC_SIM_GUI="omniverse://airlab-nucleus.andrew.cmu.edu/Library/Assets/Pegasus/iris_with_sensors.pegasus.robot.usd"
# Set to "true" to launch Isaac Sim using a standalone Python script instead of a USD file
ISAAC_SIM_USE_STANDALONE_SCRIPT="false"  # "true" or "false"
# Script name (must be in /AirStack/simulation/isaac-sim/launch_scripts/)
ISAAC_SIM_SCRIPT_NAME="example_one_px4_pegasus_launch_script.py"
PLAY_SIM_ON_START="false"  # Not supported in standalone script mode

There are two modes for launching Pegasus simulations: - Load an existing USD file (e.g. *.pegasus.robot.usd) — to simulate a prebuilt robot/environment setup. - Use a standalone Python script — to dynamically generate a USD and configure the world from scratch.

This is toggled by the ISAAC_SIM_USE_STANDALONE_SCRIPT variable in the .env file. If set to false, the file specified by ISAAC_SIM_GUI is loaded directly. If set to true, the script named in ISAAC_SIM_SCRIPT_NAME is executed to generate the scene. It is generally recommended to use the scripted approach to generate new scenes (or at least the pegasus drone) since it handles a lot of the OmniGraph and sensor configurations automatically, then saving it, for future reuse and setting ISAAC_SIM_USE_STANDALONE_SCRIPT to false afterwards with ISAAC_SIM_GUI pointing to your saved file.

Scripted Scene Generation

Example scripts for generating a scene are provided in AirStack/simulation/isaac-sim/launch_scripts/ You can also write your own scripts for your own custom scenarios.

When using the scripted launch method:

  1. Define your desired script in the .env file and run 

    airstack up isaac-sim
    This will start Isaac Sim and generate the drone(s) within the scene.

  2. Stop the simulation by pressing the square “Stop” button on the left side of the Isaac Sim interface. This is necessary before editing the OmniGraph.

Stop Sim Button

  1. Open and connect the OmniGraph:
  2. Right-click the graph and choose “Open Graph.”

  3. Connect the ROS2Context node so that it feeds into all desired sensor subgraphs (e.g., cameras, LiDARs).
    Connected Subgraphs

    This manual step is temporarily required due to a known bug. Automation of this is planned in a future update.

  4. Save the scene and drones once everything is connected.

  5. You can save both the stage and the drone USDs for future reuse.
    Saving Scene

  6. Update your environment variables:

  7. Set the ISAAC_SIM_GUI variable to point to your newly saved .usd file (make sure to put the path within the docker container or in the omniverse server)
  8. Set ISAAC_SIM_USE_STANDALONE_SCRIPT to "false" to load this saved environment directly next time.

Known bugs and workarounds for Scripted Scene Generation

  • Stage Offset Bug
    In some cases, the saved USD file may contain an offset in the stage transform.

  • After launching the stage, manually set all position offsets to zero in the Property panel of Isaac Sim.

  • Pegasus Physics Update Prompt
    When reloading a saved stage, Isaac Sim may display a popup asking to update to new Pegasus Physics.

  • Accept the update to ensure compatibility with the new Pegasus physics scene.
    Update Physics

  • Stereo Camera Initialization
    Occasionally, the right camera in a stereo camera pair may fail to initialize.

  • When starting the sim, press Start → Stop → Start again in the Isaac Sim toolbar.

  • This will refresh the right camera node and restore proper stereo output.

  • Pegasus Node Not Recognized (Drone Not Arming/Taking Off) The drone not arming/taking off can be a symptom of the PX4Multirotor Node not being recognized in omnigraph. This may be due the pegasus.simulator extension not being loaded.

  • To fix, launch the simulator with airstack up isaac-sim, in the toolbar, click Window -> Extensions -> Third Party, serach for "pegasus", select the "PEGASUS SIMULATOR" and enable "AUTOLOAD"
  • Restart your docker container by running airstack down isaac-sim && airstack up isaac-sim and the extension should load every time now.