SITL

Software-in-the-Loop (SITL) is a simulation approach used in aerospace, robotics, and autonomous systems. In SITL, the software component under development—typically an autopilot flight stack such as PX4 or ArduPilot—is executed on a standard computer while interacting with a simulated environment that emulates the surrounding hardware and dynamics. The flight control software communicates with virtual sensors (IMU, GPS, barometer, magnetometer) and actuators through the same interfaces used on real hardware, commonly via MAVLink.

A SITL setup combines a physics or flight dynamics simulator with the autopilot. The simulator provides a virtual airframe, environment, and sensor data; the autopilot runs its control loops and mission logic as if it were mounted on a physical vehicle. Popular configurations use Gazebo, RotorS, or JSBSim for physics, along with MAVProxy or QGroundControl for ground-station interaction. SITL supports real-time or faster-than-real-time execution and can be integrated into automated testing pipelines.

Purpose and use: SITL enables development, testing, and validation of control algorithms, sensor fusion, planning, and fault handling without risking hardware. It is widely used in research, product development, and CI workflows, allowing repeatable experiments and scenario playback. It also serves as a bridge for integrating software-in-the-loop with hardware-in-the-loop (HITL) testing.

Limitations: While SITL can closely model many aspects of flight dynamics and sensors, it may not capture all real-world nonidealities, hardware quirks, or radio link issues. Careful validation against real flights remains necessary before field deployment.

See also: ArduPilot, PX4, MAVLink, Gazebo, JSBSim, hardware-in-the-loop.