Simulators let you test a robot that you have not built, or test a risky move without breaking anything. Gazebo is the simulator most often used with ROS 2.
Key idea
A simulator advances the world in fixed time steps $\Delta t$: compute forces, update speeds, update positions, repeat. The real-time factor is simulated time divided by clock time: below 1 it runs slower than life, above 1 faster.
Why it works
Smaller steps follow the true motion more closely but take more work; too large a step can make a simulation blow up. A slow real-time factor does not make the physics wrong, it only takes longer to watch. The catch with all simulation: real motors, friction and sensors differ from the model, so a design must still be tested on hardware.
Worked example: Steps
1 ms steps, 10 s simulated.
- $10 / 0.001 = 10{,}000$ steps.
Answer: 10,000 steps.
Common mistake
Thinking a slow simulation gives wrong answers, or that a large time step is always fine.
Try it
Spawn a simple wheeled robot in Gazebo following the ROS 2 and Gazebo tutorials, drive it from the keyboard, and record its odometry topic. Then run your encoder-square test from the Navigation course in simulation and compare the drift with your real robot's.