An optical flow sensor tracks ground movement using a downward-facing camera, allowing drones to hold position without GPS. It can be used to determine speed when navigating without GNSS — in buildings, underground, or in any other GNSS-denied environment. The video below shows PX4 holding position using the Ark. Optical flow is foremost a human phenomenon, and it refers to our visual perception of motion, caused by either the movement of the observer or the motion of the objects in our environment. It works indoors, in urban canyons, and anywhere satellite signals are unreliable. To summarize, it is a locationing sensor, similar to a GPS. Why not just use a GPS you may ask? Well, if you plan on flying indoors, your GPS isn't going to work.