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Skydio has something like six cameras around it for this purpose, in addition to the main, gimble camera for footage.

Using a single downwards-facing camera for position tracking via optical flow pre-dates Skydio (the Parrot AR-Drone had it in 2012 I think)

Nearly all consumer drones have them.

It's a common technique.

visual inertial odometery can look at the ground to determine attitude along with imu