[ceptimus]

You can buy modules for the Arduino and other systems that use low power radar or sonar to measure the height above ground more accurately than GPS or barometric pressure allows. They only work up to a range of a few metres but are useful when trying to get planes to auto-land.

There are also optical flow sensors (similar to the technology used by optical mice) that allow ground proximity and direction of travel to be monitored when getting close to the ground.

The USA spread the propaganda that its original cruise missiles used terrain recognition and other difficult-to-do stuff, but we now know that they were just dumb blind GPS-steered things - and that technology was perfectly good enough to do the job,